The basis and basics of mechanical ventilation

THE BASIS AND BASICS OF MECHANICAL VENTILATION

ABSTRACT.-The development of mechanical ventilators and the procedures for their application began with the simple foot pump developed by Fell O’Dwyer in 1888. Ventilators have progressed through three generations, beginnhxg with intermittent positive pressure breatig units such as the Bird and Bennett device in the 1960s. These were followed by second-generation units -represented by the Bennett MA-2 ventilator-in the 197Os, and the third-generation microprocessor-controlled units of today. During this evolutionary process clinicians recognized Types I and II respiratory failure as being indicators for mechanical vent&tory support. More recently investigators have expanded, clarified, and clinically applied the physiology of the work of breathing (described by Julius Comroe and other pioneers) to muscle fatigue, requbing ventilatory support. A ventilator classification system can help the clinician understand how ventilators function and under what conditions they may fail to operate as desired. Pressure-support ventilation is an example of how industry has responded to a clinical need-that is, to unload the work of breathing. All positive pressure ventilators generate tidal volumes by using power sources such as medical gas cylinders, air compressors, electrically driven turbines, or piston driven motors. Positive end-expiratory pressures! synchronized intermittent mandatory ventilation, pressure support ventilation, pressure release ventilation, and mandatory minute ventilation, are examples of the special functions available on modern ventilators. Modern third-generation ventilators use microprocessors to control operational functions and monitors. Because UXI,

June

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these units have incorporated the experience learned from earlier ventilators, it is imperative that clinicians understand basic ventilator operation and application in order to most effectively prescribe and assess their use. IN BRIEF Since 1888, mechanical ventilators have undergone three generations of development. First-generation devices included the basic intermittent positive pressure breathing units such as the Bird Mark 7 and Bennett PR2. Second-generation units were represented by ventilators such as the Bennett MA-2. Third-generation ventilators are microprocessor-operated and have more extensive monitoring and alarm packages than did earlier units. During the century of ventilator evolution, medicine gained more insight into the recognition and treatment of respiratory failure. Recent investigations have shown that providing a patient with ventilator support to relieve the work of breathing is not an all or none process and requires clinical judgment as to the levels of support that will not allow ventilatory muscles to atrophy. Total or partial muscle unloading through pressure supported ventilation is a useful means of decreasing the work of breathing. Pressure supported ventilation (PSV) is a technique in which the ventilator provides and maintains a preset positive pressure level throughout the inspiratory cycle. A contact (point) level of ainvay pressure is maintained by the ventilator through solenoid valves that automatically increase and decrease respiratory flow rates to produce a desired tidal volume. A delivered tidal volume of 10 to 12 mWkg is referred to as PSV-. Once this level of pressure support is reached, the work of breathing and the oxygen consumption by ventilatory muscles are all but eliminated. The new third-generation ventilators are much more sophisticated and require that operators have thorough understanding of theory of operation as well as various special modalities. In order to accomplish this clinicians must be able to classify ventilators. Ventilators are classified on the basis of several characteristics: method of generating the tidal volume; mechanism of triggering fmm exhalation to inhalation; mechanism for cycling fkom inhalation to exhalation; assessment of the inspiratory and expiratory phases of the breathing cycle; mechanism by which tidal volume is delivered to the patient; and special functions that can be performed (these may include positive end expiratory pressure or continuous positive airway pressure, intermittent mandatory ventilation IIMVJ, synchronous intermittent mandatory ventilation [SIMV], mandatory minute ventilation, PSV, and airway pressure release ventilation). 32s

DM,

June1991

Weaning patients from ventilators may be supported with the use of special ventilator functions such as IMV, SIMV, and PSV. Weaning techniques may include: l

l

l

l

Cold Turkey-Patient is removed from the ventilator and observed. Intermittent T-tubes-Patient using a T-tube is alternately placed off and on the ventilator, with each “off period” extended until ventilatory support is no longer required. IMV-Patient is placed on a specified IMV rate. The rate is then reduced 1 to 3 breaths per minute every 30 minutes until all mandatory breaths are eliminated and all breaths are spontaneous. PSV-Similar principle to IMV, except that the patient’s pressure support level is decreased 3 to 5 cm H,O at 30-minute intervals until a pressure level of 3 to 5 cm H,O is reached. At that point, the patient can be successfully removed from the ventilator. In all of the above techniques, it is assumed that the patient will be closely observed clinically and arterial blood gases will be monitored as necessary to confirm the patient’s physiological status.

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1991

323

ErnewnPOST-OP

Gwen

& Severs

FIG 1. Growth

and

development

of first-generation

ventilators.

and warning systems of the ventilator. These units also provided special functions such as pressure support ventilation, airway pressure release ventilation, and guaranteed minute ventilation. Thirdgeneration ventilators include the Bennett 7200, Bear 5, Bird 6400 ST, Hamilton Veolar, Amadeus, Engsttim Erica, and Siemens 9OOC. Fig 3 depicts the growth and development of third-generation ventilators.

-

L IMV Wwnxd monitoring

WC Pneumotron - Engstrom ECS 2000 - Dr;iger Spiromat XOK - SiemensSOOB - Monogham 225 SIMV - Gill 1 IMV

. r

2nd Generation

FIG 2. Growth 332

and

features

of second-generation

ventilators. DA4, June

1991

E 0 .E 2 2 zi ; 3 y

Neural based computing Artificial intelligence integral LO2 Integral metabolic5 Feedback control of function Inverse I:E Flow-by Dlsplayxree” MMV Apnea ventilation Digital communications Respiratory mechanics Pressure support Extensive monitoring The tntelligent ventilator -

r

3

Siemens 900C Puritan-Bennett

7200 series

- Air shields Oracle - Biomed K-S - Ohio CPU-l

I

.

- Hamilton Veolar Bird 6400ST Driger IRISA +- Infrasonic Adult Star Y I 1930

FIG 3. Growth

and

development

of third-generation

ventilators.

The use of computers in the design and subsequent operation of ventilators undoubtedly will result in development of even more advanced models that will incorporate patient trending data, automatically making adjustments to the ventilator (i.e., closing the loop). Technically, certain ventilators already exist that can provide closed loop ventilation. Hesitation on the part of clinicians in using this capability is based on suspicion of the accuracy of the data; for this reason the use of closed loop or “smart” ventilators will probably be delayed another decade. It should be noted, however, that many of the earlier ventilators (units such as the Bennett MA-l, Emerson Post-Op or Bear 2) are capable of supporting most patients who require mechanical ventilation.’ Even the negative pressure ventilators such as the tank and thoracoabdominal cuirass models have been known to be useful in treating patients with recurring episodes of acute hypercapnic ventilatory failure or dyspnea following exertion in home care settings.’ IDENTIFICATION RESPIRATORY

AND TREATMENT FAILURE

OF

Because it would require volumes to cover the entire topic of respiratory failure, this monograph focuses only on the use of mechanDM,

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1991

333

ical ventilation in that condition. There are, however, no clear-cut definitions that identify respiratory failure. Instead, there are generally agreed upon conditions that usually are present together or individually when a patient is diagnosed as being in respiratory failure. These include: acute dyspnea; PaO, < 50 mm Hg (with the patient on room air); PaCO, > 50 mm Hg; and pH < 7.35. PaO, is arterial oxygen pressure and PaCO, is arterial carbon dioxide pressure. There are two primary types of acute respiratory failure. Type I respiratory failure includes patients with hypoxemia and normocapnia or hypocapnia. These patients usually have acute lung injury such as adult respiratory distress syndrome. Type II failure describes patients with hypoxemia and hypercapnia. These patients primarily have chronic obstructive pulmonary disease or central causes of hypoventilation.

CAUSES

OF RESPIRATORY

FAILURE

Respiratory failure can be traced more of the following: l l l

l

l

l

l

to conditions

that affect one or

Brainincluding drug overdose, trauma, poliomyelitis Spinal cord-including Guillain-Barre syndrome, trauma Neuromuscular system-including myasthenia gravis, tetanus, neuromuscular blocking agents, antibiotics Thorax and pleura-including massive obesity, trauma, pneumothorax, pleural effusion Upper airway-including tracheal obstruction, sleep apnea, epiglottitis, laryngotracheitis Cardiovascular system-including pulmonary embolism, cardiogenie pulmonary edema Lower airway and alveoli-including aspiration, sepsis, bronchiolitis, chronic obstructive pulmonary disease, atelectasis, bronchiectasis

MUSCLE

FATIGUE

Historically, Gad in 1880 described how the Hering-Breuer reflex regulated the work of the respiratory system to achieve the most effective alveolar ventilation with the least amount of muscular effort. Julius H. Comroe, in his now classic 1965 text Physiology offlespiration discussed the work of breathing and its relationship to oxygen consumption3 Collectively, recent investigators have arrived at a similar conclusion: increasing fatigue and oxygen consumption by the muscles of inhalation will cause ventilatory failure. 334

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1991

AS early as 1973, Proctor and Woolson reported the relationship of muscle fatigue as a precipitating factor in respiratory failure.4 More recent studies by MacIntyre and his colleagues have verified the deleterious effects of muscle fatigue on the ventilatory pump.5 Still other investigations by Marini have demonstrated how the forces that increase the work of breathing can be quantified and minimized.” These and other publications have stimulated renewed interest in identifying and controlling the factors that individually or in combination lead to respiratory failure. Muscle fatigue and failure occur because of the involved work of breathing and, concomitantly, the ever-decreasing energy reserves of the compromised patient. In physics, work is described as force times distance or pressure times volume. The combined product of pressure and the volume of gas moved at a given instance is work (W = s PdV). The efficiency of muscles to perform this work may be described as E(S) = UF/TE X 100 where E = efficiency, UF = useful work, and TE = total energy expended. It is important to realize that energy (work) can be expended without actual movement occurring. This is the basis for isometric exercise. At the bedside, it is important for the clinician to understand the factors that influence ventilation. John Marini summarizes most eloquently the total relationship between the work of breathing and energy expended in the following quote. The connection between perceived effort and the external work accomplished in ventilation is a very loose one, depending on the neural stimulation to contraction, the force reserves, the coupling efficiency of the neural signal to muscular generation, the structural integrity of the ventilatory pump, the pattern (coordination) of muscular contraction, and the impedance to airflow. However, for any given individual, if all of these remain intact and unchanging (so that the overall efficiency of the system is unchanged), then the mechanical work of breathing correlates quite well with the oxygen consumed by the ventilatory task and with perceived effort. Although external work measurements are linked in a circuitous way to perception of effort, they bear an exact relationship to alveolar ventilation and inflation impedance. The “bottom line” regarding CO, exchange is to move the lung, and the energy cost to an external power source of achieving this goal is precisely correlated with the mechanical work of breathing measurenlent.7 One must not assume that it is always desirable to shift as much of the work of breathing as possible to a ventilator with the net result of totally unloading ventilatory muscles. Clinically this is not the case, and studies have shown that it may or may not be therapeutically desirable to eliminate all work during mechanical ventilation if the goal is to strengthen muscles to prevent atrophy.7 On the other

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hand, if the patient has ventilatory muscle fatigue as manifested clinically by tachypnea, use of abdominal muscles, retraction of soft tissues of the clavicle or intercostal spaces, and a rise in CO,, one may wish to completely unload the work of breathing by mechanical ventilation. Physiologically, muscle fatigue persists as an inability of the patient to maintain adequate diaphragmatic tension and a shift to a lower frequency on the electromyogram. One way of minimizing the work of breathing is to totally or partially unload ventilatory muscles during spontaneous ventilation through pressure support ventilation (PSV). According to MacIntyre and his group, total unloading is accomplished by providing the patient with enough PSV to negate the need for muscle contraction during inspiration and to produce a tidal volume of 10 to 12 mI.&g, defined as PSV,,.8 Once PSV,, is achieved, oxygen consumption by the ventilatory muscles to support the work of breathing is virtually eliminated. Partial unloading refers to a PSV level that reduces but does not completely eliminate the active use of the patient’s inspiratory muscles. Partial unloading is indicated to prevent atrophy in patients who may require ventilatory support for periods of 72 to 96 hours.’ It is suggested that up to 10 J/min electromyogram may be well-tolerated without the patient demonstrating adverse effects.8 This partial unloading approach enables one to customize the patient’s work of breathing to desired levels from initiation of mechanical ventilation to weaning. As with any treatment, exact levels of PSV (cm H,O) should be set to accomplish a specific task. For example, a PSV of 5 cm H,O is sufficient to overcome the resistance to air flow created by most adult-size endotracheal tubes. Details regarding the mechanics of PSV are presented in a subsequent section of this monograph.

TREATMENT

OF RESPIRATORY

FAILURE

Treatment of respiratory failure consists primarily of correcting the hypoxemia and/or hypercapnia without causing additional complications secondary to oxygen toxicity and/or barotrauma (associated with mechanical lung inflation). When mechanical ventilation is used to treat respiratory failure, two major goals are pursued: (11 alveolar ventilation must be maintained and (2) hypoxemia must be corrected. The decision to initiate mechanical ventilation is a clinical one, and should not rest on any isolated criterion. Criteria have, however, been used as general guidelines. These criteria vary, but the following are representative:

336

DA4, June

1991

l l l

l

l l l

Decreasing level of consciousness Vital capacity < 15 mIJkg body weight PaO, < 70 ton [with fraction of inspired oxygen (FIO,) = 0.4 on mask)] Alveolar to arterial diffusion of oxygen (A-aDO, > 400 torr (with FIO, = 1.0) PaCO, > 50 torr (in a previously normocapnic patient1 Dead space/tidal volume ratio Wd/Vt) > 0.60 Inspiratory force < -25 cm H,O Once a patient has been placed on mechanical ventilation, arterial blood gases must be measured to determine effectiveness of the ventilator in providing alveolar ventilation. Blood should be drawn approximately 30 minutes after the patient has been placed on mechanical ventilation. In order to assess changes, arterial blood gas values should be compared to preventilator measurements. A PaO, level of 50 to 60 mm Hg is considered acceptable with a PaCO, of 40 to 50 mm Hg and a pH of 7.35 to 7.50. Some patients, however, normally function with abnormally high PaCO, values, so a rapid depletion of PaCO, to laboratory normals may be hazardous. If changes are required in the ventilation values expected, the following simple equation may be used to determine new ventilatory parameters. Desired respiratory

rate = Prior rate X (Prior PaCO,/Desired

PaCO,)

If supplemental oxygen is required, the lowest possible FIO, to attain a PaO, of 50 to 60 mm Hg should be used. An FIOz greater than 0.5 for prolonged periods may cause oxygen toxicity. If an FIO, of 0.5 does not lead to adequate PaO,, one should consider the use of continuous elevated baseline pressure, such as CPAP or PEEP. One way of assessing the effectiveness of oxygen therapy is to determine the arterial/alveolar oxygen gradient. A useful abbreviated formula is: PAO, = PIO, - PaCO,/R where PIO, = barometric pressure (PB) minus water vapor pressure of (PB - 47) and R = respiratory quotient (assumed to be 0.8). Once the PAO, is determined, the a/A gradient can be calculated by: a/A gradient

DM,June

1991

= PaO,/PAO,

337

A normal a/A gradient is greater than 0.75. Given this gradient, the new PaO, that will result from a change in the FIO, can be predicted by using the following equation: Prior PaO,/Prior

THE PHYSICIAN’S

ORDER

PAO, = New PaO,/New

FOR RESPIRATORY

PAO,

CARE

The first step in meeting a patient’s ventilatory needs is to complete the physician’s order sheet. Standing or verbal orders should nor be used for implementing mechanical ventilation, except in emergency situations when no other option is available. A physician’s order for continuous ventilation should provide the following routine information: the type of ventilator (i.e., volume constant or pressure); cycling frequency (cycles per minute); oxygen percentage (FIO,); maximum pressure limit (cm H,O/torr); inspiratory to expiratory (I : E) ratio; and PaO, and PaCO, levels to be maintained. The following special information, where appropriate, should also be documented in the physician’s order: IMV rate (mechanical breaths per minute); PEEP level (cm H,O); levels of pressure support or pressure release; sigh volume and frequency; expiratory retard; medication administration (dose and frequency,; special drugs and instructions for delivery; and weaning instructions.

PREPARING

THE PATIENT

FOR MECHANICAL

VENTILATION

Patients must be prepared psychologically and physically for mechanical ventilation. Lack of control of the breathing process, combined with inability of the intubated patient to speak, is very frightening to both patient and family. Too frequently in the rush to provide quality care, clinicians overlook the patient’s needs as a feeling human being. The conscious patient must be carefully informed of the purpose of the ventilator, how it will aid in recovery, and the limitations it will place on the ability to talk, eat, or move about. The patient should also be gently coached in the best methods of breathing in conjunction with the ventilator and given other instructions on how to cough, move, acquire assistance, and go about other necessary daily tasks. The family or others who may be with the patient should also be informed of the process, its benefits, and possible complications. To gain the patient’s confidence, it is probably desirable to ventilate the patient by hand before switching to the ventilator. The patient who has been attached to the ventilator should not be left until stabilization has occurred and anxiety has diminished somewhat. Ideally, patients on ventilators should be in a special care unit or 338

DIM

June

1991

have special duty personnel to monitor their progress and make any necessary adjustments in the ventilator’s operation. The patient should be placed in a supine position initially, conditions permitting. Movement should be carefully planned so that stress is not placed on the airway or the patient’s attachment to the ventilator. For example, very small movements by the patient (e.g., flexion of the head) can cause the tracheal tube to move approximately 1.9 cm toward the carina. Head extension can cause the tube to move approximately 1.9 cm away from the carina. Lateral head rotation can cause the tube to move approximately 0.7 cm away from the carina. Once a patient has been positioned, bilateral breath sounds must be auscultated to determine that the tracheal tube has not been accidentally misplaced from a functional location in the trachea. If there is any doubt, tube placement should be radiographically confirmed. Patient comfort is an important consideration for ventilator patients who spend hours, days, or even weeks in the same location. Patients can lose their contact with reality and themselves. When this occurs, their will to live diminishes, they frequently become uncooperative, and their personalities weaken. The term most frequently used to describe this behavior is intensive care unit stress or syndrome. The most important service to the ventilated patient is a caring attitude, as demonstrated by frequent visits and special actions (e.g., rearranging a pillow or positioning breathing tubes so they are not resting on the patient’s chest). Not only are the visits necessary to assess the ventilator and to aid in patient comfort, they also help the patient’s mental state. METHODS

OF GENERATING

MECHANICAL

BREATHS

The principles applied for creating an artificial breath are not complicated. During inspiration, gas flow occurs and a tidal volume is delivered as a result of a difference in pressure (gradient) between two points (i.e., the ventilator and the lungs). Pressure gradients can be generated by many methods including: l

l

l

l

Squeezing (emptying) a compressible container (Figs 4 and 5) that has an opening to the atmosphere. An example would be a bulb, bellows, or bag-type ventilator (resuscitator). Rapidly spinning vanes, such as fan blades in a turbine; these unidirectional blades (Fig 6) push air from one point to another. Movement of a rigid piston causes air to leave a fixed-sized cylinder (Fig 7). The controlled release of gas (Fig 81 from a high pressure source such as a cylinder or wall outlet.

KJM, June

19Yl

339

FIG 4. Creating

l

a tidal

volume

by manually

squeezing

a gas-filled

bag.

Fluidics-an application of the Coanda effect, whereby a continuous flow of gas is switched back and forth by other gas impulses to cause inspiration and expiration. A fluidic ventilator uses gas channels and gas impulses to generate patient volumes and control functions.

PHASES OF VENTILATION The mechanical tificial ventilation l

ar-

Inspiration is the point at which a ventilator causes an exhalation valve in the patient’s breathing circuit to close and allows a flow of gas to pressurize the patient’s system.

FIG 5. Creating 340

actions performed by the ventilator to provide can be divided into four distinct phases.

a ttdal

volume

by compressing

a bellows. DA4, June

1991

FIG 6. Creating

l

l

a tidal

volume

with

unidirectional

rapidly

spinning

vanes.

Changeover from inspiration to expiration (cycling) is the point at which the ventilator interrupts the main flow of gas to the patient’s system and opens an exhalation valve-releasing pressure from the patient’s system and allowing the patient to exhale. Expiration begins and continues from the point when the main ventilator flow is stopped (or interrupted) and the exhalation valve in the patient’s breathing circuit is opened until the exhalation valve is once again closed.

FIG 7. Creating Uhf, June

a tidal 1991

volume

by a piston

pushing

air from

a cylinder. 341

FIG 8. Creating outlet.

l

a tidal volume

by controlling

gas

delivery

from

a compressed

gas cyhnder

or wall

Changeover from expiration to inspiration or “triggering” occurs whenever a ventilator switches from exhalation to inhalation. This can be automatic or can be initiated by a spontaneous breathing effort. Either way, the point at which the exhalation valve is closed in the patient’s breathing system and the ventilator’s main flow begins is the changeover phase.

The various phases of a mechanical ventilation cycle can be seen and/or recorded by attaching the patient to a strip chart recorder or oscilloscope. Fig 9 represents a typical pressure curve for a pressurecycled ventilator attached to a spontaneously breathing patient. This pressure curve can be used to visualize the various phases of a mechanical breath. When using an x-y graph, pressure points above the baseline (marked zero) are positive ( +) and, below the baseline, subambient I- ). At point 1 in Fig 9, the patient has taken a spontaneous breath, causing the curve to drop below baseline. At this time, the exhalation valve has closed but no gas is flowing from the ventilator. This conclusion is based on the continued downward movement of the pressure curve. At point 2 the exhalation valve is closed and a mechanical breath has begun. This is represented by the fact that the pressure curve has started an upward movement. The time elapsed between point 1 and point 2 is the ventilator’s response time, or the patient’s triggering effort. This time delay between initial inspiratory effort and the beginning of ventilation is critical, because it repre-

MechanIcal

Mechamcal begms

breath

I Mechanical breath begins patient initiation

breath (2)

at (5)

FIG 9. Typical

pressure

curve

generated

by a pressure-cycled

ventilator

durmg

Inspiration.

sents patient work and wasted ventilatory volume. Muscles moved to generate a breath, but breathing did not occur. This is a primary clinical consideration when one is concerned about unloading the work performed by ventilatory muscles. The time delay factor can be adjusted in most ventilators by adjusting the sensitivity control to a more sensitive position. Once begun, inspiration continues until the main flow of gas is interrupted (point 3). At this time, the exhalation valve opens and the patient begins to exhale passively, as represented by a downward movement of the curve. Exhalation (point 4) is the point at which the curve starts downward. This phase continues until the patient takes another breath (point 51. In apneustic patients, the pressure pattern appears as represented in Fig 10. The primary difference between Figs 9 and 10 is displayed at points 1 and 2. In the controlled situation, the patient is not breathing and the pressure curve does not drop below baseline levels because the ventilator is automatically cycled. In positive-to-positive phase ventilators, pressurization begins at the baseline (point 6) during the control mode of operation. Here, exhalation is defined as the point at which flow is interrupted (point 7) and continues until flow begins again (point 8).

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1991

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Mechkical breath begins

(6)

FIG 10. Typical

pressure

CLASSIFYING

curve

for a positive

pressure

ventilator

operating

as a controller

VENTILATORS

BASIC PERFORMANCE

DIFFERENCES

Most simply defined, mechanical ventilators are devices that help patients breathe. They accomplish this by assisting the spontaneously but inadequately breathing patient to take a deeper breath or by automatically delivering a breath (controlled breathing) to the apneustic patient. Therefore, by function and by definition, a ventilator can be called an assistor if it helps one who is already breathing and a controller if it assumes the breathing function. When a ventilator is referred to as an assistor or a controller, the clinician can immediately identify one of two important operational features of the ventilator. Such a verbal description of a ventilator’s operational characteristics is referred to as “classiIication of the ventilator.” The following descriptions present various methods for classifying ventilators. It is important that clinicians understand these methods so that they will be able to assess the functional and operational characteristics of a ventilator based on its design specifications before it is purchased and before it is used on a patient. Understanding classification also allows one to evaluate how a ventilator should perform on a patient and therefore to assess whether the ventilator is operating properly under clinical conditions. All mechanical devices can and will fail at some point as they reach and exceed their designed performance limits and as malfunctions occur. It is only 344

DM,

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1991

through careful observation and applied knowledge of ventilators that these potentially dangerous patient care situations can be prevented or corrected. The specific method for classifying a ventilator varies according to the point of view of the person describing the ventilator. For this reason there are numerous classification methods. Most describe how the ventilator generates the tidal volume and. the characteristics of gas flow that result in delivery of this volume to the patient, as well as other performance characteristics. The following classification terminology is general enough that it can be used to describe (classify) most of the ventilators available for patient use today. CLASSIFICATION

OF VENTILATORS

Ventilators are generally classified according to: their method of generating tidal volume; the mechanism by which they trigger from exhalation to inhalation; the mechanism by which they cycle from inhalation to exhalation; assessments of the inspiratory and expiratory phases of the breathing cycle; the mechanism by which the tidal volume is delivered to the patient; and special functions they can perfonng Methods of Generating Tidal Volume During inspiration, a ventilator may be classified as a constantnonconstant-flow generator or a constant or nonconstant-pressure generator.

or

Constant-Flow Generator.-The pure constant-flow generator (CFG) is a type of unit that produces and maintains a continuous rate of gas flow to the patient (measured in liters per minute) during inspiration, regardless of increasing impedances in the ventilator/patient system distal to the ventilator. To accomplish this, the ventilator must use an unlimited high pressure source (head) that will always be greater than any distal impedances to gas flow created by ventilator circuits and the patient’s airways. To protect the patient against direct exposure to a high pressure source, the ventilator system incorporates a fixedsized orifice that causes the internal pressure to decrease as it crosses the orifice and enters the patient’s breathing circuit. Even though the internal ventilator pressure is decreased on the distal side of the orifice, the potential pressure head on the proximal side of the orifice is such that a large pressure gradient is always present between the ventilator and the patient’s airway. This pressure gradient, as long as it is maintained, results in a constant gas flow to the patient. In Fig 11, graphs A through C show typical pressure, flow, and volUA4, June

1991

345

FIG 11. Pressure,

flow,

and

volume

wave

forms

for a constant

flow generator.

ume patterns for a CFG when plotted on an x-y axis. In graph A, a pressure is generated and constantly maintained to generate the high level of flow shown in graph B. Because this flow rate is constantly maintained throughout inspiration, a straight line is plotted on the flow graph. By subtracting the portion of the tracing related to the initial opening of the valve (point 1) and the venting of the valve causing exhalation (point 21, one can easily see the square wave flow pattern that is characteristic of constant-flow generators. Volume ventilators such as the Bennett MA and 7200 series and the Bourns BEAR series generate this type of flow pattern and therefore are classified as constant-flow generators. It is important that clinicians understand that a constant-flow generator delivers a constant flowhence constant volumeonly as long as the internal driving pressure of the ventilator exceeds the distal impedances to gas flow. Whenever distal impedances due to airway obstruction, bronchial spasm, patient’s body weight, or even mechanical blockage in the ventilator tubing or patient’s tracheal tube become excessive (i.e., equal to or greater than the internal driving force of the ventilator), the constant-flow generator will cease to 34s

DM,June

1991

operate as such, and a desired tidal volume may not be delivered. As this point is reached, the flow pattern will become less of a square wave and will come to resemble the peaked sinusoidal wave of a constant pressure generator. All ventilators have a maximum performance level at which point the ventilator ceases to operate as it was designed. Clinicians must be aware of a ventilator’s limitations and performance characteristics under varied clinical conditions so that dangerous assumptions will not be made as to the ventilator’s performance capabilities under changing clinical conditions. Nonconstant-Flow Generator.-A nonconstant-flow generator is a type of ventilator that delivers the same inspiratory flow pattern to the patient regardless of changing airway characteristics. The Emerson 3-W and Engstrom ventilators are examples of nonconstant-flow generators. The consistency of this pattern is created because a piston powered by an electric motor is used to deliver a preset volume of ait from a rigid cylinder (Fig 1.2). Because the piston (11 is linked to a fixed rotating wheel by a rigid rod (21, the flow pattern that is created is always the same as the piston moves up and down causing inspiration and exhalation. The gas volume is varied by changing the distance that the piston travels within the cylinder to empty the preset volume. As the cam, which is attached to a wheel, rotates (31, the piston movements are smaller at the beginning of inspiration and at the end of inspiration. This causes a gradually increasing flow rate as

Piston

(1)

Wheel

(3)

Rod (2)

FIG 12. Mechams DMJune

of a nonconstant 1991

flow generator

piston 347

the wheel continues to rotate through 90 degrees and a decreasing flow rate as it reaches the end of its compression stroke at 180 degrees. The greatest volume of air is moved as the wheel and cam pass through 90 degrees of arc. This type of wheel rotation produces a sine wave-type flow pattern such as that illustrated in Fig 13. Here, inspiration is represented by the (+ 1 sign and the gradually and decreasing half- sine wave pattern above the baseline (11. Exhalation is represented by the (-1 sign and the half-sine wave below the baseline (2). The peak flow rate (3) is generated during inspiration as the piston in the ventilator reaches its point of greatest movement. Once peak flow has been delivered, the piston in the ventilator continues to travel, but it travels a much shorter distance, causing a decaying flow-wave pattern that eventually reaches the baseline or zero as the piston reaches the end of its preset stroke distance (4). Exhalation occurs as the piston withdraws (5) to pull in fresh gas for another inspiration. This portion of the waveform is not generally displayed by most textbooks because during the recovery period of the piston, the patient is exhaling and the breathing circuit is vented to the atmosphere. Fig 14 represents a basic design for a piston-type ventilator such as the Emerson 3-W. In this illustration, the patient’s volume is determined by the distance the piston (1) moves upward in the cylinder (2). The inspiratory flow rate is determined by the speed at which the piston moves upward, so that the maximum volume that could be delivered by this ventilator is 2000 ml or a full stroke of the piston. As the piston starts upward, an increasing gas pressure in the cyl-

Piston withdraws

(5)

FIG 13. Flow 348

wave

pattern

of nonconstant

flow

generator. DM,

June

1991

One-way valve open allowing gas flow to patient (5)

Breathing

Air entrainment valve (closed) (4) pushed

Piston up (1)

Cylinder

(2) -

A

B

FIG 14. Mechanics

of a nonconstant

flow

generator.

inder causes gas to leave a port (3) to close the patient’s exhalation valve located in the breathing circuit, to close the air entrainment valve (41, and to open a one-way valve (5) that allows the main gas to flow to the patient. As the wheel cam (6) continues to rotate, the piston is pulled downward 01, causing room air to be entrained at the valve (8). Simultaneously, the one-way valve (91 is seated to prevent a negative pressure from being pulled into the patient’s airway by the descending piston, and the exhalation valve is opened (101, allowing air to leave the patient’s lungs. Inspiration begins again as the wheel continues to rotate, causing the piston to start upward. One of the theoretical physiologic advantages of this type of ventilator is that maximum gas flow occurs simultaneously with a maximum distending airway pressure. Also, inspiration does not end abruptly; rather it decays as the descending side of the sine wave is generated. This exposes the airways to a slowly decreasing pressure and flow, which may increase gas distribution and delay premature closure of small airways. It is important to note that with this type of ventilator, if the pressure relief (“pop off’) valves are bypassed, the patient will receive tidal volumes at a pressure equal to the force generated by a powerful electrical motor-a clearly hazardous situation. Constant-Pressure Generator.-A constant-pressure generator (CPG) is a type of ventilator that produces and maintains the same ventilator system pressure level throughout inspiration, regardless of DM,

June

1991

349

changes in the patient’s airway (i.e., the pressure is preset). To accomplish this, the ventilator must use an unlimited source of pressure when compared to possible alveolar pressure so that a pressure gradient exists and gas flow occurs. In a true CPG, the ventilator pressure and internal resistance are usually low and approximate those in the patient’s lungs at the end of an inspiration. The inspiratoty flow rate begins high, gradually decreasing during inspiration as the pressure gradient declines between the patient and the ventilator. In this type of device, flow can be interrupted by equalization of the pressures between the ventilator and the patient or by time. In either case, a predetermined volume may or may not have been delivered. Typical pressure, flow, and volume curves for a CPG, when plotted on an x-y axis, are presented in Fig 1.5. In graph A, the ventilator produces a high initial flow rate (11, which slows as the pressure gradient decreases within the lung during inspiration (2). In graph B, as the ventilator cycles on, the high inspiratory flow rate results in an increasing airway pressure, which causes an increasing lung volume. IPPB devices, such as the Bird Mark 7 or Bennett valve-type ventilators, perform as constant pressure generators whenever they are operated without air entrainment. The operation of a CPG is not to be confused with PSV, in which the gas flow is automatically adjusted by a servo mechanism to maintain a preset pressure level from the beginning to the end of an inspiratory cycle. Nonconstant-Pressure Generator-.-A nonconstant-pressure generator is a ventilator that automatically changes the pressure of gas delivered to the patient’s breathing circuit in response to changing

FIG 15. TypIcal 350

pressure,

flow,

and

volume

curves

for a constant

pressure

generator. Uhf,

June

1991

airway conditions. In this type of device the pressure patterns remain the same even though the airway pressure levels vary. This type of ventilator is rarely used today. The Mechanism for Cycling from EFhalation to Inhalation A general method of classifying a ventilator is to describe the mechanism by which inspiration is initiated. As previously described, ventilators may be identified as assistors, controllers, or assist-controllers. Most ventilators today have both assist and control capabilities. The Mechanism for Changing from Inhalation to Exhalation The variables that can cause a ventilator to change from inhalation to exhalation (cycling or switchover) include time, pressure, and volume. One or more of these variables are incorporated by various Qypes of ventilators. Time-cycled Ventilators.-Time is a constant that can be determined by the operator. In a time-cycled ventilator, inspiration is allowed to continue for a preset time interval. Exhalation begins when the inspiratory gas flow is blocked or stopped, regardless of airway pressure or the volume delivered. In a true time-cycled device, inspiration will end regardless of whether the ventilator is attached to the patient and regardless of the patient’s lung characteristics. A time-cycled ventilator can function as a volume ventilator provided a long enough period is allowed for inspiration to occur and enough pressure is generated to cause a volume to be delivered. To accomplish this, time-cycled devices usually incorporate a high working pressure that allows the ventilator to create an airway pressure gradient necessary to generate a gas flow, irrespective of changing airway impedances. Examples of time-cycled ventilators are the Emerson 3-W volume ventilator and the Siemens Servo 900 series ventilators. It must be remembered that with these units all ventilation occurs as a function of time. This time function results in calculation of a ratio between the time allowed for inspiration and that for exhalation and is known as the I : E ratio. This ratio is usually not allowed to become less than 1 to 1.5, except in special circumstances in which inverse I: E ratios are established by the operator (e.g., to allow for better diffusion and increased oxygenation). Clinically, it is important to realize that a time-cycled ventilator may provide a complete ventilatory cycle without generation of an actual tidal volume. For this reason, exhaled volume must be accurately monitored whenever the unit is being used on a patient. Pressure-cycled pressure, describes OM,

June

1991

Ventilators.-Switchover a situation in which

(cycling), caused by inspiration continues until 351

chamber such as a bag or bellows. A ventilator that delivers gas to the patient directly from its power generator (gas or piston) is a single-circuit device. Characteristically, this type of ventilator can generate greater pressure levels than a double-circuit system. Also, the wave patterns are different from those generated by a double-circuit device because of the dampening effect caused by the bellows, bag, or other type of intermediate chamber. A double-circuit ventilator delivers gas from its power source to empty a bag or bellows that, in turn, delivers its contents to the patient. In this context a double-circuit device employs a figurative “pneumatic hand” to compress a bellows (as in the Bennett MA-l or 2 ventilator) or to squeeze a bag (as in the Engstrijm ventilator). Understanding the mechanism by which the tidal volume is delivered to the patient is clinically significant, since it influences mechanical deadspace (i.e., compressible volumes) and determines how the ventilator will perform when impedance increases. Classification According to Special Functions Special ventilator functions include IMV, SIIW, CPAP, PEEP, and more recently with the new third-generation devices, pressure support ventilation, airway pressure release ventilation, and mandatory minute ventilation. Intermittent Mandatory Ventilators.-IMV describes a ventilator function that allows the patient to breathe spontaneously without assistance and, at predetermined intervals, delivers a mechanical breath. The rationale for using this type of system is that it: allows patients to adjust their own PaCO,; decreases the need for suppressant drugs; facilitates transition from mechanical ventilation to spontaneous breathing (weaning); can be used with high and low levels of PEEP; helps stabilize FIO,; decreases the duration of ventilator use by reducing psychologic dependence and helps patients maintain ventilatory muscle tone; and decreases 0, consumption caused by the patient “fighting the ventilator.” Fig 17 illustrates a typical Ih4V breathing pattern, showing the patient’s spontaneous breath (11, followed by a periodic mechanical breath delivered by the ventilator (2). After this breath, the patient returns to spontaneous breathing (3). Technical details on the use of IMV are presented below. Synchronized Intermittent Mandatory Ventilators.-SIMS, sometimes called intermittent demand ventilation, describes another technique for applying IMV. With this method, the delivery of the mechanical breath is synchronized to the patient’s spontaneous breathing attempts. This prevents the possibility of “stacking a breath” (i.e., a mechanical breath delivered on top of a spontaneous inspiration). Breath stacking can be especially dangerous because it 354

OM, June

1991

P

Mechanical breath (2)

3

Time Spontaneous breath (1)

FIG 17. Typical

IMV breathing

pattern

can lead to barotrauma pressure.

secondary

to excessive

lung

volume

and

Continuous Positive Airway Pressure.-CPAP may be used in conjunction with a mechanical ventilator or without it. A CPAP system uses a high-pressure reservoir and a constant flow of gas that exceeds the patient’s inspiratory peak flow demands. With this system the patient breathes a constant elevated baseline pressure. An example of a typical face mask CPAP system is presented in Fig 18. In this example, an air/oxygen blender (21 is connected to source gases (1) that are administered via a flow meter (3) that is capable of delivering 70 to 90 IJmin through an adapter (41 to a 5 L anesthesia bag (6). The flow rate is adjusted to keep the anesthesia bag fully distended, forcing gas to flow past a one-way valve (51 to the inlet side of a heated humidifier (7). Humidified gas leaves the humidifier via the outlet (81 and is breathed by the patient, who is attached to an anesthesia-type mask (9). This mask is held in place by a four-prong retaining strap (10). During inspiration, gas is inhaled by the patient at a preset positive pressure level, as determined by the distention (fullness) of the anesthesia bag and height of the water column in the PEEP valve (13). Exhalation is permitted only to the point that the positive airway pressure force created by the patient during exhalation is able to overcome the weight of the water holding the exhalation valve closed (14). At this point, the rubber diaphragm is lifted and exhalation occurs (15). When the airway pressure generated by exhalation no longer exceeds the downward resistance caused by the weight of the water on the valve, the valve closes, causing the breathing circuit to equalize with the force generated by the patient through distension of the anesthesia bag. Nasal CPAP units are commercially available that contain valves UM, June

1991

355

FIG 20. Pressure

tracing

of a ventilator

dung

PEEP

patient will pull gas at the preset FIO, from the reservoir. As the FIO, is enriched, the reservoir tube needs to be lengthened. A rule of thumb is to keep the volume of the reservoir tube at approximately one and a half times the patient’s tidal volume. If a decrease in FIO, is noted at the patient’s airway, the reservoir volume should be increased until the desired FIO, is achieved. When PEEP is used with a mechanical ventilator (Fig ZO), the pressure baseline shifts upward from ambient (1) so that the inflation pressure caused by the mechanical breath begins at the PEEP level (2). In this example, the PEEP level is 10 torr. Inflation pressure caused by the mechanical breath becomes a combination of the PEEP pressure and the pressure required to deliver the tidal volume (3). As the volume is being delivered, the ventilator system manometer reflects an inflation pressure of 25 torr. During inspiration and exhalation the baseline does not return to ambient, causing some to comment that this method should be called CPAP instead of PEEP. In cases such as this, PEEP describes any method of attaining an expiratory elevated baseline pressure whenever the patient is attached to a ventilator. One needs to be aware that high levels of PEEP combined with large tidal volumes, can cause excessive airway pressures, leading to possible barotrauma, decreases in cardiac output, and other pressure-related complications. TRADITIONAL DISTENDING

SYSTEMS FOR GENERATING AIRWAY PRESSURE

CONSTANT

Constant distending airway pressure (either CPAP or PEEP) is generated by systems that cause exhaled gases to be directed into a tube that is submerged in water (Fig 21); against an exhalation diaphragm that is regulated by a spring (Fig 22); in opposition to an exhaled gas flow (Fig 23); or against a weighted exhalation diaphragm

358

Uhl, June

1991

OW-wa~ valve16)

lOcmH,O

I

FIG 21. TradItIonal

method

of generatlng

CPAP

and

PEEP with

tube

under

water

Adiustable spring (1)

FIG 22. Generating

llM,June

CPAP

1991

and

PEEP with a spring-loaded

exhalation

diaphragm

359

Expiratory breathing

limb of tube (2)

FIG 23. Generating

CPAP

and

PEEP with

exhaled

gas

dtrected

against

opposing

gas

flow.

(Fig 24). The following explanation and illustrations focus primarily on the expiratory side of a breathing circuit. Fig 21 (1) represents a patient who is exhaling past an open gas collection-type exhalation valve (2) into a tube with one end attached to the expiratory port of the exhalation valve (3) and the

lScmH,O

.Rubberdiaphragm (closed position1

(21

FIG 24. Generating 360

CPAP

and

PEEP with

a weighted

exhalation

diaphragm UM,

June

1991

other submerged in water (41. To exhale to the atmosphere, the patient must first generate enough positive pressure on exhalation to force water out of the submerged tube so that air can escape to the surface. The amount of PEEP or CPAP generated is equivalent to the distance, in centimeters, that the tube is submerged under the water. In this model, the tube is submerged 10 cm, as measured by a metric ruler (5). This requires the patient to generate enough positive force on exhalation to displace the mass caused by 10 cm of water (i.e., generating 10 cm H,O pressure CPAP or PEEP during exhalation with each breath). The one-way valve (6) prevents the ambient pressure from being directed back into the ventilator breathing circuit . In Fig 22, the principle is the same, except that the patient must now exhale against the force caused by an adjustable spring (I) that attempts to hold the valve closed during exhalation. The amount of expiratory positive pressure the patient must generate to overcome this resistance (2) and open the closed valve can be read directly from a pressure manometer inserted in the circuit (calibrated in cm H,O or ton-J. Positive end expiratory pressure generated by an opposing gas flow is a concept incorporated in the Bird CPAP or PEEP system. In Fig 23, a gas flow (1) is directed against the expiratory gas as it enters the expiratory limb of the breathing circuit (2). The opposing gas flow causes a resistance to exhalation that the patient must overcome. Expiratory pressure generated to overcome the increased resistance results in a positive force that can be read on the system manometer (in cm H,O or torr). CPAP or PEEP levels can be increased or decreased by altering the flow rate of the opposing gas flow. Fig 24 represents a system that is a variation on the “tube under water.” The principle of operation is the same, except that the weight of the water is determined by its height in a column (1) supported by a rubber diaphragm (2). As the patient exhales, the diaphragm is held closed by the water until enough expiratory pressure is generated by the patient to force the diaphragm with its weight of water away from its seat (31, allowing exhalation to occur. In this system, the higher the water in the column, the greater the PEEP or CPAP in cm H,O. Valves used to generate PEEP or CPAP can be classified as either threshold-resistor or flow-resistor valves. Most ventilator systems use threshold-resistor valves because they allow expiratory pressure to remain relatively constant in the face of changing expiratory flows. This is not the case with flow-resistor valves, in which expiratory positive pressure varies directly with changing flow rates (i.e., the greater the expiratory flow rate, the higher the levels of CPAP or PEEP). Examples of threshold-resistor valves include the Bird CPAP

demand system and the Emerson water column PEEP valve. Examples of flow-resistor valves include the Siemens ventilator PEEP valve and the Babybird outflow valve. Fig 25A illustrates a traditional IMV setup. Understanding this type of system enables one to better comprehend the newer systems that employ more sophisticated flow demand valves. In this type of system, an oxygen-air blender (1) delivers a constant flow of gas at a predetermined FIO, to a reservoir bag (2) and the IMV circuit to the point of the one-way valve (3). As the patient takes a breath (41, the pressure in the breathing circuit decreases, causing gas to flow (5) through the one-way valve to the patient. This flow continues with the patient drawing gas from the reservoir bag (2) until spontaneous inspiration ends. The ventilator (6) has been adjusted not to respond to the patient’s inspiratory efforts and remains off. As inspiration ends, expiration begins (Fig 25B), with the patient exhaling (7) to close the one-way valve (3) that stops gas flow from the reservoir bag. The ventilator (6) is still cycled off and the patient is forced to exhale against a fixed resistance on the expiratory limb of the breathing circuit, which causes CPAP to be generated (8). Fig 25C illustrates the ventilator, which has been set at a slower cycling rate than the patient is breathing. Cycling the ventilator on

FIG 25. Traditional mechanlcai 362

tMV system. breath.

A, during

inhalation;

B, during

exhalation;

and

C, with

DXI,

controlled

June

1991

causes a mechanical breath to be delivered to the patient (9). As the positive pressure breath is delivered, the one-way valve (3) is forced closed, preventing pressure from escaping into the reservoir bag. Simultaneously, the patient receives a preset tidal volume and the ventilator cycles off. This breath is subsequently exhaled against the fixed resistance, causing PEEP to occur, since the patient is not permitted to exhale to ambient before the resistance valve closes. With the ventilator off once again, the patient reverts to a spontaneous breathing mode incorporating the IMV system described in Fig 25A. If a ventilator is not equipped for delivering IMV at the factory, there are many commercial systems available. As previously stated, the primary difference between these systems and the traditional one described above is the use of a demand valve to provide the constant gas flow in lieu of the 3-L anesthesia bag and flow meters.

PRESSURE

SUPPORT

VENTILATION

Pressure support ventilation (PSV) describes a procedure that was introduced in the mid-1980s to complement the ventilatory efforts of a spontaneously breathing patient. This technique requires a ventilator that has a pressure-support feature and a patient that is capable of triggering his or her own ventilator breath. Simply described, once inspiration is triggered by the patient, the ventilator automatically adjusts gas flow rates to provide and maintain preset inspiratory support pressure (through the use of flow sensing devices). Fig 26A shows the preset PSV level of 15 cm H,O. In addition, a $5

PWSSUW (cm HzO)

FIG 26. Pressure graph. A shows wave with PSV of 15 cm H,O; B shows and C shows wave generated by a conventional IPPB unit. m4,June

1991

wave

with PEEP added;

363

cm H,O PEEP is added (B). The dotted line (C) represents the waveform generated by a conventional IPPB unit. Fig 27A shows that flow rates are delivered by the ventilator to initiate and maintain the preset level of PSV throughout the inspiratory phase. The ventilator rapidly responds to the patient’s spontaneous effort to initiate inspiration. PSV levels can vary between 1 and 100 cm H,O pressure depending on the ventilator model. The inspiratory phase of PSV is terminated when a certain minimum inspiratory flow rate is reached or whenever an excessive airway pressure is detected. For example, on the Servo 9OOC ventilator, inspiration ends when the flow decreases to 25% of the peak flow generated for the inspiratory phase or the inspiratory pressure rises to +3 cm H,O above the preset inspiratory pressure. PSV differs from the traditional assisted ventilator mode in several ways. With the beginning of inspiration, an accelerated inspiratory flow rate is delivered by the ventilator so that the PSV level is immediately reached (Fig 27A). As inspiration continues, flow rates are automatically adjusted to maintain a constant preset pressure level throughout inspiration. Also, with conventional IPPB or controlled mechanical ventilation, airway pressure levels gradually increase during inspiration, reaching peak pressure just before the end of the inspiratory cycle. During inspiration, airway pressure fluctuates with airway patency and compliance and resistance changes (Fig 27B). The physiologic rationale for PSV includes the fact that it interacts

mspwatory effort

-v

FIG 27. A IS a line graph flow rate pattern 364

showing flow of a conventlonal

rates to provide IPPB unit.

PSV;

and

B shows

the accelerating

gas

DA4 June1991

with mechanoreceptors (pressure receptors) located in the lungs and chest wall to enable patients to establish a more physiologic ventilatory control over rate, tidal volume, and flow rate. It also causes patients to actively participate in their ventilation. PSV enables patients to maintain control over ventilation and to use their muscles without the undesired stress of fatigue that may occur with unassisted and controlled work of breathing. PSV used with intermittent mandatory ventilation combines the benefits of having the patient breathe independently, with the added assurance that a minimum level of minute ventilation can be mandated by the ventilator. Clinically, PSV may be most useful for patients who are being weaned and in situations in which it is desirable for a patient to assist in ventilation without the added fatigue that accompanies the work of breathing. PSV alone probably should not be used on patients with an unstable ventilatory drive, excessive secretions, bronchospasm, frequently changing airway resistance and compliance, or severely weakened physical condition. Further research is necessary to document the various clinical applications of PSV. AIRWAY PRESSURE

RELEASE VENTILATION

The purpose of airway pressure release ventilation is to allow the lungs of a spontaneously breathing patient being maintained by CPAP to briefly vent to ambient pressures at the end of a ventilatory cycle, then rapidly reinflate to regenerate the elevated baseline with the subsequent inspiration. Theoretically, the physiologic advantage of this modality is that the brief interruption of elevated airway pressure at the end of each exhalation facilitates venous return and elimination of CO,. An important consideration is that with airway pressure release ventilation peak airway pressure never exceeds the CPAP level. This should result in fewer incidents of barotrauma than occur with conventional ventilation. MANDATORY

(MINIMUM)

MINUTE

VElNTILATION

Mandatory minute ventilation is a relatively new modality that enables the ventilator to automatically adjust a patient’s IMV rate or level of pressure support to maintain a preset minute volume. The purpose of mandatory minute ventilation is to decrease the weaning time for a ventilator patient. Examples of patients in whom mandatory minute ventilation may be useful include those with drug overdose or postoperative respiratory depression. It is important to note that mandatory minute ventilation does not substitute for mandated breaths in situations in which apnea may occur. DM,

June

1991

365

Third-generation ventilators have monitors that constantly or periodically assess the operation of the ventilator and the patient’s status. These monitors are generally linked to alarms that notify the operator audibly or visually whenever a variance from a norm occurs. A problem is that a “Peter-and-the-Wolf’ syndrome has developed as a result of the number and frequency of false positives that occur with the alarms. All too frequently clinicians either ignore or bypass the alarms to avoid needless disruptions. Kacmarek and Meklaus have estimated mathematically that a 0.64 probability exists that any actiTABLE

1.

Monitors

and Alarms

Monitoring

Function

Source

pressure

Failure

to cycle

Standby

Reason

(air/oxvgen

or power)

iventilation

High-system

pressure

Low-system

pressure

Low

PEEPCPAP

Low

exhalation

volume

Apnea

I: E ratio

Oxygen

percent

Temperature Auto-PEEP

only

366

level

the Omeda

Advent

and Servo YOOC and

for Alarm

Drop in inlet pressure caused by cylinder depletion or wall piped service or loss of electrical power. Electrical failure or ventilator failure. Some ventilators incorporate battety backups. Ventilator “on” but not delivering breaths for a set time period (e.g., 60 s) Breathing system pressure exceeds preset level and is being dumped on inspiration. Breathing system pressure has not been reached or has dropped below a preset level during inspiration (such as occurs with a patient disconnect). PEEP or CPAP levels are less than that preset by the control. Exhaled tidal volume or minute volume is less than selected for the patient. Failure of the patient to breathe spontaneously, or of ventilator to deliver a breath in a preset time period. Ventilator fails to maintain preset inspiratory to expiratory (I:E) ratio, or ratio becomes less than 1: 1. Failure of ventilator to deliver preset FIO, or FIO, drops below 20% (such as occurs with connection to gas source other than oxygen). ‘Temperature of gas delivered to patient is not within the range preset on the humidifier. Inadvertent elevated airway pressure is maintained on exhalation, causing the patient’s functional residual capacity to be accidently elevated. 900E monitor

auto-PEEP.

DM, June

1991

vated alarm is a false positive.‘” This is extremely disruptive, especially in critical care units, where the potential exists for activation of such a wide variety of alarms. Typical monitoring and alarm systems on ventilators are shown in Table 1. For details regarding the characteristics and performance of specific ventilators, the reader is referred to factory operating manuals. It is axiomatic that patients on life support must be watched by qualified people and not just by alarms and monitors that can and do fail. VENTILATOR

VARIABLES

Modern ventilators offer the user numerous combinations ational variables. All of these can be confusing to clinicians not familiar with a particular unit or who do not frequently different clinical situations. Most volume ventilators offer combinations of operational variables; the controls for some are listed and explained below.

of operwho are use it in various of these

TYPICAL CONTROLS Mode control (also called function) enables the operator to select how the ventilator will function, i.e., in which mode. By turning the labeled rotary knob, the operator can cause the ventilator to operate as a controller, assistor, assistor-controller, IMV generator, CPAP/ PEEP generator, or other configuration. Respiratory rate control (also called normal rate, rate-cycle per minute, or breaths per minute [bpm] allows the operator to select a breathing frequency for the patient. A calibrated rotary knob controls this function on most ventilators, allowing the operator to select a controlled frequency between 1 and 80 bpm. Tidal volume control (also called normal volume, preset inspiration minute volume). Depending on the ventilator, this control enables the operator to dial in a desired preset tidal volume (breath) for the patient. This function may also be selected by turning a calibrated rotary knob to the desired gas volume in milliliters, liters, or liters/minute if the minute volume can be preset. On most adult ventilators, tidal volume can be adjusted to 2 L (2000 mL) or greater. It is also important to remember that if the minute volume (gas moved per minute) is not preset, tidal volume X rate = minute volume. Normal pressure limit (also called high pressure limit) limits the maximum pressure level that can be generated in the patient breathing circuit during a mechanical breath (inspiration) before allowing it to escape to the atmosphere. The purpose of this control is to enable the operator to set safe maximum pressure limits that should not cause barotrauma to the patient with known or suspected weakDA4 June

1991

367

ened pulmonary structures. This is usually set by a calibrated rotary knob and is adjustable to pressures up to 100 cm H,O. An undesirable aspect of this control is the fact that when pressure is released, so is the tidal volume. This means that continued elevation of the system pressure, for that breath or subsequent breaths, probably would prevent the patient from receiving the desired and necessary tidal volume. Uncorrected, this situation will cause respiratory failure, even though the patient is still connected to a cycling ventilator. Sigh volume (also called sigh function) is the control that enables the operator to dial a preset sigh volume. A sigh breath may be periodically prescribed to hyperinflate a patient’s lung, which will open atelectatic (partially collapsed) alveoli or prevent atelectasis from occurring. This function is usually combined with a sigh rate and a sigh pressure limit, which serve the same function as those described for normal ventilator operation. Adult sigh volumes are adjustable to 2000 mL or greater at rates of 1 per minute to 2 per hour, in combinations of single or multiple breaths. As an example, most sigh volumes are 1.5 to 3 times the prescribed tidal volumes at rates of one every 4 to 6 minutes. Sigh pressure limits are adjustable to 100 cm H,O or greater, depending on the ability of the patient’s lungs to tolerate these periodic hyperinflations. Waveform control is an option on some ventilators. Selection of a wave form requires the operator to decide the shape of the gas flow pattern to the patient if it were traced on an electrocardiograph-type chart paper. Most volume ventilators deliver either a square wave or a half-sine wave. Clinically, there is still much debate about the value of various wave forms for different pulmonary conditions. I: E ratio control, found on some ventilators, allows the operator to adjust the length of inspiration to the length of expiration (I : E ratio). Most ventilators do not have a specific control for the I : E ratio; these types require the operator to establish the ratio by adjusting other controls (e.g., inspiratory flow rate) or by actually setting the duration of inspiration and exhalation. A normal I : E ratio for most adults is 1: 2 or 1: 3, i.e., more time is allowed for exhalation to occur than for inhalation. Positive end e2cpirator-y pressure is used with or without a mechanical ventilator. In either situation, during exhalation, the patient is allowed only to exhale to a preset airway pressure level. This pressure level in the lung is positive when compared to the atmosphere. Continuous positive airway pressure control. Many ventilators incorporate PEEPCPAP controls as a single function. The difference is that CPAP describes positive pressure that is held in the airway during both inspiration and exhalation. For this reason, CPAP is used to maintain continuous positive airway pressure during intermittent

mandatory ventilation operation of the ventilator, when the patient is allowed to breathe spontaneously. As previously described, intermittent mandatory ventilation is a technique whereby the patient is allowed to breathe spontaneously for a period of time, and then the ventilator automatically delivers a mechanical breath. Clinical advantages of this procedure are that it: psychologically involves patients in their own recovery; forces them to use their respiratory muscles; allows them to adjust their respiratory rate and depth to maintain blood gases; and causes less interference with cardiac output than does the traditional method of ventilation. Most ventilators with IMV capability have adjustable cycling rates down to 1 per minute, so that a full range of combinations of spontaneous breaths to controlled breaths can be adjusted. The owgen percentage controZ is usually a calibrated rotary knob that enables the operator to “dial in” a prescribed FIO, (oxygen percentage) level for both CMV and during IMV. Oxygen percentages are adjustable between 21% and 100%. It is important to ascertain if a particular ventilator monitors FIO, by determining gas pressure of air and oxygen flow relationships or by actual measurement of PO, (such as with the Clark electrode). Ventilators that monitor FIO, by measuring gas pressures or flows may give erroneous readings because the pressure in the oxygen reservoir may be created by room air or even by some other gas. Many ventilators control FIO, through the use of an oxygen blender. These are precise instruments that enable the operator to set an exact percentage of oxygen for patient use. Blenders accept independent external sources of air and oxygen at 50 psi and mix them to provide the desired FIO,. These units are used with ventilators and oxygen therapy systems in neonatal units or whenever exact levels of oxygen must be delivered. Some manufacturers build in oxygen blenders as part of their equipment; in other units, the blender must be purchased separately. Most oxygen blenders use balance valves to provide the desired FIO,. These valves allow a specified blend (x parts air with y parts oxygen) to render the preset FIO, at a given flow rate. Pressure support ventilation is used by the operator to reduce the patient’s work of breathing. As previously described, PSV,, refers to total unloading of the work of breathing. Levels of 5 to 6 cm H,O PSV are probably adequate to reduce the increased work of breathing caused by airflow resistance through adult endotracheal tubes. THE BENNETT

MA-1 VENTILATOR

The Bennett MA-1 ventilator is used here as an example of classification because it is still the ventilator of choice in many hospitals DM,

June

1991

369

and because it is easy to conceptually explain how it generates a tidal volume and other mechanical features. This unit is a first-generation, electrically powered, volume-cycled, double-circuit, constant-flow generator that can be operated as an assistor, a controller, or an assist-controller. Special features available on this unit include PEEP, IMV, and a servo-controlled humidifier. The Bennett MA-1 ventilator can also be pressure-cycled, causing it to perform as a constant pressure generator instead of a constant-flow generator. This is an important point, because it demonstrates how a ventilator’s performance and expected outcome can be altered intentionally or accidently by the operator. Under certain conditions, an accidental change can result in inadequate ventilation of the patient. Following is an explanation of the Bennett MA-1 ventilator classification terminology. Note that the principles detailed in this explanation can be applied to other ventilators that have similar structural and performance characteristics. ELECTRICALLY POWERED The Bennett MA-1 ventilator has two air compressors that are powered by electricity (Fig 28). The main compressor is a Bell and Gossett O.&HP, 115-volt, AC, 60-cycle, single-phase, rotary motor, which is capable of delivering a maximum tidal volume of 2200 mL at a peak flow of 100 IJmin. The second compressor is much smaller and is used to generate a gas flow of 6 to 8 Wmin to operate a small volume nebulizer located at the breathing manifold. The ventilator also has an oxygen inlet with a high-pressure hose attached to the rear panel (1 in Fig 29). This hose is connected directly to a 50-psi oxygen source by means of an adapter (for use when an FIO, greater than room air is desired). The ventilator will not operate unless the electrical cord on the rear panel (21 is plugged into a 115-volt, 60-cycle AC outlet and the off-on switch is activated, even though oxygen may be flowing through the highpressure hose.

VOLUME-CYCLED

SYSTEM

The Bennett MA-1 ventilator is said to be a volume-cycled ventilator because a desired tidal volume can be preset by rotating the normal volume control on the front panel of the ventilator. Tidal volume may be adjusted in lOO-mL increments to a maximum of 2200 mL. When the ventilator is operational, inspiratory pressure increases automatically to provide power necessary to deliver the preset tidal volume, if a pressure limit is not reached. 370

DM, June 1991

II

FIG 28. Schematic

DXJ, June

of gas

I991

flow through

Bennett

MA-1

ventilator

FIG 29. Exposed

rear panel

of Bennett

MA-1

ventilator.

A “pop-off’ pressure limit can be established by adjusting the normal pressure limit control knob on the face of the ventilator panel. If the pressure limit is set too low, the ability of the ventilator to function as a volume-cycled device may be severely restricted if airway impedance increases. This is true, however, of all volume ventilators that incorporate a pressure-limiting device. DOUBLE-CIRCUIT

SYSTEM

The Bennett MA-1 ventilator incorporates two independent gas systems, which interact with each other to generate the patient’s tidal volume. In Fig 28, gas flow during inspiration is generated by the main compressor (1). This gas passes through an outlet filter (3) and proceeds upward through a main solenoid (41 to the peak flow control (51. As gas enters the peak flow control assembly, it is directed through a Venturi (6). At this point, room air is entrained to supplement (increase) the gas flow rate from approximately 30 Wmin 372

DM,

June

1991

proximal to the Venturi to as much as 100 Wmin after air entrainment. The exact flow rate is adjusted from a minimum of 15 Wmin to a maximum of 100 Wmin by adjusting the peak flow control on the panel of the ventilator. The adjusted peak flow then proceeds down a connecting tube (7) to open a one-way valve (8) and enter the bottom of the bellows chamber (9). At this point, the positive pressure created in the chamber forces the bellows IlO) upward, causing its gas contents (preset tidal volume) to pass through an outlet valve (11) and enter a main tube that carries the tidal volume (dark line) to the patient (12). Although unlikely, if a hole were to occur in the bellows (101, the ventilator would function as a single-circuit device with gas moving directly from the compressor, through the hole, to the patient’s lungs. This is not desirable and could cause excessive airway pressure and inaccurate FIO,.

CONSTANT-FLOW

GENERATOR

The Bennett MA-1 ventilator performs as a constant-flow generator at system pressures less than 40 cm H,O gauge pressure. Because of the low internal driving force of the bellows, the gas flow decreases as impedance to gas flow increases. This causes the characteristic square flow wave of a constant-flow generator to taper from a sinusoidal wave form characteristic of a constant pressure generator. When the ventilator begins to function like a constant pressure ventilator, preset volumes may not be delivered. If this situation develops, either the impedance to flow must be reduced or the ventilator must be exchanged for one that generates more power. The Bennett MA-1 ventilator flow rate is set by the operator by adjusting the peak flow control.

ASSISTOR

FUNCTIONS

The Bennett MA-1 ventilator triggers on in response to a patient’s spontaneous inspiratory efforts, provided the solenoid valve is set to be sensitive enough to detect the decrease in system pressure caused by spontaneous breathing. The ability of the ventilator to detect a patient’s spontaneous inspiratory effort is called sensitivity. The degree of sensitivity can be set by adjusting the sensitivity control on the panel of the ventilator. The more sensitive a ventilator is, the faster it will respond to spontaneous breathing. If a ventilator is made too sensitive, however, it will self-cycle. This uncontrolled cycling, referred to as “chattering,” is not desirable since it will result in diminished minute ventilation and subsequent hypoventilation of the patient. DAL June

1991

3i3

CONTROLLER

FUNCTIONS

The Bennett MA-1 automatically cycles to ventilate apneic patients at rates adjustable from 6 to 60 times per minute. Rates are set by adjusting the “rate in cycles per minute” control on the panel of the ventilator. ASSIST-CONTROL

MODE

The Bennett MA-l can be operated as an assist-controller by adjusting the rate control to cycle the ventilator at a preset frequency. This rate will be maintained should the patient fail to breathe at a frequency equal to or greater than the preset control rate. IDENTIFICATION AND FUNCTION BENNETT MA-1 VENTILATOR

OF THE CONTROLS

OF A

The control panel of the MA-1 ventilator is conveniently arranged into a left and right grouping of controls. The controls comprising the left-hand grouping, with the exception of the oxygen percentage controls, are used for general operation of the ventilator. The righthand control grouping consists of special function controls. BASIC

CONTROLS

The pressure manometer and light indicators (Fig 30) are horizontally arranged in a row across the top of the panel. These are used to monitor the patient and the ventilator functions. SENSITIVITY

CONTROL

The sensitivity control adjusts the ventilator to respond to the patient’s spontaneous breathing attempts. The range of inspiratory effort required to trigger the machine on can be adjusted from -10 cm H,O gauge pressure to -0.1 cm H,O gauge pressure and to an oversensitive (self-cycling) level. The greater the sensitivity, the easier it is for a spontaneously breathing patient to trigger the ventilator. PEAK

FLOW

CONTROL

The peak flow control establishes the maximum flow (in Wmin) to the patient during inspiration. This control is adjustable from 15 to 100 Urnin. Adjustment of this control, combined with the cycling frequency, establishes the length of the inspiratory phase and the I : E ratio for a given tidal volume. A normal peak flow range for most 374

mu,

June

1991

FIG 30. Bennett

MA-1

control

panel

adults, using a 12 to 15 mWkg tidal volume less, is 40 to 50 Umin. ON/OFF

and an I : E ratio of 1: 2 or

SWITCH

The power switch turns the electrical functions of the machine on and off. This is the master switch that must be activated before the ventilator will perform any electrical function. NORMAL.

PRESSURE

LIMIT

CONTROL

The normal pressure limit control allows the operator to set maximum inspiratory pressure limits from 20 to 60 cm H,O as monitored by the system manometer. Maximum pressure limits are critical in determining the ventilator’s ability to deliver a predetermined tidal volume under conditions of increasing airway impedances. As previously stated, when the normal pressure limit is reached, the ventilator will vent its system pressure to the atmosphere. This action ends the inspiratory cycle regardless of whether a desired preset tidal volume was delivered by the ventilator. At this point the ventilator may or may not be functioning as a volume ventilator, deUM,

June

1993

375

pending on whether the preset volume was delivered before the normal pressure limit was met. Pressure limit is not to be confused with pressure support, a more recent mode of ventilation.) The normal pressure limit control should be set approximately 10 cm H,O greater than the system pressure required to deliver the desired tidal volume or at a level that will prevent pressure trauma to patients with known pulmonary structural weaknesses such as blebs. A ventilator that is venting due to the normal pressure limit setting may indicate that the initial pressure limit is set too low for the desired tidal volume or for dynamic increases in system and/or airway resistance (including increases in resistance resulting from a kinked breathing circuit, mucus in the tracheal tube, bronchospasm, or decreases in pulmonary compliance). For these reasons, the system manometer and the normal pressure limit can be clinically useful for monitoring changes in the ventilator-patient system. NORMAL VOLUME The normal volume control allows the operator to set a desired tidal volume (in milliliters) to be delivered by the ventilator. This volume is calibrated between 0 and 2200 mL on inspiration. Exhaled volume may be measured by placing a spirometer distal to the exhalation valve. It is important that the operator realize that the tidal volume delivered by the ventilator and measured by the spirometer is not the volume that participated in alveolar ventilation. The alveolar gas volume is less than the delivered tidal volume because of gas compression in ventilator circuitry and accessories such as humidifiers. This compressed volume is referred to as ventilator dead space and tubing compliance. RESPIRATORY RATE The frequency control establishes the number of respirations (cycles) per minute when the ventilator is operated in the control or assist-control mode. If the ventilator is operated as a pure assistor, the rate control should be off. Frequency is calibrated between 6 and 60 cycles per minute in the standard MA-1 model. Frequencies less than 6 cycles/min for IMV can be obtained by changing the rate card, although most new models already have this modification. As a safety function, in clinical use it may be desirable to set a control rate at a value less than the spontaneous breathing rate. When establishing the cycling frequency, the clinician must consider norms from a particular age group, as well as the desired I : E ratio, the tidal volume, and the peak flow. All of these must be adjusted to accommodate a particular cycling frequency, since time of inspiration and exhalation constitutes a single cycling frequency. 376

DM,

June

1991

Clinically, a respiratory rate is measured for a l-minute time frame. This rate times the tidal volume determines the minute volume (ventilation). SIGH PRESSURE LIMIT This is the first control that constitutes the special function controls of the ventilator (right side of the control board). The sigh pressure limit functions the same way as the normal pressure limit control, except that it controls the maximum system pressure delivered during a sigh breath. The sigh function is an artificial way of providing periodic lung hyperinflations larger than that of a normal tidal volume. Theoretically, it is used to prevent patchy microatelectasis that may occur with constant volume ventilation. The effectiveness of routine sighing is not considered to be necessary by many physicians when adult tidal volumes are provided at levels of 10 to 15 mIJkg body weight. However, recent work with pressure support systems have led to recommendations that sigh breaths be interspersed as sampling breaths whenever PETCO, is used to titrate PSV minimal limits.8 The sigh pressure limit is adjustable between 20 and 80 cm H,O. It is normally set at 5 to 10 cm H,O higher than the normal pressure limit. It is believed that artificial sighs may not be indicated if an initial tidal volume is established on the basis of 12 to 1.5 mWkg of the patient’s body weight. SIGH VOLUME This control sets the volume of gas (in milliliters) that is to be delivered during an artificial sigh-up to a maximum of 2000 mL. This volume is normally set at twice the normal tidal volume being delivered to the patient; however, all special functions of the ventilator should be prescribed by the physician. SIGHS PER HOUR This control regulates the number of sighs that are to be delivered per hour. It can be adjusted to deliver 2 to 15 sighs/hour. This means that at 2 sighs/hour the ventilator delivers one sigh every 30 minutes and, at 15 sighs/hour, one sigh every 4 minutes. A normal range is one sigh every 15 minutes. In addition, a function lever located on the control allows the operator to program the ventilator to deliver 1, 2, or 3 sighs sequentially. For example, if the control is set to deliver 4 sighs/hour and the function lever is set at 2, the patient receives 2 sighs in sequence every 1.5 minutes. DM,

June

1991

377

OXYGEN PERCENTAGE

This control establishes the oxygen percentage of the patient’s inspired air (FIO,). It is calibrated and can be set between 21% and 100% oxygen. It is important for the operator to be aware that on the MA-l ventilator, oxygen-air mixing occurs as the result of a Venturi that entrains oxygen from an accumulator (reservoir). This oxygen mixes with air that is pulled into the bellows during its descent following an inspiration (ascent). If the control is set at 100% oxygen, no room air is entrained, and the bellows fills with gas drawn entirely from the accumulator. Air-oxygen mixing is based on a proportion theory, which is also used with air-oxygen blenders. The critical consideration is that the oxygen accumulator must be connected to an oxygen source (not some other gas such as compressed air) by means of the high-pressure hose. It is essential that operators understand that if such a mistake were to occur, the oxygen alarm would not be activated, since it only monitors the physical gas pressure (cm H,O) in the accumulator. It does not test the air in the accumulator for the actual presence or absence of oxygen molecules (i.e., a patient could be receiving compressed air even though the oxygen control is set at 100%). EXPIRATORY RESISTANCE

This control retards exhalation by holding the exhalation valve inflated (closed) at the end of a mechanical inspiration. If the control is adjusted to “full increase,” the exhalation valve will momentarily completely block exhalation. This technique is used to create an inspiratory plateau for measuring static compliance. Partial adjustment of the control only partially holds the expiratory valve closed, resulting in expiratory retard. The effects of expiratory resistance can be observed on the system manometer by noting a delayed return of the indicator to zero at the end of inspiration. When using expiratory retard, the cycling frequency must be adjusted to allow the patient a complete exhalation (i.e., the pressure indicator returns to zero before heginning a subsequent inspiration). Failure to allow adequate time for a complete exhalation results in the creation of an inadvertent PEEP condition. NEBULIZER

CONTROL

The nebulizer off and on switch controls the small compressor used to provide gas at 8 to 10 psi to power a small volume medication nebulizer located at the breathing manifold. When this compressor is on, gas is pulled from the bellows volume by the compres378

LAM June

1991

sor to power the nebulizer. This approach cause no additional gas volume is added by the nebulizer flow. lzlANUAL

is clinically important, beto the preset tidal volume

SWITCH

Two push-button controls allow the operator to manually initiate a normal or a sigh mechanical breath. This control is used to cycle the ventilator during a check before putting the ventilator on a patient and in clinical situations for which manual control of the patient is desired. Pushing this control overrides any previous frequency control setting, as long as the button is depressed. Care must be taken when manually cycling the ventilator to allow time for a complete exhalation before manually starting another inspiration. OPERATING AND MONITORING MA-1 VENTILATOR

THE BENNETT

One of the outstanding features of any modern ventilator is its increased capabilities to monitor the patient. The Bennett MA-l ventilator uses indicator lights and/or audible alarms to inform the operator of patient and ventilator status (see Fig 30). ASSIST

LIGHT

An amber light flashes on each time a spontaneous breath is generated or if the sensitivity control is overset (causing the machine to self-cycle). PRESSURE

LIGHT

A red light illuminates (combined with a buzzing alarm) when a preset pressure level is reached and vented. This situation occurs whenever airway resistance becomes so high that the system pressure required to deliver the preset tidal volume exceeds the preset pressure limit. The cause of increased resistance should be immediately located and corrected (e.g., by suctioning the patient or unkinking tubing) and/or the pressure limit should be increased. RATIO

LIGHT

A red light appears when the I : E ratio falls below 1: 1. Once the ventilator has been initially set up, this is usually caused by reduced compliance and increased resistance. Usually, the undesirable 1:E OM,

June

1991

37Y

ratio can be corrected by clearing the patient’s airway, increasing the flow rate, decreasing the cycling rate, or decreasing the tidal volume. SIGH LIGHT A white light flashes on each time the patient sighed by machine or manually by the operator.

is automatically

OXYGEN LIGHT A green light appears to indicate that the high-pressure hose has been attached to an external gas source of at least 40 psi, regardless of the composition of the gas. A red light appears and a high-pitched alarm sounds if the desired oxygen inlet pressure is not being delivered. This may mean a low source pressure or a leak in the system. SPZROMETER ALARM A shrill, whistle-like sound is heard if the desired tidal volume is not being reached; the sound continues until the problem is corrected or the alarm is shut off. This alarm usually signals a disconnect or a gas leak in the tubing system. Often, the problem can be quickly corrected by checking the patienthrentilator system, beginning with the patient and working backwards toward the ventilator. Bennett ventilators sold before 1981 were equipped with Bennett SA-1 or SA-2 monitoring spirometers and spirometer alarms. Under certain conditions of flow, volume, and rate, the alarm on these devices does not sound if the expiration diaphragm tube becomes disconnected and/or the exhalation diaphragm becomes damaged and nonfunctional. When this situation occurs, it was possible for the spirometer to alternately fill and empty in reverse order, simulating ventilation even though the patient was receiving little or no ventilation. In 1981, the Puritan-Bennett Corporation replaced the SA-1 and SA-2 models of spirometers on new ventilators with an SA-3 model that does alarm to indicate that preset tidal volumes are not being exhaled and/or there is inadequate positive pressure being generated. This alarm properly indicates a leak in the system and/or an inoperative exhalation valve. In 1985, a letter was mailed to hospitals indicating that the model SA-2 spirometer was obsolete and would no longer be serviced by the Puritan-Bennett Company. In today’s environment of intensified litigation, it is important to note that whenever a ventilator or its breathing circuit is modified in the field from the factory model, it 380

DM,

June

1991

may cause the ventilator and its monitoring fail or perform inaccurately. CASCADE

and alarm functions

to

HUMIDIFZER

This device provides a continuous source of warm, moist gas to the patient. The thermostat control on top of the cascade electrical unit can be adjusted to provide temperature ranges between room temperature and 50°C (120°F). A thermometer or electric thermistor is inserted into the breathing tube circuit to monitor the gas temperature before it enters the airway. This servo system automatically adjusts the heater temperature based on the temperature of the effluent gas. The type of cascade alarm depends on the MA-l model; several are available. NEW GENERATION

VENTILATORS

The use of microprocessors to control ventilator function has resulted in a new third generation of ventilators. These units produce and deliver tidal volume in the same manner as their predecessors-except for high-frequency devices, which deliver smaller tidal volumes at rates greater than 60 breaths per minute. The primary differences are in the ways in which function modes are set and monitored. New third-generation ventilators use electronic central processing units to automatically scan ventilator circuits for proper function and to monitor the unit’s ability to meet preset clinical parameters. Any variations from factory performance specifications and/or preset clinical parameters activate warning systems at the bedside and even at remote stations. With these devices, the need for ventilator checks by personnel is diminished. However, the need for patient rounds is not diminished and may in fact become more frequent as the new and sophisticated technology gives the practitioner many more options for controlling a patient’s ventilation. Some of these options are listed below. Although there are numerous devices that could qualify, the following ventilators are examples of a new generation of devices that undoubtedly will become even more automated as the blending of microprocessors with ventilators is more fully accepted as a safe and dependable method for controlling a ventilator’s function and for monitoring patients. Although these third-generation devices are available from several manufacturers, each incorporates similar operational features: microprocessor control; electromechanical valves to control and adjust waveforms for gas flow; numerous ventilatory

DM, June

1991

381

modalities monitoring

SIEMENS

(including pressure support and alarm packages.

SERVO 900B,900C,AND

ventilation);

and extensive

SOOE

The Siemens Servo ventilators, purely by definition, do not qualify as third-generation devices because microprocessors are not used to control the operational and monitoring-alarm functions of these units. Even without microprocessors, these ventilators control input gas flow based on output delivery, with automatic adjustment to meet preset volumes. These units are therefore on the leading edge of the transition to computer-controlled devices. The primary difference is that microprocessor-controlled ventilators use preprogrammed software to direct the functional logic of the ventilator. The Siemens 900B series ventilators are time-cycled, pneumatically powered, and electronically controlled. They can be operated as assistors, controller, or assistor-controllers. Special functions of the Siemens units include SIMV, CPAP, and PEEP. The principle of operation for all of the models is the same, though there are some differences in the features offered by the more modern model 9OOC and 900E ventilators. The 900E model is a lower priced unit that does not offer a pressure control mode, a wide range of pause time, pediatric alarm scale, or IMV rates greater than Wmin. The pneumatic power system of the Servo Ventilator is unique (Fig 31) in that it incorporates mechanical force generated by spring tension (1) to compress (empty) a gas-filled bellows (21. As the bellows is

FIG 31. Gas flow diagram 382

for Slemens

Servo

ventilator. m4,

June

1991

compressed (moved up), gas flows out past an electronically controlled inspiratory valve (3) at a constant flow rate. This flow rate is ensured by the constant spring tension against the bellows and by the opening and closing of the electronic inspiratory valve. Siemens ventilators incorporate electromechanical scissors (chopper) valves that open and close in response to electrical signals. These valves control tidal volume, flow rates, flow pattern, inspiratory time, expiratory time, and, on the 9OOC, expiratory pressure. Fig 32A and B illustrate a scissors valve in the opened and closed position. These inspiratory and expiratory valves are electrically opened and closed for given periods of time to allow the tidal volume to be delivered to the patient. During mechanical inspiration, the expiratory valve closes and the inspiratory valve opens. During expiration, the reverse is true. Both valves respond to preset signals for time, flow rate, flow patterns, and tidal volume. A motor on the inspiratory valve constantly corrects the valve opening to obtain preset values. The use of a spring-driven bellows between the service gas and the patient makes the ventilator a double-circuit system and causes it to perform as a constant-flow generator. Such a classification is appropriate because the force of the spring against the bellows plate creates a constant force, hence gas flow, throughout the inspiratory cycle. This performance classification remains valid as long as the working pressure exceeds airway resistance and other opposition to gas flow. The combination of feedback electronics (Fig 31) integrated into the inspiratory and expiratory gas circuits enables the ventilator to continuously electronically monitor, compare, and adjust inspiratory gas flow rates. Flow rates are adjusted as needed to deliver the preset minute volume (4). If resistance to gas flow increases inspiration, causing expiratory flow to be reduced, the ventilator’s integrated electronic control system automatically increases inspiratory flow rates to produce the tidal volume necessary to achieve a preset minute ventilator CV, X f). In other words, gas flow rate from the

FIG 32. Scissor DM, June

valve. 1991

A. opened

and

B. closed 3s3

ventilator is increased or decreased (“servoed”) as needed the preset minute volume, giving the ventilator its name. VENTILATOR

to deliver

CONTROLS

Since the Siemens 900B and 9OOC ventilators are very similar, the 900B will be discussed first, followed by an explanation of changes made in the 9OOC. For the reader to acquire a better understanding of the relationship of the controls and their locations on the control panel, the following explanations are coordinated with Fig 33A and B. The preset working pressure control sets the maximum pressure available to empty the bellows that delivers gas to the patient (adjustable to 100 psig). The preset inspiratory minute volume control establishes the desired minute volume to be delivered to the patient. It can be adjusted from 0.5 L/min to 2.5 Umin for neonatal or adult ventilation. The breaths per minute control sets the cycling frequency. It can be adjusted from 6 to 60 bpm. The inspiratory time percentage control adjusts the length of inspiration, which varies with the preset cycling frequency. It is ad-

FIG 33. Control panel Schaumburg.

384

for Siemens IL )

9008

ventilator.

(Courtesy

of Siemens

Life

Support

Systems,

OA4, June

1991

justable between 15% and 50% of the breathing cycle. This control adjusts the peak flow rate as a function of time. The pause time percentage control sets the end expiratory pause as a percentage of the respiratory cycle. This pause occurs at the end of an inspiration just before the beginning of an exhalation. It is adjustable between 0% and 30% of the cycle. During the pause, the lungs remain inflated, causing a plateau pressure wave form. The flow pattern selector switch selects (Fig 34) a square wave (A), accelerating flow wave (B), or decelerating flow wave (Cl. The ma,ximum expiratory flow control creates expiratory retard. When activated, this control prolongs the length of exhalation by causing the patient to exhale against a preselected resistance (Fig 34, D1. The sigh finction presets periodic sighs, which are delivered when the sigh system is activated and following every 100 breaths. On this unit, this function is activated by the same knob as the IMV control. The intermittent mandatory ventilation control sets the rate at which the machine-delivered breath is provided to the patient. In the intervals between machine-delivered breaths, the patient breathes spontaneously without ventilator support. It should be noted that the 900B delivers IMV in a manner synchronized with patient effort. If the patient has already begun a spontaneous breath when it is time for the machine breath, the machine cycle is delayed until it no longer conflicts with the patient’s spontaneous breathing efforts. The continuous positive airway pressure and positive end expira-

FIG 34. Flow wave burg, IL.) Uhf,

June

forms

1991

for Slemens

9008.

(Courtesy

of Siemens

Life Support

Systems,

Schaum-

385

tory pressure control is an external (spring-loaded PEEP) valve. It must be attached to the expiratory outlet (flap valve) of the ventilator to generate desired PEEP or CPAP levels (Fig 33, B). Two valves are available: 0 to 20 cm H,O and 0 to 50 cm H,O. The resulting pressure is then read on the airway manometer. The triggering level Cl’rig level”) knob adjusts the sensitivity of the ventilator when it is operated in the assisted or IMV mode. It determines the sensitivity level (patient effort) that will be required to open the inspiratory valve, which delivers a mechanically assisted or spontaneous IMV breath. It is usually set at 2 to 3 cm below the baseline working pressure. The Siemens 900B has no specific control for setting desired inspired oxygen (FIO,) percentage levels. Desired oxygen concentrations are adjusted on an air-oxygen blender that is attached to the lower inlet of the source gas inlet connection. Also, the Siemens 900B does not include a humidifier. Humidity may be provided by a Cascade-type unit or other acceptable humidification system. The Servo has monitors available to show expired minute volume and airway pressure. An optional monitor is available to provide end tidal CO, values. This ventilator has visual and audible alarms to indicate high or low minute volumes in variance with preset values. These alarms are TABLE

2.

Specifications Minute Rate

for Siemens

Servo

volume

Inspiration

percent

Pause

percent

time

Sensitivity (patient triggering) Pressure limit Working pressure (driving force) Flow pattern switch Sigh system

Ventilator

900B

0.5 to more than 25 Wmin 6 to 60 BPM on assist-control mode or 6 to 60 BPM divided by 2, 5, or 10 on IMV 15%, 20%, 25%, 33%, or 50% of set ventilatory cycle time established by rate control OS%, lo%, 20%) or 30% of set ventilatory cycle time established by rate control Variable from -20 to +45 cm H,O pressure Adjustable up to 100 cm H,O pressure Adjustable up to 100 cm H,O pressure Square wave or sine wave One sigh every 100 breaths at double tidal volume 01 Off

Displays Pressure Expired

mete1 minute

volume

meter

-20 to 100 cm H,O 0 to 30 Wmin

AhIllS

High and low minute volume High pressure limit Z-min alarm silence Electric power disconnect Courtesy

Siemens-Elema

Ventilator

Audio and visual Audio and visual Automatically reset Approximately 1 min Systems.

audio

Elk Grove Village, IL. Table

signal 23-4 from

Eubanks

and &me.’

Uhf. June

IYYl

useful in detecting hyperventilation, patient disconnects, cuff leaks, and high airway pressure (secondary to kinked tubing or other problems). Specifications for the Siemens Servo Ventilator 900B are presented in Table 2. THE SIEMENS

SERVO VENTILATOR

SOOC

The 9OOC is an updated model of the 900B ventilator. Although the operating principles are the same, a number of features have been added to the 9OOC ventilator (Fig 35) that were not available on the 900B model: l l

l

l

Working pressure has been increased to 120 cm H,O. A new mode was added to give pressure support during spontaneous breathing in the SIMV mode. Once a preset pressure level has been set, it is maintained throughout the inspiratory cycle. The advantage to this system is that it helps the patient to overcome tubing and other circuit resistance during spontaneous breaths. Respiratory rate is now adjustable between 0.5 and 120 bpm compared to a maximum of 60 bpm on the 900B. The 9OOC ventilator may be operated as a pressure-cycled unit by

FIG 35. Control panel for Slemens tems, Schaumburg, IL.)

DIM, June

1991

Servo

Ventilator

900C.

(Courtesy

of Siemens

Life Support

Sys-

387

l

l

l

l

l

l

l

setting the mode control to “Press Contr” and adjusting inspiratory pressure control to the desired pressure level. Minute volume has been increased to 40 Urnin from a range of 25 to 30 Wmin for the 900B. Inspiratory time percent has been increased to 67% and 80% (from a maximum of 50% 1, enabling reverse I : E ratios to be delivered. Triggering levels (sensitivity) can be set at a level below the present PEEP level. As PEEP is changed, the triggering level is automatically compensated to maintain the same degree of inspiratory triggering effort. PEEP capability is built into the unit and can be electronically adjusted from 0 to 50 cm H,O. Direct in-line monitoring of FI02, inspired and expired tidal volume, breaths per minute, expired minute volume, peak pause, and mean airway pressure is available by turning a selector knob. A selection switch allows for expired volume alarms based on an adult or infant scale. A flush button is available to allow injection of a completely fresh concentration of gas into the patient.

Performance specifications for the Siemens 9OOC ventilator are the same as those for the 900B model, except for the changes noted. In 1980, the Puritan-Bennett Corporation introduced the first third-generation, microprocessor-controlled ventilator (Fig 36). Currently, three models are available, the Bennett 72OOA, AE and 7200 Special. The two units offer similar standard operational and monitoring options, but the 7200 Special cannot be upgraded. The Bennett 7200 Series unit is ‘an electrically powered, pneumatically driven, microprocessor-controlled volume ventilator.” It can be used for pediatric patients and adults, but not neonates. It functions primarily as a constant-flow generator, delivering the characteristic inspiratory square wave flow pattern (Fig 37A). In addition, the operator may select either a descending ramp wave form or a sine wave (Fig 37B and Cl. These ventilators offer four operational modes: CMV, SIMV, CPAP, and SIMV with pressure support (optional). During CMV the ventilator functions as an assistor or a controller. As a controller the tidal volume may be machine initiated or manually cycled by the operator. During SIMV, breaths may be manually (operator) generated, machine initiated, or triggered by the spontaneously breathing patient. If pressure supported SIMV is used, flow accelerates to the preset pressure supported level whenever airway pressure is dropped by the spontaneously breathing patient to the sensitivity level of the triggering flow. Once the preset pressure support level is reached, the inspiratory flow rate is automatically adjusted to maintain this pressure level throughout inspiration. Inspi-

388

LAM, June

1991

FIG 36. Bennett

7200

ventilator

ration ends whenever the flow rate decreases to 5 Urnin or when airway pressure reaches 1.5 cm H,O above the preset pressure support level. During CPAP, breaths are primarily generated by the spontaneous breathing efforts of the patient. The exception is a manually cycled breath by the operator. Regardless of the mode of operation, all normal inspiratory gas flows are generated from pressurized air and oxygen. The source for these gases may be a wall piping system or, in the case of air, an internal compressor (optional). The operating pressure for the Bennett 7200 ventilator is 35 to 100 psig for both air and oxygen, resulting in a maximum flow rate of 190 Urnin unrestricted flow. The peak inspiratory flow rate during CMV is 120 Wmin; 180 Wmin during spontaneous breathing. Tidal volumes may be adjusted from 0.10 to 2.50 L at rates of 0.5 to 70 bpm and maximum pressure limits of 120 cm H,O. Both CPAP /X&June

1991

389

stepup

Time

lb

ramp dawn

Tb

FIG 37. Bennett

7200

respiratory

flow

wave

patterns.

and PEEP are created by restricting deflation of the exhalation balloon that covers the exhalation port of the patient breathing circuit. These elevated baseline pressures are adjusted by turning a knob from 0 to 4.5 cm H,O. Technical data and specifications for the model 7200 microprocessor ventilator are presented in Table 3, which points out this model’s many control, monitoring, and safety features. Mechanically, the unit is similar to other ventilators in that the tidal volume and variations to the pressure and flow patterns are available on other devices. The uniqueness of this ventilator is the electronic memory and control of ventilator functions by the microprocessor. For example, gas flow through the ventilator is controlled by the coordinated operation of seven different solenoids (electromechanical valves). Once the operator has set the wave form, tidal volume, respiratory rate, peak inspiratory flow, plateau, and oxygen concentration the microprocessor automatically regulates the opening and closing of the proportional solenoid valves to deliver the specified tidal volume. The microprocessor then continuously monitors gas flow during subsequent breaths and converts it to a measurement at body temperature, ambient pressure, saturated with water, with 100% humidity. This flow is then compared to actual flow, and adjustments are automatically made as needed to provide the preselected tidal volume. Other functions controlled by the microprocessor include: the en-

TABLE

3.

Technical

Data

1 PHYSICAL

and

Snecifications

for the Model

7200 Microorocessor

Ventilator

CHARACTERISTICS

Dimensions Ventilator

module

Ventilator

module

with

compressor

Assembly weight Ventilator module Ventilator module with pedestal Ventilator module with compressor Pedestal Compressor pedestal Shipping weight lapproximate] Ventilator module Ventilator module with pedestal’ Ventilator module with compressor Pedestal Compressor pedestal 2 ENVIRONMENTAL

pedestal

pedestal

Height: Depth: Width: Height: Depth: Width:

41.9 56.5 55.9 102 64.8 55.9

50.8 95.3 114 44.5 62.6

I112 lbl 1210 lbl (250 lb) 198.0 lb, (138 lb)

kg kg kg kg kg

cm cm cm cm cm cm

(16.5 i22.5 122.0 140.0 12.5.5 122.0

79.4 kg 1175 lbt pedestal

172 kg 1380 lbr 72.6 Q 1160 lbl 90.8 kg 1200 lbl

IWQUIREMENTS

Altitude Operating Storage/shipping Environmental temperature Operating Storage Relative humidity Operating Storage Clearances for air circulation Storage lxquirements Less than 200 days More than 200 days

3048 m 110,000 ftl 15.240 m 150,000 ftl 16” to 41’C (60’ to 105’FI -34O to 71°C (-30° to 160°FJ 0% to 90% noncondensing 0% to 100% noncondensing Minimum of 15 cm 16.0 in) on all vertical None Replace

to use

Voltage Amperes (AC) CrmsJt Ventilator 115 + 10% 2.8 100 + 10% * 100 f 10% 3.4 220 + 10% * 240 5 10% 1.6 Compressor pedestal 115 k 10% 4.7 Courtesy Puritan-Bennett Carp, G&bad. CA The ventilator module and pedestal will be shipped separately tPower consumption and amperage assume the connection of a Cascade II or equivalent *Values not specified at this printing. §Aiway pressure is measured at the patient Y. Table 23-5 hm Eubanks and BorxY

Frequency (Hz1 60 k 5% 60 k 5% 50 + 3% 50 -f 3% 50 t 3% 60 t 5%

Model module

batteries

(21 before

sides

returning

3 ELECTRICAL

inl ini in) ini inl inl

SPECIFICATIONS

humidifier.

Continued. DM, June

1991

391

TABLE

3 (cont’d.).

Leakage current: module with sor pedestal Power cord Internal

batteries

100 * 100 + 220 + 240 t Less than

ventilator compres-

* 6.4 * 2.6 at 115 V

60 50 50 50

* * + 2

5% 3% 3% 3%

125 (240) V AC hospital grade, UL and CSA approved, 305 cm 110.0 ft) Lead acid, 2.1 V DC typical, General Electric, sealed X-cell, 5 ampere-hour rating

121

4 PNEUMATIC SPECIFICATIONS Source pressure Oxygen (DISS 9/l&181, medical grade, dry Air (DISS 314-161, medical grade, dry Source flow: air and

10% 10% 10% 10% 100 Pg amperes

241 to 689 kilopascal to 100 PSIGJ 241 to 689 kilopascal to 100 PSIG) 190 Wmin, minimum PSIG

oxygen 5 VENTILATOR DATA Gas inlet protection: Filtering capability, air and oxygen Water filter Operator-selected parameters Tidal volume Respiratory rate Peak inspiratory flow, maximum Sensitivity, inspiratory O,% Plateau PEEPKPAP pressure Operator-selected alarm thresholds High-pressure limit Low inspiratory pressure Low PEEP/CPAP pressure Low exhaled tidal volume Low exhaled minute volume High respiratory rate Operator-selected modes CMV SIMV CPAP § Inspiratoty flow waveforms square Descending ramp Sine

135 (35 at 35

Particle size 0.3 Pg with Not intended to remove gas. Use dry gas only.

99.8% water

efficiency vapor from

0.10 to 2.50 L 0.5 to 70 BPM 10 to 120 Wmin, operator selected 180 L/min, during spontaneous breathing 0.5 to 20 cm Ha0 below PEEP 21% to 100% 0.0 to 2.0 seconds 0 to 45 cm H,O 10 to 120 cm H,O 3 to 99 cm H,O 0 to 45 cm H,O 0.00 to 2.50 L 0.00 to 60.0 L 0 to 70 BPM

Selects [Reserved

Selects

modes

of ventilation

for future

waveform

enhancements]

for mandatory

DM,

breaths

June

1991

Operator-selected submodes: 100% 0, suction Manual inspiration Manual

Switches 0,W to 100 for 2 min Commands the delivery of one mandatory breath Commands the delivery of one mandatory sigh breath 11.5 X Tidal volume) One sigh breath every 100 breaths Activates nebulizer for 30 min

sigh

Automatic sigh Nebulizer Operator-selected alarm control Alarm silence Alarm reset Alarm indicators High-pressure limit Low exhaled tidal volume Low pressure 0, inlet Low inspiratory pressure

keys

Low exhaled minute volume Low pressure air inlet Low PEEPCPAP pressure High respiratory rate Low

battery

Apnea I:E Exhalation

valve

leak

Power disconnect alarm Alarm summary display Ventilator inoperative (red) Alarm (red1 Caution (yellow) Backup ventilator (redt Safety valve open bed) Normal (blue) Operator-selected or monitored parameters Airway pressure Exhaled volume Breath-type indicator lights lautomaticl Assist Spontaneous Sigh Plateau Mean airway Peak airway PEEPCPAP

pressure

Silences audible Resets ventilator

alarm for 2 min to prealarm state

of alert

Airway pressure* exceeds alarm threshold Tidal volume is below alarm threshold Supply 0, pressure is below 35 PSIG Airway pressure during delivery of a mandatory breath is below alarm threshold Minute volume is below alarm threshold Supply air pressure is below 35 PSIG Airway pressure is below alarm threshold Actual respiratory rate exceeds alarm threshold Less than 1 hr reserve power for audible alarm No breath detected for 20 s Actua value greater than 1: 1 Gas flow past the exhalation flow. Sensor during breath delivery is 50 mL or 10% of delivered volume, whichever is greater AC power to the ventilator is interrupted

Illuminates

to indicate

Continuous Continuous

display, display,

Illuminates during breath cycle

ventilator

status

breath-by-breath breath-by-breath

appropriate

breath

or

In cm H,O: three-digit display (maximum two digits to the right of the decimal)

of

pressure pressure Continued.

DM, June

1991

393

TABLE

3 kont’d.).

Plateau pressure Kespiratory rate

In breaths per minute: three-digit display (maximum of one digit to the right of the decimal) Two-digit display (maximum of one digit to the right of the decimal1 In liters: three-digit display (maximum of two digits to the right of the decimal)

I : E ratio Tidal

volume

Minute volume Spontaneous minute volume Safety modes of operation Apnea ventilation

Temporary preset

Backup ventilator Disconnect ventilation Safety valve open

Patient breaths ventilator

Self-diagnostics Power-on self test (POST) Extended self-test K~TI Ongoing checks I: E ratio check Lamp test Output signals Remote nurse’s Analog signals

call for pressure

ventilatory parameters

room

support

with

air unassisted

factory

by

Automatic after power on I10 s duration) Operator selected C-3 min duration Automatic, continuous during ventilator operation Automatic, with parameter changes Operator selected

and

flow

For remote For display recording

indication of alarm of parameters on separate device

tire pneumatic system that, through the proportional solenoid valves, mixes and shapes the wave forms of inspired gases; the correction circuit that converts delivered gases to BTPS; memory storage of operational data; and monitoring alarm and digital and message display systems. An example of the safety features incorporated into this unit includes the microprocessor self-testing systems that automatically search for any operational problems and activate relevant corrective or safety overrides. These systems are called power-on self-test, extended self-test, lamp test ongoing checks, I : E ratio check, and nebulizer flow check. Both power-on and extended self-tests should be used to determine whether the ventilator is operational. If a major problem is detected, the ventilator automatically switches to a standby mode until the difficulty is overcome. The extensive options offered by this ventilator are presented in Fig 38, which shows the control panel. It is beyond the scope of this work to discuss further details about this ventilator. Additional information can be obtained by contacting the manufacturer. 394

DM, June

1991

FIG 38. Control

panel

OTHER

for Bennett

7200

ventilator.

THIRD-GENERATION

ADULT

VENTILATORS

Other third-generation ventilators that are growing in popularity but have not been covered in detail in this monograph include: InterMed Bear 5 (InterMed Bear Corporation, Riverside, CA); Bird 6400 ST (Bird Products Corporation, Palm Springs, CA); Hamilton Veolar/ Amadeus (Hamilton Medical Corporation, Reno, NV); Advent (Ohmeda Corporation, Madison, VW; Irisa (PPG Biomedical Systems, Lenexa, KS); Adult Star (Infrasonico Inc., San Diego, CA); and Engstrom Erica (Engstrom Medical, Sweden). NONCONVENTIONAL HIGH-FREQUENCY

VENTILATORS VENTILATION:

HISTORY

High-frequency ventilation (HF’V) is a generic term describing the operational performance of ventilators that deliver rapid respirator DMJune

1991

395

rates at small tidal volumes. The exact cycling rates that qualify a device as a high-frequency ventilator are still undecided, but the Food and Drug Administration has established 150 bpm to qualify a ventilator’s performance as high frequency. High-frequency mechanical ventilation is the use of mechanical ventilators that can deliver cycling rates greater than 60 bpm. HFV is still an evolving technology that has been used primarily in rigidly controlled laboratory and clinical studies. Its specific clinical applications are still to be defined and documented. The history of HFV can be traced back to 1949, when James L. Whittenberger, M.D., worked with panting dogs. As early as 1954, Virginia Apgar, M.D., and Richard Day, M.D., tried high-frequency ventilation on newborns with atelectasis. In the early 1970s the Swedes experimented with reverse I: E ratios for the ventilation of neonates. More recently, Kim Bland, M.D., et al reported a 92% survival rate with the use of HFV on neonates with respiratory distress syndrome. In 1980 and 1981, Forrest Bird, M.D., expanded the concept of HFV to include intermittent percussive ventilation with monopulsing and counterpulsing intrapulmonary percussion on adults and neonates. This approach not only provides alveolar ventilation but also causes internal percussion of the airways to promote clearance of secretions.

TERMS

AND

DEFINITIONS

Some terms and definitions that are currently HFV and its application include: l l l l l l

used to describe

Wave- complete respiratory cycle Amplitude-size of the wave (force) Frequency-number of waves passing a given point in one second HFPPV/HFV- high-frequency positive pressure ventilation cps-cycles per second (rate) Hz-Hertz, cycling frequency (one wave/s)

Although there is disagreement as to what cycling rates constitute a high-frequency ventilator, there is general acceptance that HFV is primarily generated by one of three methods discussed below. HIGH-FREQUENCY

POSZTZVE

PRESSURE

VENTILATION

HFPPV describes a ventilator that cycles in the bpm and has adjustable I : E ratios. To prevent the dioxide, the delivered tidal volume approximates the patient. Inspiration is active and is followed 336

range of 60 to 150 buildup of carbon the dead space of by passive exhalaDMJune

1991

tion. A conventional type of device.

ventilator

can be modified

to function

as this

A high-frequency jet ventilator is a custom-designed device that delivers small tidal volumes of 3 to 6 mhkg at 60 to 900 bpm. As with the HFPPV, inspiration is active, and is followed by passive exhalation. HFJV (Fig 391 is based on the principle of a source gas delivered to a rapidly cycling electromechanical valve called a solenoid. When the valve is open, gas streams through a narrow cannula, which is placed inside a tracheal tube. As gas exits the cannula opening (jet), it is accelerated and entrains supplemental gas before entering the trachea. In this mechanism, inspiratory gas travels through the center of the tube while exhalation occurs simultaneously along the walls of the tube (see coaxial flow in Figs 39 and 40). A stated advantage of this type of device is the fact that adequate ventilation may be achieved in patients with major airway complications using relatively low peak inspiratory pressures. This technique may offer an alternative to conventional ventilation, in which high peak airway pressures interfere with cardiac output.

FIG 39. Gas flow OM.

June

through 1991

high frequency

let ventilator. 397

FIG 40. Coaxial flow concept

HIGH-FREQUENCY

OSCKLATION

High-frequency pulse generators provide the greatest range of cycling frequencies. These devices can deliver rates of 3000 to 4000 bpm. Tidal volumes are small, and oscillation occurs during both inspiration and exhalation. Oscillation is created by a rapidly vibrating radio-type speaker, a rubber diaphragm, or a rapidly cycling piston. DYNAMICS

OF GAS FLOW

An exact explanation for the dynamics of gas flow during HFV has not been formulated, but two theories have been postulated. Augmented DifSusion or Enhanced Diffusivity HFJV causes turbulence that excites proximal molecules, which in turn excite distal molecules until diffusion gradients are satisfied throughout the lung. Cowial Flow With HFV, gas flow is laminar and bidirectional. In coaxial flow, inspiration occurs through the center of the airway while simultaneous exhalation (CO,) is leaving the lung along the walls of the airway (Fig 401. 398

mt

June

1991

POTENTIAL

BENEFITS OF HFV

Noted benefits of HFV include: reduced barotrauma; reduced interference with intracranial pressure; reduced interference with cardiovascular pressure; prevention of aspiration during cardiopulmonary rescusitation (with expiratory rates greater than 66% of the total cycle and cycling rates greater than 60 bpm); increased ventilation in noncompliant lungs with bronchopulmonary fistulas; and more uniform ventilation in patients with obstructed airways.

DISALIVANTAGES

OF HFV

Disadvantages include inadvertent (auto) PEEP and CO, retention at cycling rates greater than 200 bpm; retardation of mucocilliary transport; poor humidification systems; and the lack of understanding of HF’V.

FUTURE OF HFV It is still too early to determine exact indications and contraindications for HFV. Human and animal studies are still under way at selected medical centers, and data are being gathered and compared. The technology is new and problems such as poor humidification and medication administration by means of aerosols still must be resolved. In general, clinical data have shown that in certain patients HW works better than other methods of ventilation. Perhaps HFV may be compared to the use of CPAP, PEEP, and IMV just a few years ago. Clearly, it is an interesting technology that should be developed and used when indicated; for example, when more conventional ventilatory methods do not achieve desired results.

WEANING

THE PATIENT

Maintenance of patients through mechanical ventilation should be regarded as a very sophisticated procedure that is unnatural and psychologically frightening to the patient, and financially costly to the hospitalll For these and other reasons, patients should be weaned as soon as possible. The literature has numerous articles listing criteria or variables useful in predicting successful weaning of patients from ventilators. General criteria for weaning are listed in Table 4. Once these criteria have been met, one of the following techniques may be selected

DIM,

June

1991

399

TABLE General

4. Weaning

Criteria

Spontaneous tidal volume 3-5 mUkg Vital capacity >lO- 15 mWkg body weight Maximum negative inspiratov force >25-30 cm H,O Minute ventilation >lO L’min Maximum voluntary ventilation > 2X resting V, PaO, SO mm Hg with FIO, 5 0.35 A-aD0, <400 mm Hg with FIO, = 1.0 PaCO,
= alveolar

to arterial

PaO,

= arterial

of inspired diiiksion

oxygen A = of oxygen.

oxygen;

for the discontinuance of mechanical ventilation. With each of the techniques, it is assumed that the patient is removed from the ventilator with an endotracheal tube in place with the cuff inflated. It is also assumed that proper humidification of inhaled gases is provided.

COLD TCJRKEY The patient is connected to a T-tube (blow-by system) or open ventilator circuit for 20 to 30 minutes. During this period, the patient’s clinical response is constantly monitored and is confirmed by arterial blood gases after 30 minutes. If patient response is unsatisfactory, the ventilator is reconnected. An important consideration with this technique is that ventilation circuits and humidifiers have increased airflow resistance and will increase the patient’s work of breathing.

INTERMITTENT

T-TUBE

The patient is connected to a T-tube (blow-by system) for undetermined trial periods covering 5 to 30 minutes every 30 to 120 minutes. Each trial period is gradually increased by 5 to 10 minutes until the patient can tolerate 30 minutes without mechanical ventilation. The patient is carefully monitored during each trial period and reattached to the ventilator if indicated. After a 30-minute trial is at400

DM, June

1991

tained, arterial blood gases are drawn to confirm the patient’s ventilatory status. If gases are satisfactory, the regime can be expanded with even longer time intervals until weaning is completed. INTERMl7TENT

MIWDATORY

VENTILATION

The use of this modality was previously explained in detail. A typical weaning protocol includes reducing the IMV rate 1 to 3 bpm every 30 minutes until all mandatory mechanical breaths are eliminated. As the IMV rate is reduced, the patient should be carefully monitored and confirmed by arterial blood gases. PRESSURE SUPPORT VENTILATION The theory and use of PSV to reduce the work of breathing and to facilitate weaning was previously discussed. The technique employed for weaning the patient is similar to that for IMV with PSV levels reduced in increments of 3 to 5 cm H,O at 30-minute intervals, until a PSV level of 3 to 5 cm H,O is reached. At this level, weaning normally is complete. As with other techniques, the patient must be carefully monitored during the weaning procedure and arterial blood gases drawn as required. AUTO-PEEP: A POTENTIAL MECHANICAL VENTILATION

COMPLICATION

OF

The complications most frequently observed during mechanical ventilation are related to pressure and/or volume. Complications include interference with cardiac output, mechanical injury to lung structure, massive gastric distension, increased intracranial pressure, and ventilator misuse or malfunction. One area that is more subtle yet potentially hazardous is the presence of an unintentional elevated positive pressure on exhalation. This phenomenon has been called inadvertent PEEP, occult PEEP, intrinsic PEEP, and more recently, auto-PEEP. Mechanically, auto-PEEP describes a condition of accidental lung hyperinflation in which the lung volume at end expiration is greater than the relaxation volume of the respiratory system. Physiologically, this results in gas trapping that may be dynamic, with pressure levels increasing with each mechanical inflation until functional residual capacity exceeds inspiratory reserve capacity. At this point, cardiac output is drastically reduced due to transmural pressure generated across the superior and inferior vena cavae. As venous return is hindered, pulmonary blood flow is reduced and cardiac output drops. DM,

June

1991

401

Auto-PEEP, therefore, is a physiological considered whenever a patient receives Mathematically, it may be described as: Trapped Gas Volume Compliance Volume

condition that must be mechanical ventilation.

= auto-PEEP

Auto-PEEP is most frequently caused by rapid respiratory rates and/or minute volumes that are excessive for the patient’s expiratory flow capacity. It is imperative that the lung be given adequate time for emptying before receiving a subsequent inspiration. Auto-PEEP is not easily observed because air trapping is not recorded on the ventilator pressure monitor, which is opened to the atmosphere during exhalation. However, if the ventilator’s exhalation valve is blocked at the end of exhalation, just prior to a subsequent inspiration, the manometer will display the level of auto-PEEP. As auto-PEEP levels increase, ventilation efficiency decreases and the advantages of modalities such as pressure support ventilation to reduce the work of breathing may be neutralized. Clinicians must therefore consider the presence of auto-PEEP whenever a patient’s total lung volume is consideredtotal airway pressure includes auto-PEEP. PTOT

= P,,

+ P, +

A,

Mechanisms to reduce auto-PEEP include: reducing minute ventilation, increasing inspiratory flow rates to extend the time for expiration, decreasing expiratory airflow resistance, and adding CPAP as a counter-balance. The amount of CPAP required should be sufficient to decrease the inspiratory threshold load, to decrease the work of breathing, and to decrease the minute ventilation CV,). The addition of CPAP to the spontaneously breathing patient suspected of having auto-PEEP is currently a research procedure and should be done only in extreme circumstances. CONCLUSIONS The development of mechanical ventilators has been an evolutionary process, with each new generation of device building on the technology and experience gained from previous ones. The transition of ventilators from second- to third-generation units is probably the most dynamic because of the integration and utilization of microprocessors to control the ventilator and monitor its function and patient responses. In order for practitioners to folly understand the operation of ventilators they must understand the logic used as the basis for their development and operations. 402

DM.

June

1991

This understanding is best achieved by applying the principles of operation learned with first- and second-generation ventilators to third-generation units. Failure to fully comprehend how early devices worked and the rationale for their clinical utilization will and has resulted in information gaps that prevent practitioners from making informed decisions about the operation of the newer ventilators or to interpret and apply the data presented by these sophisticated monitoring systems. This monograph presented much of the theory that is required to understand how ventilators work and, perhaps of equal importance, when they may not work. Ventilators are mechanical tools that can and do fail. They are designed to function within certain limits and will not ventilate patients when these limits are exceeded. It is imperative that informed users of ventilators recognize this failure to perform and take appropriate life-saving action. REFERENCES

1. Kacmarek RM, Meklaus GJ: The new generation of mechanical ventilators. o-it care czin 1990; 6:561. 2. Levine S, Levy S, Henson, D: Negative pressure ventilation. Crit Care Cfin 1990; 6:505-527. 3. Comroe JH: Physiology of Respiration. Chicago, Year Book Medical Publications, 1975. 4. Proctor HJ, Woolson R: Prediction of respiratory muscle fatigue by measurements of the work of breathing Surg Gynecol Obstet 1973; 136:367-370. 5. Maclntyre NR: Weaning from mechanical ventilatory support: Volume-assisting intermittent breaths versus pressure-assisting every breath. Respir Care 1988; 33:121-122. 6. Marini JJ: Breathing effort and work of breathing during mechanical ventilation. Probl Crit Care 1990; 4:184-l!%. 7. Marini JJ: Strategies to minimize breathing effort during mechanical ventilation. Crit Care C/in 1990; 6:637. 8. MacIntyre N, Nishimura M, et al: Special report: The Nagoya conference on system design and patient-ventilator interactions during pressure support ventilation. Chest 1990; 97:1463-1465. 9. Eubanks D, Bone R: Mechanical ventilation. In: Comprehensive respiratory care. A learning system. 2nd ed. St Louis, CV Mosby Co, 1990, pp 664-669. 10. Kacmarek RM, Meklaus GJ: Micro-processor-controlled ventilators. Crit Care C!in 1990; 6:167. 11. Rosen RL, Bone KC: Economics of mechanical ventilation. Clin Chest Med 1988:9:163-168. RECOMMENDED

READINGS

Banner MJ: Expiratory positive pressure valves: Flow resistance and work of breathing. Respir Care 1987; 32:431-439. Banner MJ, Downs JB, Kirby RR, et al: Effects of expiratory flow resistance on inspiratory work of breathing. Chest 1988; 93:795. Beaty CD, Ritz RH, Benson MS: Continuous in-line nebulizers complicate pressure support ventilation. Chest 1989; 96:1356-1360. DIM, June 1991

403

Benson MS, Pierson DJ: Auto-PEEP during mechanical ventilation of adults. Respir Care 1988; 33:557. Black JW, Grover BS. A hazard of pressure support ventilation. Chest 1988; 93:333-335. Bone RC: Pulmonary barotrauma complicating mechanical ventilation. Am Rev Respir Dis 1976; 113tSuppl):l88. Abstract. Bramson RD, Hurst JM, Davis K, et al: Measurement of maximal inspiratory pressure: A comparison of three methods. Resp Care 1989; 34:789-794. Braun NMT: Intermittent mechanical ventilation. Ckn Chest Med 1988; 9:153-162. Braun NMT, Faulkner J, Hughes RL, et al: When should respiratory muscles be exercised? Chest 1983; 84:76. Brochard L, Pluskwa F, LeMaire F: Improved efficacy of spontaneous breathing with inspiratory pressure support. Am Rev Respir Dis 1987; 136:411-415. Brown DG, Pierson DJ: Auto-PEEP is common in mechanically ventilated patients: A study of incidence, severity, and detection. Respir Care 1986; 31:1069- 1674. Calderini E, Petrof B, Gottfrled SB: Continuous positive airway pressure (CPAP) improves efficacy of pressure support (PS) ventilation in severe chronic obstructive pulmonary disease (CQPD). Am Rev Respir Dis 1989; 139Al55. Carlson CA, Boysen PG, Banner MJ, et .al: Conventional vs high frequency jet ventilation for extracorporeal shock wave lithotripsy. Anesthesiology 1985; 63i4.530. Chang HK: Mechanisms ofgas transport during xentibtion by high frequency oscillations. J A@ Physior 1984; 56553. Fiastro JF, Habib MP, Quan SF: Pressure support compensation for inspiratory work due to endotracheal tubes and demand CPAP. Chest 1988; 93:499. Giron AE, Sasson CSH, Ely EA, et al: Inspiratory work of breathing on demandflow and flow-by-continuous positive airway pressures. Am Rev Respir Dis 1987; 135A55. Gurevich MJ, Gelmont D: importance of trigger sensitivity to ventilator response delay in advanced chronic obstructive pulmonary disease with respiratory failure. Crit Care Med 1989; 17:354. Haake R, Schlichtig R, Ulstad DR, et al: Barotrauma: Pathophysiology, risk factors, and prevention. Chest 1987; 91608. Ho L, MacIntyre N: Pressure supported breaths: ventilatory effects of breath initiation and breath termination design characteristics. Crit Care Med 1989; 1726.

Kacmarek R: The role of pressure support ventilation in reducing work of breathing. Respir Care 1988; 33:99. Kacmarek RM, Spearman CB: New generation mechanical ventilators: Siemens Servo SOOC. Respir Times 1987; 2:7-S. Kacmarek RM, Spearman CB: New generation mechanical ventilators: The Puritan-Bennett 7200/72OOA. Respir Times 1988; 3:7-9. Kacmarek RM, Spearman CB: The new generation mechanical ventilators: The Hamilton Medical Veolar. Respir Times 1388; 38-S. Kimball WR, Leith DE, Robins AG: Dynamic hyperinflation and ventilator dependence in chronic obstructive pulmonary disease. Am Rev Respir Dis 1982; 126:391-995. Leatherman J, Ravenscraft SA, Iber C, et al: High peak inflation pressures do not predict barotrauma during mechanical ventilation of status asthma. Am Rev Respir Dis 1989; 139Al54. LeMaire F, Teboul JL, Cinotti L, et al: Acute left ventricular dysfunction during a04

DA!, June 1991

unsuccessful weaning from mechanical ventilation. Anesthesiology 1988; 69:171- 179. MacIntyre NR: Respiratory function during pressure support ventilation. Chest 1986; 89:677-683. MacIntyre NR, Leatherman NE: Mechanical loads on the ventilatory muscles: A theoretical analysis. Am Rev Respir Dis 1989; 139:968. MacIntyre NR, Leatherman NE: Ventilatory muscle loads and the frequency-tidal volume pattern during inspiratory pressure-associated (pressure-supported1 ventilation. Am Rev Respir Dis 1990; 141:327-331. Marini JJ: The role of the inspiratory circuit in the work of breathing during mechanical ventilation. Respir Care 1987; 32:419. Marini JJ: Monitoring during mechanical ventilation. Clin Chest Med 1988; 9:73-100. Marini JJ: Work of breathing. In: Kacmarek RM led,: Current Respiratory Care. Philadelphia, BC Decker, 1988, pp 188-194. Marini JJ: Mechanical ventilation. In: Simmons DH (edl: Current Pulmonology. Chicago, Year Book Medical Publishers, 1988, p 164. Marini JJ: Should PEEP be used in airflow obstruction? Am Rev Respir Dis 1989; 140 :1 Marini JJ, Crooke PS, Truwit JD: Determinants and limits of pressure preset ventilation. A mathematical model of pressure control. J Appr Physiol 1989; 67:1081. Marini JJ, Kirk W, Culver BH: Flow resistance of the exhalation valves and PEEP devices used in mechanical ventilation. Am Rev Respir Dis 1985; 131:850-854. Marini JJ, Rodriguez RM, Lamb VJ: The inspiratory workload of patient-initiated mechanical ventilation. Am Rev Respir Dis 1986; 134:902-909. Marini JJ, Rodriguez RM, Lamb VJ: Bedside estimation of the inspiratory work of breathing during mechanical ventilation. Chest 1986; 8956-63. Milic-Emili J: Is weaning an art or a science? Am Rev Respir Dis 1986; 134:1107. Milic-Emili J, Gottfried SB, Rossi A: Dynamic hyperinflation: Intrinsic PEEP and its ramifications in patients with respiratory failure. In: Vincent JL led): Update in Intensive Care and Emergency Medicine; Update 1987. Berlin, Springer-Verlag, 1387, pp 192-198. Pepe PE, Marini JJ: Occult positive end-expiratory pressure in mechanically ventilated patients with airflow obstruction. Am Rev Respir Dis 1982; 126:166- 170. Petrof BJ, Legare M, Goldberg P, et al: Continuous positive airway pressure reduces work of breathing and dyspnea during weaning from mechanical ventilation in severe chronic obstructive pulmonary disease. Am Rev Respir Dis 1990; 141:281-289. Rossi A, Gottfried SB, Zocchi L, et al: Measurement of static lung compliance of the total respiratory failure during mechanical ventilation: The effect of intrinsic positive end-expiratory pressure. Am Rev Respir Dis 1985; 131:672-677. Sasson CSH, Te ‘IT, Mahutte CK, et al: Airway occlusion pressure. An important indicator for successful weaning in patients with chronic obstructive pulmonary disease. Am Rev Respir Dis 1987; 135:107-113. Shikora SA, Bristrian BR, Borlase BC, et al: Work of breathing: Reliable predictor of weaning and extubation. Crit Care Med 1930; 18:157-162. Smith TC, Marini JJ: Impact of PEEP on lung mechanics and work of breathing in severe airflow obstruction. The effect of PEEP on auto-PEEP. J Appl Physiol 1988; 65:1488. Stock MC, Downs JB, Frolicker DA: Airway pressure release ventilation. Crit Care Med 1987; 15:462-466. Tobin MJ: Respiratory muscles in disease. C/in Chest Med 1988; 9:263-286. DM, June 1991

405

Tobin MJ, Lodato RF: Editorial: PEEP, auto-PEEP, and waterfalls. Chest 1989; 96:449-451. Tobin MJ, Perez W, Guenther SM, et al: The pattern of breathing during successful and unsuccessful trials of weaning from mechanical ventilation. Am Rev Respir Dis 1986; 134:1111-1118. Tuxen D: Detrimental effects of positive end expiratory pressure during controlled mechanical ventilation of patients with severe airflow obstruction. Am Rev Respir Dis 1989; 140:5. ViaIe JP, Annat G, Bertrand 0, et al: Additional inspiratory work in intubated patients breathing with continuous positive airway pressure systems. Anesthesiology 1985; 63:536. Wright PW, Marini JJ, Bernard GR: In vitro versus in vivo comparison of endotracheal tube airflow resistance. Am Rev Respir Dis 1989; 140:10-16. Younes M, Roberts D, Light RB, et al: Proportional assist ventilation (PAV): A new approach to ventilatory support. Am Rev Respir Dis 1989; 139A363.

406

DM,

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1991