Mechanical Ventilation in Respiratory Failure

Symposium on Respiratory Failure

Mechanical Ventilation in Respiratory Failure MD.,* and Gregg Stober, Stoher, R.R.T.t Roger C. Bone, M.D.,*

limits of a mechanical A thorough knowledge of the capabilities and lilnits ventilator is necessary before initiating treatlnent treatment of a patient. Through this knowledge, patient safety can be maximized and length of comlnitcommitment to a ventilator can be kept to a minin1Uln. minimum. The purpose of this article with several of the n10re more con11non common ventilais to familiarize the practitioner \tvith management of the patient requiring a ventilator. tors and techniques for managelnent A detailed presentation of all ventilators on the market is found f(mnd in Respi13 McPherson. 13 Equipment by :'vlcPherson. ratory Therapy Equiprnent

MECHANICAL VENTILATORS by the Inechanisln mechanism used for terMechanical ventilators are classified. bv mination of inspiration-cycling. A flow-cyc"'led flow-cycled ventilator ends inspiration common when a preset flow rate is reached. Few of these ventilators are in comlnon use. Pressure, volume, and time-cycled time-cvcled ventilators are the Inost most comn1only commonlv used. . . used. A pressure-cycled ventilator terlninates terminates inspiration when a preset maximum pressure is reached. Because of this, anything that affects ventilating pressure also affects the volume delivered. If airway resistance or volume changes. As resistance increases thoracic compliance changes, tidal volulne volume delivered is decreased as the preset presor compliance decreases, volulne volume cannot sure is reached more quickly. As a result, a consistent tidal volulne be guaranteed. For this reason, it is not the ideal type of device for longterm ventilator Inanagelnent. management. Pressure-cycled ventilators are Inore more frequently used for short-term post-anesthesia post-ancsthesia recovery. In either case, close observation and frequent measurement of tidal volume must be Inaintained. maintained. Pulmonary and Critical Care Medicine, Uni*Professor of Medicine, and Chief, Division of Pulnl0nary versity of Arkansas for Medical rv1edical Sciences, Little Rock, Arkansas \!edical Sciences, Little Rock, Hock, tDirector of Respiratory Therapy, University of Arkansas for Medical Arkansas lK07 HL00.518-01 HL00.51S-01 and a Puhnonary Pulmonary Chair Supported in part by NIH Academic Career Award 1K07 from the Arkansas Lung Association.

America-Vo!' 67, No. 3, May 1983 Medical Clinics of North America-Vo!.

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volume-cycled ventilator vcntilator ends cnds inspiration when a predetennined predetermined The volulne-cycled volume is delivered. Tilne-cycled Time-cycled nlachines machines stop flo\v flow after a certain tilne time determined along with a flow rate so that a has elapsed. This time limit is deterlnined consistent volume is delivered in each cach tiIne time frame. Both volulnevolume- and tilTIetimecycled ventilators deliver a relatively constant tidal volunle volume in spite of compliance. For this reason, rcason, these ventilators changes in resistance and conlpliance. are optimal for the treahnent treatment of respiratory failure and will be elnphasized emphasized in this article.

VENTILATION MODES Three lTIodes modes of lTIechanical mechanical ventilation are COnl1TIOnlv commonlv described: Assist/Control Ventilation. A predeterlnined Uln respiratory predetermined lnininl minimum rate is set to be delivered to the patient. The patient lnay may initiate additional breaths at his or her own rate. This lTIinimunl minimum rate will be delivered made, the patient is given an "asif there is no patient effort. If effort is lnade, sisted" breath, and the minimum nlininlUlTI rate will be exceeded. (lMV). A predeterlnined predetermined lnemeIntermittent Mandatory Ventilation (IM"l). chanical breath rate is set and between those breaths the patient is allowed to take a normal spontaneous breath. 99 machine delivers a preset respiratory Controlled Ventilation. The nlachine eHort. rate irregardless of patient effort.

VENTILATORY SETTINGS

Tidal Volume. Tidal volunle volume is the volunle volume delivered to the patient volume should be set during a normal ventilator breath. Normal adult tidal volunle between betw~en 8 and 13 lTIl ml per kg body weight. Respiratory Rate. The respiratory rate is the rate at which the venvolume breaths. In the assist/control lnode, modc, the tilator will deliver tidal volulne may override this rate and breathe nlore more rapidly at any tilne. timc. In patient nlay may take periodic spontaneous breaths, and the IMV mode, the patient lTIay respiratory rate set on the lnachine . nical breath rate. machine will be the lnechc. mech'·.nical Total respiratory rate equals mechanical nlechanical breaths plus spontaneous breaths. min and can be A starting point for respiratory rate is 12 to 14 breaths per nlin PaCO 2z or pH. adjusted according to desired PaC0 Peak Flow Rate. Peak flow rate is the nlaxilTIUlTI maximum flow rate at which the machine delivers the tidal volume. volulne. Most ~lost conlnl0nly, comlllonlv, ventilators deliver a square flow wave, and peak flow is reached very vcry quickly and ll1aintained maintained throughout inspiration. Tapered flow waves are available on sonle some ventilabc adequate to allow the tidal volulne volume to he tors. The flow rate should be be delivered quickly enough to allow for conlplete complete exhalation. Expiratory tilne time time and, preferably, double it. This should at least exceed inspiratory tilne allows for complete exhalation so that no air is "trapped." It also allows venous return to the right heart to return to nornlal, normal, helping to lnaintain maintain nlore normal cardiac output. As tidal volulne a more volume and/or respiratory rate increases, the peak flow must increase to lnaintain maintain a proper inspiratory/

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expiratory ratio. Using norlnal normal vollllnes volumes and rates, a peak flo\v flow of 40 liters (lpm) is usually adequate. per min nlin (lpnl) determines the ventilator's response to paSensitivity. This control deterlnines tient effort. The Thc sensitivity scnsitivity level is the point at which the ventilator \vill will assist the patient in the assist/control lnode modc or allow spontaneous breaths mode, sensitivity is turned off so that the Inachine machine in IMV. In the control lnode, does not respond to patient effort. Oxygen Concentration (FIO:J: nlay be set to deliver (FrOJ: This control may delivcr a ccnt oxygen. Oxygen toxicity is considered concentration of 21 to 100 per cent a possible side effect of a concentration greater than 50 .50 per cent for lnore more than two to three days. Care lnust must be taken to keep the concentration to maintaining adequate oxygenation. Unless the lowest possible level while nlaintaining Fr022 will be suffithe previous clinical setting assures you that a lower :FI0 cient, the initial setting should be 100 per cent oxygen. If that proves cient' excessive, the percentage should be tapered to avoid the danger of oxygen toxicity. Sighs. A breath larger than normal tidal volume (sigh) should be delivered if volumes less than 7 ml per kg are being used. If tidal volumes greater than 7 ml per kg are used, sigh breaths are probably not necessary. The purpose of the sigh is to prevent microatelectasis mieroatelectasis and its resultant normal sigh breath is one and one-half to two intrapulmonary shunting. A norlnal 1.5 times per hour. times the tidal volume delivered 10 to 15 Humidification. As the natural humidification mechanism of the upHunlidification. must be per airway is hypassed bypassed by the endotracheal tube, all gas delivered Inust humidified. Telnperature Temperature of the inspired gas must be moniwarmed and hUlnidified. tored to prevent tracheal and bronchial drying from excessive heat or mucosal drying from inadequate heat and humidity. It is recommended to keep inspired temperature close to hody body temperature without exceeding it. Positive End-Expiratory Pressure. Positive end-expiratory pressure ventilators.!l This increases functional or "PEEP" may be added to most ventilators. residual capacity and often allows for better hetter gas exchange. 33 It also tends to increase the effect of Inechanical mechanical ventilation on venous return as a positive pressure is maintained during expiration instead of a pressure of zero. PEEP he discussed in greater detail in the section on oxygenation. PE E P will be mechanism to vent exPressure Limits. All ventilators should have a lnechanism cessive pressure, if necessary. Inspiration is terminated when a certain maximum pressure is reached and exhalation is ilnlnediately immediately begun. This maxilnUln possihility of pulmonary barotrauma (i. (i.e., is to protect the patient from the possibility e., pneulnothorax, pneumothorax, pneumolnediastinuln, pneumomediastinum, and subcutaneous emphysema). Normally, this limit is set 15 1.5 to 20 Cln cm H 220 above normal ventilating pressure. An audible and visual alarlTI alarm should be present to alert the staff that excessive pressure prcssure has been generated. The causes of this can be many, including secretions, biting the endotracheal tube, fighting the respirator, pneumothorax, and right mainstem intubation. Monitors. All ventilators Inust must be equipped with a device to monitor must be observed ohserved frequently as decreased volvolumes delivered. This lTIUSt umes can result from leaks in the ventilator circuit, inadequate cuff volume, ventilator malfunction, pneulnothorax, pneumothorax, and so forth. 44 Any significant drop in volulTIe volume must be detected and corrected. An audible alarm should sound if a decrease in exhaled volume occurs.

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Alarms. In addition to the alarms for excessive pressure and reduced volumes, other alarm mechanisms must be present. There should be alarms to indicate insufficient oxygen line pressure and electrical failure. All alarms must be responded to immediately and the cause determined. Care should be taken not to set volume and pressure alarms too closely so that they sound or flash frequently when the patient may have caused some minor fluctuation by moving or not exhaling fully. When alarms occur frequently with no true problem occurring, they tend to be ignored by staff managing the ventilator. Expiratory Resistance. Resistance to exhalation is available on many ventilators. This forces the patient to exhale against a resistance much as a patient does with pursed-lip breathing. This is not frequently used for current ventilator management. BENNETT MA-l MA-I AND MA-2 VENTILATORS MA-I and its second generation, MA-2, are both volume-cycled The MA-1 ventilators. We will deal with the two ventilators separately as there are distinct differences. The MA-1 MA-I ventilator and control panel is shown in Figure 1. The specifications for the MA-1 MA-I ventilator are shown in Table 1. MA-I Ventilator MA-l This ventilator provides assist/control and controlled ventilation. Various adaptations are available to provide ventilation in the IMV mode. Mode. 1. Control. The ventilator may be operated in the control mode by turning the sensitivity control knob counterclockwise as far as possible. This causes the ventilator to be insensitive to patient effort and gives the preset respiratory rate only. may be operated in the assist/control 2. Assist/control. The ventilator 'may later in this mode by adjusting the sensitivity in the manner explained l~ter Table 1.

Specifications for Bennett MA-l Ventilator*

Tidal volume (normal and sigh): Rate: Peak flow rate: Pressure limit (normal and sigh): Sigh rate: Oxygen percentage: Nebulizer: PEEP: NEEP: Pressure manometer: Sensitivity:

Up to 2200 ml Calibrated from 6 to 60 breaths per min; uncalibrated from 60 to 100 + + breaths per luin. min. Modified for IMV rate from less than 1 to 60 breaths per min Approximately 15 to 100 liters per min 20 to 80 cm H 2 0 1, 2, or 3 sighs, 4, 6, 8, 10, or 15 times per hour or off Continuously variable from 21 to 100 per cent Inhalation only Approximately 0 to 15 cm H 220 Approximately 0 to - 9 cm H 220 - 10 to + 80 cm H 22 0 -10 +80 - 10 cm H 2 0 or self-cycle Stable from - 0.1 to -10 (+ pressure level) (+

*Abbreviations: PEEP == positive end-expiratory pressure; NEEP == negative end-expiratory pressure; IMV = intermittent mandatory ventilation. (Courtesy of Bennett Respiration Products, Inc., Los Angeles, California.)

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Figure 1. The MA-l ventilator and control panel. (Courtesy of Bennett Respiration Products, Inc., Los Angeles, California.)

section. The ventilator will then deliver a minimum mllllmum number of breaths determined by the rate knob. If patient effort is sufficient to reach the sensitivity level, additional breaths may be obtained by the patient. In this case, the total respiratory rate will vary dependent on patient demands. 3. IMV. The ventilator has no built-in mechanism for IMV ventilation. However, several mechanisms are available to provide this mode. One method is the insertion of a "demand" valve into the circuit. The sensitivity control is turned off as in the control mode. This assures that the ventilator delivers the predetermined amount of mechanical breaths only. In between mechanical breaths, the "demand" valve senses a drop in pressure in the ventilator circuit and adds flow to the system in order to compensate for this drop. This flow goes to the patient in the form of a spontaneous breath as it is given at a low enough flow rate to prevent a positive

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pressure breath. When the pressure in the circuit returns to baseline, the demand valve closes and the patient exhales. Another less expensive method is the insertion of a T-piece with a one-way valve into the circuit. A source gas at the desired Fr02 FI0 2 is fed into The one-way valve prevents mechanical breaths a reservoir at the T-piece. 'The from escaping the circuit and entering the reservoir. When the patient exerts effort for a spontaneous breath, the one-way valve opens and gas enters the circuit to provide the patient with fresh gas. Either of these methods is acceptable. One must keep in mind that this machine has no mechanism to detect patient effort when the sensitivity is turned off. Therefore, the ventilator delivers mechanical breaths at the preset rate irregardless of patient effort. Thus, mechanical breaths can be "stacked" on spontaneous breaths or delivered during exhalation and thus cause high pressure in the circuit. Most patients tend to synchronize with the ventilator, and this problem is usually minimal. Tidal Volume. The tidal volume control may be set between 100 and 2200 ml. This volume should always be confirmed with an accurate respirometer as all ventilators can be out of calibration and the volume delivered may be different than indicated on the knob. Respiratory Rate. The respiratory rate can be set from 0 to 60 breaths per min. Again, actual rate should always be timed and confirmed. Flow Rate. The peak flow rate is adjustable between 15 and 100 liters per minute. The ventilator delivers a peak flow determined by this knob. As airway pressure increases near the end of inspiration, the flow rate begins to taper toward zero. Sensitivity. Sensitivity may be adjusted from a self-cycling range down to -10 cm H 220. In the self-cycling range, the ventilator delivers one breath after another as the pressure needle approaches zero. In order to adjust the sensitivity to a point where the most minimal patient effort is required, the knob should be turned clockwise until the machine begins to self-cycle. Then the knob should be turned counterclockwise to the point where the machine ceases to self-cycle. This enables the patient to obtain an assisted breath with a minimal amount of effort. Oxygen Concentration Control. The MA-1 contains a built-in compressor unit. It may provide 21 per cent oxygen with only an electrical supply. To obtain percentages greater than 21 per cent, an oxygen source FIO"2 from of 50 psi must be provided. Then the machine can deliver an Fr0 oxygen 0.21 to 1.0. This setting should be confirmed with the use of an ;xygen analyzer. Sigh. Sighs may be given in multiples of 1, 2, or 3 at a rate of 4, 6, 8, 10, or 15 times per hour. The volume is variable up to 2200 ml. Humidification. Humidity is generally provided with a cascade humidifier. This is equipped with a heater so that adequate temperatures may be maintained. Temperature should be continually monitored with a thermometer placed in the inspiratory line as close as possible to the patient's airway. PEEP. PEEP may be added by the use of a valve mounted on the Hp. side of the ventilator. It is adjustable from 1 to 15 cm H 2 0. Higher pressures can be obtained depending on the condition of the valve. PEEP is

MECHANICAL VENTILATION IN RESPIRATORY FAILURE

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set by turning the valve and watching the airway pressure manometer. A A simple way of obtaining PEEP in the absence of a valve is by submerging the exhalation tubing in a column of water equal to the desired PEEP level. Expiratory Resistance. Expiratory resistance may be applied by turning that control knob clockwise. This is a variable resistance and cannot be set at a particular flow rate. Pressure Limit. The pressure limit is set independently for normal and sigh breaths at a level from 20 to 80 cm Hp. H 20. A good range for this limit is 15 to 20 cm H 220 above normal ventilating pressure. When this pressure is reached, the ventilator terminates inspiration. Monitors. The MA-1 MA-l is supplied with a spirometer to monitor exhaled volumes. It may be used to detect patient disconnects and leaks in the ventilator circuit. It also possesses an airway pressure manometer to observe ventilating pressure. Alarms. The ventilator has a visual and audible alarm to indicate that the pressure limit has been reached. A red light illuminates and a loud buzz sounds. The spirometer alarm is a continual high-pitched alarm which sounds when a predetermined minimal exhaled volume is not obtained. This alarm may be silenced for 20 seconds and automatically resets. The alarm may be turned off completely, and this switch must be checked frequently to ensure that the alarm is on. The I:E ratio alarm sounds when I:E time required for exhalation exceeds time for inspiration, an I: E ratio greater than 1:1. MA-2 Ventilator

The MA-2 ventilator has the following features that are different from the MA-I. Synchronized IMV. Synchronized IMV (SIMV) is provided. With SIMV the machine synchronizes with patient respiratory effort to deliver a mechanical breath simultaneously with patient effort. The machine determines an interval in which a meohanical breath must be delivered to achieve the preset minimum respiratory rate. If the patient produces negative pressure during that interval, the machine gives a breath. The patient may then take spontaneous breaths until it is time for the next mechanical breath. If no effort is made by the end of this interval, the mechanical breath is automatically delivered. A A rate from 0 or 1 per 3 minutes or up to 30 per min may be given. "Demand" Valve. The patient may take a breath larger than the preset volume if sufficient effort is exerted. If the patient continues to inhale at the end of inspiration, a drop in pressure is sensed and the "demand" valve opens, allowing flow until the pressure in the circuit becomes positive as the patient begins exhalation. Continuous Positive Airway Pressure (CPAP). (CPAPj. The ventilator may be set to an IMV=O IMV = setting, the PEEP mechanism turned on, and CPAP delivered. PEEP. With the use of the manufacturer's circuit, PEEP of up to 45 cm H 220 may be given. The PEEP mechanism is built in, and no external valves are necessary.

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Inspiratory Hold. The patient may be forced to hold the inspired breath for a period of time up to 2 seconds prior to exhalation. Humidification. The ventilator is provided with a cascade humidifier. This contains a probe inserted at the patient airway which monitors temperatures and feeds back to the heater to control temperature. Monitors and Alarms. A variety of digital readouts are present on the MA-2. They include actual respiratory rate, temperature, and oxygen concentration. An alarm sounds to indicate fluctuations of FI0 22 with preset limits. In addition to the normal high pressure alarm, the MA-2 has a low pressure alarm which sounds if pressure during mechanical breath does not exceed 10 cm H 220 above baseline pressure. It also sounds if pressure falls to 5 cm H 22 0 below baseline pressure for more than one second. An audio alarm bypass mechanism silences all alarms with the exception of the spirometer alarm for a period of 2 minutes. SIEMENS 900B AND 900C SERVO VENTILATORS 900C Servo ventilators are time-cycled ventiThe Siemens 900B and 900e lators. We will cover the 900B first, then outline the changes made on the 900C. 900e.

Siemens 900B Ventilator Figure 2 shows the Siemens 900B ventilator, and the specifications are shown in Table 2. This ventilator provides assist/control, synchronized intermittent, or continuous positive airway pressure modes of ventilation. Mode. 1. Assistlcontrol. Assist/control. The ventilator may be operated in the assistl assist/ control mode by turning the lower right hand knob to the left side of the vertical line marked "sigh." intermittent 11Wndatory mandatory ventilation (SIMV). The Servo 2. Synchronized intennittent . delivers IMV in a synchronized manner. With SIMV the. mechanical breaths are delivered in synchrony with patient effort. This mode may be obtained by turning the knob to the right side of the vertical line to the f/2, f/5, or fl10 fllO setting. This programs the ventilator to divide the preset respiratory rate by the number corresponding with the setting. For example, a respiratory rate of 14 and an IMV setting of f/2 would deliver a mechanical breath rate of seven breaths per minute. The machine delivers synchronized breaths by determining a time frame in which mechanical breaths must be delivered to guarantee a minimum respiratory rate. At the beginning of the segment of time when a mechanical breath is due, the machine senses patient effort and delivers the breath at the time effort is made. If no effort is made by the end of the time segment, the machine then delivers a mechanical breath. In between the mechanical breaths, the inspiratory valve is opened when patient effort drops the pressure in the tubing to the "triggering level" and allows gas to flow to the patient. The valve shuts and the expiratory valve opens to allow exhalation when positive pressure due to respiratory muscle relaxation and when increased intrathoracic pressure occur. The predetermined number of mechanical breaths will always be delivered irregardless of presence or lack of patient effort. The patient may take spontaneous breaths at any rate desired.

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Figure 2. The Siemans 900B ventilator. (Courtesy of Siemens-Elema Ventilator Systems, Elk Grove Village, Illinois.)

3. Continuous positive airway pressure (CPAP). CPAP may be delivered by turning the mode knob to the "0" setting on the "IMV" side. The amount of CPAP exerted is determined by the PEEP valve to a setting equal to the desired CPAP level. The pressure is read on the airway pressure manometer. The triggering level is then set at a level 2 cm H 220 below the CPAP level. The machine then allows complete spontaneous ventilation in the same manner it uses in the SIMV mode. Tidal Volume. The Servo provides ventilation based on minute vol-

Table 2.

Specifications for Siemens Servo Ventilator 900B*

Minute volume: Rate: Inspiration per cent: Pause time per cent: Sensitivity (patient triggering): Pressure limit: Working pressure (driving force): Flow pattern switch: Sigh system: Displays: Pressure meter: Expired minute volume meter: Alarms High and low minute volume: High pressure limit: 2-minute alarm silence: Electric power disconnect:

IllQre than 25 liters per min 0.5 to more 6 to 60 breaths per min on assist-control mode or 6 to 60 divided by 2, 5, or 10 on IMV 15, 20, 25, 33, or 50 per cent of set ventilatory cycle time established by rate control 0.5, 10, 20, or 30 per cent of set ventilatory cycle time established by rate control Variable from - 20 to + 45 cm H 2 0 pressure Adjustable up to 100 cm H 2 0 pressure Adjustable up to 100 cm H 20 pressure Square wave or sine wave One sigh every 100 breaths at double tidal volume or

off ofl'

- 20 to 100 cm H 20

o to 30 liters per min Audio and visual Audio and visual Automatically reset Approximately 1 min audio signal

Siemens- Elema Ventilator Systems, Elk Grove Village, Illinois. *Courtesy of Siemens-Elema

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volume. It does possess a ume. It possesses no knob to directly set tidal volulne. volume lnay may be calculated by dividing the nlinminminute volume knob. Tidal volulne volume is guaranteed in the assist/ ute volume by respiratory rate. Tidal volunle "Serv(/' lllechanisln mechanism that gives the ventilator its name. control mode by the "Servo" The ventilator monitors delivered flow rate before it leaves the ventilator. It continually compares this to the flow rate required to deliver the tidal volume. If the flow rate drops in the face of increased back pressure, the inspiratory valve opens further to maintain the necessary flow rate. As a result, tidal volumes are guaranteed in spite of fluctuating compliance and volume can be set from 0.5 to 30 liters per min. airway resistance. Minute volullle may be set between 6 and 60 Respiratory Rate. The respiratory rate lnay breaths per min in the assist control mode. Rates between 0.6 and 30 breaths per min may be given in SIMV. Flow Rate. The peak flow rate is determined by the inspiratory time percentage control knob. The lnachine machine delivers a flow rate based on what percentage of the minute is to be spent in inspiration. The flow rate may minute volulne volume by the reciprocal be calculated by multiplying the preset lllinute of the inspiratory time percentage, which can be set at 15, 20, 25, 33, or 50 per cent. The flow wave can be set in a square or accelerating flow pattern. Sensitivity. Sensitivity is set on the "triggering level" knob. This determines at what pressure level the inspiratory valve will open to deliver a mechanical or spontaneous breath. It should be set at a level 2 to 3 cm H 220 below the baseline pressure level. Oxygen Concentration Control. The Servo contains no built-in commay be delivered by use of an pressor unit. Varying percentages of oxygen lnay oxygen air blender connected to the high pressure inlet or by low flow vVhen using flowlneters, flowmeters, the peak flowmeters into the low pressure inlet. When limited to the flow provided. Percentages of oxygen flow of the machine is liInited must be analyzed may be set from 21 to 100 with the use of a blender and Inust for confirmation. . Sigh. Sighs may be given in the assist/control mode only. Upon turnmode the Inachine machine delivers a norlnal normal tidal volume ing the knob to this Inode volume-sized sigh breath. breath. Then the next breath is a double tidal volulne-sized After this sigh, every hundredth breath will be a sigh breath. . Working Pressure. The available working pressure Inay may be adjusted up to 100 cm H 220. If pressure limited ventilation is desired, the working pressure may be lowered to the prescribed pressure limit. Gas flow will cease when this pressure is reached. Because of this, the preset working pressure should always be above the pressure limit when using volumelimited rather than pressure ventilation. Otherwise, flow will be stopped at the working pressure and inspiration will cease. No alarm will indicate this until the low minute volume lilnit limit is reached. Humidity may be provided with a cascade type huHumidification. HUlnidity midifier. Siemens has also recently marketed a hygroscopic humidifier which traps the patient's exhaled moisture and re-moisturizes inspired gas with it. PEEP. The PEEP valve is located on the expiratory outlet. It may be adjusted from 0 to 50 cm PEEP. PEEP level is adjusted by manipulat-

YiECHANICAL VENTILATION IN RESPIRATORY RESPIHATOHY FAILURE FAILt:RE MECHANICAL

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ing the valve while watching the point where the airway pressure needle rests on end-exhalation. limit is set on the knob so indicated. Pressure Limit. The pressure liInit When this level is reached, inspiration is tern1inated. terminated. Inspiratory Pause Time. An inspiratory pause time percentage may be set at 5, 10, 20, or 30 per cent of each ventilatory cycle. he Maximum Expiratory Flow. Maximum expiratory flow rate can be from infinity to one liter per min. adjusted froln Monitors. The Servo is equipped with monitors to display expired minute volume and airway pressure. A separate monitor can be added to read end tidal carbon dioxide. Alarms. The ventilator has visual and audible alarms to indicate high or low minute volumes adjustable between 1 and 25 liters per lnin. min. The low volume will detect circuit leaks, patient disconnects, cuff leaks, and so forth. The high limit will detect hyperventilation; the light illuminates and an intermittent beeping is audible. When the high pressure limit is reached, a solitary beep is sounded and a red light flashes. When a blender is in use, audible, low source gas pressure alarms are present. An alarm is also present to detect electrical power failure. Siemens 900C Ventilator

gooe. It Siemens has updated the Servo ventilator and marketed the 900C. has the following additional features. mode has been added to give Mode. 1. Pressure support. A new lnode "pressure support" during inspiration. A preset pressure is set on the "Insp. Press. Level" knob. That pressure is then maintained throughout volume is dependent on patient effort. This helps to overinspiration. The volulne come system resistance during spontaneous ventilation. It can also be given in conjunction with SIMV. In that mode the pressure support is given only during the spontaneous breaths. cotltrolled ventilation lnay may be delivered 2. Pressure control. Pressure cOlltrolled by setting the mode knob on "Press. Contr." and turning the "Insp. Press. Level" to the desired pressure. Then, routine pressure limited ventilation is given. This may be useful when pressure limit ventilation is desired for neonates. Flow decreases as the lungs become filled, and volume is dependent on cOlnpliance compliance and patient effort. Respiratory Rate. The respiratory rate is variable between 0.5 and 120 breaths per min. SIMV breaths are on a separate knob so labeled, and no division calculations need to be made. n1ade. A switch next to the SIMV breath rate knob changes the scale from 0.4 to 4 or 4 to 40 breaths per min. Minute Volume. The Ininute minute volume limit has been increased to 40 liters per min. Respiratory Time Per Cent. Inspiratory time per cent can be increased to 67 and 80 per cent to allow for inverse I:E ratios. Triggering Levels. Triggering level can be set at a fixed level below PEEP. Then as the baseline pressure is changed by reducing or increasing PEEP, the triggering level is automatically adjusted.

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Working pressure is adjustable up to 120 cm

PEEP. PEEP is given by electronic control of exhalation valve and not a mechanical valve attachment. It may be directly set on the "PEEP" knob with increments up to 50 cm H 220. Monitors. An in-line FI0 22 digital readout is included with high and low oxygen per cent alarms. (The digital displays located in the lower right hand corner can be changed to read breaths per min, inspired tidal volume, expired tidal volume, expired minute volume, FI0 22 , and peak, pause, and mean airway pressure by turning the knob below the readout.) Alarms. The expired minute volume alarms have an infant and adult scale that changes the readout approximately by flipping the switch. An apnea alarm has been added to sound if a breath has not been taken in 15 A gas supply alarm indicates a decrease in available working presseconds. A sure below acceptable levels. Gas Change. A button may be depressed to flush the system of available gas so that a new concentration of oxygen may be given immediately. This may also be used to flush anesthetic gases if the ventilator is used for that purpose. BEAR I AND 11 VENTILATORS The Bear I and 11 ventilators are also volume-cycled ventilators. Figure 3 shows the Bear I ventilator, and specifications are shown in Table 3. 3.

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The Bear 1 ventilator. (Courtesy of Bear Medical Systems, Riverside,

MECHANICAL VENTILATION IN RESPIRATORY FAILURE

Table 3. Modes: Tidal volume: Rate: Pressure limits: Sigh volume: Sigh rate: Waveform: Peak flow: PEEP: Inspiratory pause: Oxygen concentration: Assist: Nebulizer: Ratio limit: Alarms Low pressure: Minimum exhaled volume: PEEP/CPAP: Apnea: Ventilator inoperative:

611

Specifications for Bear II Ventilator* Control, assist-control, SIMV, and CPAP 100 to 2000 ml 5 to 60 or 0.5 to 6 with divide by 10 switch o to 100 cm H 220 pressure separately adjustable for normal and sigh breaths 150 to 3000 ml 1, 2, or 3 consecutive sighs from 2 to 60 times per hour or off Constant flow or decelerating flow taper 20 to 120 liters per min o to 30 cm H 20 pressure o to 2 seconds 21 to 100 per cent (,50 msec response time) Less to more sensitive (.50 On or off, inspiratory phase only On or off, stop inspiratory phase when half of ventilatory cycle time (set by rate control) is reached Audiovisual, variable from 0 to 50 cm H 220 Audiovisual, variable from 0 to 2 liters, must sense 3 consecutive low volumes exhaled Audiovisual, variable from 0 to 30 cm H 220 Audiovisual, activates if inspiratory phases, pressurized or spontaneous, are not sensed for 20 seconds Audiovisual, indicates machine has electric failure or loss of gas supplies

*Abbreviations: PEEP == positive end-expiratory pressure; SIMV == synchronized intermitCPAP=continuous tent mandatory ventilation; CPAP = continuous positive airway pressure. (Courtesy of Bear Medical Systems, Riverside, California.)

Bear I Ventilator This ventilator provides control, assist/control, synchronized IMV, and CPAP modes. The display panel on top of the machine will light to indicate which mode the machine is set to deliver. Mode. 1. Control. The ventilator may be placed in the control mode by turning the "mode" knob to "control." The ventilator is then insensitive to patient inspiratory effort and will deliver the preset respiratory rate only. 2. Assist/control. The assist/control mode may be obtained by turning the knob to "assist/control" as above. The Bear II will deliver assist/control ventilation as explained previously. A minimum respiratory rate is guaranteed and higher rates may be achieved through patient effort of greater than 1 cm H 220. 3. SIMV. The Bear I provides synchronized intermittent mandatory ventilation. The mechanism is much like the MA-2 SIMV mode in that the machine monitors patient effort in a time period determined by the preset minimum respiratory rate. The first patient effort in that time frame will receive an assisted breath. All subsequent efforts before the end of the time frame will result in spontaneous breaths.

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4. 4. CPAP. CPAP. CPAP may be achieved by turning the mode control to knoh. CPAP and dialing in the desired pressure on the "PEEP" knob. Tidal Volume. The tidal volume knob may be set from 100 to 2000 ml and, as always, must be periodically confirmed with an accurate spirometer. Respiratory Rate. The respiratory rate may be adjusted from 5 to 60 breaths per min. By using ": cc: 10" toggle switch rates, 0.5 to 6 breaths per min may n1ay be given. The toggle switch must be pulled and moved downward to activate. This prevents inadvertent use of the": the ": 10" mechanism. Flow Rate. The peak flow may be set from 20 to 120 liters per min. The flow wave form rnay may be set at either square (constant flow) or decelerating tapered flow. Sensitivity. The sensitivity may be adjusted from - 5 cm H Hp 2 0 in the - 1 cm H 220 when turned to "more." "less" position and from -1 Oxygen Concentration Control. The Bear I contains a built-in air compressor or may be connected to 50 psi compressed air. It delivers an FIOz2 range of 0.21 to 1.0. Again, all oxygen percentages should be conFI0 firmed with an in-line oxygen analyzer. Sigh. Sighs may be given in multiples of 1, 2, or 3 at a rate of 2 to 60 times per hour. The volume is variable up to 2200 ml. The single sigh button may be pressed to deliver a sigh at any given time. Humidification. Humidity is generally provided with a heated cascade-type humidifier. Temperature must be monitored with a thermometer in-line near the patient airway. PEEP/CPAP. The PEEP/CPAP level may be set by adjusting the "PEEP" knob until the desired level is read on the manometer. PEEP levels up to 30 cm H Hp 20 can usually be reached with high quality disposable circuits. Pressure Limit. The pressure limit may be set independently for cm H 220. normal and sigh breaths up to 100 Cln Monitors. The Bear I provides a digital display of exhaled tidal volume. A "minute volume" button may be depressed, and the readout accumulates exhaled volumes for one minute and displays exhaled minute volume. It also has a display of average respiratory rate. An airway presaiIway pressures. vVhen sure manometer monitors machine and proximal airway the "pressure" toggle switch on the left is in the straight position, machine pressure is displayed. If proximal airway pressure is desired, the toggle switch must be moved to the right or left. An "inspiratory source" indicator determines and displays whether the previous breath was assisted, controlled, or spontaneous. Alarms. The Bear I possesses a low pressure alarn1 alarm which determines a minimum pressure that must be achieved on mechanical breaths or an audible alarm will sound and a visual indicator will light. It is adjustable between 0 and 50 cm H 22 0 and is based on machine pressure. The minimum exhaled volume alarm determines a Inandatory mandatory exhaled volume that must be reached at least one of every everv three breaths. Three breaths in a row that do not exceed the alarm limit will result in an audio and visual alarm. The alarm may be set from 0 to 2 liters. The alarm is

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reset when the first breath exceeds the limit. When the preset pressure limit is exceeded, inspiration ends and an audiovisual alarm activates. An alarm silence button may be pressed to silence audible alarms for 60 seconds. It automatically resets in 60 seconds or may be manually reset by pressing it again. A "ventilator inoperative" alarm will sound when a flow rate exceeding 20 liters per min is sustained for 9 to 12 seconds. This indicates gross leaks or patient disconnect. Other causes could be electrical or gas power failure. It cannot be silenced with the alarm silence button. A PEEP/CPAP alarm may be adjusted from 0 to 30 cm H 22 0. An audiovisual alarm will result if pressure in the proximal airway drops below the preset limit. The apnea alarm occurs when no inspiratory activity is made in 20 seconds. All alarms reset automatically when the situation has been corrected. The machine possesses alerts that give audiovisual indications of low oxygen and air pressure. In addition, an alert for inverse I:E ratios activates if expiration time exceeds inspiration.

II Ventilator Bear 11

Respiratory Rate. The The": ": 10" toggle switch has been eliminated and the rate can be directly dialed from 0.5 to 60 breaths per min. Sigh. Maximum sigh volume has been increased to 3000 ml and the pressure limit to 120 cm H 220. Humidification. An LS 420 humidifier can be added to the ventilator. By means of an in-line thermometer probe, digital display of temperature of inspired gas is provided. An overtemperature alarm activates if the temperature exceeds 41 00 C or the probe becomes disconnected. PEEP. PEEP may be adjusted up to 50 cm ClTI H 22 0. (This requires a high quality ventilator circuit, usually the permanent type.) Pressure Limit. The pressure limit for normal and sigh breaths has been increased to 120 cm H 220. tfie alarln alarm system must be manually reAlarms. The visual aspect of tlle set each time an alarm occurs. The audible alarms reset automatically when the cause is corrected. Time of allowable apnea may be adjusted from 2 to 20 seconds. The number of breaths required below the alarm setting for activation of the between 2 and 5 consecminimum exhaled volume alarm may be adjusted bet\veen utive breaths. Changes have been made in the low inspiratory pressure alarm so that it cannot be turned off and must be set at a level above baseline. An alarm has been added to detect hyperventilation. It must be adjusted from 10 to 80 breaths per minute. MANAGEMENT OF MECHANICAL VENTILATION management of paWhen mechanical ventilation is indicated in the lnanagement tients with respiratory failure, two lnajor major goals are involved. 22, 3 First, alveomust be maintained appropriately to meet the metabolic lar ventilation lTIUst

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hypoxemia Inust must be corrected needs of the patient, and, second, arterial hypoxelnia and oxygen transport ll1axilnized. maximized. This section outlines techniques in nlanmanaging ventilation and oxygenation. Arterial Blood Gases Following the initial setup, a measurelnent measurement of arterial blood gases is mandatory. Blood gas evaluation is the only way to assess the adequacy of oxygenation and ventilation. The first sample should be drawn approximately 30 minutes after initiation of ventilator support. The adequacy of oxygenation is determined from the Pa0 22 values. A Pa0 22 of 50 to 60 mm acceptable.!4 The adequacy of alveolar ventilation is Hg is considered to be acceptable. determined by the amount of carbon dioxide in the arterial blood. The optimal Pa0 22 value depends on the patient's pH. Alveolar Ventilation. If the patient's ventilation is fully controlled by the machine (controlled ventilation), achieving the desired PaC0 22 and alveolar ventilation is relatively easy. Once the PaC0 22 has been determined, volume or respiratory frequency to changes can be made in the tidal volulne achieve the desired PaC0 22 • Alveolar ventilation is inversely related to PaC0 PaCO z2 •' To lower the PaC0 PaCO 2z,, the alveolar ventilation lnust must be increased, and vice versa. After the initial setup, the changes in ventilation required to attain the required PaC0 22 are relatively easy to calculate. A simple equation to use is: · t Previous PaC0 22 P reVlOUS Desired respiratory rate revlOUS ra e X P PaCO z2 Desired PaC0 New rate = == 12 breaths X 80 mm Hg mm Hg 60 mn1 16 === 12 X 4/3 PaCO z2 of 80 mln mm In the example, 12 breaths per min resulted in a PaC0 PaCO 2z of 60 mm Hg, the new respiratory rate .would would be Hg. Desiring a PaC0 16. One factor to consider when trying to stabilize alveolar ventilation is physiologic deadspace. Physiologic deadspace consists of that portion of the tidal volume which does not come in contact with pulmonary perfusion, i. e., conducting airways, unperfused or poorly perfused alveoli, and any volume added by the ventilator circuitry. Production of carbon dioxide (the other main factor in determining PaC0 PaCO 2z)) and deadspace ventilation may be evaluated by making a threeminute collection of expired gas and a simultaneous measurement of PaC0 22 . Production of carbon dioxide per minute is calculated from the expired volume and the expired carbon dioxide concentration. The wasted (deadspace) ventilation, as a proportion of deadspace to total ventilation, is PaCO z2 and the carbon dioxide tension in the mixed calculated from the PaC0 expired gas (peC0 (PeCO 2z).)' This proportion is called the VD/VT VDiVT ratio. The Bohr equation is used and is as follows:

MECHANICAL \lENTILATION IN RESPIRATORY FAILURE YlECHANICAL VENTILATIO'" FAILCRE

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Physiologic deadspace dead space is increased in respiratory failure due to obstructive lung disease and adult respiratory distress syndrome. It is also emboli and those whose lungs are increased in patients with pulmonary en1boli overinHated overinflated during positive pressure ventilation. Deadspace generally remains constant so the previously mentioned respiratory rate equation may be used to adjust PaC0 22 . If a change in deadspace is suspected as a result of suddenly increased PaC0 PaCO 2z,, the VD/VT VnlVT equation may be used. 44 Care must be taken when adjusting PaC0 PaCO 2z to attain the patient's normal PaC0 PaCO z2 ,, not necessarily textbook values. If a patient with a normal down to a PaC0 PaCO z2 of 40 mIn mm Hg, PaC0 22 of 65 mm Hg is rapidly ventilated do\vn an acute alkalosis will occur along with its accompanying side effects. This PaCO 2z patient will also have difficulty weaning from the ventilator. The PaC0 should be adjusted to normalize pH, not to attain general population norms for PaC0 22 • Occasionally, patients allowed to trigger (assist) the ventilator will spontaneously hyperventilate. In these cases, a simple decrease in the set respiratory rate will not suffice. There are three alternatives when serious alkalosis results. One alternative is sedation so that the patient no longer assists. Another is to place the patient on IMV and reduce the set rate. By doing so, when the patient desires additional breaths, he or she receives generally slTIaller smaller spontaneous breaths rather than machine breaths. A third choice is to add mechanical deadspace to the system; however, with this n1ethod, method, many patients increase their triggering rate and maintain the alkalosis. Arterial Oxygenation. To establish and maintain adequate arterial PaO 2z of 50 to 60 mm Hg. Care must be taken oxygen saturation requires a Pa0 PaO 2z as an FI0 FIOz2 greater than to use the lowest possible FI0 22 to attain this Pa0 50 per cent for prolonged periods can result in oxygen toxicity. toxicity . .33 If a Pa0 22 greater than 50 mm Hg is not achieved with 50 per cent oxygen or less, PEEP may be cautiously added in patients with acute diffuse restrictive disease such as the adult respiratory distress syndrome. PEEP increases the end-expiratory lung volume or functional residual capacity. While oxygen is carried in bulk'combined with hemoglobin, delivery to tissues depends on its partial pressure in the blood, which is a reflection of oxygen available to be delivered by hemoglobin. A fall in Pa0 PaO 2z without PaCO 2z is indicative of deteriorating oxygenation in spite of cona rise in PaC0 stant alveolar ventilation. In the acutely ill patient, this is usually due to mismatch or intrapulnlonary intrapulmonary shunting. An important ventilation-perfusion misluatch feature of shunting is failure to relieve hypoxemia by giving 100 per cent oxygen because blood that has not come in contact with alveolar gas is mixed with oxygenated blood and a resultant decrease in Pa0 22 is seen. PaC0 PaCO 2z is rarely affected because the diffusion rate of carbon dioxide is much higher than that of oxygen. For this reason carbon dioxide can diffuse into the ventilated and perfused alveoli lTIuch much Inore more rapidly. PaC0 22 is also nornlalized normalized as a result of the chelTIoreceptors' chemoreceptors' response to rising PaC0 PaCO 2z,, which is to increase ventilation. A useful equation for calculation of physiologic shunt is:

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Qs refers to physiologic shunt, Qt to the total lung blood flow, and CaO z, and CvO z refer, respectively, to the oxygen content of ideal, CIO z, CaOz, arterial and mixed venous blood. 4I A normal Qs/Qt value would be 5 per cent. The shunt calculation should be Inade made using 100 per cent oxygen. Unless 100 per cent oxygen is given for an adequate amount of time, overestimation of shunt will result. This is caused by insufficient nitrogen washout of poorly perfused alveoli. The magnitude of physiologic shunt is related to cardiac output. A decrease in flow or cardiac output will decrease the shunt. Increasing cardiac output by volume expansion or pharmacologic means increases shunt in septic shock. Thus, the magnitude of shunt should always be interpreted based on correlation with cardiac output. Arterial/alveolar oxygen gradient is another useful tool in managing patients on mechanical ventilation. 44 The alveolar oxygen tension can be formula which is adequate for calculated from the following abbreviated forlnula clinical purposes: PAO

z

PaC0 2 PIOz - R -

barometric pressure (PE) minus water vapor pressure PIO Z is equal to barolnetric PIOZ (44 mm Hg at body teluperature) temperature) multiplied by FI0 FIOz' 2 • The respiratory quotient (R) is approximately 0.8 in the steady state resting condition. It is assumed to be 0.8 in respiratory failure, although this is not always the assurned case. The arterial oxygen tension divided by the alveolar tension is called lO This ratio is relatively stable with varying FIO , unlike the ratio.;o the alA ratio. z classic alveolar/arterial gradient. This is a useful index of changes in lung function. The normal alA ratio is > O. 75. The ratio can be used to predict 0.75. the new Pa0 22 that will result from a change in FIO FIOz' z' Simply figure the alveolar oxygen content of the new and old FIO FIOzz and, using the Pa0 PaO 2z on FIO 2z,, set up the following equation: the old FI0 PaO z2 __ New PaO z Old Pa0 Old PAO New PAO z PA0 z2 Assuming a stable alA, a relatively accurate prediction of the new Pa0 PaO 2z may be made. Oxygen Delivery. In the patient \vith with severe respiratory failure, iInimproved delivery of oxygen to the tissues is the therapeutic goal. Positive end-expiratory pressure (PEEP) is often given to increase arterial oxygen content. Since oxygen delivery is the product of cardiac output and oxygen deliverv can occur if the PEEP causes a decrease in content, a decrease in delivery us~ of PEEP is dependent on serial measurecardiac output. Successful use maximized. If PEEP ment of cardiac output so that oxygen delivery can be Inaximized. results in decreased cardiac output, the patient may experience cardiac filling pressure too low to be maintained in the presence of PEEP. In this case, intravascular volume replacen1ent replacement is indicated. Alternatively, the thc patient Inay may have primary cardiac dysfunction and require inotropic agents. commonly for patients with adult respiratory disPEEP is used most cOlnmonly

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PaO 2z is not iInproved improved and lnay may even tress syndrome. In some situations, Pa0 be worsened by PEEP if blood flow is diverted from ventilated to unventilated lung units. This can occur when lung injury is unequal. (PvO 2z)) is a useful monitor of oxygen The mixed venous oxygen tension (Pv0 delivery.15 If Pv0 22 decreases as PEEP is increased, you can assume that PvO 2z is to be used for assessrnent assessment of oxygen delivery has been reduced. If Pv0 two other factors nlust must be considered: Pa0 PaO 2z and circulatory function, t"vo hemoglobin concentration. Infusion of large alnounts amounts of colloid or saline may alter the hematocrit. Thus, the Pa0 PaO z2 and henlatocrit hematocrit must be assessed along with pv0 22.• Pv0 22 has gained popularity as an index of tissue oxygenation, a trend facilitated by the relative ease of Pv0 PvO z2 measurement with mixed venous blood can only the use of a pulmonary artery catheter. True lnixed be obtained with the use of pulmonary artery blood. If the Pv0 22 value is less than 30 mm Hg, severe tissue hypoxia is present. presenP·3, 4 We and others PvO 2z is an accurate lneasure measure of tissue oxygen in have recently shown that Pv0 hemorrhagic or hypoxic shock but is falsely high with endotoxin shock beperipherally.l1. cause of arteriovenous shunting peripherally. 11, 12 Recently, Danek et al. have shown that the Pv0 22 is not a good predictor of cardiac output changes during a PEEP trial in patients with ARDS.7 ARDS.' In the absence of a pulmonary artery catheter, PEEP should be employed only if blood pressure, urine flow, and central nervous systeln system status are monitored to ensure that adequate tissue perfusion is maintained. In summary, all the above-mentioned calculations can be made and considered along with other clinical signs to provide optimal ventilatory support to the patient with respiratory failure. Weaning. The following can generally be used as criteria for initiation of weaning (all need not be satisfied): volume 4 to 5 1nl ml per kg. 1. Vital capacity 10 to 15 ml per kg; tidal volulTIe (VE) less-than 10 liters, with maximum 2. Minute ventilation (VE) n1axilnUlTI voluntary ventiVE. lation double the YE. 3. Inspiratory forces (peak negative pressure) 20 cm H 22 0 or greater. 4. A-aD0 22 below 350 mm Hg (100 p~r per cent FI0 FlOe)' 2). 5. PEEP below 10 cm H H,O. 2 0. 6. Stable cardiovascular systelTI. system. VD/VT < 0.6. 7. YD/YT s 40 per cent and PaC0 22 to preill8. Correction of pH, Pa0 22, PaC0 22 : FI0 22 :::; ness ne ss condition. 9. Close supervision by physician. 10. Correction of compliance abnormality. 11. Chest wall flailing not severe. 12. Correction of conditions which complicate the primary cause of respiratory insufficiency: anemia, reduced cardiac output, fluid balance, shock, state of conarrhythmias, fever, infection, sciousness, pain, renal failure, energy depletion, arrhythlnias, acid-base abnormalities, electrolyte abnormalities, hyperglycemia, and sleep deprivation.

(i.e., When it has been determined that a patient is ready to be weaned (i. e., the underlying disease process has sufficiently resolved), the following procedure may be followed: Patient Preparation. One major consideration is the avoidance of any medications that could affect the patient's mental or ventilatory function.

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bcilitating a smooth weaning An alert, cooperative patient is helpful in facilitating process. The patient should be instructed about the procedure and its purpose. Equipment must be set up prior to weaning. A T-tube setup which provides heated aerosol with oxygen enrichnlent enrichment is necessary. The Siemens and Bear ventilators provide T-tube by using a CPAP setting of zero. It must be remembered that none of these systems are resistance-free, and the only way to provide zero resistance is a real T-tube. Prior to weaning, draw an arterial blood gas and measure tidal volume, maximum voluntary ventilation, negative inspiratory force, and vital Fr0 22 should be signs. Elevate the head of the bed 20 to 40°. The initial FI0 10 per cent greater than the ventilator percentage. The patient should then be removed froln from the ventilator and placed on the T-tube, and the following should be monitored: 1. Level of consciousness, apprehension, and diaphoresis. cvery 5 minutes until the patient can tolerate 15 to 30 min2. Blood pressure every paticnt to the utes off the ventilator; then every 10 to 15 minutes. n1inutes. Reconnect the patient ventilator if a significant rise or fall in blood pressure occurs. 3. Pulse and cardiac rhythm (same as 2). Reconnect the patient paticnt if an increase of 20 beats per min or arrhythmias occur. 4. Respiratory rates (same as 2). Reconnect the patient if an increase of luore more than 30 breaths per min occurs. 5. Tidal volume should exceed 300 m!. 6. Arterial blood gases first 15 minutes, then every hour. Reconnect the paPaCO 2e increase), pH < 7.25, or hypoxemia. tient with hypercapnia (> 8 mm Hg PaC0

The patient can be extubated when the data collected show no change increments at inin 30 minutes to 1 hour. Sometimes increasing time increlnents creased frequency are necessary to gradually wean a patient. When difficulties with T-tube trials occur, IMV can be used. Initially reduce the original assist/control rate 2 to 4 breaths per lninute minute every 1 to 2 hours as tolerated. This can usually be done in a short period of time (4 to 8 hours) and should not require days. T-tube trials should start when min. the rate has been reduced to less than 4 to 5 breaths per Inin. Complications that may occur include carbon dioxide retention, hypoxemia, and cardiovascular instability. These should always be watched for during and after weaning. Weaning can be made difficult by the following problems: 1. Increased work of breathing: (a) low lung cOlupliance compliance (stiff lung), (b) increased airway resistance (bronchospaslu, (bronchospasm, small tracheostomy tube, and bronchial obstruction), and (c) reduced chest wall compliance (obesity, abdominal distention). 2. Reduction in ventilatory reserve: (a) prolonged negative nitrogen balance, electrolytc disturbance (hypomagnesen1ia, (hypomagnesemia, hypokalemia), (d) (b) muscle wasting, (c) electrolyte disuse atrophy of respiratory muscles, and (e) respiratory muscle discoordination, sedation, or neuromuscular disorder. VDiVT and (b) increased 3. Increased ventilatory requirement: (a) increased VD/VT production of carbon dioxide (fever, shivering, high glucose feedings).

Extubation should be carried out in the following steps: 1. Explain the procedure to the patient and prepare the equipment: laryngomask, wall scope and endotracheal tube for reintubation, resuscitation bag and n1ask, suction with sterile catheter, 10 ml syringe, scissors, and aerosol mask setup.

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2. Suction the endotracheal tube thoroughly. 3. Suction the oropharynx to prevent secretions entering the airway upon cuff deflation. 4. Instruct the patient to take a deep breath and cough on demand. 5. Have the patient take a deep breath, deflate the balloon, and withdraw the tube as the patient coughs. 6. Immediately place the patient on an aerosol mask on FI0 22 equal to that before extubation.

Immediately after extubation, the clinician should be aware of the possible complications of laryngeal edema. One possible treatment is the administration of 0.25 to 0.5 ml racemic epinephrine by hand-held nebulizer. Reintubation may be necessary if laryngeal edema is unresponsive to therapy.

REFERENCES 1. Ashbaugh, D. G., Bigelow, D. B., Petty, Y. L., et al.: Acute respiratory distress in adults. Lancet, 2:319, 1967. 2. Bone, R. C.: Treatment of respiratory failure due to advanced chronic obstructive lung 40:1018--1021, 1980. disease. Arch. Intern. Med., 40:1018-1021, c.: Treatment of severe hypoxemia due to the adult respiratory distress syn3. Bone, R. C.: drome. Arch. Intern. Med., 140:8fr-89, 140:85-89, 1980. 4. Bone, R. C.: Monitoring respiratory function in the patient with the adult respiratory Sem. distress syndrome. In Hudson, L. (ed.): Adult Respiratory Distress Syndrome. Sen1. 2:140-150, 1981. Chest Med., 2:140--150, 5. Cain, S. M., and Chopler, C. K.: O 22 extraction by hind limb versus whole dog during J. Appl. Physiol., 45:96~968, 45:96~968, 1978. anemic hypoxia. J. 6. Carlon, G. C., c., Ray, C., Klein, R., et al.: Criteria for selective positive end-expiratory pressure and independent synchronized ventilation of each lung. Chest, 74:501-505, 1978. J., Lynch, J.J. P., Weg, J. G., et al.: The dependence of oxygen uptake on 7. Danek, S. J., Am. Rev. Respir. Dis., oxygen delivery in the adult respiratory distress syndrome. Aln. 122:387-395, 1980. 8. Dantzker, D. R., Brook, C. H., DeHart, ,P., et al.: Gas exchange in adult respiratory distress syndrome and the effects of positive end-expiratory pressure. An1. Am. Rev. Respir. Dis., 120: 1039, 1979. J. B., Perkins, H. M., and Modell, Model!, J. H.: Intermittent mandatory ventilation. 9. Downs, J. Arch. Surg., 109:519, 1974. 10. Gilbert, F., and Keighley, J. F.: The arterial/alveolar oxygen tension ratio: An index of gas exchange applicable to varying inspired oxygen concentrations. Am. Rev. Respir. Dis., 109:142-145, 1974. 11. Hiller, F. C., and Bone, R.: Assessment of correlation between tissue oxygen and mixed venous oxygen during hemorrhage and hypoxemia. Clin. Res., 26:48A, 1978. 12. Jordan, E., Eveleigh, M. C., Gurdjion, F., et al.: Venous admixture in septic shock. Circulation, 60: 60:155-160, 15fr-160, 1979. 13. McPherson, S. P.: Respiratory Therapy Equipment. St. Louis, Missouri, C. v. V. Mosby Co., 1977. 14. Shimada, Y., Yoshiya, I., Tanaha, K., et al.: Evaluation of the progress and prognosis of 1., Tanaba, adult respiratory distress syndrome: Simple physiologic measurenlent. measurement. Chest, 70: 180-180184, 1979. 15. Tenney, S. M.: A theoretical analysis of the relationship between venous blood and mean tissue oxygen pressures. Respir. Physiol., 20:283, 1974. Division of Pulmonary and Critical Care Medicine University of Arkansas for Medical Sciences 4301 W. Markham Little Rock, Arkansas 72205