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Mechanical ventilators: Part 2
Mechanical Ventilators: Part 2
M. K. Sykes
The physiological basis of mechanical ventilation was discussed in an earlier issue of this journal.’ This is the second of two articles. The first described the newer ventilatory modes used in the Intensive Care Unit and showed how mechanical ventilators have evolved in response to the physician’s search for improved methods of respiratory support. This second article describes the physical basis of mechanical ventilation followed by a discussion of the special characteristics of some of the more commonly used microprocessor-controlled machines designed for adult use.
Ventilator mechanics There are a number of methods of classifying ventilators, but the system which has proved most satisfactory is that devised by Mapleson.* This analysis ignores both the mechanical design of the ventilator and the source of its motive power, but classifies the machine according to its functional characteristics. Two aspects are considered: first, the way in which the ventilator forces gas into the patient, and second, the mechanisms which cause the ventilator to cycle between the two phases of respiration (Table 1). The first characteristic, the method by which the gas is driven into the patient, divides ventilators into two groups- flow generators and pressure generators. The classification is based solely on the results of the interaction between the ventilator driving mechanism and the impedances to respiration produced by the lungs and chest wall. If the driving mechanism generates a fixed
pattern of flow, which is maintained despite changes in airway resistance and compliance, the ventilator is classified as a flow generator. It should be noted that it does not matter whether the pattern of flow is constant throughout inspiration, or whether it follows any other pattern. What matters is whether the pattern is maintained in the face of a change in impedance. A pressure generator, on the other hand, generates a relatively low pressure (which again may be constant or may vary in a defined pattern), but the pattern of flow which results will depend on the shape of the pressure waveform and on the compliance and resistance encountered as the gas flows into the lungs. It follows that changes in impedance result in changes in airway pressure with flow generators and changes in flow with pressure generators. These are illustrated in Figures 1 and 2. Since expiration is usually passive, expiratoty flow will depend on alveolar pressure and the resistance of the expiratory pathway. Most ventilators may, therefore, be classified as pressure generators during expiration. The second characteristic, the method of cycling from inspiration to expiration, is often under the control of the operator. In most machines the cycling is controlled by some sort of timing device so that the machine is timecycled. In a volume-cycled machine inspiration ceases when a pre-set volume has been delivered, irrespective Table
1 - Classification
Insptiatory
Pressure generator Fitzherbert
Close, Iffley, Oxford, OX4
I, inspiration;
E, expiration.
Cycling
phase
Flow generator
Professor Sir Keith Sykes,10 4EN, UK
of ventilators.
-
constant variable constant variable
I to E
E to I
Time Pressure Volume Flow Mixed
Time Patient Mixed
MECHANCIAL VENTILATORS: PART 2
P
cm H20
165
Pressure A
20
Flow P 20
1
cm H20
B
10
Volume
Set
pl
cm H20
C
20
Normal 10
I
E
Fig. 1 - Effects of changes in airway resistance and compliance on airway pressure (PI during ventilation with a constant flow generator. Tidal volume 1000 ml, inspiratory time 1 set, pause time 0.3 sec. (A) Resistance (R) = 0.5 kPa L-l s (5 cm H,O/L/sec), Compliance (Cl = IL kPa-’ (0.1 Urn H,O); (6) R = 1 kPa L-l s (IO cm H,O/L/sec), C = 1 L kPa-’ (O.lL/cm H,O); (C) R = 0.5 kPa L-’ s (5 cm H,O/L/sec), C = 0.5 L kPa-’ (0.05 L/cm H,O).
of the time taken to achieve this, whilst in a flow-cycled machine inspiration is terminated when flow has fallen tb a given level. A common mistake is to classify a machine which delivers a preset volume as volume-cycled when it is, in fact, time-cycled. Pressure-cycling is another mode which may be used to ensure that a preset inflation pressure is not exceeded. The change over from expiration to inspiration is usually time- or patient-cycled though in some modes (e.g. SIMV) a combination of the two is used. Patient-cycling will only be effective if the trigger mechanism responds to a decrease in pressure of 0.050.1 kPa (0.5-l .O cm H,O) and has a response time of less than 200 msec. In some machines, such as the Engstrijm Erica and Elvira and the Drager Evita, the trigger mechanism is flow-cycled, the mechanism being capable of detecting inspiratory flows down to about 1 L/min. The Puritan Bennett flow-by system also uses a flow sensor, but this detects a transient decrease in a constant, low flow of gas which circulates continuously through the inspiratory and expiratory tubes. This decreases the time delay and work done by the patient to a minimum.3 Few machines function as perfect flow or pressure generators and their performance under clinical conditions often differs markedly from the manufacturer’s specification, For this reason the British and Intemational Standards Organisations have produced recom-
Increased Resistance
Decreased compliance
Fig. 2 - Recordings of airway pressure, flow and lung volume with a constant pressure generator. Left: normal resistance and compliance. Centre: increased resistance, normal compliance. Right: normal resistance, decreased compliance. Note that increased resistance results in a decrease in tidal volume when inspiratory time is limited. It may also lead to incomplete emptying of the lung and PEEPi.
mended test procedures which are designed to reveal deficiencies in performance when the machines are stressed as they might be in clinical practice.4 To accomplish this the ventilator is connected to a model lung with variable compliances and resistances which enables pressure, flow and volume traces to be recorded at frequencies and tidal volumes which are appropriate for the sphere of use (i.e. adult, paediatric, or neonatal). In a more recent document the testing procedure has been broadened to include spontaneous breathing modes.5 The tests not infrequently reveal unsuspected deficiencies in design which might have unfortunate clinical consequences.6,7
Ventilator design Most ventilators consist of a mixing device which controls the composition of the inspired gas, a driving mechanism, which forces the gas into the patient and a breathing system, which connects the ventilator with the patient and ensures proper separation of inspired and expired gas streams. The breathing system usually incorporates some form of gas conditioning system for humidifying the inspired gas and may be separated from the ventilator by bacterial filters. In most ventilators the driving mechanism acts directly on the respired gas, but in some machines a separate source of driving gas is used to compress a flexible reservoir containing the respired gas. This facilitates sterilization of the respired gas pathway and the control of gas concentrations.
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CURRENT ANAESTHESIA AND CRITICAL CARE
Gas mixing devices In the earlier ventilators utilising a mechanically driven bellows, oxygen concentrations were controlled by admitting oxygen to the proximal end of a large diameter air inlet tube. This acted as a reservoir for the oxygen which flowed in a retrograde fashion towards the air inlet whilst the bellows was inflating the lungs. During the refilling phase oxygen was preferentially drawn into the bellows, the balance of the mixture being air. A bacterial filter was often fitted to the air inlet. This system was not suitable for machines which required compressed gas to provide the motive power, and these were supplied with compressed air and oxygen which was metered by flow meters. Most modern ventilators require compressed gases which are usually supplied either directly from pipelines or from oxygen blenders which provide accurate concentrations at suitable pressures. However, it is important to ensure that such blenders do not allow retrograde flow of one gas into the other’s pipeline. In many modern ventilators the gas mixing mechanism is contained within the ventilator and oxygen concentration is then monitored with a fuel cell or similar analyzer.
Driving mechanisms: flow generators The earliest flow generators, such as the Beaver, SmithClarke and Engstrom ventilators, utilised a bellows or piston which was connected to an approximately sinusoidal drive mechanism powered by an electric motor. With such a mechanism inspiration and expiration would each have occupied half of the cycle (180”). In order to produce a more generally acceptable inspiratory-expiratory ratio of 1:2, the inspiratory phase was often shortened by mechanically-controlled valves. In the Smith-Clarke (and later Cape ventilators) the inspiratory valve was not opened until the bellows had been compressed for 60” of the cycle so that inspiration occupied 120”. Expiration was permitted during the 60” period so that the expiratory phase occupied 240°, thus giving an 1:E ratio of 1:2. The Engstrom 200 ventilator used air from a large piston to compress the respired gas in a reservoir bag and achieved the 1:2 ratio by spilling the gas during the later portion of the compression phase. These mechanisms obviously modified the original sine wave pattern of flow but should have allowed the machine to function as a perfect flow generator during the remainder of the cycle. However, such perfection was never attained because of the presence of a large compressed gas volume or internal compliance. There were two sources of inefficiency.8 First, a proportion of the gas issuing from the bellows was compressed in the ventilator tubes and humidifier during inspiration and so did not reach the lungs. With rigid delivery tubes the loss of delivered volume was proportional to the internal volume of the tubes and humidifier and could be calculated from Boyle’s law. However, if corrugated rubber tubes were used there was an additional volume loss due to the increase in their internal volume with increased inflation
pressure. Under the worst conditions the volume loss might amount to 400-500 ml at an inflation pressure of 5 kPa (50 cm H20). This volume of gas re-expanded on expiration so that a spirometer placed in the expiratory limb of the breathing system overestimated the volume delivered to the lung (Fig. 3). The second source of inefficiency was the volume of gas left in the bellows at the end of each stroke. This caused the delivered volume to be less than that predicted from the displacement of the bellows. This volume was greatest when small tidal volumes were used, and decreased when the bellows was more completely emptied by the delivery of large tidal volumes. In later designs the problem was overcome by modifing the bellows drive so that the bellows was completely emptied whatever the stroke. Both these gas volumes were subject to compression during inspiration so that the flow pattern generated by the driving system was not transmitted to the lungs. Thus a machine which should have been a perfect flow generator often displayed some of the characteristics of a pressure generator. In a number of machines, such as the Bennett MAl, the gas flow was generated by a fan or blower which compressed a bellows containing the respired gas, and the duration of inspiration and expiration was controlled by mechanical valves which intermittently connected the source of pressure to the bellows chamber. Unfortunately, most fan or blower units are sensitive to back pressure so that flow decreases as the pressure in the airways increases. Consequently, machines powered by these units showed many of the characteristics of a pressure generator with flow and tidal volume decreasing as airway pressure increased. Machines incorporating venturi devices to economise on driving gas often behaved similarly since the entrainment characteristics of most venturi devices are affected by back pressure. In many modem machines such problems are minimised by the changes in design. For example, in the Penlon Oxford
Swept volume
+ I.:r
Patient
I Breathing system compartment,
2 .‘.
1.
-ln .;..; ~.~.:.‘..;.~.,:..‘.:.~~..~ y.:;.:..;. ....,.., .:.;.,;.;.,: .;.,,.,;.,,.,; .;,;.,..; _ u Fig. 3 - Sources of internal compliance. The gas compressed in the breathing system compliance during inspiration re-expands during expiration and causes the expired volume meter to over-estimate the tidal volume. The gas compressed in the bellows causes the volume delivered to be less than that calculated from the swept volume.
MECHANCIAL VENTILATORS: PART 2
ventilator the bellows is driven by a link to a small piston which is moved at constant speed by a high pressure gas source controlled by a variable high resistance. Other machines control the gas flow from high pressure gas sources with precision inspiratory and expiratory valves. Others store the mixed gas in a reservoir and incorporate flowmeters to measure the pattern of inspiratory flow: this is then controlled by altering the orifice of the inspiratory valve to ensure that the preset volume is delivered to the patient. The internal compliance of tubing and humidifier is minimised and, in some machines, the measurement of flow is effected at the patient Y-piece, thus eliminating the effects of tubing compliance on the tidal volume display. Pressure
generators
Because of fears that airway pressures above 3 kPa (30 cm H,O) might cause lung damage, most of the early flow generators were fitted with safety valves to prevent high pressures reaching the patient, and it was because of these considerations that pressure generators or pressurecycled machines were widely used in the early days of mechanical ventilation. One of the earliest pressure generators was the Radcliffe ventilator. This derived its motive force from weights situated on the top of the Oxford inflating bellows. The bellows was expanded by a forklift mechanism which raised the upper end of the bellows and weights during expiration and then released them to produce inspiration. The pressure was constant throughout inspiration and was adjusted by altering the number of weights. The inspiratory:expiratory ratio was fixed at 1:2 by the cam drive controlling the lifting mechanism whilst the frequency could be changed in 8 steps by the use of a 4-speed bicycle gear box and a two-speed clutch knob. Other devices, such as the Barnet ventilator, used gases from a compressor or pipeline to inflate the bellows against the pressure exerted by a spring. Electronically timed valves were then used to control the gas flow to and from the patient. This was one of the first ventilators to permit inspiratory and expiratory times to be adjusted independently. Tidal volume was determined by the volume of fresh gas fed into the bellows and by the respiratory frequency. Because the pressure exerted by the spring decreased somewhat as the bellows emptied, this behaved as a decreasing pressure generator. As a consequence, flow decreased more rapidly during inspiration than with the Radcliffe ventilator. It was also necessary to ensure that the spring tension was adequate to drive the gas into the patient during the preset inspiratory time. Another ventilator which functioned as a pressure generator and minute volume divider was the Manley. In this machine a controlled flow of fresh gas was fed into a weighted bellows and the expiratory time adjusted by a valve mechanically linked to a timing bellows. One other way of overcoming the problems associated with high pressure was to use pressure-cycling. This approach was used in the Bird series of ventilators. In these devices a constant flow of gas from a high pressure
167
source (4 atmospheres) was directed into a chamber in the machine and then into the patient through a nonrebreathing valve, the expiratory valve of the latter being closed by pressure from the fresh gas line. As the pressure in the patient and chamber built up during inspiration it pressed on one side of a diaphragm (which constituted one wall of the chamber), so tending to displace this laterally against the attraction of a magnet. The position of the magnet determined the cycling pressure and when this had been reached the diaphragm separated from the magnet and flipped into the expiratory position. Since the diaphragm was also attached to a sliding ceramic valve which controlled the inspired gas flow, it switched this off and expiration then occurred. The expiratory time was controlled by a magnet acting on the opposite side of the diaphragm and by a controlled leak of driving gas into this second chamber. When this built up sufficient pressure, the diaphragm flipped back to the inspiratory position and the cycle was repeated. The machine was, therefore, a flow generator with pressure-cycling from inspiration to expiration, but time-cycling from expiration to inspiration. The machine could also be set to patient-cycling by adjusting the position of the second magnet so that a small drop in pressure in the patient’s inspiratory tube initiated the next inspiration. Whilst the machine behaved as a flow generator when it was delivering only the driving gas (oxygen), the performance was impaired when the air-mix venturi was activated because this was affected by back pressure from the patient. As a result, flow decreased and oxygen concentration increased as airway pressure increased during the inspiration. Microprocessor-controlled
ventilators
Although there is, as yet, little firm evidence that any of the newer ventilatory modes has any effect on mortality, manufacturers have been forced to produce ventilators capable of satisfying the demand for them. This could only be achieved by adopting completely new design concepts. In some modem machines (for example the Drlger Evita) rapidly acting valves control the flow from high pressure oxygen and air sources. thus directly controlling both the mixing of gases and the pattern of flow. In other machines (for example the Servo 900) compressed gases from pipelines are mixed by calibrated blenders and collected in a reservoir which is pressurized to 6-12 kPa (60-120 cm H,O). Flow from this reservoir to the patient is then controlled by altering the aperture of a rapidly acting inspiratory valve (Fig. 4). The inspiratory flow rate is measured continuously by a flowmeter placed just after the inspiratory valve and the orifice of the latter is then varied by a stepper motor to produce the desired flow pattern and tidal volume within the preset inspiratory time. This system is controlled by one or more microprocessors and enables the ventilator to adjust the resistance of the inspiratory valve to maintain a preset flow pattern despite changes in the patient’s compliance and resistance. The microprocessor units not
168
CURRENT ANAESTHESIA
AND CRITICAL
CARE
Pressurized
Patient
Fig. 4 - Basic design of a modern microprocessorcontrolled ventilator. Information from the flow and pressure sensors is fed into the microprocessor unit, which then adjusts the position of the valves to produce the desired pattern of ventilation.
only control the inspiratory flow pattern but may also be programmed to produce an inspiratory hold or to synchronise a mandatory breath with the patient’s inspiration. A second valve on the expiratory side can be used to control expiratory time and flow and can be linked to the airway pressure so that it can be used to maintain a given level of PEEP.i3 This system provides great flexibility and enables the ventilator to be converted into a pressure generator by reducing the driving pressure and opening the inspiratory valve, so that flow is determined by the pressure and the characteristics of the patient’s lungs. However, the requirement for many different ventilatory modes inevitably increases the number of controls and increases the risks of improper adjustment. To guard against this hazard manufacturers have had to add more monitors and complicated alarm systems. Since many of the controls are rarely used this makes the machine expensive and much less attractive to medical and nursing staff. Unfortunately, this trend to multiplicity of controls originated from, and has been encouraged by, the respiratory therapy technicians in the USA. These are highly intelligent and skilled personnel who take full responsibility for maintaining the equipment and for adjusting the ventilators. However, the resulting complexity in design now prevents all but the most dedicated physicians and nurses from gaining any insight into the workings of the modem machines and, as a result, there are few well-designed studies which document the advantages and disadvantages of any of the modes of ventilation currently employed.
Ventilators in current use In such a brief review it is not possible to describe the special characteristics of all the newer machines. Readers desiring further information are referred to other and to the manufacturer’s literature. publications 9~10~“,‘Z However, it may prove helpful to compare some of the basic features of some of the newer machines such as the Siemens Servo 9OOc, the Hamilton Veolar, the Engstriim Erica and Elvira, the DrHger Evita, the Puritan Bennett 7200 range and the Bear Mk V. These have much in
common. They all provide a full range of settings for the CMV mode and they all permit the standard ventilatory modes of PEEP, assisted ventilation, IPS, IMV, and SIMV to be used. Most provide a range of flow wave forms (Table 2). However, there are significant differences in the way they are controlled (Table 3) For example, on the 900~ the primary settings are minute volume, frequency and inspiratory and pause times as a percentage of cycle time. These settings define the expiratory time. The ventilator then computes the peak flow necessary to generate the preset minute volume with the chosen flow pattern and inspiratory time. On the Veolar the same principles apply but tidal volume replaces the minute volume setting. The method of setting the inspiratory and pause times also differs. The inspiratory pointer is first set to the inspiratory time % and the expiratory time pointer then set to the % of the cycle time at which expiration is required to start. The gap between these pointers then defines the pause time. The advantage of this rather unusual procedure is that it permits the operator to read off the 1:E ratio from the position of the expiratory pointer on a second background scale. For the same tidal volume and inspiratory time the peak flow for a sine wave will be 1.57 times that for a square wave,13 whilst ramp waveforms will have peak flows double the square wave flow. Since peak flow rate is dependent on waveform and, within reasonable limits, is of little physiological importance, the method of setting up these two ventilators seems to be eminently sensible for routine use. The other four manufacturers all use peak flow as one of the primary variables to be set on the machine. In the Engstrijm Erica the other settings are tidal volume, frequency and 1:E ratio, whilst on the Elvira the pause time is substituted for the 1:E ratio. Changing the waveform thus alters the inspiratory pause time on the Erica and 1:E ratio on the Elvira. The Drager Table 2 - Electronic ventilators: availability of inspiratory flow waveforms in flow generation mode. All ventilators produce a decreasing flow pattern in the pressure generation mode Flow waveforms available Sinusoidal Increasing
Ventilator Constant 9oOc Veolar Erica/Elvira 7200 Bear V Evita
_
+ + + + + +
+ + + _
+ _ + + _
+ _
Decreasing _ + + + +
Table 3 -Effect of changing flow waveform on other variables during controlled ventilation with the flow generation mode. V,, V,, minute and tidal volume; f, respiratory rate; T,,T,,T,, inspiratory, expiratory and pause times; V,, inspiratory flow rate Ventilator
Controls
Variable
servo 9ooc Hamilton Veolar Engstriim Erica Engstrom Elvira Drlger Evita Puritan Bennett 7200 Bear V
V, V,
f f
V, V, V, V, V,
f f f f f -.
T,% T,% T,:TE TP T,:T, T, T,
Tp% Tp% V, VI V, V, V,
VI V, TP T,:T, TP T,:T, T,:T,
MECHANCIAL
Evita is set up in a similar manner to the Erica but there is no direct control of flow waveform: to change from constant to decelerating flow it is necessary to lower the peak pressure so that the machine becomes a pressure generator. In the 7200 and Bear V the settings are tidal volume, frequency, pause time and peak flow so that an alteration in waveform causes an alteration in inspiratory time and 1:E ratio. If the wrong peak flow is chosen by the operator when setting up these two machines undesirable 1:E ratios may result.
Monitoring The Servo 900~ displays airway pressure 2nd expired minute volume on analogue meters and there are adjustable high/low alarm limits for volume. There is a control which permits the operator to display digitally one of the following: inspired or expired volume, peak, mean or plateau airway pressure, frequency or oxygen concentration. There are controls to set alarm limits for these variables and there are also alarms for gas and power supply failure. This is a very practical monitoring system which does not need to be very complex because of the logical setting up procedure and the presence of built in safety factors such as the mechanical limitation to a maximum 1:E ratio of 4: I. At the other end of the scale is the Puritan Bennett 7200 which has a most complex monitoring system to ensure that the operator does not make inappropriate settings and that the performance of the machine corresponds to the settings on the controls. There are three major areas on the control/display panel of this ventilator. The first contains 12 specific keypads and a numerical keyboard for setting the pattern of ventilation. This procedure is aided by a series of prompts and the continuous digital display of set tidal volume, frequency, peak flow rate and oxygen concentration. The second area displays data from the patient. It contains an analogue display of airway pressure or exhaled volume and three digital displays: peak, mean, plateau or PEEP pressure; frequency or 1:E ratio; and tidal, minute or spontaneous minute volume. The third area displays various aspects of ventilator status and 12 alarm conditions, varying from high airway pressure to leakage at the expiratory valve. The ventilator itself has a number of features not found on some of the other machines, such as back-up ventilation in the event of microprocessor failure, and this sophistication is reflected in the extremely comprehensive monitoring incorporated in the machine. The sensors built into modern machines can also be used to generate other data. The measurement of dynamic compliance and resistance from the end-inspiratory pause pressure has already been described. Intrinsic PEEP can be measured similarly by recording the airway pressure when this equilibrates with alveolar pressure during an added end-expiratory pause. Another option is the measurement of Pu.,-a measure of the neural drive to respiration. The measurement is made by recording the decrease in pressure in the breathing system whilst the inspiratory valve is held closed during the first 0.1
VENTILATORS:
PART 2
169
second of a spontaneous inspiration. However, the accuracy of the measurement depends on the correct functioning of the valve and on its proximity to the patient and it seems probable that the measurement would be downgraded considerably by the internal compliance of most ventilator breathing systems. It is also possible to calculate 0, consumption and CO, output from measurements of inspired and expired volume and gas concentrations. However, the accuracy of such measurements must always be questioned because of the extreme accuracy required in flow and concentration measurements and the possibility of errors introduced by small leaks in the system.
Conclusion In 1952 the introduction of controlled ventilation resulted in a reduction in the mortality rate in patients with severe bulbo-spinal paralysis from over 80%-40%14. Similar improvements were observed when neonatal tetanus was treated with muscle relaxants and mechanical ventilation.‘5 However, the improvement in results was less dramatic when mechanical ventilation was used in patients with severe lung disease. The use of mechanical ventilation is associated with a number of complications due to immobility, the tracheal tube or tracheostomy, infection and barotrauma, and there have been few controlled studies demonstrating any definite advantage of one ventilatory mode over another. Indeed, the only randomised, prospective trials of the use of PEEP showed that its use was associated with an increase in mortality. KJ’ New ventilatory modes should therefore be employed with caution. Since positive airway pressure is associated with barotrauma, a decrease in cardiac output and other undesirable changes. it would seem logical to employ techniques which reduce peak and mean airway pressures as much as possible. A spontaneous breathing component may also help to preserve respiratory muscle function and so facilitate weaning. However, such techniques may lead to diaphragmatic fatigue and an inefficient pattern of breathing which may, in turn, cause a deterioration in the patient’s condition. In the light of present knowledge it must be concluded that the majority of patients will derive optimum benefit from a simple flow generator which has facilities for altering 1:E ratio and providing SIMV and PEEP. The function and method of use of such a machine can be easily understood by medical and nursing staff and there is less likelyhood of complications due to operator error. If it is decided to invest in one of the more comprehensive machines it should be chosen with care and thorough training in its use given to all staff. The most comprehensive monitoring system is no substitute for close observation of the patient at all times.
References 1. Grummitt RM, Jones JG The physiology of artificial ventilation. CUIT Anaes Crit Care 1989; 1: 3-10 2. Mapleson WW. The effect of changes of lung characteristics on the functioning of automatic ventilators. Anaesthesia 1962; 17: 300-314
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AND CRITICAL
CARE
3. Cox D, Tinloi SF, Farrimond JG. Investigation of the spontaneous modes of breathing in different ventilators. Intensive Care Med 1988; 14: 532-7 4. IS0 5369 Breathing machines for medical use-lung ventilators. International Organization for Standardization, Geneva (1987) 5. British Standard 5724. Medical Electrical Equipment. Part 3. Particular requirements for performance. Section 3. 12: 1991 6. Loh L, Sykes MK, Chakrabarti MK. The assessment of ventilator performance. Br J Anaesth 1978; 50: 63-71 7. Health Equipment Information No 119. Evaluation of lung ventilators: first report. Department of Health and Social Security: London, 1983 8. Loh L, Chakrabarti MK. The internal compliance of ventilators. Anaesthesia 197 1; 26: 414-20 9. Mushin WW, Rendell-Baker L,Thompson P, Mapleson WW. Automatic Ventilation of the lungs. 3rd Ed. Oxford: Blackwell Scientific Publications, 1980 10. Kirby RR, Banner MJ, Downs JB. Clinical Applications of ventilatory support. Edinburgh: Churchill Livingstone, 1990
11. Hayes B. Ventilation and ventilators-an update. 3 Med Eng Technol 1988; 12: 197-218 12. Bersten AD, Skowronski GA, Oh TE. New generation ventilators. Anaestb Int Care 1986; 14: 293-305 13. Sykes MK. Mechanical ventilators: Part 1. Curr Anaes Crit Care 1993; 4: 114-120 14. Lassen HCA. A preliminary report on the 1992 epidemic of poliomyelitis in Copenhagen with special reference to the treatment of acute respiratory insufficiency. Lancet 1953; 1: 3741 15. Wright R, Sykes MK. Jackson BG, Mann NM, Adams EB. Intermittent positive pressure respiration in tetanus neonatorum. Lancet 1961; ii: 678-80 16. Pepe PE, Hudson LD, Carrico CJ. Early application of positive end-expiratory pressure in patients at risk for the adult respiratory distress syndrome. New Engl J Med 1984; 311: 2816 17. Carroll GC, Tuman KJ, Braverman B, Logas WG, Wool N, Goldin M, Ivankovitch AD. Minimal positive end-expiratory pressure (PEEP) may be ‘best PEEP’. Chest 1988; 93: 102&5
M. K. Sykes
The physiological basis of mechanical ventilation was discussed in an earlier issue of this journal.’ This is the second of two articles. The first described the newer ventilatory modes used in the Intensive Care Unit and showed how mechanical ventilators have evolved in response to the physician’s search for improved methods of respiratory support. This second article describes the physical basis of mechanical ventilation followed by a discussion of the special characteristics of some of the more commonly used microprocessor-controlled machines designed for adult use.
Ventilator mechanics There are a number of methods of classifying ventilators, but the system which has proved most satisfactory is that devised by Mapleson.* This analysis ignores both the mechanical design of the ventilator and the source of its motive power, but classifies the machine according to its functional characteristics. Two aspects are considered: first, the way in which the ventilator forces gas into the patient, and second, the mechanisms which cause the ventilator to cycle between the two phases of respiration (Table 1). The first characteristic, the method by which the gas is driven into the patient, divides ventilators into two groups- flow generators and pressure generators. The classification is based solely on the results of the interaction between the ventilator driving mechanism and the impedances to respiration produced by the lungs and chest wall. If the driving mechanism generates a fixed
pattern of flow, which is maintained despite changes in airway resistance and compliance, the ventilator is classified as a flow generator. It should be noted that it does not matter whether the pattern of flow is constant throughout inspiration, or whether it follows any other pattern. What matters is whether the pattern is maintained in the face of a change in impedance. A pressure generator, on the other hand, generates a relatively low pressure (which again may be constant or may vary in a defined pattern), but the pattern of flow which results will depend on the shape of the pressure waveform and on the compliance and resistance encountered as the gas flows into the lungs. It follows that changes in impedance result in changes in airway pressure with flow generators and changes in flow with pressure generators. These are illustrated in Figures 1 and 2. Since expiration is usually passive, expiratoty flow will depend on alveolar pressure and the resistance of the expiratory pathway. Most ventilators may, therefore, be classified as pressure generators during expiration. The second characteristic, the method of cycling from inspiration to expiration, is often under the control of the operator. In most machines the cycling is controlled by some sort of timing device so that the machine is timecycled. In a volume-cycled machine inspiration ceases when a pre-set volume has been delivered, irrespective Table
1 - Classification
Insptiatory
Pressure generator Fitzherbert
Close, Iffley, Oxford, OX4
I, inspiration;
E, expiration.
Cycling
phase
Flow generator
Professor Sir Keith Sykes,10 4EN, UK
of ventilators.
-
constant variable constant variable
I to E
E to I
Time Pressure Volume Flow Mixed
Time Patient Mixed
MECHANCIAL VENTILATORS: PART 2
P
cm H20
165
Pressure A
20
Flow P 20
1
cm H20
B
10
Volume
Set
pl
cm H20
C
20
Normal 10
I
E
Fig. 1 - Effects of changes in airway resistance and compliance on airway pressure (PI during ventilation with a constant flow generator. Tidal volume 1000 ml, inspiratory time 1 set, pause time 0.3 sec. (A) Resistance (R) = 0.5 kPa L-l s (5 cm H,O/L/sec), Compliance (Cl = IL kPa-’ (0.1 Urn H,O); (6) R = 1 kPa L-l s (IO cm H,O/L/sec), C = 1 L kPa-’ (O.lL/cm H,O); (C) R = 0.5 kPa L-’ s (5 cm H,O/L/sec), C = 0.5 L kPa-’ (0.05 L/cm H,O).
of the time taken to achieve this, whilst in a flow-cycled machine inspiration is terminated when flow has fallen tb a given level. A common mistake is to classify a machine which delivers a preset volume as volume-cycled when it is, in fact, time-cycled. Pressure-cycling is another mode which may be used to ensure that a preset inflation pressure is not exceeded. The change over from expiration to inspiration is usually time- or patient-cycled though in some modes (e.g. SIMV) a combination of the two is used. Patient-cycling will only be effective if the trigger mechanism responds to a decrease in pressure of 0.050.1 kPa (0.5-l .O cm H,O) and has a response time of less than 200 msec. In some machines, such as the Engstrijm Erica and Elvira and the Drager Evita, the trigger mechanism is flow-cycled, the mechanism being capable of detecting inspiratory flows down to about 1 L/min. The Puritan Bennett flow-by system also uses a flow sensor, but this detects a transient decrease in a constant, low flow of gas which circulates continuously through the inspiratory and expiratory tubes. This decreases the time delay and work done by the patient to a minimum.3 Few machines function as perfect flow or pressure generators and their performance under clinical conditions often differs markedly from the manufacturer’s specification, For this reason the British and Intemational Standards Organisations have produced recom-
Increased Resistance
Decreased compliance
Fig. 2 - Recordings of airway pressure, flow and lung volume with a constant pressure generator. Left: normal resistance and compliance. Centre: increased resistance, normal compliance. Right: normal resistance, decreased compliance. Note that increased resistance results in a decrease in tidal volume when inspiratory time is limited. It may also lead to incomplete emptying of the lung and PEEPi.
mended test procedures which are designed to reveal deficiencies in performance when the machines are stressed as they might be in clinical practice.4 To accomplish this the ventilator is connected to a model lung with variable compliances and resistances which enables pressure, flow and volume traces to be recorded at frequencies and tidal volumes which are appropriate for the sphere of use (i.e. adult, paediatric, or neonatal). In a more recent document the testing procedure has been broadened to include spontaneous breathing modes.5 The tests not infrequently reveal unsuspected deficiencies in design which might have unfortunate clinical consequences.6,7
Ventilator design Most ventilators consist of a mixing device which controls the composition of the inspired gas, a driving mechanism, which forces the gas into the patient and a breathing system, which connects the ventilator with the patient and ensures proper separation of inspired and expired gas streams. The breathing system usually incorporates some form of gas conditioning system for humidifying the inspired gas and may be separated from the ventilator by bacterial filters. In most ventilators the driving mechanism acts directly on the respired gas, but in some machines a separate source of driving gas is used to compress a flexible reservoir containing the respired gas. This facilitates sterilization of the respired gas pathway and the control of gas concentrations.
166
CURRENT ANAESTHESIA AND CRITICAL CARE
Gas mixing devices In the earlier ventilators utilising a mechanically driven bellows, oxygen concentrations were controlled by admitting oxygen to the proximal end of a large diameter air inlet tube. This acted as a reservoir for the oxygen which flowed in a retrograde fashion towards the air inlet whilst the bellows was inflating the lungs. During the refilling phase oxygen was preferentially drawn into the bellows, the balance of the mixture being air. A bacterial filter was often fitted to the air inlet. This system was not suitable for machines which required compressed gas to provide the motive power, and these were supplied with compressed air and oxygen which was metered by flow meters. Most modern ventilators require compressed gases which are usually supplied either directly from pipelines or from oxygen blenders which provide accurate concentrations at suitable pressures. However, it is important to ensure that such blenders do not allow retrograde flow of one gas into the other’s pipeline. In many modern ventilators the gas mixing mechanism is contained within the ventilator and oxygen concentration is then monitored with a fuel cell or similar analyzer.
Driving mechanisms: flow generators The earliest flow generators, such as the Beaver, SmithClarke and Engstrom ventilators, utilised a bellows or piston which was connected to an approximately sinusoidal drive mechanism powered by an electric motor. With such a mechanism inspiration and expiration would each have occupied half of the cycle (180”). In order to produce a more generally acceptable inspiratory-expiratory ratio of 1:2, the inspiratory phase was often shortened by mechanically-controlled valves. In the Smith-Clarke (and later Cape ventilators) the inspiratory valve was not opened until the bellows had been compressed for 60” of the cycle so that inspiration occupied 120”. Expiration was permitted during the 60” period so that the expiratory phase occupied 240°, thus giving an 1:E ratio of 1:2. The Engstrom 200 ventilator used air from a large piston to compress the respired gas in a reservoir bag and achieved the 1:2 ratio by spilling the gas during the later portion of the compression phase. These mechanisms obviously modified the original sine wave pattern of flow but should have allowed the machine to function as a perfect flow generator during the remainder of the cycle. However, such perfection was never attained because of the presence of a large compressed gas volume or internal compliance. There were two sources of inefficiency.8 First, a proportion of the gas issuing from the bellows was compressed in the ventilator tubes and humidifier during inspiration and so did not reach the lungs. With rigid delivery tubes the loss of delivered volume was proportional to the internal volume of the tubes and humidifier and could be calculated from Boyle’s law. However, if corrugated rubber tubes were used there was an additional volume loss due to the increase in their internal volume with increased inflation
pressure. Under the worst conditions the volume loss might amount to 400-500 ml at an inflation pressure of 5 kPa (50 cm H20). This volume of gas re-expanded on expiration so that a spirometer placed in the expiratory limb of the breathing system overestimated the volume delivered to the lung (Fig. 3). The second source of inefficiency was the volume of gas left in the bellows at the end of each stroke. This caused the delivered volume to be less than that predicted from the displacement of the bellows. This volume was greatest when small tidal volumes were used, and decreased when the bellows was more completely emptied by the delivery of large tidal volumes. In later designs the problem was overcome by modifing the bellows drive so that the bellows was completely emptied whatever the stroke. Both these gas volumes were subject to compression during inspiration so that the flow pattern generated by the driving system was not transmitted to the lungs. Thus a machine which should have been a perfect flow generator often displayed some of the characteristics of a pressure generator. In a number of machines, such as the Bennett MAl, the gas flow was generated by a fan or blower which compressed a bellows containing the respired gas, and the duration of inspiration and expiration was controlled by mechanical valves which intermittently connected the source of pressure to the bellows chamber. Unfortunately, most fan or blower units are sensitive to back pressure so that flow decreases as the pressure in the airways increases. Consequently, machines powered by these units showed many of the characteristics of a pressure generator with flow and tidal volume decreasing as airway pressure increased. Machines incorporating venturi devices to economise on driving gas often behaved similarly since the entrainment characteristics of most venturi devices are affected by back pressure. In many modem machines such problems are minimised by the changes in design. For example, in the Penlon Oxford
Swept volume
+ I.:r
Patient
I Breathing system compartment,
2 .‘.
1.
-ln .;..; ~.~.:.‘..;.~.,:..‘.:.~~..~ y.:;.:..;. ....,.., .:.;.,;.;.,: .;.,,.,;.,,.,; .;,;.,..; _ u Fig. 3 - Sources of internal compliance. The gas compressed in the breathing system compliance during inspiration re-expands during expiration and causes the expired volume meter to over-estimate the tidal volume. The gas compressed in the bellows causes the volume delivered to be less than that calculated from the swept volume.
MECHANCIAL VENTILATORS: PART 2
ventilator the bellows is driven by a link to a small piston which is moved at constant speed by a high pressure gas source controlled by a variable high resistance. Other machines control the gas flow from high pressure gas sources with precision inspiratory and expiratory valves. Others store the mixed gas in a reservoir and incorporate flowmeters to measure the pattern of inspiratory flow: this is then controlled by altering the orifice of the inspiratory valve to ensure that the preset volume is delivered to the patient. The internal compliance of tubing and humidifier is minimised and, in some machines, the measurement of flow is effected at the patient Y-piece, thus eliminating the effects of tubing compliance on the tidal volume display. Pressure
generators
Because of fears that airway pressures above 3 kPa (30 cm H,O) might cause lung damage, most of the early flow generators were fitted with safety valves to prevent high pressures reaching the patient, and it was because of these considerations that pressure generators or pressurecycled machines were widely used in the early days of mechanical ventilation. One of the earliest pressure generators was the Radcliffe ventilator. This derived its motive force from weights situated on the top of the Oxford inflating bellows. The bellows was expanded by a forklift mechanism which raised the upper end of the bellows and weights during expiration and then released them to produce inspiration. The pressure was constant throughout inspiration and was adjusted by altering the number of weights. The inspiratory:expiratory ratio was fixed at 1:2 by the cam drive controlling the lifting mechanism whilst the frequency could be changed in 8 steps by the use of a 4-speed bicycle gear box and a two-speed clutch knob. Other devices, such as the Barnet ventilator, used gases from a compressor or pipeline to inflate the bellows against the pressure exerted by a spring. Electronically timed valves were then used to control the gas flow to and from the patient. This was one of the first ventilators to permit inspiratory and expiratory times to be adjusted independently. Tidal volume was determined by the volume of fresh gas fed into the bellows and by the respiratory frequency. Because the pressure exerted by the spring decreased somewhat as the bellows emptied, this behaved as a decreasing pressure generator. As a consequence, flow decreased more rapidly during inspiration than with the Radcliffe ventilator. It was also necessary to ensure that the spring tension was adequate to drive the gas into the patient during the preset inspiratory time. Another ventilator which functioned as a pressure generator and minute volume divider was the Manley. In this machine a controlled flow of fresh gas was fed into a weighted bellows and the expiratory time adjusted by a valve mechanically linked to a timing bellows. One other way of overcoming the problems associated with high pressure was to use pressure-cycling. This approach was used in the Bird series of ventilators. In these devices a constant flow of gas from a high pressure
167
source (4 atmospheres) was directed into a chamber in the machine and then into the patient through a nonrebreathing valve, the expiratory valve of the latter being closed by pressure from the fresh gas line. As the pressure in the patient and chamber built up during inspiration it pressed on one side of a diaphragm (which constituted one wall of the chamber), so tending to displace this laterally against the attraction of a magnet. The position of the magnet determined the cycling pressure and when this had been reached the diaphragm separated from the magnet and flipped into the expiratory position. Since the diaphragm was also attached to a sliding ceramic valve which controlled the inspired gas flow, it switched this off and expiration then occurred. The expiratory time was controlled by a magnet acting on the opposite side of the diaphragm and by a controlled leak of driving gas into this second chamber. When this built up sufficient pressure, the diaphragm flipped back to the inspiratory position and the cycle was repeated. The machine was, therefore, a flow generator with pressure-cycling from inspiration to expiration, but time-cycling from expiration to inspiration. The machine could also be set to patient-cycling by adjusting the position of the second magnet so that a small drop in pressure in the patient’s inspiratory tube initiated the next inspiration. Whilst the machine behaved as a flow generator when it was delivering only the driving gas (oxygen), the performance was impaired when the air-mix venturi was activated because this was affected by back pressure from the patient. As a result, flow decreased and oxygen concentration increased as airway pressure increased during the inspiration. Microprocessor-controlled
ventilators
Although there is, as yet, little firm evidence that any of the newer ventilatory modes has any effect on mortality, manufacturers have been forced to produce ventilators capable of satisfying the demand for them. This could only be achieved by adopting completely new design concepts. In some modem machines (for example the Drlger Evita) rapidly acting valves control the flow from high pressure oxygen and air sources. thus directly controlling both the mixing of gases and the pattern of flow. In other machines (for example the Servo 900) compressed gases from pipelines are mixed by calibrated blenders and collected in a reservoir which is pressurized to 6-12 kPa (60-120 cm H,O). Flow from this reservoir to the patient is then controlled by altering the aperture of a rapidly acting inspiratory valve (Fig. 4). The inspiratory flow rate is measured continuously by a flowmeter placed just after the inspiratory valve and the orifice of the latter is then varied by a stepper motor to produce the desired flow pattern and tidal volume within the preset inspiratory time. This system is controlled by one or more microprocessors and enables the ventilator to adjust the resistance of the inspiratory valve to maintain a preset flow pattern despite changes in the patient’s compliance and resistance. The microprocessor units not
168
CURRENT ANAESTHESIA
AND CRITICAL
CARE
Pressurized
Patient
Fig. 4 - Basic design of a modern microprocessorcontrolled ventilator. Information from the flow and pressure sensors is fed into the microprocessor unit, which then adjusts the position of the valves to produce the desired pattern of ventilation.
only control the inspiratory flow pattern but may also be programmed to produce an inspiratory hold or to synchronise a mandatory breath with the patient’s inspiration. A second valve on the expiratory side can be used to control expiratory time and flow and can be linked to the airway pressure so that it can be used to maintain a given level of PEEP.i3 This system provides great flexibility and enables the ventilator to be converted into a pressure generator by reducing the driving pressure and opening the inspiratory valve, so that flow is determined by the pressure and the characteristics of the patient’s lungs. However, the requirement for many different ventilatory modes inevitably increases the number of controls and increases the risks of improper adjustment. To guard against this hazard manufacturers have had to add more monitors and complicated alarm systems. Since many of the controls are rarely used this makes the machine expensive and much less attractive to medical and nursing staff. Unfortunately, this trend to multiplicity of controls originated from, and has been encouraged by, the respiratory therapy technicians in the USA. These are highly intelligent and skilled personnel who take full responsibility for maintaining the equipment and for adjusting the ventilators. However, the resulting complexity in design now prevents all but the most dedicated physicians and nurses from gaining any insight into the workings of the modem machines and, as a result, there are few well-designed studies which document the advantages and disadvantages of any of the modes of ventilation currently employed.
Ventilators in current use In such a brief review it is not possible to describe the special characteristics of all the newer machines. Readers desiring further information are referred to other and to the manufacturer’s literature. publications 9~10~“,‘Z However, it may prove helpful to compare some of the basic features of some of the newer machines such as the Siemens Servo 9OOc, the Hamilton Veolar, the Engstriim Erica and Elvira, the DrHger Evita, the Puritan Bennett 7200 range and the Bear Mk V. These have much in
common. They all provide a full range of settings for the CMV mode and they all permit the standard ventilatory modes of PEEP, assisted ventilation, IPS, IMV, and SIMV to be used. Most provide a range of flow wave forms (Table 2). However, there are significant differences in the way they are controlled (Table 3) For example, on the 900~ the primary settings are minute volume, frequency and inspiratory and pause times as a percentage of cycle time. These settings define the expiratory time. The ventilator then computes the peak flow necessary to generate the preset minute volume with the chosen flow pattern and inspiratory time. On the Veolar the same principles apply but tidal volume replaces the minute volume setting. The method of setting the inspiratory and pause times also differs. The inspiratory pointer is first set to the inspiratory time % and the expiratory time pointer then set to the % of the cycle time at which expiration is required to start. The gap between these pointers then defines the pause time. The advantage of this rather unusual procedure is that it permits the operator to read off the 1:E ratio from the position of the expiratory pointer on a second background scale. For the same tidal volume and inspiratory time the peak flow for a sine wave will be 1.57 times that for a square wave,13 whilst ramp waveforms will have peak flows double the square wave flow. Since peak flow rate is dependent on waveform and, within reasonable limits, is of little physiological importance, the method of setting up these two ventilators seems to be eminently sensible for routine use. The other four manufacturers all use peak flow as one of the primary variables to be set on the machine. In the Engstrijm Erica the other settings are tidal volume, frequency and 1:E ratio, whilst on the Elvira the pause time is substituted for the 1:E ratio. Changing the waveform thus alters the inspiratory pause time on the Erica and 1:E ratio on the Elvira. The Drager Table 2 - Electronic ventilators: availability of inspiratory flow waveforms in flow generation mode. All ventilators produce a decreasing flow pattern in the pressure generation mode Flow waveforms available Sinusoidal Increasing
Ventilator Constant 9oOc Veolar Erica/Elvira 7200 Bear V Evita
_
+ + + + + +
+ + + _
+ _ + + _
+ _
Decreasing _ + + + +
Table 3 -Effect of changing flow waveform on other variables during controlled ventilation with the flow generation mode. V,, V,, minute and tidal volume; f, respiratory rate; T,,T,,T,, inspiratory, expiratory and pause times; V,, inspiratory flow rate Ventilator
Controls
Variable
servo 9ooc Hamilton Veolar Engstriim Erica Engstrom Elvira Drlger Evita Puritan Bennett 7200 Bear V
V, V,
f f
V, V, V, V, V,
f f f f f -.
T,% T,% T,:TE TP T,:T, T, T,
Tp% Tp% V, VI V, V, V,
VI V, TP T,:T, TP T,:T, T,:T,
MECHANCIAL
Evita is set up in a similar manner to the Erica but there is no direct control of flow waveform: to change from constant to decelerating flow it is necessary to lower the peak pressure so that the machine becomes a pressure generator. In the 7200 and Bear V the settings are tidal volume, frequency, pause time and peak flow so that an alteration in waveform causes an alteration in inspiratory time and 1:E ratio. If the wrong peak flow is chosen by the operator when setting up these two machines undesirable 1:E ratios may result.
Monitoring The Servo 900~ displays airway pressure 2nd expired minute volume on analogue meters and there are adjustable high/low alarm limits for volume. There is a control which permits the operator to display digitally one of the following: inspired or expired volume, peak, mean or plateau airway pressure, frequency or oxygen concentration. There are controls to set alarm limits for these variables and there are also alarms for gas and power supply failure. This is a very practical monitoring system which does not need to be very complex because of the logical setting up procedure and the presence of built in safety factors such as the mechanical limitation to a maximum 1:E ratio of 4: I. At the other end of the scale is the Puritan Bennett 7200 which has a most complex monitoring system to ensure that the operator does not make inappropriate settings and that the performance of the machine corresponds to the settings on the controls. There are three major areas on the control/display panel of this ventilator. The first contains 12 specific keypads and a numerical keyboard for setting the pattern of ventilation. This procedure is aided by a series of prompts and the continuous digital display of set tidal volume, frequency, peak flow rate and oxygen concentration. The second area displays data from the patient. It contains an analogue display of airway pressure or exhaled volume and three digital displays: peak, mean, plateau or PEEP pressure; frequency or 1:E ratio; and tidal, minute or spontaneous minute volume. The third area displays various aspects of ventilator status and 12 alarm conditions, varying from high airway pressure to leakage at the expiratory valve. The ventilator itself has a number of features not found on some of the other machines, such as back-up ventilation in the event of microprocessor failure, and this sophistication is reflected in the extremely comprehensive monitoring incorporated in the machine. The sensors built into modern machines can also be used to generate other data. The measurement of dynamic compliance and resistance from the end-inspiratory pause pressure has already been described. Intrinsic PEEP can be measured similarly by recording the airway pressure when this equilibrates with alveolar pressure during an added end-expiratory pause. Another option is the measurement of Pu.,-a measure of the neural drive to respiration. The measurement is made by recording the decrease in pressure in the breathing system whilst the inspiratory valve is held closed during the first 0.1
VENTILATORS:
PART 2
169
second of a spontaneous inspiration. However, the accuracy of the measurement depends on the correct functioning of the valve and on its proximity to the patient and it seems probable that the measurement would be downgraded considerably by the internal compliance of most ventilator breathing systems. It is also possible to calculate 0, consumption and CO, output from measurements of inspired and expired volume and gas concentrations. However, the accuracy of such measurements must always be questioned because of the extreme accuracy required in flow and concentration measurements and the possibility of errors introduced by small leaks in the system.
Conclusion In 1952 the introduction of controlled ventilation resulted in a reduction in the mortality rate in patients with severe bulbo-spinal paralysis from over 80%-40%14. Similar improvements were observed when neonatal tetanus was treated with muscle relaxants and mechanical ventilation.‘5 However, the improvement in results was less dramatic when mechanical ventilation was used in patients with severe lung disease. The use of mechanical ventilation is associated with a number of complications due to immobility, the tracheal tube or tracheostomy, infection and barotrauma, and there have been few controlled studies demonstrating any definite advantage of one ventilatory mode over another. Indeed, the only randomised, prospective trials of the use of PEEP showed that its use was associated with an increase in mortality. KJ’ New ventilatory modes should therefore be employed with caution. Since positive airway pressure is associated with barotrauma, a decrease in cardiac output and other undesirable changes. it would seem logical to employ techniques which reduce peak and mean airway pressures as much as possible. A spontaneous breathing component may also help to preserve respiratory muscle function and so facilitate weaning. However, such techniques may lead to diaphragmatic fatigue and an inefficient pattern of breathing which may, in turn, cause a deterioration in the patient’s condition. In the light of present knowledge it must be concluded that the majority of patients will derive optimum benefit from a simple flow generator which has facilities for altering 1:E ratio and providing SIMV and PEEP. The function and method of use of such a machine can be easily understood by medical and nursing staff and there is less likelyhood of complications due to operator error. If it is decided to invest in one of the more comprehensive machines it should be chosen with care and thorough training in its use given to all staff. The most comprehensive monitoring system is no substitute for close observation of the patient at all times.
References 1. Grummitt RM, Jones JG The physiology of artificial ventilation. CUIT Anaes Crit Care 1989; 1: 3-10 2. Mapleson WW. The effect of changes of lung characteristics on the functioning of automatic ventilators. Anaesthesia 1962; 17: 300-314
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CARE
3. Cox D, Tinloi SF, Farrimond JG. Investigation of the spontaneous modes of breathing in different ventilators. Intensive Care Med 1988; 14: 532-7 4. IS0 5369 Breathing machines for medical use-lung ventilators. International Organization for Standardization, Geneva (1987) 5. British Standard 5724. Medical Electrical Equipment. Part 3. Particular requirements for performance. Section 3. 12: 1991 6. Loh L, Sykes MK, Chakrabarti MK. The assessment of ventilator performance. Br J Anaesth 1978; 50: 63-71 7. Health Equipment Information No 119. Evaluation of lung ventilators: first report. Department of Health and Social Security: London, 1983 8. Loh L, Chakrabarti MK. The internal compliance of ventilators. Anaesthesia 197 1; 26: 414-20 9. Mushin WW, Rendell-Baker L,Thompson P, Mapleson WW. Automatic Ventilation of the lungs. 3rd Ed. Oxford: Blackwell Scientific Publications, 1980 10. Kirby RR, Banner MJ, Downs JB. Clinical Applications of ventilatory support. Edinburgh: Churchill Livingstone, 1990
11. Hayes B. Ventilation and ventilators-an update. 3 Med Eng Technol 1988; 12: 197-218 12. Bersten AD, Skowronski GA, Oh TE. New generation ventilators. Anaestb Int Care 1986; 14: 293-305 13. Sykes MK. Mechanical ventilators: Part 1. Curr Anaes Crit Care 1993; 4: 114-120 14. Lassen HCA. A preliminary report on the 1992 epidemic of poliomyelitis in Copenhagen with special reference to the treatment of acute respiratory insufficiency. Lancet 1953; 1: 3741 15. Wright R, Sykes MK. Jackson BG, Mann NM, Adams EB. Intermittent positive pressure respiration in tetanus neonatorum. Lancet 1961; ii: 678-80 16. Pepe PE, Hudson LD, Carrico CJ. Early application of positive end-expiratory pressure in patients at risk for the adult respiratory distress syndrome. New Engl J Med 1984; 311: 2816 17. Carroll GC, Tuman KJ, Braverman B, Logas WG, Wool N, Goldin M, Ivankovitch AD. Minimal positive end-expiratory pressure (PEEP) may be ‘best PEEP’. Chest 1988; 93: 102&5