Ventilation—which mode?

8 Ventilation

which mode?

E. S. LIN T. E. OH

The o p t i m u m choice of ventilation therapy usually arises as a compromise dictated by patient, clinical context, and ventilator availability. Many different modes of ventilation have been applied in response to various clinical indications (Table 1). These involve a wide range of machinery relying On different basic mechanisms to achieve gas exchange. Selecting the best m o d e and ventilator settings can be viewed as a matching process between patient and machine. Positive pressure ventilation (PPV) techniques are available to the majority of clinicians and will form the basis of this review.

Table 1. Indications for mechanical ventilation of the lungs. During general anaesthesia when muscle relaxants are used to facilitate surgery To maintain gas exchange in acute and chronic respiratory failure To increase carbon dioxide excretion in hypermetabolic states or in intracranial hypertension To reduce work of breathing in cardiorespiratory failure During cardiopulmonary resuscitation To reduce pulmonary complications postoperatively* To reduce afterload in severe left ventricular failure* * Based on anecdotal evidence only.

PHYSIOLOGICAL CONSIDERATIONS The main difference between P P V and spontaneous ventilation is the use of positive airway pressures to inflate the lungs during inspiration. This results in positive m e a n intrathoracic pressures during mechanical ventilation.

Respiratory system A reduction in functional residual capacity (FRC) has been noted in patients who are sedated or anaesthetized and subjected to mechanical ventilation (Froese and Bryan, 1974; Hedenstierna et al, 1985). This may be associated with changes in muscle tone or posture but the mechanism remains poorly understood (Hillman, 1986). The decreased F R C gives rise to a change in the distribution of compliance within the lungs by shifting the position of BailliOre' s Clinical A naesthesiology--

Vol. 4, No. 2, September 1990 ISBN 0-7020-1465-6

441 Copyright9 1990, by Bailli~re Tindall All rights of reproduction in any form reserved

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E. S. LIN A N D T. E. OH

TLC -

C~

E >

c

FRC a

J

FRC~ RV

Transputmonary pressure Figure 1. Apical and basal lung units (a and c) in relation to the pressure-volume curve of the lung. The effect of reducing FRC is to shift the individual lung units further down the pressure-volume curve (to b and d), thus reversing their relative compliances and preferentially distributing ventilation to the apical units. From Oh (1988) with permission.

individual lung units on the pressure-volume curve (Figure 1). The dependent regions of the lung thus become less compliant compared with the upper lung. This results in a distribution of ventilation that is greatest in the uppermost and least in the dependent parts of the lungs, the reverse of the situation at normal FRC with spontaneous ventilation. Since the distribution of fresh gases is opposite to the distribution of lung perfusion which remains gravity dependent, an increase in ventilation/perfusion (fz/Q) mismatch results. This mismatch, however, may be reversed to some extent by increasing FRC with the use of positive end-expiratory pressures (PEEP). PPV can also affect V/Q ratios adversely by increasing dead space (Vd/Vt).This may occur due to the mechanical effects of positive intraalveolar pressures developed during mechanical ventilation (Douglas and Downs, 1980). The application of PEEP in normal lungs can accentuate this effect by overdistending alveoli, compressing pulmonary capillaries and

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M O D E .9

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decreasing cardiac output (Hammon et al, 1976). On the other hand, the use of PEEP in diseased lungs may improve V/Q mismatch by increasing FRC and recruiting collapsed alveoli (Hedenstierna et al, 1979). A further effect that can worsen V/Q ratios depends on the relationship between closing volume and FRC. Several studies have demonstrated that closing volume can become greater than FRC in anaesthetized humans (Gilmour et al, 1976). When closing volume exceeds inspiratory volume, a significant increase in physiological shunt can result, as demonstrated in animals undergoing PPV (Weenig et al, 1973). Cardiovascular system

A commonly observed effect of positive intrathoracic pressures on the cardiovascular system is a reduction in cardiac output. This tends to be more marked in patients with less active cardiovascular reflexes such as the elderly, or hypovolaemic patients. The mechanism underlying this reduction is primarily a reduction in right ventricular preload due to the raised intrathoracic pressure (Qvist et al, 1975). There is also a decrease in end-diastolic and stroke volumes following decreased cardiac transmural pressures (Raj agopalan et al, 1982). At higher intrathoracic pressures associated with the application of PEEP right ventricular afterload increases and left ventricular distensibility decreases (Robotham et al, 1980). It is also possible that neural reflexes (Daly et al, 1967) or the release of humoral agents play a part in cardiac depression (Patten et al, 1978). These side-effects can usually be ameliorated by augmenting the intravascular volume of patients. They may also be significantly offset by reduced lung compliance in diseased lungs. Even in healthy lungs only a fraction of the raised intra-alveolar pressure is transmitted to the intrapleural space (Nunn, 1984). In some patients with severe cardiac failure it has been found that left ventricular performance improved with the use of IPPV (Mathru et al, 1982). This effect may occur due to a reduction in left ventricular afterload following the increased intrapleural pressures associated with PPV (Robotham et al, 1983). Recent animal studies also suggest that left ventricular work and myocardial oxygen demand can be reduced by the application of positive end-expiratory pressure (Hevroy et al, 1989). Renal function

Renal function is reduced in PPV secondary to a decrease in renal cortical blood flow and renal perfusion pressure. These changes are primarily due to the effective reduction in intravascular volume and increased renal vein pressure associated with positive intrathoracic pressures (Priebe and Hedley-Whyte, 1981). However stimulation of aortic arch baroreceptors (Fewell and Bond, 1980) and increased levels of antidiuretic hormone (Hemmer et al, 1980; Annat et al, 1983), may also play a part. Barotrauma

Pulmonary barotrauma is a direct result of positive airway pressures being

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employed and the production of excessive pressure gradients between alveolar spaces and interstitial spaces. Studies of the pathogenesis of pulmonary barotrauma (Macklin and Macklin, 1944) show that graded changes occur starting with alveolar rupture leading to interstitial emphysema and the dissection of air into the perivascular spaces. This can then progress to pneumomediastinum, pneumopericardium, pneumoperitoneum, pneumothorax or subcutaneous emphysema. Less commonly, barotrauma may result in the formation of a bronchopleural fistula or venous and arterial air embolism. Although many centres report an incidence of barotrauma between 5 and 10% (Cullen and Caldera, 1979), variability is very high. The most significant factors predisposing patients to barotrauma appear to be pre-existing lung disease, hypovolaemia, young age and unsynchronized breathing during PPV. Although some studies have shown that the use of high peak airway pressures (60-80 cm H20), large tidal volumes and high levels of PEEP also increase the incidence of barotrauma (Petersen and Baier, 1983; Slavin et al, 1982), limiting these parameters does not necessarily affect the incidence of barotrauma in the face of severe tung disease (Kirby et al, 1975).

RESPIRATORY SYSTEM MECHANICS

Mechanical ventilation involves a dynamic mechanical interaction between both the ventilator and the respiratory system. Thus, the mechanical properties of both the ventilator and the respiratory system will affect the movement of gas in and out of the respiratory system. The movement of gas into the lungs is active and requires a suitable output of energy to produce the required volume transfer between ventilator and lungs. The work done by the ventilator is expended in overcoming the flow-resistive properties of the airways, storing potential energy in the elastic components of the respiratory system and storing kinetic energy in accelerating the masses in the system. The expiration of gas from the lungs is usually passive and dependent only on the properties of the lungs and the expiratory circuitry. If a simplified model of the respiratory system is assumed the concepts outlined above can be summarized by using a simple combination of resistance (R), compliance (C) and inertance (/) to represent the mechanical properties of the respiratory system. The applied airway pressure, PA(t), in delivering a volume, V, can then be equated to the sum of the pressures generated in overcoming the elements defined above: d2V dV + 1 I--d7 + R dt ~ V = PA(t)

(1)

At conventional ventilation frequencies flow accelerations and respiratory system mass accelerations are low enough for inertance effects to be negligible. Thus, the respiratory system reduces to a simple resistance-compliance model with the following response as in conventional respiratory mechanics:

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VENTILATION--WHICH MODE?

dV+ 1

R--~

{ V = PA(t)

(2)

Thus, solving equation (2) yields the time response of the lungs at conventional ventilation rates. The increase in lung volume, AV, produced by a constant inspiratory pressure, P0, will be given by: AV = P0 C (1 - e -'/Rc) It can be seen that inflation volume during inspiration is determined by the applied inspiratory pressure, compliance and time constant of the lungs, "r ('r = RC, Figure 2). A time constant for a normal respiratory system can be obtained by taking values of 0.051cmH20 -1 for compliance and 5 cmH20 1-1 s -1 for airways resistance. These give "r = 0.25 s. Thus for an inspiratory time, Ti, greater than i s (i.e. >4~-), it can be seen that tidal volumes will not increase significantly with Ti.

E o > r e"

c

&

l"t

J

I

2X

3X

|

4x

time Figure 2. The increase in volume of the lung with time when a constant inflationary airway pressure is applied to the airway.

However, if ,r increases, as may occur with severely increased airways resistance, or Ti is reduced by increasing ventilation frequencies, so that Ti <'r, tidal volumes may be significantly reduced (Figure 3). During expiration, the volume of the lungs decreases with an exponential decay dependent on the expiratory compliance and expiratory airways resistance. Expiratory time, Te, should be > 5"r, in order to allow adequate deflation of the lungs, otherwise air-trapping or an 'auto-PEEP' effect may result (Figure 4). Regional variations in compliance-resistance time constants may occur in severely diseased lungs where some areas may become emphysematous or replaced by bullae, while others become fibrosed or oedematous. Functional lung units can then be identified as 'fast' or 'slow' according to whether their

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E . S . LIN AND T. E. OH

E D 0

1~

2"~

3'1~

4~

inspiratory time Figure 3. The variation of tidal volume with inspiratory time with high airways resistance when a constant inflationary pressure is applied. - . . . . , normal airways resistance; - - - , high airways resistance.

airway pressure

~

_."auto-PEEP" pressure

0 time Figure 4. Breath stacking and 'auto-PEEP' effect when expiratory times are too short or ventilation frequencies are too high to allow complete lung deflation.

respective time constants are short or long. This may lead to significant maldistribution of inspired gases. At higher ventilation frequencies such as might be employed in HFV techniques the inertial properties of the respiratory system become increasingly significant in affecting the mechanical response of the lungs. Under such conditions individual responses may demonstrate resonant phenomena, i.e. natural modes of oscillation depending on the flow and mechanical resistive effects, which can dampen resonant behaviour (Smith and Lin, 1989). The mechanical response of the respiratory system can be quite variable at higher frequencies (Figure 5). Resonant frequencies will be dependent on the values of compliance and inertance and resonant behaviour may be exhibited by the respiratory system as a whole or by regions of the lungs. The mechanics of the

447

VENTILATION--WHICH MODE? 10~

8

62 kc /%N

E >

O

E

6

r--

4

0

0

t

i

I

i

I

l

2

4

6

8

10

12

ventilation frequency (Hz) Figure 5. Mechanical resonance effects on chest wall movement at high ventilation frequencies in anaesthetized animals of differing body weights (after Smith and Lin, 1989).

respiratory system may also interact with the mechanics of the abdomen to form an interconnected system of masses and compliances with a complicated mechanical frequency response (Lin et al, 1988). This may make the selection of ventilation frequency during HFV more critical than at conventional ventilation rates. The mechanics of the respiratory system should not be considered in isolation from those of the ventilating system (Harrison, 1987). Flow resistances, compliances and inertances of the ventilator and the interfacing circuitry with the patient may affect time constants and frequency responses. These are not normally significant but may affect the optimum choice of parameters during ventilation of lungs with grossly abnormal mechanics or during HFV (see The mechanics of patient and ventilator during HFV).

VENTILATION MODES Intermittent positive pressure ventilation (IPPV) IPPV or controlled mechanical ventilation (CMV) remains the mainstay of artificial ventilation practice. Optimal choice of the basic ventilator parameters of frequency, tidal volume, inspiratory to expiratory time (I : E) ratio, maximum flow rate and maximum airway pressure can help to minimize the adverse physiological side-effects of IPPV. In addition, the

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LIN A N D T. E. O H

level of patient distress or discomfort can vary with ventilator settings which in turn can affect levels of sedation required.

Ventilation frequency Ventilation frequency during IPPV normally ranges between 0.15 and 0.7 Hz (10-40 bpm). Normally ventilation rates are selected to determine minute volume with a preset tidal volume, or to control tidal volume if minute volume is preset. In the presence of severe bronchospasm frequencies may have to be decreased to allow for prolonged inspiratory and expiratory times. Decreased respiratory rates may also be required if prolonged inspiratory hold is used to minimize gas maldistribution.

Inspiratory time Inspiratory time is not commonly used as a preset parameter in flow generators or volume-cycled ventilators. Reductions in these cases may lead to unacceptably high flow rates and increased inspiratory airway pressures if tidal volumes are to be maintained. However, in a time-cycled ventilator or pressure generator as used for infant ventilation, tidal volumes will be largely determined by inspiratory time. The relationship between inspiratory time, tidal volumes and tung time constants for the lungs as a whole and regional time constants may limit tidal volumes and gas distribution as discussed earlier.

Inspiratory: expiratory (I: E) ratio The I : E ratio should be adjusted to allow adequate time for inspiration (see earlier) and for expiration. Expiratory times should be long enough to allow deflation of the lungs (> 5 times the expiratory time constant), and an I : E ratio of less than 1:1 will usually be adequate. With severe obstructive airway disease I : E ratios < 1 : 3 may be required. Inversed ratio ventilation reduces expiratory times by using I ~E ratios > 1 : 1, and is discussed further below.

Reversed I: E ratio This technique uses shortened expiratory times so that expiratory times are longer than inspiratory times. It is applied in cases of severe hypoxaemia to improve oxygenation (Tharratt et al, 1988). Suggested optimum I : E ratios are between 1.1:1 and 1.7: 1, and may be combined with low ventilation frequencies, inspiratory hold and PEEP (Cole et al, 1984). The mechanism of action is thought to be similarlto that of PEEP in increasing FRC by preventing full expiration of the lungs. If used the ratio should be increased by small increments while monitoring oxygenation, oxygen delivery and cardiovascular parameters. Disadvantages will be similar to those of PEEP.

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449

Flow rate High flows promote gas maldistribution, but low flow rates are associated with longer inspiratory times leading to increased patient discomfort. No inspiratory flow patterns offer any particular advantage over others (Cameron and Oh, 1986), but accelerating flow rates appear to produce significantly higher peak airway pressures and should be avoided if possible (Smith and Venus, 1988). Few ventilators have adjustable flow rates, although some offer alternative patterns of flow (sinusoidal or accelerating).

Airway pressure Increased airway pressures carry an increased risk of barotrauma in patients with diseased lungs. Peak and mean airway pressures should therefore be kept below 40 cmH20 (3.92 kPa) whenever possible. Airway pressure is not directly adjustable in flow generator ventilators and will reflect the mechanical impedance of the lungs. However, when using a pressure generator the airway pressure is primarily determined by the preset inflation pressure.

Tidal volume The use of large tidal volumes (10-15 ml kg -1) tends to reduce physiological shunt in normal lungs. However, in diseased lungs with a wide variation in regional compliance, the overall compliance of the lungs can vary with inspiratory flow rate (Bake et al, 1974). Large tidal volumes delivered rapidly may then overinflate more compliant regions leading to increased dead space. Gas distribution may be improved in these cases by reducing tidal volumes and applying positive end-expiratory pressure (PEEP, see later).

End-inspiratory hold/plateau~pause This prolongs inspiration by keeping the lungs inflated for a preset time (Figure 6). It may allow better gas exchange (Lindahl, 1979), but its advantages have not been clearly established. The potential disadvantages of this technique are increased risk of barotrauma and cardiac depression. (a)

(b)

/---I

+

E ~0 Ct-

Time

Figure 6. Airwaypressurewaveformswheninspiratoryhold is used. (a) Inspiratoryvolume

hold; (b) inspiratorypressurehold. From Spearmanand Sanders(1985) withpermission.

450

E. S. LIN AND T. E. OH

Sighs~mechanical yawns These are periodic hyperinflations of the lungs delivered by the ventilator. It has been suggested that they improve gas exchange by causing a transient increase in lung compliance and preventing atelectasis by stimulating surfactant production from type II alveolar cells. The benefits of a sigh function remain inconclusive and it is not widely used.

Positive end-expiratory pressure (PEEP) Applying PEEP increases transpulmonary pressure at the end of expiration (Figure 7). This has the potential advantages of increasing FRC, reexpanding collapsed alveoli, and decreasing intrapulmonary shunt, thus improving oxygenation and reducing levels of FIO 2needed. The increase in lung volume may also increase lung compliance thus ultimately reducing peak inspiratory airway pressures required (Duncan et al, 1986). airway pressure

IPPV

o

airway pressure PEEP level

IPPV + PEEP

o

Figure 7. Airway pressure profiles comparing IPPV and IPPV with PEEP.

The most generally accepted indication for the use of PEEP is in patients with the adult respiratory distress syndrome (ARDS) (Mathru, 1987). These patients often have refractory hypoxaemia and are at risk of pulmonary oxygen toxicity. PEEP is also useful in the treatment of pulmonary oedema possibly due to a redistribution of intralveolar oedema (Pare et al, 1983) although total water content of the lungs is not significantly reduced (Hopewell, 1979). Animal studies suggest that PEEP can also slow the accumulation of extravascular lung water in the presence of crystalloid infusion (Myers et al, 1988).

VENTILATION--WHICH MODE.9

451

A prophylactic role for PEEP has been claimed in ARDS, due to effects in preventing surfactant aggregation (Wysogrodski et al, 1975), enhancing the pulmonary degradation of bradykinin and reducing protein flux into the alveoli (Pang et al, 1982). However, it has not yet been convincingly demonstrated that PEEP can alter or avert the disease process causing respiratory failure. The major disadvantages in using PEEP are a reduction of cardiac output and decreased organ perfusion (Beyer et al, 1982). Thus, although arterial blood oxygenation may be improved, actual oxygen delivery to organs can be decreased (Dorinski et al, 1987). Recent studies, however, suggest that reduced myocardial oxygen consumption may be a result of reduced ventricular workload during PEEP due to the effect of PEEP in reducing the ventricular transmural pressure gradient (Hevroy et al, 1989). The positive intrathoracic pressures associated with PEEP produce an increased risk of barotrauma as in other modes of PPV. Specific problems may be encountered in patients with right to left intracardiac shunts which may be worsened by the increase in pulmonary vascular resistance produced by PEEP. Intrapulmonary shunting can also deteriorate with the application of PEEP in patients with non-uniform lung disease or in the application of PEEP to the dependent ventilated lung during one-lung anaesthesia. PEEP can also reduce urine output secondary to reduced cortical perfusion and increased secretion of antidiuretic hormone (Hemmer et al, 1980; Annat et al, 1983). Different workers have defined the most suitable PEEP level for individual patients by titrating applied PEEP against a chosen physiological parameter such as FRC, static respiratory compliance, work of breathing and shunt fraction until a preselected endpoint is achieved. The 'best' P E E P level, as defined by Surer et al (1975), was taken as the best compromise between maximum oxygen transport and minimum dead space effect. This was also found to coincide with maximum total compliance. Use of total compliance does not separate the contributions of thoracic wall compliance and lung compliance and this may have significant implications for cardiac depression when selecting PEEP levels. In the presence of low lung compliance higher levels of PEEP will only be fractionally transmitted to the intrapleural space and cardiac depression will be attenuated. Whereas, where chest wall compliance is low in the presence of normal lung compliance higher levels of PEEP may significantly depress cardiac output. 'Optimal PEEP' was defined by Civetta et al (1975) as the level achieving maximal reduction in intrapulmonary shunt without significant reduction of cardiac output. Kirby et al (1975), used high levels of PEEP and titrated the levels against shunt fraction, and defined the 'best' PEEP level when the shunt was reduced to 15% or less. In acute severe lung injury the most common clinical endpoint will be maximum improvement in oxygenation with minimum risk of pulmonary oxygen toxicity. For most practical purposes PEEP levels employed are usually between 5 and 15 cmH20. It is only following failure of therapy at these conventional levels that progression to higher levels should be tried.

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E. S. LIN A N D T. E. OH

Differential lung ventilation

Conventional mechanical ventilation of lungs with severe unilateral disease can cause an unacceptable mismatch of ventilation and perfusion. This situation can be exacerbated further by the application of PEEP which may overdistend the more compliant side without restoring the FRC of the pathological lung. Independent or differential lung ventilation has been advocated in such cases with the use of a double-lumen endobronchial tube and two ventilators (Carlon et al, 1978). The tidal volume and PEEP levels to each lung can be adjusted for optimum gas exchange and synchronization is not necessarily required (Hedenstierna et al, 1984). Different modes of ventilation may be employed for each lung, as in the case of unilateral bronchopleural fistula, where high-frequency jet ventilation may be used to ventilate the side with the air leak, while IPPV is applied to the normal side. Combined spontaneous and mechanical modes of ventilation

The most commonly used techniques in this group are intermittent mandatory ventilation (IMV) and mandatory minute volume (MMV). These have both advantages and disadvantages, as shown in Table 2. Their most common applications lie in weaning patients from CMV, or to avoid the use of CMV in patients with moderate lung disease who retain some spontaneous respiratory effort. Table 2. Advantages and disadvantages of intermittent mandatory ventilation (IMV) and mandatory minute volume (MMV) modes.

Advantages Lower mean airway pressures with better cardiac and renal function Reduced sedation requirements Reduced likelihood of respiratory alkalosis More uniform intrapulmonary gas distribution Easier weaning from mechanical ventilation Reduced respiratory muscle atrophy and incoordination Reduced risk of cardiac decompensation during weaning

Disadvantages Increased oxygen demand due to work of breathing Risk of respiratory acidosis if respiratory muscle fatigue develops Prolongation of weaning if IMV is not decreased aggressively Dependence on low resistance in spontaneous breathing circuit to minimize work of breathing

However, conflicting results may be obtained with their application. The use of these modes may decrease weaning time, but if mechanical support is not withdrawn aggressively enough weaning times can become unduly prolonged due to patient dependence on mechanical support. Similarly, oxygenation can be improved by more uniform gas exchange using combined modes but in severe hypoxaemia the oxygen costs due to work of breathing may become unacceptable (Field et al, 1982). The successful application of these modes is also dependent on the suitable design of ventilators, their demand valves, and associated

V E N T I L A T I O N - - W H I C H M O D E .9

453

humidifiers. Thus, although clinical impressions of these techniques are favourable, conclusive evidence of the superiority of one mode over the others or indeed CMV, remains unavailable.

Intermittent mandatory ventilation (IMV) In this mode of ventilation the patient receives a preset number of 'mandatory' breaths per minute but between these mandatory breaths is allowed to breathe spontaneously. Synchronized intermittent mandatory ventilation (SIMV) is a modification of IMV in which the mandatory breaths are delivered at suitable intervals, as assisted breaths by the ventilator on detection of an inspiratory effort from the patient. However, if no inspiratory effort is detected within a predetermined period (time 'window') a breath is still delivered to maintain the preset mandatory respiratory rate. A potential disadvantage is that patient discomfort may be greater during unsynchronized IMV due to excessive tidal volumes if patient and mandatory breaths coincide. However, patients' breathing can usually synchronize with the ventilator and there are no proven advantages of SIMV compared with IMV. IMV and SIMV are now accepted modes Of ventilation in non-stuporose patients and patients who can generate adequate tidal volumes but have inadequate respiratory rates (Cameron and Oh, 1986). These modes are also particularly useful in the weaning process, when a suitable preset mandatory minute ventilation can be selected and progressively decreased as the patient's spontaneous contribution improves.

Mandatory minute volume (MMV) This mode of ventilation provides a preset minute volume which is composed of a spontaneous ventilation fraction and a machine-delivered fraction. In the original system described, the mandatory minute volume was into a reservoir from which the patient breathed spontaneously. When the patients spontaneous minute volume was less than this preset minute volume, the remainder was diverted through a minute volume divider which delivered this fraction to the patient mechanically. Thus, a constant minute volume was maintained even in the presence of a varying level of spontaneous ventilation (Hewlett et al, 1977). Extended mandatory minute volume (EMMV) was a later term, which described a modification of the system to allow the spontaneous minute volume to exceed the preset value. A theoretical advantage of this method is that there is less likelihood of hyperventilation occurring. Also, since the mechanical ventilatory support decreases automatically as the spontaneous minute ventilation of the patient increases, the weaning process is self-regulating. However, because Vd : Vt ratios may increase significantly with a change from spontaneous to mechanical ventilation (Douglas and Downs, 1980) this may offset the apparent self-weaning nature of the mode. MMV has been used successfully in the management of patients with myasthaenia and drug overdose (Willatts, 1985).

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A significant problem with the use of MMV arises in cases where the work of breathing places excessive metabolic demands on the patient. These would include those with parenchymal disease (e.g. ARDS) and rapid shallow breathing patterns. This disadvantage may not become apparent in IMV where a level of mechanical support can be preset helping to reduce the maximum work of breathing done by the patient. A key function of the ventilator is to measure periodically the patient's spontaneous minute volume and then deliver the difference between that and the preset level. Few ventilators offer MMV mode and, when available, success depends largely on the ventilator design.

Inspiratory pressure support (IPS) During IPS (sometimes called inspiratory assist) ventilation the patient triggers the ventilator which provides a preset pressure-limited breath. This mode was originally developed as an adjunct to MMV in order to augment ventilation during the spontaneous breathing periods (Norlander, 1982). It then evolved as a ventilatory support mode to be used as a weaning technique. Pressure support may be maintained either for a preset time (or fraction of the inspiratory period) or until the flow rate has fallen to a preset level (or fraction of the preset flow rate). As the patient's ventilatory function improves the pressure support level can be progressively reduced. The main characteristic of pressure support ventilation is that a fixed amount of mechanical assistance can be given per breath, which is adjustable by the preset pressure support level. This leaves the patient to adjust his own respiratory rate and duration of inspiration as he adapts to changing physiology and respiratory mechanics during weaning. Thus, theoretically, rapid shallow breathing can be augmented with reduced likelihood of respiratory alkalosis if suitable levels of support are selected. Pressure support levels used in practice during weaning lie between 5 and 30 cmH20. A recommended approach (McIntyre, 1986) is to set initial levels of support to give tidal volumes of 10 ml/kg and then to reduce support as clinical status improves. IPS has also been applied to patients with acute respiratory failure, where the patients retain intact respiratory drives and require low levels of sedation. It is of particular use in patients where volume-limited ventilation is poorly tolerated requiring high peak airway pressures or high levels of sedation and paralysis. In these cases, improved gas exchange can be achieved with lower airway pressures and lower levels of sedation. Clinical studies of pressure support ventilation are few, but report lower patient oxygen consumption (Kanak et al, 1985), decreased work of inspiratory muscles (Brochard et al, 1987; Kacmarek, 1988), and shorter weaning times in cardiac surgical patients (Prakash and Meij, 1985).

Assist control ventilation (ACV) This mode of ventilation (also called assisted mechanical ventilation), similar to IPS, is another patient-triggered mode in which the triggered

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VENTILATION--WHICH MODE.9

breath is volume limited. It has been applied in similar cases to those in which IPS is used. In a recent comparison with IPS (Tokioka et al, 1989) IPS was tolerated better by patients with acute respiratory failure, achieving larger tidal volumes with lower airway pressures and better synchronization.

Spontaneous PEEP Spontaneous PEEP (sPEEP) adds a positive end-expiratory pressure during spontaneous breathing. This can be achieved by incorporating an expiratory resistance in the breathing circuit. Inspiration remains dependent on the development of negative airway pressures (Figure 8). Care must be taken to airway

spontaneous breathing

pressureI 0 [~<_./

CPAP

airway pressure

PEEP level

airway spontaneous PEEP

pressure

- 9PEEP level

0 Figure 8. Airway pressure profiles comparing spontaneous breathing, CPAP and spontaneous PEEP.

select suitable demand valves in this mode if unacceptable increases in inspiratory work are to be avoided. The beneficial effects of sPEEP stem from an increase in FRC and increase in compliance which potentially improves gas exchange and decreases work of breathing. Application of this technique remains very limited, since patients may generate their own endogenous spontaneous PEEP if required by pursing their lips, but it may have a place during weaning in patients who remain intubated or with tracheostomies.

Continuous positive airway pressure (CPAP) In continuous positive airway pressure (CPAP) breathing, a preset positive pressure is maintained during both inspiration and expiration (Figure 8). The inspiratory airway pressure is maintained using a pressurized fresh gas source, while the expiratory pressure requires a PEEP mechanism. This

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technique improves gas exchange because of the increased FRC. Inspiratory work of breathing may also be decreased, particularly compared with sPEEP (Schlobohm et al, 1981), although this will depend on individual circuitry and fresh gas flows available (Kirby, 1985). CPAP can be applied to intubated patients during weaning from mechanical ventilation. Alternatively, it may be applied using a tight-fitting CPAP mask enabling some patients to avoid endotracheal intubation and/or mechanical ventilation (Duncan et al, 1986). Use of a CPAP mask, however, is often found to be too uncomfortable to be tolerated for prolonged periods and may also increase the risks of gastric distension, regurgitation, aspiration and facial skin pressure effects. The side-effects of the technique include the risks of cardiovascular depression and barotrauma, but these must be balanced against the other disadvantages of IPPV that are avoided if mechanical ventilation is avoided. Normally, levels of CPAP (and sPEEP) between 0 and 15 cmH20 are used. Optimum levels, as with PEEP during mechanical ventilation, have been defined according to various criteria including PaOz, SaO2, and physiological shunt. Minimizing inspiratory work of breathing may also be used to select the optimum level of CPAP and this may correspond to maximum pulmonary compliance (Katz and Marks, 1985). In patients with chronic chest disease who are presenting weaning problems, inspiratory work may become the most significant factor. Decreasing the work of breathing in spontaneously ventilating patients can often help to reduce inspired oxygen concentrations. These modes need not be used as exclusive modes but can also be applied in between mandatory breaths during IMV and MMV. CPAP has been demonstrated to be beneficial in the management of pulmonary oedema, hyaline membrane disease (Gregory et al, 1971) and obstructive sleep

()

2 PAT

Figure 9. The airway pressure release ventilation circuit. An oxygen-powered Venturi flow generator (1) creates a high flow of gas that traverses a heated humidifier (2) and exits through a threshold resistor (3) creating continuous positive airway pressure when the pressure release valve (4) is closed. When the timer (5) opens the pressure release valve (6), airway pressure and lung volume decrease abruptly. When the pressure release valve closes, CPAP is re-established and lung volume returns to baseline. P A T = patient. From Rasanen and Downs (1987) with permission.

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apnoea (Sullivan et al, 1981). CPAP may also be useful during acute asthma by producing passive hyperinflation and thus reducing work of breathing (Martin et al, 1982). However, further studies are needed to define the therapeutic role of CPAP in asthma. Airway pressure release ventilation (APRV) A P R V was originaily designed to augment alveolar ventilation in patients who have already been established on CPAP (Downs and Stock, 1987; Stock et al, 1987) and can be applied by either endotracheal tube or face mask (Jousela et al, 1988). In this technique a periodic release of the CPAP pressure is produced using a release valve, which is controlled by a preset timer (Figure 9). The duration, frequency and release pressure level are all controlled during APRV. A critical factor determining performance is the design of the release valve, which should have a rapid response and low resistance. (Also see Chapter 7.) The main differences between APRV and other combined modes such as IMV with CPAP is that peak airway pressures never exceed CPAP level and augmented ventilation is achieved by decreasing lung volume below baseline FRC. Although theoretical advantages should include a lower risk of barotrauma and reduced depression of cardiac output, so far no significant differences in gas exchange or cardiovascular function have been found and the technique requires further clinical assessment. VENTILATORS There are a wide variety of makes and models of ventilators available today. They all represent a major expense and offer a proliferation of accessories and features. While many new features rely on advanced technology, their clinical usefulness may be difficult to assess. Specifically designed paediatric (neonatal) ventilators are significantly different from adult ventilators and are not interchangeable in function. The new generation of ventilators incorporate more advanced monitoring and a significant degree of microprocessor control. Relatively simple mechanical and pneumatically controlled regulators and valves have been superseded by more sensitive and accurate electr0nically controlled devices. The most obvious areas of improvement lie in greater reliability, faster response times and greater sensitivity in demand valves, and more conprehensive monitoring with gas analysis and measurement of lung mechanics (Bersten et al, 1986). The desirable features of modern ventilators are summarized in Table 3. Ventilator classification A ventilator's performance is dependent on the mechanics of the machine and its interaction with the patient's respiratory system. Various schemes have been suggested for the classification of ventilators according to their functional characteristics in an attempt to facilitate an understanding of their capabilities and limitations (Smallwood, 1986).

458

E. S. LIN AND T. E. OH Table 3. Desirable features of a mechanical ventilator.

Ability to ventilate a wide range of patient sizes Versatility of operation, offering various modes including PEEP and CPAP Ability to deliver preset volumes despite changes in patient lung characteristics Good monitoring facilities and alarms, including optional analysis of inspired oxygen concentration, expired carbon dioxide concentration and lung mechanics calculations Low resistance and rapid response valves in spontaneous breathing circuits Ability to nebulize drugs in-circuit Reliability and simplicity of use, maintenance and sterilization, with electrical and gas safeguards

Mapleson's scheme (Mapleson, 1980) is well accepted, and considers each ventilator cycle as four components: inspiratory phase, inspiratory to expiratory cycling, expiratory phase, and expiratory to inspiratory cycling. Using this scheme ventilators can be labelled as 'pressure generators' or 'flow generators', depending on whether flow or pressure profiles are defined during inspiration. Relative advantages and disadvantages of each type of machine are summarized in Table 4. Classification according to cycling conditions may be more complex in modern machines. Inspiratory to expiratory cycling is often time cycled but tidal volumes can be preset. Thus, terms such as 'volume-limited time cycling' have arisen, since although expiratory to inspiratory cycling is usually time-cycled triggering may be employed (as in IMV or MMV). Similarly, inspiratory to expiratory cycling may also be time-cycled but overlaid by volume or pressure limiting. The effects of lung mechanics on ventilator function

The concepts of a 'pressure generator' and a 'flow generator' may be applied to examine the effects of varying patient lung characteristics on ventilator Table 4. Advantages and disadvantages of pressure and flow generators.

Pressure generators Advantages: Preset pressure limit reduces risk of barotrauma Low mechanical source impedance, more efficient mechanically Shunt compliance has less effect on tidal volume provided preset airway pressure is maintained Disadvantages: Tidal volume delivered depends on the mechanical characteristics of the respiratory system Flow generators Advantages: Preset tidal volumes are. delivered irrespective of mechanical characteristics of the respiratory system Disadvantages: High airway pressures may be generated increasing the risk of barotrauma Accurate monitoring of small tidal volumes and flow rates for ventilation of infants is difficult High mechanical source impedance, less efficient mechanically

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VENTILATION--WHICH MODE?

performance. In practice real ventilators only approximate to ideal pressure or flow generators since a ventilator cannot be expected to maintain preset pressure or flow patterns irrespective of widely varying lung characteristics. In the case of a pressure generator the ventilator attempts to maintain a preset pressure at the patient's airway during inspiration. Inspiratory gas flow is then determined by the mechanical input impedance seen at the airway, which includes the combined effects of flow resistance and compliance of the lungs. Thus, both increases in resistance and decreases in compliance will cause high impedances. The effects of increasing resistance or compliance are shown in Figure 10. It can be seen that the use of a A Inspiration

B

Expiration

Inspiration

Expiration

C Inspiration

I l I

~ \ Decreasilng '~~ compliance

o~ \

\

Expiration I I l

Increasing o~ ~,\ resistance

1 I

\

Time

Time

Time

Figure 10. The effects of altered lung compliance and airways resistance on inspiratory flow rate with a constant-pressure generator. From Lin and Hanning (1979) with permission.

pressure generator to ventilate lungs with high resistance or low compliance can readily result in inadequate tidal volumes being delivered. Hence, the usefulness of pressure generator-type ventilators is very limited when ventilating lungs with grossly abnormal mechanics as seen in many ICU patients. However, in patients with less severe lung pathology who retain an intact respiratory drive, pressure generators may have some advantages over flow generators in IPS mode (see earlier). In the case of flow generators a preset flow rate pattern is maintained into the airway, irrespective of respiratory system impedance. If the impedance of the lungs increases, then the airway pressure required to produce the preset will be proportionately higher. The effects of increases in resistance and compliance are shown in Figure 11. The clinical consequences are that in Inspiration

Expiration

Inspiration

Expiration

Inspiration

'l

l

1 I

I

1 jl/I o u_

/ /

E O aA4 o 1 ~r E I

Time

Time

Expiration ]

] I

//

I

~,m

ii ~3ca I ..

Time

Figure 11. The effects of altered lung compliance and airways resistance on airway pressure with a constant-flow generator. From Lin and Hanning (1979) with permission.

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LIN AND T. E. OH

diseased lungs with low compliance, maintenance of adequate tidal volumes may result in high airway and raised intrapulmonary pressures with an increased risk of barotrauma. Intrapulmonary pressures may not be raised to the same extent in patients with increased impedance due to upper airway obstruction but with normal compliance. In such a case, a significant proportion of the applied airway pressure may be dropped across the increased airway resistance. The presence of a shunt compliance may also affect ventilator performance. Such an effect may arise from the connecting circuitry between ventilator and patient or from the ventilator compliance itself. Internal compliance values of < 10 ml cmH20-1 have been determined for different ventilators and circuits (Loh and Chakrabarti, 1971). These would not be significant during IPPV applied to adults with normal respiratory compliances, but may become significant during high-frequency ventilation or ventilation of patients with greatly reduced compliances and small tidal volumes (e.g. neonates). Since pressure generators should ideally produce a preset airway pressure irrespective of shunt compliances and do not exceed the preset pressure, they are accepted as being safer for neonatal ventilation. Selection of a ventilator

Ideally, selection of a ventilator should depend on the clinical requirements and the demands of the working environment. However, choice is often dictated by cost, personal bias and availability. The majority of modern conventional ventilators are flow generators and offer most, if not all, of the modes and features outlined earlier. Some machines, however, do present serious inadequacies in the components forming their spontaneous breathing circuits. In these the work of breathing may be considerably increased by demand valves that require high negative pressures to activate them. Similarly, supplying fresh gas flow via a demand valve rather than as a continuous flow from a reservoir bag can also lead to increased work of breathing (Gibney et al, 1982; Katz et at, 1985). Inappropriately designed humidifiers can also give rise to unacceptable increases in the resistance of spontaneous breathing circuits. Thus, anecdotal reports of poor performance with certain ventilators in IMV, MMV and CPAP modes may well relate to the unsuitable designs of these components. The flow resistors or vanes used to preset levels of PEEP or CPAP may also cause problems by restricting flow during spontaneous breathing or causing excessively high airway pressures during coughing or straining. HIGH-FREQUENCY VENTILATION High-frequency ventilation is a generic term describing a group of techniques that use ventilation frequencies greater than four times the spontaneous breathing rate at rest (Drazen et al, 1984). Each technique has been identified essentially with the type of mechanism used to produce the ventilation breaths or pulses. The ventilation cycles produced by different techniques vary in

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frequency, inspiratory to expiratory time (I : E) ratio, amplitude of airway pressure swing (i.e. baseline to peak pressure), the presence of entrained gas, the use of a steady bias flow of fresh gas, and the use of active or passive expiration. Table 5 outlines some of the methods that have been described. The following three methods of HFV have evolved into clinically applied techniques. Table 5. Modes of high-frequency ventilation.

Mode High-frequency positive pressure ventilation High-frequency jet ventilation High-frequency oscillatory ventilation High-frequency flow interruption High-frequency chest wall compression

Frequency ( H z ) 1-2 1-3 3-40 1-20 3-5

Comments

Conventionalventilatormechanism running at fast rates High pressure gas deliveredvia cannula to airway Reciprocatingpump superimposing oscillationson bias flowof freshgas High pressure gas flowinterrupted by rotating valve Pressure oscillationsappliedexternally to chestwallvia pneumaticcuff

High-frequency positive pressure ventilation (HFPPV) This was the first practical form of HFV reported, and was applied by Oberg and Sjostrand (1969). They used ventilation rates of 60-100 breaths min -1 and tidal volumes of 2-3 mlkg -1, which were applied to dogs using conventional endotracheal tubes. Ventilators used are essentially conventional ventilators, which have been modified to operate at high rates and with low compliance output circuitry. The potential advantages with this technique are lower airway pressures, increased cardiovascular stability and reduced movement of the operative field during surgery. HFPPV has been applied successfully in laryngoscopy, bronchoscopy, general tracheal, cardiac and thoracic surgery, and in adult and neonatal intensive care (Sjostrand, 1980).

High-frequency jet ventilation (HFJV) HFJV has become the most widely used form of HFV in clinical practice (Carlon et al, 1981; Gallagher, 1985). In this technique, high-pressure gas (30-300 kPa) is delivered into the airway of the patient via a small bore catheter or via a purpose-designed endotracheal tube, which incorporates a jet delivery tube in its wall. Ventilation frequencies range between 1 and 5 Hz and expired minute volumes may be between 10 and 501/min. Entrainment of gases is often used but a closed system is also possible. During surgery HFJV has found particular application in maintaining ventilation in the presence of an open or disrupted airway, as in bronchoscopy, laryngoscopy and laryngotracheal operations (Turnbull et al, 1981; Vourc'h et al, 1983). In the ICU environment, HFJV has been applied to cases of massive air leak, bronchopleural fistula, and in cases of severe acute respiratory failure (Cation et al, 1983). Although HFJV has been success-

462

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fully applied to a wide variety of cases, definite advantage only appears to have emerged in the management of massive air leaks and disrupted airways. Its superiority over other methods of ventilation in the management of severe acute respiratory failure remains unproven (Schuster et al, 1982). Problems have been encountered with inadequate humidification of inspiratory gases although humidification of the injected gas and entrained gas may overcome this shortcoming (Carlon et al, 1985; Smith, 1985). Further specific disadvantages with HFJV are the increased risk of barotrauma and tracheal mucosal injury when the jet impinges directly on the tracheal wall. High-frequency oscillatory ventilation (HFOV) In HFOV an oscillating pressure waveform is applied to the airway using a reciprocating mechanism such as a piston pump, loudspeaker or diaphragm. This applied pressure waveform is superimposed on a bias flow of fresh gas, which supplies oxygen and washes out carbon dioxide. Frequencies used range between 5 and 50 Hz, although the most commonly used frequencies are 10-20 Hz (Kolton, 1984). The technique has been applied successfully to animal models of surfactant-deficient lung injury (Thompson et al, 1982; Truog et al, 1983). No definitive studies in humans are available although preliminary observations have been reported (Butler et al, 1980). The most promising application for this technique appears to lie in the management of infant hyaline membrane disease where use of HFOV may reduce the incidence of longterm pulmonary sequelae. Its clinical application, however, remains restricted as yet and further assessment is required. Combined modes of HFV An increasing practice is the use of modes comb!ning the above HFV techniques with conventional ventilation, either applied commonly to both lungs (Kopotic et al, 1986; Barzilay et al, 1987), or applied separately to each lung, as in asynchronous differential lung ventilation (Feeley et al, 1988). HFOV may be superimposed on conventional IPPV, sometimes limited to the inspiratory phase only, to improve oxygenation in cases of severe respiratory failure (Boynton et al, 1984). High-frequency percussive ventilation (HFPV) is a technique claimed to improve oxygenation in patients with severe respiratory distress who require high FIO2 and PEEP levels. HFPV produces a mechanical breath in patients by 'stacking' high-frequency breaths (300-450 bpm) during inspiration and then interrupting the sequence to allow expiration to preset PEEP levels. Significantly enhanced PaO2 levels have been produced compared with conventional IPPV with the same levels of FIO2 and PEEP (Gallagher et al, 1989). The use of HFV and IPPV modes in differential lung ventilation is described for successful management of a unilateral bronchopleural fistula

VENTILATION--WHICH

463

M O D E .9

with a relatively normal contralateral lung. HFJV was applied to the lung with the air leak while maintaining the opposite lung with IPPV. Mechanisms of gas exchange in HFV When using HFJV and HFPPV at ventilation frequencies <2.5 Hz, tidal volumes may approach those employed with conventional ventilation. Gas flow waveforms produced in the trachea are shown in Figure 12. Under these conditions gas exchange may rely mainly on direct alveolar ventilation as in IPPV (Rouby et al, 1985). Direct alveolar ventilation can still occur with tidal volumes less than but comparable with the anatomical dead space, because some alveoli will have shorter than average pathways connecting them to the main airway (Ross, 1957). Bulk convective flow of gas can also occur between different alveolar regions of the lungs at high frequencies (> 5 Hz), as described by the term 'Pendelluft'. This may occur as a result of asynchronous oscillation of flow rate

HFPPV

o

~ow rate

HFJV

flow rate

~ HFOV

_ _ _

bias flow

o

Figure 12. Airwayflowprofilesfor three modesof high-frequencyventilation,

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E . S . LIN AND T. E. OH

,o2

Unsteady, laminar

1

~'~'~,~

1.0 ~

0.1 1.0

Steady,

f~

10

laminar

,\~,,

Turbulent ~\"~

,

,

10z

103

104

Re Figure 13. Diagram illustrating the different gas transport modes in different regions of the bronchial tree. Gas flow is characterized by two parameters, Re = Reynolds number, and c~=Womersley parameter plotted on the abscissa and ordinate axes respectively. These parameters tend to be high in the larger central airways and low peripherally. Thus, anatomically large airways are located in the top right corner of the diagram becoming smaller towards the lower left corner. From Kamm et al (1984) with permission.

communicating alveolar regions as complex modes of oscillation are excited in the lungs. Pendelluft may play a role in improving intrapulmonary gas mixing during HFV. As ventilation frequencies increase, tidal volumes decrease further and alternative mechanisms are required to explain gas transport. A threshold of 2.8Hz has been suggested above which these alternative mechanisms predominate (Venegas et at, 1986). The character of the gas movement produced in the airways varies in different regions of the lung (Figure 13). In the larger airways gas flow is turbulent or unsteady laminar. Here convective streaming is used to describe the resultant axial displacement of gas as it oscillates backwards and forwards. This net transport occurs as a result of asymmetry between inspiratory and expiratory velocity profiles. This asymmetry produces a net distal (inspiratory) displacement of fresh gas in the centre of the airway and a net proximal (expiratory) displacement of the peripheral gas layers with each cycle (Figure 14). Cross-stream mixing may occur in the larger airways due to turbulence. This lateral mixing of the gas streams serves to reduce the net axial transport of gas (Slutsky et al, 1985). In the smaller airways, flow velocities decrease and velocity profiles become more symmetrical. Here again a combination of axial displacement and lateral mixing occurs. In this case, however, the lateral mixing may enhance gas transport which becomes more effective than diffusion alone.

465

VENTILATION--WHICH MODE?

a)

b)

c)

Figure 14. (a) Inspiratory and (b) expiratory gas velocity profiles in convective streaming, as modelled in a diverging duct. (c) Profile showing resultant net displacement of gas across lumen. From Kamm et al (1984) with permission.

This may be described as 'augmented diffusion'. Mechanisms responsible for lateral mixing may include Taylor dispersion (Taylor, 1953, 1954), cardiogenic oscillations (Slutsky, 1981) or acoustic resonances (Lin and Smith, 1986). Gas transport in the alveolar spaces is diffusion dependent as in the case of conventional tidal ventilation. Physiological effects of HFV

Gas exchange at the lower frequencies and larger tidal volumes is determined by principles similar to those operating during CMV. A prospective trial in patients with adult respiratory failure, failed to demonstrate any significant advantage in gas exchange with HFJV when compared with conventional ventilation (Carlon et al, 1983). At higher frequencies the situation is different. Several models of alveolar ventilation attempt to predict the dependency of COz elimination, 12co2, on tidal volume, VT, and frequency, f, based on the transport mechanisms outlined above. These models are complex and differ in conclusion. In general it appears that 12co2 is dependent on the following product: IZCO2 oc f a . VTb

(where a -----1, 1 < b > 2)

rather than individual values of f o r VT (Kamm et al, 1984).

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Oxygenation appears to be largely dependent on FRC and studies have related oxygenation to mean airway pressure during HFOV applied to lung-injured animals (Mansel and Gillespie, 1986). In lungs prone to atelectasis the recruitment of collapsed alveoli by maintaining raised mean airway pressures during HFV may explain improvements in oxygenation achieved. Lung re-expansion and the use of small tidal volumes may also increase ventilation-associated lung damage such as epithelial necrosis, hyaline membrane formation and bronchopulmonary dysplasia (Kolton et al, 1982). The various methods of HFV have been demonstrated to be capable of maintaining gas exchange with lower proximal peak airway pressures than in IPPV (Hamilton et al, 1983). However, no significant advantage in terms of reduced depression of the cardiovascular system has been demonstrated with the use of HFV techniques. Adverse mechanical pressure effects may occur when using HFV. A significant risk of b arotrauma is still present with ttFJV due to the injection of high-pressure inspiratory gases (Crawford, 1986). Although proximal peak airway pressures are lower, mean pressures may be greater in the peripheral airways and alveolar spaces. Intrapulmonary pressures may be increased due to an 'auto-PEEP' effect at frequencies >2.SHz. Raised alveolar pressures can also be produced by 'gas trapping' when an active expiratory phase is used (Egol et al, 1985). Peripheral pressure amplification has been demonstrated at frequencies close to local resonant frequencies in the lungs and may result in alveolar amplification of the central airway pressure swings by factors greater than two (Allen et al, 1985). The mechanics of ventilator and patient during HFV

A wide variety of devices has been developed to deliver HFV but so far standardization is very limited. Changes in ventilator settings may have effects that are not always easy to predict. During HFJV, alterations of I : E ratios can affect minute ventilation by varying entrained minute volume as well as injected pulse volume (Seigneur et al, 1986). Varying patient mechanics may affect the efficacy of HFV more than CMV. The frequency response of the respiratory system may affect the choice of ventilation frequency, since if resistive damping is low, resonant phenomena can be produced (Smith and Lin, 1989). Resonant phenomena may affect tidal volumes, gas exchange and intrapulmonary pressures (Brusasco et al, 1986; Allen et al, 1985). Different regions of the lung will respond with different amplitudes to high-frequency excitation, either due to variations in their intrinsic properties or interactions with neighbouring structures. MONITORING IN MECHANICAL VENTILATION Optimal use of artificial ventilation not only involves the initial choice of mode and ventilator settings, but also requires repeated reassessment of the

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Table 6. Escalating tiers of cardiorespiratory monitoring for ventilated patients according to clinical status (after Lysak and Prough, 1987). Tier 1

Tier 2

Tier 3

Short-term ventilatory support for stable patients on PPV Ventilator pressures and disconnect alarms Analysis of FIO2

Potential physiologicshifts or active therapeutic intervention All devices under Tier 1

Continuous ECG Intermittent BP

Pulse oximetry Capnography

Chest X-ray

Intra-arterial pressure monitor Pulmonary compliance*

Critically-ill patients with risk of haemodynamic instability All devicesunder Tier 1 and Tier 2 Pulmonary artery catheterization Continuous Sv02* Continuous wave Doppler ultrasound cardiac output* Extravascular lung water*

Arterial blood gases

Transcutaneous gas tensions*

Transcapillary flux measurement* Nuclear cardiographic studies* Transoesophageal echocardiography*

* Not of proven clinical value. patient and readjustment of ventilation parameters. Monitoring of ventilator parameters usually includes proximal airway pressures (peak, mean, plateau, end-expiratory), tidal volume, ventilation rate, minute volume, and FIO2. During the use of H F V techniques standard measurements may not be adequate for safe and predictable results when adjusting ventilator controls. Airway pressure may vary in an unpredictable way depending on location in the lungs (Smith et al, 1988). In H F J V measurements of entrained volumes, backspill volumes and tidal volume are necessary to assess delivered minute volumes and newer measurement techniques are required (Young and Sykes, 1988). Patient monitoring can be stratified into the routine non-invasive techniques adequate for stable short-term patients, more specialized techniques for patients requiring therapeutic intervention, and finally highly technical and invasive techniques for critically ill patients (Table 6, after Lysak and Prough, 1987). The monitoring applied to a patient thus expands to include higher level modalities as the patient's condition demands. Increasing use is being made of respiratory mechanics measurements such as pulmonary compliance, airways resistance and pulmonary inertance, as improved techniques are being developed (Masters et al, 1988; Fouke and Wolin, 1989),4n-the wake of an increasing awareness of the importance of the relationship between ventilator and patient mechanics (Shannon, 1989).

SUMMARY: CHOOSING VENTILATION MODE AND SETTINGS IPPV remains the mainstay of ventilation therapy for patients with no

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spontaneous respiratory activity. These will be patients undergoing routine anaesthesia, comatose patients or sedated patients withrespiratory failure. The use of PEEP is indicated when oxygenation is inadequate or in order to reduce high FIO2 levels. The level of PEEP applied is chosen as a compromise between achieving satisfactory arterial blood gases and minimizing cardiovascular depression. Ventilator rates are adjusted during IPPV to achieve the required minute ventilation with given tidal volumes. Limiting factors include the absolute values of expiratory times in relation to the RC constant for the individual respiratory system, and the airway pressures produced with the given tidal volumes. In severe cases of asthma, controlled hypoventilation may be acceptable in order to allow full deflation of the lungs and avoid excessively high airway pressures. If full deflation of the lungs is not allowed, an 'auto-PEEP' effect is produced as with the use of inverse ratio IPPV or with high ventilation frequencies as in HFV. Improved oxygenation in acute severe asthma with reduced hyperinflation and pleateau airway pressure, has been reported with the use of high inspiratory flows and low tidal volumes, in order to maximize expiratory times at a constant respiratory rate (Tuxen and Lane, 1987). Severe unilateral lung problems or mechanical mismatch may require the use of differential lung ventilation, possibly with mixed modes, in order to achieve satisfactory gas exchange. Such conditions include the occurrence of a unilateral bronchopleural fistula, a large unilateral bulla or severe unilateral collapse or consolidation. HFPPV or HFJV techniques in the past have been applied in extreme cases of lung pathology where refractory hypoxaemia is a problem, or gross leaks from bronchopleural fistula have made IPPV ineffective. Certainly in cases of upper airway disruption the ability of HFJV to maintain ventilation in the absence of a sealed delivery circuit via a catheter makes it a technique of choice. The ability of HFJV to deliver high minute volumes also enables it to cope in the presence of large shunts. However, in cases of severe hypoxaemia, although HFV modes may offer improved oxygenation by promoting non-tidal mechanisms of gas transport at the higher frequencies (e.g. HFOV), or by taking advantage of grossly altered lung frequency responses, the consistent superiority of any one mode or combined mode has yet to be proven. Mixed spontaneous and mechanical modes of ventilation find their main application in patients with spontaneous respiratory activity or in weaning from ventilation. IMV is commonly used in patients with an inadequate respiratory drive but who can generate adequate tidal volumes. While MMV may be more suitable for patients with variable respiratory drive, this mode still requires further evaluation. Pressure support appears to be more suitable for patients with adequate drive but poor respiratory effort, i.e. in conditions giving rise to low vital capacities and tidal volumes. During weaning from mechanical ventilation, the choice of mode and CPAP or sPEEP levels has to be made in a context that may not necessarily place PaO2 or PaCO2 level as the primary criteria. The mechanical ability of the patient to cope with the work of breathing may be a major problem and a

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compromise between gas exchange and work of breathing capability may have to be reached. Additional factors such as nutrition and patient psychology can also become important during prolonged weaning. CPAP and sPEEP techniques may also have a prophylactic role enabling patients with moderate respiratory dysfunction to avoid endotracheal intubation and/or mechanical ventilation. Accurate and adequate monitoring, including the measurement of respiratory mechanics, is essential for the optimum choice and adjustment of ventilation therapy.

REFERENCES Allen JL, Fredberg J J, Keefe DH et al (1985) Alveolar pressure magnitude and asynchrony during high-frequency oscillations of excised rabbits lungs. American Review of Respiratory Disease 132: 343. Annat G, Viale JP, Bui Xuan Bet al (1983) Effect of PEEP ventilation on renal function, plasma renin, aldosterone, neurophysins and urinary ADH, and prostaglandins. Anesthesiology 58: 136-141. Bake B, Wood L, Murphy Bet al (1974) Effect of inspiratory flow rate on regional distribution of inspired gas. Journal of Applied Physiology 37: 8--17. Barzilay E, Lev A, Ibrahim M & Lesmes C (1987) Traumatic respiratory insufficiency: comparison of conventional mechanical ventilation to high-frequency positive pressure with low-rate ventilation. Critical Care Medicine 15(2): 118-121. Bersten AD, Skowronski GA & Oh TE (1986) New generation ventilators. Anaesthesia and Intensive Care 14: 293-305. Beyer P, Beckenlechner P & Messmer K (1982) The influence of PEEP ventilation on organ blood flow and peripheral oxygen delivery. Intensive Care Medicine 8: 75. Boynton BR, Mannino FL, Davis RF et al (1984) Combined high-frequency oscillatory ventilation and intermittent mandatory ventilation in critically ill neonates. Journal of Pediatrics 105: 297. Brochard L, Pluskwa F & Lemaire F (1987) Improved efficacy of spontaneous breathing with inspiratory pressure support. American Review of Respiratory Disease 136: 411415. Brusasco V, Beck KC, Crawford M e t al (1986) Resonant amplification of delivered volume during high frequency ventilation. Journal of Applied Physiology 60: 885. Butler WJ, Bohr D J, Bryan AC & Froese AC (1980) Ventilation by high frequency oscillation in humans. Anesthesia and Analgesia 59: 577-584. Cameron PD & Oh TE (1986) Newer modes of ventilatory support. Anaesthesia and Intensive Care 14: 258--266. Carlon GC, Ray C Jr, Klein R, Goldiner PL & Miodownik S (1978) Criteria for selective positive end-expiratory pressure and independent synchronized ventilation of each lung. Chest 74: 501-507. Carlon GC, Kahn RC, Howland WS et al (1981) Clinical experience with high frequency jet ventilation. Critical Care Medicine 9: 1. Carlon GC, Howland WS, Ray C et al (1983) High-frequency jet ventilation: a prospective randomized evaluation. Chest 84: 551. Carlon GC, Barker RL, Benua RS & Guy YG (1985) Airway humidification with high frequency jet ventilation. Critical Care Medicine 13: 114. Civetta JM, Barnes TA & Smith LO (1975) 'Optimal PEEP' and intermittent mandatory ventilation in the treatment of acute respiratory failure, Respiratory Care 20: 551-557. Cole AGH, Weller SF & Sykes MK (1984) Inverse ratio ventilation compared with PEEP in adult respiratory failure. Intensive Care Medicine 10: 227-232. Crawford MR (1986) High-frequency ventilation. Anaesthesia and Intensive Care 14: 281-292. Cullen DJ & Caldera DL (1979) The incidence of ventilator induced pulmonary barotrauma in critically ill patients. Anesthesiology 50: 185--190.

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