BJA Advance Access originally published online on May 19, 2006
British Journal of Anaesthesia 2006 97(1):57-63; doi:10.1093/bja/ael115
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New ventilators for the ICU—usefulness of lung performance reporting
Critical Care Unit, Derriford Hospital Plymouth PL6 8DH, UK
*E-mail: peter.macnaughton{at}phnt.swest.nhs.uk
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Abstract |
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Monitoring the functional and mechanical properties of the lungs during positive pressure ventilation may assist in confirming the underlying pulmonary diagnosis, allow therapeutic interventions to be accurately assessed and provide information that ensures the optimal setting of the ventilator parameters and encourages timely weaning. This article reviews the range of lung function measurements, both continuous and intermittent, that may be undertaken during mechanical ventilation. The monitoring capability of ICU ventilators is increasing in complexity.
Keywords: equipment, ventilators; monitoring, intensive care; ventilation
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Introduction |
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Although the correction of abnormal arterial blood oxygen and carbon dioxide (CO2) tensions is the main goal of mechanical ventilation, it is of note that an intervention which improves gas exchange does not necessarily improve patient outcome. Nitric oxide inhalation has been shown to improve oxygenation but not outcome.11 Similarly, prone ventilation is associated with a significant improvement in gas exchange but in a large multicentre randomized, controlled trial there was no effect upon patient mortality.9 Interventions that improve gas exchange have even been associated with an adverse effect upon outcome. In the ARDSnet trial which compared high and low tidal volumes in acute lung injury, gas exchange as reflected by the
ratio was significantly higher in the high tidal volume group up to day 7 of the trial.3 However, despite causing improved gas exchange, high tidal volume ventilation was associated with an increased absolute mortality of 8.8% compared with using low tidal volumes. This landmark study confirmed that how the ventilator is set has a significant influence upon mortality and that an improvement in blood gas tensions does not imply that the patient is receiving optimal ventilatory support. The ARDSnet tidal volume trial confirmed the large body of evidence from laboratory studies that positive pressure ventilation can itself cause and exacerbate lung injury.38 The application of excessive tidal volumes that result in hyperinflation (‘volutrauma’) appears to be the most important cause of ventilator-induced lung injury (VILI). Cyclical opening and closing of atelectatic lung regions during tidal breathing (‘atelectrauma’) appears to be the other major contributory factor. Lung protective ventilation has been described as ensuring maximal recruitment of collapsed areas of the lung, avoiding regional hyperinflation and preventing cyclical recruitment–derecruitment during tidal ventilation.2 The increasing range of measurements available during mechanical ventilation should assist the clinician in the optimal setting of the ventilator.
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Static measurements |
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Compliance and resistance
The basic mechanical properties of compliance and resistance are readily measured during volume control ventilation with constant flow inspiration (Fig. 1).10 There must be an adequate end inspiratory pause in order that the inspiratory plateau pressure can be accurately measured and there should be no leak in the respiratory circuit. If intrinsic PEEP is present, the compliance value will be underestimated if the end-expiratory pressure measured in the airway is used. Total PEEP should be measured after an end-expiratory pause (see below) as this reflects the true end-expiratory pressure within the lungs.
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A static compliance (Crs.st) less than 50 ml per cm H2O is commonly encountered in ICU and patients with severe acute respiratory distress syndrome (ARDS) may have values less than 20 ml per cm H2O.10 Monitoring compliance may be useful for assessing the effectiveness of lung recruitment or the occurrence of lung over-distension after adjustments to ventilator settings. The change in compliance that occurs after the application of PEEP may differentiate between lung recruitment or hyperinflation.32
When interpreting the measured values of Crs.st, the influence of the chest wall compliance (CW) should be considered. Chest wall abnormalities, increased muscle tone and abdominal distension may all reduce chest wall compliance. In order to measure the two components of compliance (CL and CW), an oesophageal balloon is used to estimate pleural pressure. Some ventilators provide an auxillary pressure port that can be used for this measurement, which allows a complete evaluation of the elastic properties of the respiratory system.30
Airway resistance is calculated from the airway pressure change after an airway occlusion during a constant inspiratory flow (Fig. 1). In constant flow ventilation, the airway pressure changes that occur at the end of inspiration can be used. Minimum inspiratory airway resistance is calculated from the immediate decrease in airway pressure (P1). Maximum resistance, which includes the additional resistance attributable to stress relaxation and time constant inequalities, can be calculated using the inspiratory plateau pressure (P2). If measurements of resistance are undertaken with the inspiratory flow set at 60 litre min–1 (1 litre s–1), the total airway resistance in cm H2O litre–1 s can be obtained simply from the difference between the peak and plateau airway pressures (1 cm difference=resistance of 1 cm H2O litre–1 s). Typical values of resistance measured in ventilated patients are presented in Table 1. The increase in airway resistance observed in ARDS and cardiogenic pulmonary oedema may reflect oedema in the airway wall and the presence of fluid or secretions within the airway lumen.10 An additional factor is a reduction in the number of patent airways because of the marked loss of functional lung volume. Monitoring airway resistance together with compliance is of use when interpreting the cause of an increased airway pressure during mechanical ventilation and to quantify the response to a bronchodilator.
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Static pressure–volume curve
The static pressure–volume (PV) curve provides a more complete evaluation of the elastic properties of the respiratory system. Previously this was undertaken by recording the change in static airway pressure that occurred after a series of progressive 100 ml increases in lung volume introduced with a large calibrated syringe (supersyringe) during a prolonged apnoea. Alternatively the multiple occlusion technique could be used where the individual inspiratory plateau pressures obtained after a series of random changes in tidal volume set on the ventilator between functional residual capacity (FRC) and total lung capacity (TLC) were recorded.18 A number of ventilators now offer the ability to construct a static PV curve using the slow inflation technique.34 A preset volume is applied slowly, usually over a period of 15–20 s. The low flow rate minimizes the airway pressure changes that arise from resistive forces. Accurate measurements require that the patient is deeply sedated and that a neuromuscular blocking agent has been administered in order that there are no spontaneous respiratory efforts during the manoeuvre.
From the PV curve a lower inflection point (LIP) and upper inflection point (UIP) may be determined (Fig. 2). Traditionally the LIP is thought to reflect the opening of atelectatic areas of the lung and the pressure that PEEP should be set above in order to ensure that lung recruitment is maintained throughout tidal breathing. The UIP is considered to reflect the decrease in lung compliance from hyperinflation and the upper airway pressure that should not be exceeded in order to minimize the risk of VILI.18
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There remains considerable debate as to the interpretation and utility of static PV curves.14 20 Computed tomography (CT) scan studies have revealed that recruitment occurs throughout the inspiratory phase of the PV curve above the LIP.1 7 The presence or absence of a LIP does not predict whether the application of PEEP will be effective in producing recruitment.23 Furthermore, the process of undertaking a static PV curve may induce changes that are not a true reflection of what happens to the lungs during tidal breathing. Reducing the PEEP level to zero and allowing a prolonged expiration may cause significant de-recruitment that would not be present during ventilation with a normal ventilatory frequency.35 The PV curve may also be influenced by changes in chest wall compliance caused by abdominal distension in extra pulmonary ARDS.24 It appears that ventilation during tidal breathing follows the deflation limb of the PV curve and a point of maximum curvature has been described which equates to the onset of de-recruitment which may be a more appropriate pressure to guide the setting of PEEP.12 The ability to undertake the measurement of PV curves in proprietary ICU ventilators will allow the role of this measurement in clinical practice to be established.
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Dynamic measurements |
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Ventilation is a dynamic process and although easy to undertake, static measurements may not be the most appropriate method to assess lung mechanics and performance in the clinical setting. Furthermore, static measurements usually require that the patient is sedated and ventilated and that continuous flow ventilation (volume control) is used. Many ICU ventilators continuously display pressure, volume and flow measurements and plot dynamic PV loops. Interpretation is difficult as it is impossible to differentiate the resistive and elastic components of the waveform.35 A dynamic PV loop recorded during pressure control ventilation or volume control with decelerating inspiratory flow does not give any information regarding changes in lung compliance during inspiration and no inference can be made regarding the onset of recruitment or hyperinflation. However, during constant flow ventilation (i.e. volume control) a curving of the PV plot towards the end of inspiration suggests decreasing compliance and possible over-distension.35
Dynamic measurement of compliance and resistance
The airway pressure (P) at any time point (t) during positive pressure ventilation can be predicted by the equation of motion for the relaxed respiratory system:
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As the ventilator continuously measures airway pressure (P), flow (
) and volume (Vol), in theory if three simultaneous sets of data are collected, the three unknown values of compliance, resistance and total PEEP can be calculated. A more accurate estimate of the actual values will be obtained if a large number of sets of data are processed. With a sampling rate of 60 Hz some 180 data points will be analysed over the whole respiratory cycle with a ventilatory frequency of 20 bpm. Using the least squares method the values of best fit are computed for airway resistance, respiratory system compliance and total PEEP.28 The advantage of this method is that the lung mechanical parameters can be measured during any ventilator mode with any inspiratory flow pattern provided that the patient is relaxed.13 There is no requirement for an end inspiratory or expiratory pause although it will be inaccurate if the patient is making spontaneous respiratory efforts.33 Intrinsic PEEP can be estimated from the difference between the calculated total PEEP and the applied PEEP. However, this measurement does not appear particularly accurate in patients with significant airflow limitation42 when static measurements of intrinsic PEEP are most useful clinically. If data collected from the whole respiratory cycle are used in the calculation, the assumption is that resistance and compliance remain constant. This may be a significant source of error as resistance tends to decrease with increasing lung volume, while compliance may vary during inspiration according to which part of the PV curve the lung is operating on (see below).
Stress index
Changes in compliance during inspiration can be inferred from the analysis of the slope of the airway pressure–time plot in constant flow inspiration. A linear slope implies that compliance remains constant and that recruitment or over-distension are unlikely to be occurring during tidal ventilation. If the slope increases towards the end of inspiration, this suggests that the compliance has reduced at the end of inspiration and that hyperinflation is occurring. If the slope reduces during inspiration compliance is increasing suggesting that lung recruitment is occurring. The change in the slope of the airway pressure–time curve can be expressed mathematically which has been termed a ‘stress index’ and a possible method for monitoring the optimal setting of ventilatory support.31 This analysis is limited to constant flow inspiration and to date has only been applied to animal models.
Volume-dependent compliance measurements
The equation of motion assumes that resistance and compliance are constant over the period that data are collected. This may not be valid and an analysis of the change in compliance during inspiration has been proposed where the breath is divided into six segments and the equation of motion solved for each segment.25 This allows the change in compliance during inspiration to be plotted. Compliance that increases during inspiration suggests recruitment while a compliance that decreases with increasing volume suggests over-inflation. This type of analysis may allow dynamic assessment of recruitment or hyperinflation during tidal breathing.
Tracheal PV loops and dynostatic algorithm
Some ventilators offer the facility to measure tracheal pressure directly through a tracheal catheter introduced through the tracheal tube. Measuring tracheal pressure removes the influence of the resistance of the tracheal tube and significantly changes the appearance of the dynamic PV loop.34 A more accurate estimate of end inspiratory and expiratory pressures is obtained and assuming that the airway resistance is low, tracheal pressure will be a reasonable reflection of alveolar pressure. This results in a valid estimate of compliance measured under dynamic conditions.
The difference between tracheal and alveolar pressure which arises from airway resistance can be estimated from analysis of tracheal pressure and flow data collected at matching lung volumes during inspiration and expiration.16 If inspiratory and expiratory resistance are assumed to be identical at the same lung volumes, alveolar pressure can be predicted from the equation of motion and has been termed the dynostatic pressure (Pdynostatic):
If
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The calculation is repeated at a number of different volumes and alveolar pressure is plotted against volume during dynamic ventilation. With this analysis, an estimated ‘static’ alveolar PV plot can be produced during any mode of ventilation and without the need for an end inspiratory pause (Fig. 3). The dynostatic algorithm PV curve typically shows much greater changes in volume-dependent compliance than if compliance is calculated conventionally and can be used to indicate over-distension during tidal ventilation.
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–Pinspx
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Intrinsic PEEP and gas trapping |
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Minimizing intrinsic (PEEPi) and the associated hyperinflation are central to the ventilatory strategy for patients with severe airflow limitation attributable to COPD or asthma.27 The adverse effects of PEEPi include increased inspiratory work of breathing during spontaneous ventilation, reduced ability to trigger the ventilator during assisted modes of ventilation, adverse haemodynamic effects of increased intrathoracic pressure and an increased risk of barotrauma.5 The presence of PEEPi can be inferred by inspection of the flow time waveform which will reveal that expiratory flow has not reached zero before the next inspiration. Static PEEPi is measured by applying an end-expiratory pause, which holds the ventilator in expiration with the expiratory valve closed.21 If PEEPi is present the recorded airway pressure will increase during the expiratory pause to reflect the true end-expiratory alveolar pressure. Some ventilators automate the procedure and immediately after the measurement of PEEPi, are able to quantify the hyperinflation by allowing the expiratory valve to open and measuring the volume exhaled to zero positive end expiratory pressure. Static measurements of PEEPi require that the patient is relaxed and not making any inspiratory efforts which usually means that heavy sedation and a neuromuscular blocking agent need to be administered. A dynamic measurement of PEEPi can be obtained by recording the pressure change that occurs before inspiratory flow commences during inspiration. If the patient is not making any spontaneous efforts this can be measured from the change in airway pressure. However, when the patient is spontaneously breathing, oesophageal pressure needs to be recorded (as an estimate of pleural pressure) and dynamic PEEPi is extrapolated from the change in oesophageal pressure which occurs before inspiratory flow commences. Dynamic measurements reflect the lowest regional value of PEEPi and may be considerably less than static measurements in patients with airflow limitation who typically have significant inhomogeneity.22
Measurements of PEEPi should be made without any external PEEP applied. The value of static PEEPi observed in patients with chronic airflow limitation undergoing mechanical ventilation depends upon the airway resistance and on the applied ventilatory parameters (tidal volume, expiratory time and ventilatory frequency). Values up to 20 cm H2O are not uncommon and measurements of PEEPi may be used to assess the response to bronchodilators and to titrate ventilatory settings, with a reasonable aim to maintain PEEPi less than 10 cm H2O. The increased inspiratory work of breathing caused by PEEPi is reduced by the application of external PEEP, which may be of benefit during spontaneous ventilatory modes.5 There is little evidence that matching PEEPi with externally applied PEEP has any significant benefit in patients who are fully ventilated and not making spontaneous respiratory efforts.
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Volumetric capnography |
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This describes the continuous measurement of exhaled CO2 tension combined with the simultaneous measurement of exhaled volume.4 CO2 tension is plotted against exhaled volume (Fig. 4) and displayed as the CO2 single breath test. The airway (or anatomical) dead space can be calculated from Fowlers method and if the arterial
tension is known physiological and alveolar dead space can be derived from the Bohr–Enghoff equation (Fig. 4).19 Other parameters that may be calculated include total CO2 production which is a useful measure of metabolic activity. The values obtained appear to correlate well with the traditional methods using the Douglas bag and metabolic cart.15
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)/Measurements of dead space have been shown to have an increasing number of uses in the ICU. In ARDS an increased physiological dead space has been reported to be an independent predictor of mortality.26 The mechanism for this association is unclear and may reflect that a high dead space is a marker of more extensive endothelial and vascular injury. However, increased intra-pulmonary shunt influences the measurement of the Bohr–Enghoff dead space as the shunted pulmonary venous blood which carries more CO2 is mixed with the arterial blood causing an increased arterial
. The association of dead space and mortality in ARDS may be explained by an increased apparent physiological dead space because of a more severe shunt.8 Physiological dead space measurements may be helpful in the optimal application of PEEP. Lung recruitment has been associated with a reduction in dead space,39 which may reflect a reduction in intra-pulmonary shunt. The application of PEEP which causes over-distension and impairs regional pulmonary blood flow would be expected to increase alveolar dead space.19 Volumetric capnography has also been helpful in the diagnosis of pulmonary embolism, which is associated with a large increase in alveolar dead space that dramatically improves following reperfusion after thrombolytic treatment.40
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Assessment of lung recruitment |
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An improvement in arterial oxygen tension is highly predictive of recruitment as confirmed by CT scan32 although it is non-specific and blood gas monitoring alone will not reveal recruitment occurring during tidal breathing or lung over-distension. An effective recruitment manoeuvre will be expected to result in a significant improvement in the measured static compliance. Dynamic measurements that may identify recruitment include assessment of the volume-dependent change in compliance (see above). CT scanning is considered the current gold standard for assessment of regional distribution of ventilation and for identifying recruitment but cannot be readily performed at the bedside. Electrical impedance tomography is a new non-invasive technique that allows inference of regional differences in lung ventilation (both dependent collapse and hyperinflation) from the measurement of electrical potentials on the chest wall. A series of electrodes are applied to the chest wall and a two-dimensional image of the cross-sectional distribution of thoracic impedance obtained. The changes in impedance are assessed dynamically during tidal breathing. Initial experience with this technique is promising and it may prove to be a useful way of optimizing ventilator settings to avoid VILI at the bedside.41
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Automation of ventilator setting |
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 TopAbstractIntroductionStatic measurementsDynamic measurementsIntrinsic PEEP and gas...Volumetric capnographyAssessment of lung recruitment Automation of ventilator setting ConclusionReferences |
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Some ventilators are now able to offer modes of ventilatory support where various parameters are adjusted automatically according to the ventilator's ability to monitor lung performance. Adaptive support ventilation (TMHamilton Medical, Bonaduz, Switzerland) is a fully automated mode which ensures that the desired minute volume set by the clinician is delivered either as mandatory pressure-controlled breaths or spontaneous pressure-supported breaths according to the patient's underlying respiratory effort. From continuous monitoring of dynamic lung mechanics by the least squares method, the ventilator adjusts the inspiratory pressures automatically in order to obtain the optimal combination of ventilatory frequency and tidal volume which achieves the target minute ventilation. The algorithm includes a lung protective strategy, which prevents high tidal volumes and airway pressures above 35 cm H2O. The expiratory time constant (RCe) is monitored and used to adjust the time for expiration in order to minimize the risk of PEEPi. The only parameters set by the clinician during ASV are the minute ventilation,
and PEEP. Minute ventilation is entered as the percentage predicted alveolar ventilation based on ideal body weight. Initial experience with ASV suggests that fewer ventilator manipulations are required and that weaning may be achieved more rapidly than with conventional ventilation after uncomplicated cardiac surgery.29 36 37 Smart Care (TMDrager Medical, Lübeck, Germany) is a computer-driven system that manages pressure support ventilation over a prolonged period. The level of pressure support is automatically adjusted with the aim of weaning the amount of support whenever possible. Using the measured ventilatory frequency, tidal volume and end tidal CO2 tension, the pressure support is regularly adjusted to maintain the patient in a ‘comfort zone’. While the patient remains in the comfort zone, the knowledge-based weaning system will assess the response to a reduction in the level of pressure support. Early results with this mode have reported that weaning is more rapid than compared with conventional protocol-directed weaning.6 17
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Conclusion |
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The monitoring capability of ICU ventilators is increasing in complexity. Accurate assessment of lung mechanics allows the clinician to choose ventilator settings that maximize lung recruitment and prevent over-distension thereby minimizing the risk of adverse effects. Some ventilators are now able to automatically adjust ventilator settings according to measurements of lung mechanics. There is likely to be an increasing development of such knowledge-based systems with the aim of ensuring that patients receive the optimal mode of mechanical ventilation which minimizes the risk of ventilator-induced lung and ensures that weaning occurs as rapidly as possible.
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  J. P. Thompson and R. P. Mahajan Monitoring the monitors--beyond risk management. Br. J. Anaesth., July 1, 2006; 97(1): 1 - 3. [Full Text] [PDF]
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