Measurement of pressure-volume curves in patients on mechanical ventilation: methods and significance
Critical Care volume 4, Article number: 91 (2000)
Physiological background concerning mechanics of the respiratory system, techniques of measurement and clinical implications of pressure-volume curve measurement in mechanically ventilated patients are discussed in the present review. The significance of lower and upper inflection points, the assessment of positive end-expiratory pressure (PEEP)-induced alveolar recruitment and overdistension and rationale for optimizing ventilatory settings in patients with acute lung injury are presented. Evidence suggests that the continuous flow method is a simple and reliable technique for measuring pressure-volume curves at the bedside. In patients with acute respiratory failure, determination of lower and upper inflection points and measurement of respiratory compliance should become a part of the routine assessment of lung injury severity, allowing a bedside monitoring of the evolution of the lung disease and an optimization of mechanical ventilation.
Assessment of respiratory (combined chest wall and lung) pressure-volume curves permits analysis of the static mechanical properties of the respiratory system. The static pressure-volume curves are impaired in acute respiratory distress syndrome (ARDS). Three abnormalities are characteristic : the appearance of an initial inflection point, which corresponds to the opening pressure of the collapsed bronchoalveolar zones; reduction of the slope of the ascending limb, which indicates the loss of lung aeration that characterizes acute lung injury ; and lowering of the volume that corresponds to the upper inflection point, which increases the risk of mechanical ventilation-induced over-distension . The lower inflection point determines the minimal level of PEEP at which alveolar recruitment starts . The upper inflection point determines the pressure level that is not to be exceeded in order to avoid barotrauma and/or ventilator-associated lung injury . In critically ill patients, measurement of pressure-volume curves has been suggested as a method for assessing the severity of lung injury and for monitoring the evolution of lung disease. It can also guide the ventilatory adjustments to optimize mechanical ventilation .
Elasticity and resistance of the respiratory system
The anatomy of the respiratory system comprises of three passive structures (the lung, the chest wall and the airways) and one active structure (the respiratory muscles). These structures possess the mechanical properties of elasticity and resistance. Elasticity reflects the relation between the driving pressure (ΔP) and the change in the volume (ΔV). The elasticity of the respiratory system (lung and chest wall) is quantified by compliance (ΔV/ΔP) or elastance (ΔP/ΔV). The elastance of the respiratory system is equal to the elastance of the lungs plus the elastance of the chest wall. Resistance represents the relation between the driving pressure and the gas flow (♵), and is quantified by the following equation: resistance = ΔP/Δ♵.
Inflation of the respiratory system requires opposition of the forces of resistance, inertia and elastance, which act on the chest wall and the lungs. The force required for this is generated by the respiratory muscles during spontaneous ventilation, by the ventilator during controlled ventilation, or by both when the patient is on a partial mode of ventilatory support. One of the characteristic features of the respiratory system is that the chest wall and the lung, which constitute the passive structures, cannot be dissociated anatomically from the active structure constituted by the respiratory muscles. As a result, the resistive and elastic properties of the respiratory system can be evaluated only when the respiratory muscles are inactive. In ventilated patients, this can be achieved by sedation or muscle paralysis.
The resistive forces can be measured by the end-inspiratory occlusion method . After end-inspiratory clamping, an analysis of the pressure signal reveals a decline in the airway pressure that occurs in two steps: a rapid initial decline that corresponds to the resistive forces of the airways and the endotracheal tube, and a progressive secondary decline that represents the resistive forces of the lung tissue, which depend on its viscoelastic properties. Measurement of the resistance of the respiratory system helps to evaluate the severity of the airway disease. In ARDS, the increase in the respiratory resistance is essentially due to a reduction in the aerated lung volume, which in turn modifies the viscoelastic properties of the tracheobronchial tree . In the majority of patients there is no true active bronchoconstriction and the specific respiratory resistance [the measured resistance divided by the functional residual capacity (FRC)] is normal.
In acute respiratory failure, the impairment of the respiratory mechanics involves mainly the elastic component of the respiratory system. As a consequence, the measurement of respiratory pressure-volume curves should be done under static or quasistatic conditions in order to eliminate the resistive component. To achieve this, an end-inspiratory occlusion is performed with an end-inspiratory pause that is of sufficient duration (>3 s) to equalize the pressure between bronchial and alveolar compartments. Under these conditions, the intratracheal pressure reflects the alveolar pressure and the respiratory compliance can be calculated as the change in the lung volume divided by the change in the intratracheal pressure between end-inspiration and end-expiration, both measured at zero flow.
The pressure-volume relationship permits assessment of the mechanical properties of the respiratory system at different levels of lung inflation. This can be accomplished by static methods such as the inspiratory occlusion method  and the super-syringe method , or by a quasistatic method that is based on the inflation of the lung at a constant low flow. When the latter technique is used, the resistive component must be taken into account when analyzing the pressure-volume curves [9,10].
Intrinsic positive end-expiratory pressure
Another factor that can influence the interpretation of the pressure-volume curve is the presence of an intrinsic PEEP, which is defined as the presence of a positive alveolar pressure at end-expiration. Intrinsic PEEP results from a difference between the actual expiratory time and the expiratory time required for complete expiration of the tidal volume. Intrinsic PEEP may be generated by a very short expiratory time and/or a slow expiration due to high bronchial resistance or an abnormally high respiratory compliance. The presence of intrinsic PEEP may result in an error of the measurement of the compliance if the alveolar pressure at end-expiration is higher than the intratracheal pressure. In other words, if the intrinsic PEEP is not subtracted from the measured intratracheal pressure, the difference between the end-inspiratory and end-expiratory pressures may be overestimated, and consequently the respiratory compliance may be underestimated .
Intrinsic PEEP due to a short expiratory time and an incomplete emptying of the lung at end-expiration may also interfere with the measurement of the lower inflection point on the pressure-volume curve . Kacmarek et al  compared the differences between extrinsic and intrinsic PEEP, in terms of the distribution of the end-expiratory airway pressures in a lung model with four compartments that had different time constants. Those investigators showed that, in the presence of an extrinsic pressure, the end-expiratory pressures were equal in all of the four compartments, whereas they were unequal when the same level of expiratory pressure was applied as an intrinsic pressure. At an intrinsic PEEP of 12.7 cmH2O, the slow lung unit had an end-expiratory pressure of 15.8 cmH2O, whereas the fast lung unit had an end-expiratory pressure of 10.1 cmH2O. In other words, the positive pressure generated by the intrinsic PEEP may cause an over-distension of lung areas that have a prolonged time constant and a low recruitment of lung areas that have a short time constant where PEEP is expected to keep the alveoli open. When measuring pressure-volume curves, it is essential to empty the lungs completely by a prolonged expiration before inflating the respiratory system.
Lung and chest wall
The mechanical properties of the respiratory system depend on the mechanical characteristics of the lung and the chest wall. The compliance of the chest wall is determined by the ratio ΔV/ΔP, where ΔP is the pleural pressure, which is approximated by the measurement of oesophageal pressure. The compliance of the lung is measured by the same principle, where ΔP is the transpulmonary pressure, which is defined as the difference between the alveolar and the pleural pressures. In patients with acute lung injury, the impairment of the respiratory compliance may, in part, be due to a decrease in the compliance of the chest wall .
Techniques of measurement of the pressure-volume curves
The super-syringe technique
This static method consists of inflation of the lungs in steps of50-100 ml up to 1.5 or 3.0 l starting from the FRC . Thevolume of gas administered is determined by the displacement of the piston. Theairway pressure is measured by a pressure transducer, with zero referred to theatmospheric pressure. The patients are sedated, paralyzed and ventilated at afractional inspired oxygen of 1.0 without any PEEP for 15 min before themeasurement, and the syringe is filled beforehand with humidified oxygen. A fewseconds of disconnection of the patient from the ventilator is necessary toempty the lungs completely. The syringe is then connected to the endotrachealtube and the inflation manoeuvre is started from the FRC. The interval betweentwo successive inflations should be 3 s in order to ensure a stable plateaupressure. The same maneuver can be performed during deflation in successivesteps of 50-100 ml. The pressures and the volumes are recorded simultaneouslyand the pressure-volume curve is constructed from the obtained data. The entireprocedure takes about 60 s.
The super-syringe technique was largely utilized during the 1980sto describe the different stages of ARDS [1,2]. This method has some disadvantages, however; the patienthas to be disconnected from the ventilator and there is a loss of lung volumeduring the inflation procedure due to the consumption of the oxygen containedin the syringe. The errors in measurement that occur with the use of thesuper-syringe technique have been evaluated by Dall'Ava-Santucci etal  and Gattinoni et al . Those investigators compared the variations in lungvolumes obtained using the syringe technique with those measured by inductanceplethysmography (Respitrace, NIMS Inc, Miami, FL, USA). The pressure-volumecurves obtained with Respitrace exhibited lesser degrees of hysteresis(difference between the lung volumes during inflation and deflation for thesame level of pressure), and the compliance during deflation was higher (73versus 67 ml/cmH2O). This difference was observed only if theduration of the inflation was prolonged (>45 s) and is related to the gasexchange that occurs in the lung during the manoeuvre; the loss of lung volumedue to oxygen uptake is only partially compensated for by the carbon dioxideproduction . A rapid inflation of less than 40 shelps to minimize this error . The temperature andthe humidity of the gas in the syringe may also influence the measurement ofthe pressure-volume curve. Administration of unwarmed and unhumidified gascauses a displacement of the curve to the left [15,16].
The inspiratory occlusion technique
The inspiratory occlusion technique was developed in the late1980s and was initially described by Levy et al . It consists of measurement of plateau pressures thatcorrespond to different tidal volumes during successive end-inspiratoryocclusions. This technique is performed using a mechanical ventilator equippedwith facilities for end-inspiratory and end-expiratory occlusions. It is notnecessary to disconnect the patient from the ventilator, and the loss of volumedue to lung oxygen uptake is negligible because each measurement lasts only 3 s. The patient is ventilated in a volume-controlled mode with a constant flow. Between two measurements, the ventilation is normalized by using the sameventilatory parameters. The different tidal volumes are administered in arandomized sequence. These tidal volumes are obtained by changing therespiratory rate while maintaining the inspiratory flow constant (lengtheningor shortening the duration of inflation). The intrinsic PEEP is determinedbefore each inflation to ensure that the lung volume and the end-expiratorypressure are stable. The occlusion manoeuvre is performed at end-inspirationand the plateau pressure is measured after a few seconds of occlusion. Thepressure-volume curve is constructed from the different plateau pressures thatcorrespond to the administered volumes (Fig. 1).
The inspiratory occlusion technique offers the advantage ofavoiding disconnection of the patient from the ventilator and it allows themeasurements from any level of PEEP. Since the start of the 1990s, thistechnique has been extensively used to determine the lower and upper inflectionpoints on the pressure-volume curve [4,5] and to quantify the effect of PEEP on alveolar recruitmentin patients with ARDS [17,18]. The time required to perform the manoeuvre is around 15 min, however, whichrenders the technique cumbersome in clinical practice.
The quasistatic method using a continuous inflation at a constantflow
The simplest technique to obtain the pressure-volume curve in a critically ill patient without having to disconnect the patient from the ventilator is to inflate the respiratory system by a constant flow delivered by the ventilator [9,10]. This is a quasistatic technique. It can be performed on any intensive care ventilator that is equipped with a constant flow generator, and that has software and a display screen for plotting and analyzing the pressure-volume curve. This technique is derived from a dynamic method proposed by Suratt and co-workers [19,20] and is based on the assumption that when the lungs are inflated at a constant inspiratory flow, the change in the airway pressure is inversely proportional to the compliance of the respiratory system. Those investigators compared the quasistatic technique at a constant flow with the static technique, and showed that the compliances obtained by the two methods are closely correlated. Ranieri et al  later studied the pressure-volume curves in patients with ARDS and showed that curves obtained by the constant flow technique, like those obtained by the inspiratory occlusion technique, permit the determination of PEEP-induced alveolar recruitment or over-distension. If high constant flows that range between 20 and 60 l/min are used, however, only the slope of the pressure-volume curve can be reliably measured; upper and lower inflection points are overestimated because of the resistive effect generated by the high flow [9,10]. Utilization of constant flows less than 10 l/min can reproduce conditions close to those obtained with static methods .
In the 1980s, Mankikian et al  compared the pressure-volume curves obtained by the super-syringe technique with those obtained by the constant flow technique, using a very low flow of 1.7 l/min delivered by a special flow generator that was connected directly to the patient's endotracheal tube. Those investigators showed that the curves obtained using the two methods were identical. It must be emphasized that, in order to obtain such a low flow, a period of 60 s was required to inflate the lungs, which may have resulted in a loss of lung volume during the manoeuvre caused by oxygen uptake by the lungs. In other words, this technique illustrates one of the drawbacks of the super-syringe technique, although the resistive component with such a low flow is negligible.
Fifteen years later, Servillo et al  compared this technique using a higher constant flow of 15 l/min with the inspiratory occlusion technique. The flow was delivered by a Servo 900C computerized prototype ventilator (Siemens-Elema AB, Solna, Sweden). Those authors showed that the pressure-volume curve obtained using the constant flow technique was shifted to the right when compared with the curve obtained using the inspiratory occlusion method because of the resistive factor. The slopes of the curves were similar between the two methods, but the 15 l/min constant flow method was associated with an overestimation of the upper and lower inflection points.
Two solutions have been proposed to obviate the resistive factor when quasistatic methods are used: subtraction of the resistive pressure in the connecting tubes and in the airways from the measured total pressure; and reduction of the constant flow.
The first method was described by Servillo et al  and Jonson et al  and requires a complex computerized system that includes a computer-controlled Servo Ventilator 900C (Simens-Elema AB, Solna, Sweden), a ventilator-computer interface and an IBM-compatible computer. The ventilator-computer interface allows the computer to supervise the ventilator settings and collect inspiratory and expiratory flows and airway pressure signals. A constant or oscillating low flow (200 ml/s) is then administered to the patient after a prolonged expiration (4 s). The pressure-volume curve is analyzed using the Excel spreadsheet (Microsoft Corporation) and plotted after subtracting resistive pressure in the connecting tubes and airways from the total airway pressure (P tot ), which is measured by a pressure transducer included in the ventilator. The tracheal pressure (P trach ) and the elastic pressure (P el ) are calculated using the following equations:
P trach = P tot - Pres(tube) = P tot - [(K1 × ♵) + (K2 × ♵2)] (1)
P el = P trach - P res = P trach - (R rs × V t ) (2)
In Equation 1, inspiratory flow (♵) is measured using a pneumotachograph that is included in the ventilator, and K1 and K2 are determined in vitro for each type of tube and connection in order to calculate the resistive pressure that is related to the connecting and endotracheal tubes [Pres(tube)]. In Equation 2, tidal volume (V t ) is measured using a pneumotachograph that is included in the ventilator, P res is the resistive pressure in the airway and R rs is the inspiratory resistance of the respiratory system, which can be calculated as the quotient between area of the pressure-volume loop and area of the flow-volume loop when a constant flow is administered, or as the quotient between pressure and flow when an oscillating inspiratory flow is used during the insufflation .
Servillo et al  compared the constant flow method (allowing subtraction of the resistive pressure) with the inspiratory occlusion method, and found a good agreement between both methods as far as respiratory compliance, and lower and upper inflection points are concerned.
The second method employed to obviate the resistive factor when quasistatic methods are used involves reduction of the constant flow. A recent study compared the quasistatic method using two constant flows (3 and 9 l/min) with the super-syringe technique and the inspiratory occlusion technique in patients with acute respiratory failure . The constant flows were obtained through the regulating device of a César ventilator (Taema, Antony, France) equipped with a display screen and software capable of plotting and analyzing the pressure-volume curves. The ventilator was set in a volume-controlled mode with a constant inspiratory flow, a tidal volume of 500 or 1500 ml, an inspiration:expiration ratio of 80% and a respiratory frequency of 5 breaths/min. With these particular ventilatory settings, a constant flow of either 3 or 9 l/min was delivered over a period of 9.6 s and the pressure-volume curves were displayed real-time on the screen of the ventilator. The measurement of the respiratory compliance (slope of the pressure-volume curve between the two inflection points) and the determination of the upper and lower inflection points were carried out with the help of two mobile cursors available on the ventilator display screen. The entire procedure took 2 min and was performed without disconnecting the patient from the ventilator (Fig. 2).
That study showed that the pressure-volume curves obtained at a constant flow of 3 l/min matched those obtained using the static methods. When a constant flow of 9 l/min was used, there was a slight shift of the curve to the right due to the resistive factor (Fig. 3). The lower inflection point measured using the quasistatic method with a constant flow of 9 l/min was slightly higher than that obtained using the static methods, but the difference was not statistically significant. The slopes of the curves were similar for both flows and also between the quasistatic and static methods. The resistive pressures induced by the two constant flows, which were defined as the initial increase in the airway pressure until the inspiratory flow becomes constant, were 1.0 ± 1.0 and 1.8 ± 2.1 cmH2O,respectively. These results, which have been confirmed by a second study , show that the influence of the resistive factor on the pressure-volume curves obtained using the quasi-static method is not clinically relevant if the flow administered is less than 9 l/min.
The continuous flow technique presents a number of advantages over the super-syringe and the inspiratory occlusion techniques: it does not require disconnection of the patient from the ventilator; it does not modify the lung volume before performing the manoeuvre; the construction of the pressure-volume curve on the ventilator screen takes only 10 s and the entire procedure, including the analysis of the characteristics of the pressure-volume curves, takes around 2 min; the loss of volume due to oxygen uptake by the lungs is negligible; and the technique is simple to carry out at the bedside without the need for any special equipment other than a respirator. The software for freezing and analyzing the pressure-volume curve is not available on most intensive care ventilators, however. Systems are being developed that deliver constant flows between 0 and 10 l/min and that include software that allow analysis of the pressure-volume curves; such future ventilators should facilitate routine measurement of the pressure-volume curves at the bedside
Measurement of chest wall pressure-volume curve and lungpressure-volume curve
The chest pressure-volume curve is constructed by plotting lung volumes against pleural pressures that are estimated from oesophageal pressures. Oesophageal pressure can be measured by using either a balloon or a water-filled catheter. A catheter that incorporates a thin-walled balloon inflated with air (10 cm long, 1 cm in circumference) or a water-filled catheter is inserted into the mid-oesophagus and is connected to a pressure transducer. The patient is kept in the half-sitting position in order to minimize the effect of weight of the mediastinum in the supine position. Before measurement, an 'occlusion test' consisting of a series of three to five spontaneous occluded inspiratory efforts is recommended . A ratio between oesophageal pressure changes and occluded inspiratory pressure changes that is close to 1 indicates that the catheter is properly positioned and that oesophageal pressure is an acceptable reflection of pleural pressure. The lung pressure-volume curve is constructed by plotting lung volumes against transpulmonary pressures (differences between airway and oesophageal pressures).
Clinical implications of the pressure-volume curves
General shape of the curve
In normal individuals the curve has a sigmoidal shape. It is linear in its initial part when the pressure-volume relation is measured from the FRC . During spontaneous ventilation, the total compliance of the respiratory system, including the chest wall and the lung, is 100 ml/cmH2O. The lung compliance is around 200 ml/cmH2O . In anaesthetized ventilated normal individuals, the total compliance of the respiratory system is slightly decreased (70-80 ml/cmH2O) . The respiratory compliance reflects the elastic properties of the respiratory system, and quite often that of the lungs. A stiff lung (as seen in ARDS) has a low compliance, whereas a highly distensible lung (as seen in an emphysematous patient) has a very high compliance. In healthy persons, the upper inflection point occurs at a lung volume of 3 l above the FRC, which defines the total lung capacity. The upper inflection point corresponds to the pressure above which pulmonary over-distension commences . On the pressure-volume curve, this point is situated around 30 cmH2O. Lastly, the loop formed by the pressure-volume curves in inflation and deflation indicates the presence of hysteresis .
The different components of the pressure-volume curve in acuterespiratory distress syndrome
Significance of the lower inflection point
In ARDS, the initial part of the pressure-volume curve is notlinear. The pressure corresponding to the intersection of two lines thatrepresent the minimal and maximal compliance is defined as the lower inflectionpoint [2,5]. The significance ofthe lower inflection point has been studied both in lung models and in patientswith ARDS.
Using a mathematical ARDS lung model, Hickling et al showed that the lower inflection point reflects bothgravitational superimposed pressure and alveolar threshold opening pressure,the latter playing the most important role. The lower inflection point is notable to predict optimum PEEP accurately, because there is a continuous alveolarrecruitment on the linear portion of the curve. According to Jonson etal , a marked lower inflection point indicatesthe pressure at which many collapsed alveoli are opening at the same time. Onthe other hand, the absence of a lower inflection point on the pressure-volumecurve signifies an inhomogeneous lung having many different time constants andalveolar threshold opening pressures. In this configuration, the differentalveolar compartments are opened one after another as far as the pressureincreases, so that the lower inflection point is not clearly defined on thepressure-volume curve.
In patients with ARDS, the lower inflection point can be relatedeither to the chest wall or to the lung parenchyma. Between 0 and5 cmH2O, the lower inflection point may result from the impairment ofviscoelastic properties of the chest wall due to positive fluid balance,abdominal distension, oedema of soft tissue and pleural effusion . In supine position, an upward displacement of thediaphragm resulting from an increased abdominal pressure induces an increase inthe stiffness of the chest wall and a decrease in the chest wall compliance[13,29].
Only the presence of a lower inflection point on the pulmonarypressure-volume curve identifies the existence of a massive reopening ofpreviously collapsed bronchoalveolar regions. In such a situation, applicationof a PEEP that is equal to or greater than the pressure corresponding to thelower inflection point results in significant alveolar recruitment and decreasein pulmonary shunt. It may avoid mechanical ventilation-induced lung injuryresulting from the repeated opening and closure of the terminal bronchiolesduring each respiratory cycle . Sometimes the lowerinflection point cannot be clearly identified on the respiratorypressure-volume curves in patients with a true acute respiratory failure. As aconsequence, the PEEP level is often chosen on the basis of arterialoxygenation criteria.
Vieira et al  compared therespiratory mechanics, computed tomography (CT) morphology of the lung and theradiological appearances in two groups of patients with or without a lowerinflection point on their pulmonary pressure-volume curves. In that study, theanalysis of the pulmonary morphology was performed by a technique involvingmeasurement of the CT attenuation (pulmonary density) using a fast spiralscanner. According to previous studies [32,33,34,35] lungzones with a CT attenuation between -1000 and -900 Hounsfield units (HU) areconsidered as over-distended, those between -900 and -500 HU are considered asaerated, those between -500 and -100 HU as poorly aerated, and those between-100 and +100 HU as nonaerated.
In the study by Vieira et al ,the aetiology of the lung injury and the haemodynamic and respiratoryparameters were not different between groups. However, the patients with alower inflection point were younger, their respiratory compliance was lower,and their Murray's score and mortality tended to be higher (Fig.4). The total lung volume as well as the volume of lungtissue (comprising a mixture of alveolar septa, pulmonary and bronchialvessels, bronchi, various bronchopulmonary cells and pulmonary blood volume)were similar between the two groups, suggesting that the degree of pulmonaryinflammation was similar in patients with or without a lower inflection point. On the other hand, the patients with a lower inflection point had a muchsmaller volume of normally aerated lung and a much higher volume of poorly andnonaerated lung. Their lungs were characterized by extensive diffuseradiological opacities, which were homogeneously distributed. In thesepatients, the volumetric distribution of CT attenuations was monophasic, with apeak at 7 HU (close to the CT attenuation of water) and the chest radiograph wascharacterized by diffuse pulmonary hyperdensities. In patients without a lowerinflection point, the volumetric distribution of CT attenuations was biphasicwith a peak at -727 HU and another at 27 HU. The chest radiograph showedopacities predominating in the lower lobes. In this latter group of patients,the aeration of the upper lobes appeared relatively well preserved. In bothgroups, PEEP induced an alveolar recruitment that was associated with lungover-distension only in those without a lower inflection point. Although thereasons for such differences in pulmonary morphology remain unknown, theireffect on respiratory mechanics is marked. For the managing physician, thesedifferences imply different ventilatory strategies in these two groups.
Significance of the upper inflection point
Experimental studies  have shown thatthe pulmonary lesions induced by mechanical ventilation are related to hightidal volumes. A ventilatory strategy based on the reduction of tidal volumehas been proposed to avoid this risk. This strategy consists of titrating thetidal volume in such a way that the maximal alveolar pressure does not exceedthe pressure that corresponds to the upper inflection point. The upperinflection point is defined as the pressure on the linear part of thepressure-volume curve beyond which the slope of the curve decreases. Itindicates the end of alveolar recruitment and the beginning of pulmonaryover-distension . Like the lower inflection point,however, the upper inflection point is not always present on thepressure-volume curve. This phenomenon was studied in a mathematical lung modeldeveloped by Hickling et al . Those authorssuggested that the absence of an upper inflection point does not necessarilysuggest the absence of over-distension. It is rather a result of continuedalveolar recruitment above the upper inflection point, which masks itspresence.
Respiratory pressure-volume curve, alveolar recruitment and pulmonary over-distension
Ranieri et al  showed that ananalysis of the pressure-volume curves in zero end-expiratory pressure (ZEEP)conditions permits prediction of whether PEEP induces alveolar recruitment orover-distension. A respiratory pressure-volume curve in ZEEP with upwardconvexity signifies a reduction in respiratory compliance as the lung volumeincreases. In these patients, the pressure-volume curve in PEEP conditionssuperimposes the curve obtained in ZEEP, implying the lack of any alveolarrecruitment and the presence of over-distension. Conversely, a curve with itsconvexity downward in ZEEP conditions signifies progressive alveolarrecruitment when the lung volume increases. In these patients, the applicationof PEEP induces an upward shift of the curve, indicating alveolarrecruitment.
The volume of recruited lung according to Ranieri et al can be quantified from the pressure-volume curvesmeasured in ZEEP and PEEP. After determining the difference in end-expiratorylung volume between ZEEP and PEEP, the curves are placed on the same pressureand volume axes and the recruited lung volume is calculated as the differencein the volumes between ZEEP and PEEP for the same alveolar pressure (Fig.5). An alveolar pressure of 15 or 20 cmH2O isoften chosen for this purpose [17,18]. Using this method, Jonson et al  quantified the amount of alveolar recruitment at twolevels of pressure (15 and 30 cmH2O) in a series of patients withacute lung injury. The authors showed that recruitment induced by PEEP wasgreater at 15 cmH2O than at 30 cmH2O (205 and 78 ml,respectively). The respiratory compliance in ZEEP was always higher than therespiratory compliance in PEEP. These results suggest a continuous alveolarrecruitment of previously collapsed lung units during insufflation in ZEEP anda distension or over-distension of previously open lung units at higher levelof pressure in PEEP.
In fact, the technique proposed by Ranieri et al  measures the total increase in gas volume resulting fromPEEP. It does not permit differentiation of PEEP-induced alveolar recruitmentfrom PEEP-induced distension and/or over-distension [31,35]. The measurement of pulmonary CTattenuation from a spiral CT scan permits differentiation of PEEP-inducedalveolar recruitment from over-distension. Vieira et al  quantified the decrease in lung volume with a CTattenuation between -100 and +100 HU (the recruited lung volume according toGattinoni et al ) and the increase in lungvolume with a CT attenuation of less than -900 HU (the over-distended volumeaccording to Vieira et al) following PEEP administration in patientswith acute respiratory failure. In these patients, the PEEP level was fixed at2 cmH2O above the lower inflection point. As Figure 6 shows, in some patients alveolar recruitment was accompaniedby an over-distension of the lung territories that were previously aerated. These results clearly demonstrate that PEEP can induce alveolar recruitment andlung over-distension simultaneously.
One option to avoid PEEP-induced over-distension could be torestrict the increase in lung volume in previously aerated lung areas bydecreasing the compliance of the upper part of the chest wall. Turning thepatient to the prone position results in a significant decrease in the chestwall compliance . A significant relationship hasbeen found by Pelosi et al  between proneposition-induced improvement in arterial oxygenation and the decrease in chestwall compliance observed after turning the patient. In addition, the value ofchest wall compliance in the supine position seems to be a predictive factor ofthe improvement in arterial oxygenation after the prone position is assumed;the greater the chest wall compliance in the supine position, the greater theimprovement in arterial oxygenation. In other words, by limiting the expansionof the compliant part of the rib cage, the prone position tends to limitPEEP-induced over-distension and to promote alveolar recruitment in the caudalparts of the lung.
Respiratory pressure-volume curve and optimization of ventilatory settings
The presence or the absence of a lower inflection point on thepressure-volume curve should influence the choice of ventilatory settings. In the absence of a lower inflection point on thelung pressure-volume curve, as observed when upper lobes remain fairly aeratedand lower lobes are essentially nonaerated, the patient is at risk of lungover-distension at high levels of PEEP, and a PEEP around 10 cmH2Oshould be administered because it represents a good compromise betweenPEEP-induced alveolar recruitment and over-distension. When a lower inflectionpoint is present on the lung pressure-volume curve, as observed when loss ofgas is homogeneously distributed within the lungs, a PEEP level far above thelower inflection point should be tested because the probability of asignificant alveolar recruitment is largely predominant over the risk ofover-distension.
Based on the consensus of experts, the European-American ConsensusConference of 1993  recommended limiting the plateaupressure to 35 cmH2O in patients with ARDS. Clinical studies [4,9,39] have shownthat the upper inflection point in severe ARDS is at around 26 cmH2Oand can vary from 18 to 40 cmH2O depending on the severity of thelung injury (Fig. 7). As a consequence, there is no'magic number' that defines the risk of over-distension. In eachindividual, the upper inflection point varies according to the pressure-volumecurve, and thus routine measurement of pressure-volume curves appears to be acritical element for implementing a protective ventilatory strategy.
The mortality of patients suffering from ARDS, despite a steady decline, remained high at 40-60% at the end of the 1990s. A recent randomized prospective study  has demonstrated that use of a 'protective ventilatory strategy' ameliorates lung function and decreases the mortality in patients with ARDS. This strategy combines application of a PEEP higher than the lower inflection point and administration of tidal volumes less than 6 ml/kg to limit the end-inspiratory pressures to below 40 cmH2O. Even though the results of that study need to be confirmed by larger studies, the concept of recruiting while protecting the lung is already adopted by most centres and has become an important element of the therapeutic arsenal in patients with severe ARDS. In order to achieve this, measurement of the compliance and determination of the upper and lower inflection points on the pressure-volume curves at the patient's bedside should become a part of the routine monitoring of patients with acute respiratory failure. The continuous flow technique is a simple and reliable method that facilitates routine assessment of the pressure-volume curves and should be available on most intensive care ventilators.
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Lu, Q., Rouby, JJ. Measurement of pressure-volume curves in patients on mechanical ventilation: methods and significance. Crit Care 4, 91 (2000). https://doi.org/10.1186/cc662