Bench-to-bedside review: Chest wall elastance in acute lung injury/acute respiratory distress syndrome patients

cc2854 CCJ Review Bench-to-bedside review: Chest wall elastance in acute lung injury/acute respiratory distress syndrome patients Gattinoni Luciano gattinon@policlinico.mi.it Chiumello Davide chiumello@libero.it Carlesso Eleondra Valenza Franco

Institute of Anesthesia and Critical Care, University of Milan, Policlinico – IRCCS Hospital, Milan, Italy

Critical Care 1364-8535 2004 85350355 1546959710.1186/cc2854 7 5 2004 2004 BioMed Central Ltd acute respiratory distress syndrome chest wall elastance intra-abdominal pressure pleural pressure ventilator-induced lung injury

Abstract

The importance of chest wall elastance in characterizing acute lung injury/acute respiratory distress syndrome patients and in setting mechanical ventilation is increasingly recognized. Nearly 30% of patients admitted to a general intensive care unit have an abnormal high intra-abdominal pressure (due to ascites, bowel edema, ileus), which leads to an increase in the chest wall elastance. At a given applied airway pressure, the pleural pressure increases according to (in the static condition) the equation: pleural pressure = airway pressure × (chest wall elastance/total respiratory system elastance). Consequently, for a given applied pressure, the increase in pleural pressure implies a decrease in transpulmonary pressure (airway pressure – pleural pressure), which is the distending force of the lung, implies a decrease of the strain and of ventilator-induced lung injury, implies the need to use a higher airway pressure during the recruitment maneuvers to reach a sufficient transpulmonary opening pressure, implies hemodynamic risk due to the reductions in venous return and heart size, and implies a possible increase of lung edema, partially due to the reduced edema clearance. It is always important in the most critically ill patients to assess the intra-abdominal pressure and the chest wall elastance.

Introduction

The respiratory system includes the lung and the chest wall, in series, and the overall mechanical behavior depends on the mechanical characteristics of its components and their interactions 1. The common increase in the elastance (decrease in compliance) of the whole respiratory system in acute lung injury (ALI) and in acute respiratory distress syndrome (ARDS) has traditionally been attributed to the lung component. It has long been reported, however, that the chest wall elastance was also altered in many cases 2345. Recently, mainly due to the increased concern for the abdominal pressure, more attention has been paid to the chest wall mechanics in critically ill patients 67. The problems of the mechanical impairment of the chest wall and its consequences are now widely recognized. The present review will focus on the dimension of these problems and their consequences in the critically ill patient.

Respiratory mechanics: general concepts

When partitioning the respiratory mechanics into its lung and chest wall components, it is convenient to refer to elastance instead of compliance. The total elastance of the respiratory system is the pressure required to inflate it 1 l above its resting position. This is, the applied airway pressure is spent in part to inflate the lung and in part to inflate the chest wall. The chest wall comprises the anterior and posterior thoracic cage walls and the diaphragm, which is the 'abdominal component'. Indeed, in static conditions, when the airway resistance is nil:

P_aw= P_l+ P_pl(1)

and

E_tot= E_l+ E_cw(2)

where P_awis the (static) airway pressure, P_lis the transpulmonary pressure, P_plis the pleural pressure, E_totis the total respiratory system elastance, E_lis the lung elastance, and E_cwis the chest wall elastance.

On the basis of these classical equations it is easy to grasp the mechanical interaction between the lung and the chest wall. First, however, it is important to recall that the concept of 'transmission' of alveolar pressure to the thoracic cavity is misleading 8. Let us assume that we inflate an isolated lung at an alveolar pressure of 10, 15 or 20 cmH₂O. The pressure measured at the pleural surface will always be 0 cmH₂O (i.e. atmospheric pressure) because the alveolar pressure is not 'transmitted'. When the thoracic cage surrounds the lungs as they are inflated, however, the cage has to change its volume. The lungs 'push' the thoracic cage, and the pressure generated by the interaction between the lung and the chest wall, which may have different elasticities, is the pleural pressure. If we consider that the pressure is 'transmitted', then the pleural pressure would depend on the airway pressure and the lung elasticity (the stiffer the lung, the lower the transmission). However, this approach ignores the contribution of the chest wall.

If the thoracic cage is 'soft' the pleural pressure generated to drive it will be low, but if the thoracic cage is 'stiff' then a higher pleural pressure will be needed (Fig. 1). The distending force of the lung is the pressure difference between the alveoli and pleural pressure (the transpulmonary pressure), while the distending force of the thoracic cage is the pleural pressure to which all of the intrathoracic structures, such as the heart and the intrathoracic vessels, are subjected. In mathematical terms, because of and by rearranging equations 1 and 2, it follows that:

Figure 1

Effect of different lung elastance (E_L) and chest wall elastance (E_w) on the total elastance (E_tot) of the respiratory system

Effect of different lung elastance (E_L) and chest wall elastance (E_w) on the total elastance (E_tot) of the respiratory system. An equal total elastance of the respiratory system may arise (a) from a high lung elastance and a low chest wall elastance or (b) from identical lung elastance and chest wall elastance.

P_pl= P_aw× E_cw/E_tot(3)

and

P_l= P_aw× E_l/E_tot(4)

The pleural pressure depends on the pressure applied to the airways, and on the ratio between the chest wall elastance and the total elastance of the respiratory system, which is the sum of the chest wall elastance and the lung elastance (see equation 2). In normal conditions this ratio is about 0.5 at functional residual capacity, because the chest wall elastance and the lung elastance are similar. In ARDS, however, the elastance ratio may vary from 0.2 to 0.8 689. It is thus clear that, for the same applied airway pressure (let us say 30 cmH₂O) and with a chest wall elastance/total respiratory system elastance ratio of 0.5, the transpulmonary pressure will be 15 cmH₂O and the pleural pressure will be the same. If the ratio is 0.2, however, the transpulmonary pressure will be 24 cmH₂O but the pleural pressure will be 6 cmH₂O, while if the ratio is 0.8 the transpulmonary pressure will only be 6 cmH₂O and the pleural pressure will be 24 cmH₂O.

These simple calculations illustrate the importance of knowing the mechanical characteristics of both the lung and the chest wall. The same airway pressure may generate dramatically different transpulmonary pressures and pleural pressures, with marked consequences on lung distension (mainly a function of transpulmonary pressure) and on hemodynamics (partly a function of pleural pressure).

We shall now look at the available tools to measure the pleural pressure, the causes of pleural pressure increases and the clinical consequences of elevated pleural pressure.

Measuring pleural pressure and intra-abdominal pressure

The only method available in clinical practice to measure the pleural pressure is the measurement of the changes of esophageal pressure as detected by an esophageal balloon 1011. Unfortunately, the pressure measured in the esophageal balloon does not reflect the absolute value of the pleural pressure. In animal experiments in which we measured the pleural pressures directly by wafers in non-dependent lung regions, in middle lung regions and in dependent lung regions in the supine position, in both healthy lungs and in edematous lungs, we found that esophageal pressure was a good estimate of the real pleural pressure in the middle lung but that it overestimated the nondependent pleural pressure and underestimated the dependent pleural pressure 12. It is worth pointing out, however, that the differences in pressure recorded in the esophageal balloon closely match the differences in pleural pressure 13. Although it must always be remembered that the esophageal pressure is only an estimate of the real pleural pressure, which varies in the different lung regions, we strongly believe that esophageal pressure measurement is sufficiently informative in the clinical scenario (i.e. to estimate the change in pressure).

We may also wonder when this measurement is indicated in ALI/ARDS patients. As the chest wall impairment in these patients is usually due to an abnormal increase in intra-abdominal pressure 614, in clinical practice we measure the esophageal pressure when the intra-abdominal pressure is altered. We in fact found that chest wall elastance increases linearly with the intra-abdominal pressure according to the following equation: E_cw= 0.47 × intra-abdominal pressure (cmH₂O) + 1.43 6.

Accordingly, the pleural pressure/intra-abdominal pressure relationship may be estimated as:

P_pl= P_aw[(0.47 × intra-abdominal pressure + 1.43)/(0.47 × intra-abdominal pressure + 1.43 + lung elastance)]

It is perhaps appropriate at this stage to briefly discuss the relationship between the intra-abdominal pressure and the pleural pressure. It is important to understand what the independent variable is, because the relationship is changed when the independent variable is the intra-abdominal pressure or when it is the intrathoracic pressure (pleural pressure). In addition, the diaphragm elastance plays a key role in this relationship.

We routinely measure intra-abdominal pressure in critically ill patients admitted to the intensive care unit, because the incidence of an abnormal increase in intra-abdominal pressure occurs in 24–30% of these patients 715. As the intra-abdominal pressure levels that start to impair the chest wall elastance cannot be clinically assessed 16, we estimate the intra-abdominal pressure by measuring the bladder pressure 71718, which is considered the best approach in clinical practice.

The 'normal' intra-abdominal pressure during spontaneous breathing in healthy subjects is approximately 0 mmHg, while in mechanically ventilated patients this pressure is higher (range, 5–8 mmHg) 719.

Causes of chest wall impairment

Several factors may affect the chest wall mechanics. The anatomical configuration of the thoracic cage, being overweight or pleural effusion can all increase, to various degrees, the chest wall elastance in sedated and paralyzed patients 1920212223. However, the most common causes of increased chest wall elastance in ALI/ARDS patients are abdominal diseases (such as bowel distension, ascites, hemoperitoneum).

Along with other workers 14, we have found striking differences in the chest wall mechanics between patients with pulmonary ARDS, usually due to diffuse pneumonia, and those with extrapulmonary ARDS, usually due to abdominal diseases 6. Although the total respiratory system elastance was similar in both groups, the pleural pressure was normal in pulmonary ARDS patients but abnormally high in extrapulmonary ARDS patients. This was linearly correlated with the increase of intra-abdominal pressure 2. The presence of abdominal diseases (as well as obesity) in critically ill patients with ALI/ARDS should be a drive for careful investigation of their respiratory mechanics.

Moreover, the differences we found in chest wall elastance in pulmonary ARDS patients and in extrapulmonary ARDS patients do represent a general trend. An individual patient with pulmonary ARDS may have a concomitant increase in intra-abdominal pressure.

Physiopathological consequences

For a given applied airway pressure, as previously mentioned, the pleural pressure increases when the chest wall elastance is elevated. Accordingly, the transpulmonary pressure (the distending force of the lung) drops. We shall now discuss the respiratory and hemodynamic consequences of high pleural pressure.

Respiratory system

The key point is that, for a given applied pressure, the transpulmonary pressure falls when the pleural pressure rises (see equation 1). This may have important implications in understanding some of the differences in the presentation of ALI/ARDS and in setting the ventilator in these patients.

Patients with pulmonary ARDS and patients with extra-pulmonary ARDS have different mechanical behavior, different lung morphology and different positive end-expiratory pressure response 6142242526. The chest wall elastance differences explain most of these different behavioral patterns. Extrapulmonary ARDS patients have diffuse lung edema due to inflammatory mediators originating in extrapulmonary foci 24. The increase in lung weight causes compression atelectasis of the dependent lung regions 27. Pulmonary ARDS patients, on the contrary, tend to have less homogeneous lung alteration. The main feature in these patients is the consolidation of some lung regions instead of lung collapse.

As an example, the 'safe' airway plateau pressure between 30 and 35 cmH₂O may give widely differing transpulmonary pressures 282930 in patients with normal or increased chest wall elastance. In the extrapulmonary ARDS patients, the elevated pleural pressure caused by increased chest wall elastance will cause the transpulmonary pressure to be far lower than in pulmonary ARDS patients with normal elastance.

Two factors contribute to the lung collapse in extrapulmonary ARDS patients. The first is due to the nature of the main pathological alteration (interstitial edema 31), and the second factor is due to the high chest wall elastance, leading to a lower transpulmonary pressure. This diffuse collapse associated with interstitial edema and lower transpulmonary pressure leads to a different morphological pattern (with the prevalence of ground glass opacification) in extrapulmonary ARDS patients compared with the consolidation usually prevalent in pulmonary ARDS patients 2526. The potential for recruitment is also greater in extrapulmonary ARDS patients than in pulmonary ARDS patients 63233.

The increased chest wall elastance may also be important in the pathogenesis of ventilator-induced lung injury (VILI). As this is probably due to the excessive and unphysiological strain on the lung structures, which in turn depends on the applied transpulmonary pressure 34, we may expect more VILI for a given applied pressure when the chest wall elastance is normal. VILI is in fact worse with an open chest (zero chest wall elastance 35) than in the conditions in which the chest wall elastance is increased (as in experiments in which the thoracic cage was artificially constrained) 36. It is possible, however, that the collapsed tissue would also be less prevalent in the presence of high transpulmonary pressure. Consequently, the average strain might be the same or lower.

As the transpulmonary pressure is the trigger of VILI, if we take into account the chest wall elastance then the differences between barotrauma and volotrauma vanish. The barotrauma was in fact attributed to the applied airway pressure. What is important, however, is not this pressure but the transpulmonary pressure applied to the lung structures (P_aw- P_l), which in turn causes the strain.

Some contradictory data arising from randomized studies with different tidal volumes 29373839 can be explained if we take into account the transpulmonary pressure. The same tidal volume, depending on the total elastance of the respiratory system and on the chest wall elastance/total respiratory system elastance ratio, might result in a completely different transpulmonary pressure 34, and consequently result in different VILI.

The chest wall elastance has to be taken into account when performing recruitment maneuvers. What is also important for lung opening in this case is the transpulmonary pressure and not the airway pressure. If the opening pressure of some lung regions are of the order of 25–30 cmH₂O transpulmonary pressure (sticky atelectasis 2533), then the airway pressure applied to reach this target will be completely different in patients with normal or abnormal chest wall elastance 83233.

Changes in chest wall elastance also dictate the oxygenation response to the prone position. Pelosi and colleagues 40 and Guerin and colleagues 41 showed that the greater the decrease in chest wall compliance during the prone position, the greater the increase in oxygenation. Moreover, it has been shown that extrapulmonary ARDS patients have a greater potential for oxygenation improvement in the prone position than do pulmonary ARDS patients 42. These findings are plausible in the light of the chest wall elastance changes; an increase of chest wall elastance in the prone position presumably leads to a more even distribution of the ventilation in pulmonary ARDS patients, while the changes in regional transpulmonary pressure in extrapulmonary ARDS patients explain the lung density redistributions and the better oxygenation in the prone position 43.

Hemodynamics and lung edema

The increased pleural pressure may lower the cardiac output by reducing the venous return and the cardiac volume 44. The evaluation of hemodynamics therefore calls for special care in cases of increased intra-abdominal pressure 745. Moreover, both the central venous pressure and the wedge pressure may appear 'falsely' elevated in the presence of increased pleural pressure 7.

In a recent series of experiments in pigs in which edema was induced by oleic acid and the pleural pressure was changed by pneumoperitoneum, we found a decrease in gas volume due to a decreased transpulmonary pressure 46. However, this was associated with an almost 100% increase of pulmonary edema. This effect may possibly be due to the blood shift induced by the abdominal pressure increase, which in turn may favor edema formation in a 'leaking' lung. A decrease of the edema clearance due to the increased pleural pressure is another coexisting possibility.

Conclusion

We suggest that a rational approach to the treatment of ALI/ARDS requires the knowledge of both lung elastance and chest wall elastance. Although not routinely carried out, we firmly believe that the measurement of the intra-abdominal pressure, the leading cause of chest wall impairment, should be performed.

Competing interests

None declared.

Abbreviations

ALI = acute lung injury; ARDS = acute respiratory distress syndrome; E_cw= chest wall elastance; E_l= lung elastance; E_tot= total respiratory system elastance; P_aw= airway pressure; P_l= transpulmonary pressure; P_pl= pleural pressure; VILI = ventilator-induced lung injury.

Static behaviour of the respiratory system Agostoni E Hyatt RE In Handbook of Physiology, Section 3: The Respiratory System Bethesda, MD: American Physiological Society Geiger SR 1986 III Part 1 113 130 Alterations of lung and chest wall mechanics in patients with acute lung injury: effects of positive end-expiratory pressure Pelosi P Cereda M Foti G Giacomini M Pesenti A Am J Respir Crit Care Med 1995 152 531 537 7633703 Effect of tidal volume and positive end-expiratory pressure on compliance during mechanical ventilation Suter PM Fairley HB Isenberg MD Chest 1978 73 158 162 340159 Pulmonary, chest wall, and lung-thorax elastances in acute respiratory failure Katz JA Zinn SE Ozanne GM Fairley HB Chest 1981 80 304 311 6944170 Influence of lung and chest wall compliances on transmission of airway pressure to the pleural space in critically ill patients Jardin F Genevray B Brun-Ney D Bourdarias JP Chest 1985 88 653 658 3902386 Acute respiratory distress syndrome caused by pulmonary and extrapulmonary disease Gattinoni L Pelosi P Suter PM Pedoto A Vercesi P Lissoni A Am J Respir Crit Care Med 1998 158 3 11 9655699 Abdominal pressure in the critically ill Malbrain M Curr Opin Crit Care 2000 6 1 21 Physiologic rationale for ventilator setting in acute lung injury–acute respiratory distress syndrome patients Gattinoni L Vagginelli F Chiumello D Taccone P Carlesso E Crit Care Med 2003 31 300s 304s 10.1097/01.CCM.0000057907.46502.7B Physical and biological triggers of ventilator induced lung injury and its prevention Gattinoni L Carlesso E Cadringher P Valenza F Vagginelli F Chiumello D Eur Respir J 2003 47 15s 25s 10.1183/09031936.03.00021303 Improved technique for estimating pressure from esophageal balloons Milic-Emili J Mead J Turner JM J Appl Physiol 1964 19 207 211 14155283 Choosing a catheter system for measuring intra-oesophageal pressure Maxted KJ Shaw A Macdonald TH Med Biol Eng Comput 1977 15 398 401 197332 Recruitment and derecruitment during acute respiratory failure. An experimental study Pelosi P Goldner M McKibben A Eccher G Caironi P Losappio S Gattinoni L Marini JJ Am J Respir Crit Care Med 2001 164 122 130 11435250 A simple method for assessing the validity of the oesophageal balloon technique Baydur A Behrakis PK Zin WA Jaegr M Milic-Emili L Am Rev Respir Dis 1983 126 788 791 Impairment of lung and chest wall mechanics in patients with acute respiratory distress syndrome Ranieri VM Brienza N Santostasi S Puntillo F Mascia L Vitale N Giuliani R Memeo V Bruno F Fiore T Brienza A Slutusky AS Am J Respir Crit Care Med 1997 156 1082 1091 9351606 Intra-abdominal hypertension and intensive care Sugrue M Hilman KM In Yearbook of Intensive Care and Emergency Medicine Berlin: Springer-Verlag Vincent JL 1998 667 676 Clinical examination is an inaccurate predictor of intra-abdominal pressure Sugrue M Bauman A Jones F Bishop G Flabouris A Parr M Stewart A Hillman K Deane SA World J Surg 2002 26 1428 1431 10.1007/s00268-002-6411-8 12297912 Determination of intraabdominal pressure using a transurethral bladder catheter: clinical validation of the technique Iberti TJ Lieber CE Benjamin E Anaesthesiology 1989 70 47 50 The measurement of intra-abdominal pressure as a criterion for abdominal re-exploration Kron JL Harman PK Nolan SP Ann Surg 1984 199 28 30 6691728 Positive end expiratory pressure improves respiratory function in obese patients but not in normal subjects during anesthesia and paralysis Pelosi P Ravagnan I Giurati G Panigada M Bottino N Tredici S Eccher G Gattinoni L Anesthesiology 1999 91 1221 1231 10.1097/00000542-199911000-00011 10551570 Effects of acute pleural effusion on respiratory system mechanics in dog Krell WS Rodarte JR J Appl Physiol 1985 59 1458 1463 4066576 Mechanical and morphometrical changes in progressive bilateral pneumothorax and pleural effusion in normal rats Sousa AS Moll RJ Pontes CF Saldiva PH Zin WA Eur Respir J 1995 8 99 104 10.1183/09031936.95.08010099 7744201 The effects of body mass on lung volumes, respiratory mechanics, and gas exchange during general anesthesia Pelosi P Croci M Ravagnan I Tredici S Pedoto A Lissoni A Gattinoni L Anesth Analg 1998 87 654 660 9728848 Chest mechanics in morbidly obese non-hypoventilated patients Ladosky W Botelho MA Albuquerque JP Jr Respir Med 2001 95 281 286 10.1053/rmed.2001.1035 11316110 Pulmonary and extrapulmonary acute respiratory di stress sindrome are different Pelosi P D'Onofrio D Chiumello D Paolo S Chiara G Capelozzi VL Barbas CSV Chiaranda M Gattinoni L Eur Respir J 2003 22 48s 56s 10.1183/09031936.03.00420803 What has computed tomography taught us about the acute respiratory distress syndrome? Gattinoni L Caironi P Pelosi P Goodman LR Am J Respir Crit Care Med 2001 164 1701 1711 11719313 Adult respiratory distress syndrome due to pulmonary and extrapulmonary causes: CT, clinical and functional correlations Goodman LR Fumagalli R Tagliabue M Ferrario M Gattinoni L Pesenti A Radiology 1999 213 545 552 10551239 Pulmonary densities during anesthesia with muscular relaxation Brismar B Hedenstierna G Lundquist H Strandberg A Svesson L Tockics L Anesthesiology 1985 62 422 428 3885791 Consensus conference on mechanical ventilation – January 28–30, 1993 at Northbrook, Illinois, USA. Part 2 Slutsky AS Intensive Care Med 1994 20 150 162 8201097 Ventilation with lower tidal volumes as compared with traditional tidal volumes for acute lung injury and the acute respiratory distress syndrome The Acute Respiratory Distress Syndrome Network N Engl J Med 2000 342 1301 1308 10.1056/NEJM200005043421801 10793162 Culmination of an era in research on the acute respiratory distress syndrome Tobin MJ N Engl J Med 2000 342 1360 1361 10.1056/NEJM200005043421808 10793169 The ARDS lung. New insights from computed tomography Bone RC JAMA 1993 269 2122 2127 8468768 Effects of recruting maneuvers in patients with acute respiratory disress sindrome ventilated with protective ventilatory strategy Grasso S Mascia L Del Turco M Malacarne P Giunta F Brochard L Slutsky AS Ranieri VM Anesthesiology 2002 96 795 802 11964585 Sigh in acute respiratory distress syndrome Pelosi P Cadringher P Bottino N Panigada M Carrieri F Riva E Lissoni A Gattinoni L Am J Respir Crit Care Med 1999 159 872 880 10051265 Physical and biological triggers of ventilator induced lung injury and its prevention Gattinoni L Carlesso E Cadringher P Valenza F Vagginelli F Chiumello D Eur Respir J 2003 47 15s 25s 10.1183/09031936.03.00021303 Macrophage accumulation and pulmonary edema due to thoracotomy and lung over inflation Woo SW Hedley-Whyte J J Appl Physiol 1972 33 14 21 5037402 High inflation pressure pulmonary edema. Respective effects of high airway pressure, high tidal volume, and positive end-expiratory pressure Dreyfuss D Soler P Basset G Saumon G Am Rev Respir Dis 1988 137 1159 1164 3057957 Effect of a protective-ventilation strategy on mortality in the acute respiratory distress syndrome Amato MB Barbas CS Medeiros DM Magaldi RB Schettino GP Lorenzi-Filho G Kairalla RA Deheinzelin D Munoz C Oliveira R Takagaki TY Carvalho CR N Engl J Med 1998 338 347 354 10.1056/NEJM199802053380602 9449727 Tidal volume reduction for prevention of ventilator-induced lung injury in acute respiratory distress syndrome. The Multicenter Trail Group on Tidal Volume reduction in ARDS Brochard L Roudot-Thoraval F Roupie E Delclaux C Chastre J Fernandez-Mondejar E Clementi E Mancebo J Factor P Matamis D Ranieri M Blanch L Rodi G Mentec H Dreyfuss D Ferrer M Brun-Buisson C Tobin M Lemaire F Am J Respir Crit Care Med 1998 158 1831 1838 9847275 Prospective, randomized, controlled clinical trial comparing traditional versus reduced tidal volume ventilation in acute respiratory distress syndrome patients Brower RG Shanholtz CB Fessler HE Shade DM White P Jr Wiener CM Teeter JG Dodd Almog Y Piantadosi S Crit Care Med 1999 27 1492 1498 10.1097/00003246-199908000-00015 10470755 Effects of the prone position on respiratory mechanics and gas exchange during acute lung injury Pelosi P Tubiolo D Mascheroni D Vicardi L Crotti S Valenza F Gattinoni L Am J Respir Crit Care Med 1998 157 387 393 9476848 Effects of prone position on alveolar recruitment and oxygenation in acute lung injury Guerin C Baudet M Rosselli S Heyer L Sab JM Langevin B Philit F Fournier G Robert D Intensive Care Med 1999 25 1222 1230 10.1007/s001340051050 10654205 Comparison of the response to the prone position between pulmonary and extrapulmonary acute respiratory distress syndrome Lim CM Kim EK Lee JS Shim TS Lee SD Koh Y Kim WS Kim DS Kim WD Intensive Care Med 2001 27 477 485 10.1007/s001340000848 11355115 Patient positioning in acute respiratory failure Gattinoni L Pelosi P Valenza F Mascheroni D In Principles and Practice of Mechanical Ventilation New York: McGraw Hill Tobin M 1994 1067 1077 Effect of positive end expiratory pressure on right ventricular function in humans Pinsky M Desmet JM Vincent JL Am Rev Respir Dis 1991 143 25 31 1986680 Intra-abdominal pressure in the intensive care unit: clinical tool or toy? Malbrain MLNG In Yearbook of Intensive Care and Emergency Medicine Berlin: Springer-Verlag Vincent JL 2001 547 585 An increase of abdominal pressure increases pulmonary edema in oleic-acid induced lung injury Quintel M Pelosi P Caironi P Meinhardt JP Luecke T Herrmann P Taccone P Rylander C Valenza F Carlesso E Gattinoni L Am J Respir Crit Care Med 2003