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1 observed an improvement in oxygenation in patients with acute lung injury that was managed by estimating transpulmonary pressure, there was an extrapulmonary (i.e., abdominal) cause of the acute respiratory distress syndrome (ARDS) in a high proportion of patients in their study (39%). To our knowledge, this feature has not been observed in any previously published randomized studies involving patients with ARDS. [2][3][4] The respiratory mechanics in pulmonary ARDS and extrapulmonary ARDS are different. 5 The elastance of the lung is higher in pulmonary ARDS, and the elastance of the chest wall is higher in extrapulmonary ARDS. The increase in the intraabdominal pressure, which has been shown to be greater in extrapulmonary ARDS, 6 could explain the benefit that Talmor et al. observed. It is possible that populations of patients with pulmonary ARDS, which does not involve any major changes in the compliance of the chest wall, will not benefit from measurement of esophageal pressure to set the positive end-expiratory pressure (PEEP). We do not think the results of this pilot study can be extrapolated to populations of patients with pulmonary ARDS. 1. model of pulmonary elasticity. J Appl Physiol 1970;28:596-608. Agostoni E, Rahn H. Abdominal and thoracic pressures at 2. different lung volumes. J Appl Physiol 1960;15:1087-92. Milic-Emili J, Mead J, Turner JM, Glauser EM. Improved 3. technique for estimating pleural pressure from esophageal balloons. J Appl Physiol 1964;19:207-11. Jardin F, Farcot JC, Boisante L, et al. Influence of positive 4. end-expiratory pressure on left ventricular performance. N Engl J Med 1981;304:387-92.