The current standard of practice for ventilating patients with acute respiratory distress syndrome (ARDS) is the ARDSnet protocol (N Engl J Med 2000; 342:1301-1308) which has been shown to improve survival by limiting tidal volumes and thus preventing over distention of the lungs. However, the lungs can also be damaged by under-inflation during mechanical ventilation, and current practice ignores the effects of chest wall compression of the lungs, which can cause under-inflation. The investigators have observed that a fraction of critically ill patients with apparent ARDS have lung function abnormalities caused by extrinsic compression which can be estimated by PES measurements. Changing ventilator settings to apply normal physiological pressure to the lung has been beneficial in many such patients. This protocol will formally test the clinical utility of PES measurements in patients with ARDS.