The current standard of practice for ventilating patients with 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 underinflation during mechanical ventilation, and current practice ignores the effects of chest wall compression of the lungs, which can cause underinflation. We 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 pressures 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.

This is a randomized controlled trial of therapy directed by esophageal balloon measurements (PES) vs. therapy directed by ARDSnet protocol, the current standard of care.

Inclusion Criteria:

- Patients with ALI/ ARDS according to the international consensus conference criteria.

1. PaO2 / FiO2 ratio< 300
2. Acute onset
3. Bilateral Infiltrates on chest radiography
4. PAOP < 18 or, in patients without a pulmonary artery catheter, no other evidence of abnormal cardiac function

Exclusion Criteria:

1. Patients with esophageal varices
2. Patients with esophageal trauma
3. Patients with recent esophageal surgery
4. Patients with coagulopathy (Platelets < 80k or INR> 2 )
5. Post transplant patients