Effects of Sedation on Transpulmonary Pressure and Lung Homogenous

Inclusion Criteria: 1. ARDS group patient (1) Acute onset of all of the following criteria within a 48-hour period: 1. Requires positive pressure ventilation through an endotracheal tube 2. Bilateral infiltrates consistent with edema on frontal chest radiograph, with a duration of no more than 7 days 3. PaO2/FiO2 less than 300 while receiving positive end-expiratory pressure (PEEP) at more than 5 cm H2O for at least 4 hours, with a duration of no more than 7 days 4. No clinical evidence of left atrial hypertension (2) Hemodynamics stable (dopamine \<10ug/kg•min or norepinephrine \<10ug/kg) 2. Surgical patient Surgical patient requires positive pressure ventilation through an endotracheal tube PaO2/FiO2 more than 300 Hemodynamics stable (dopamine \<10ug/kg•min or norepinephrine \<10ug/kg) Exclusion Criteria: * Age younger than 18 years or older than 85 years * Cardiac failure * Known pregnancy * Increased intracranial pressure * Severe neuromuscular disease * Recent injury or other pathologic condition of the esophagus * Pneumothorax * Pleural effusion * Diaphragmatic hernia * Severe chronic respiratory disease * End-stage chronic organ failure * Expected survival of less than 24 hours * Participation in another interventional study * Attending physician declines to give consent for participant to enroll * Patient or surrogate declines or is unable to give consent