This is a proof of concept study where the investigators aim to study the correlation between the use of a simple bedside ultrasound measurement of diaphragmatic muscle excursion with established (but time consuming) measurements made to optimize an important setting on the mechanical ventilator (positive end expiratory pressure or PEEP) in intubated adults with acute respiratory distress syndrome (ARDS) in the medical ICU.

Ventilator induced lung injury (VILI) generates morbidity and mortality in mechanically ventilated patients. The awareness of respiratory mechanics is essential in the prevention of VILI. Currently, plateau pressures are widely used as a guide to assess alveolar pressure and minimize alveolar injury. However, patients with reduced chest wall compliance can have higher plateau pressures that may not reflect true alveolar pressure. The transpulmonary pressure has been cited as the true alveolar driving pressure because it takes into account pleural pressure that reflect chest wall mechanics; however, this requires measurement of esophageal pressure.

The investigators have experienced a disproportionate degree of excursion between the posterior and anterior right hemidiaphragm on bedside ultrasound imaging in patients with ARDS, which may reflect the dependent atelectasis that occurs during low tidal volume ventilation, cardiac weight, weight of injured lung and accumulation of extravascular lung water in critically ill patients. The optimal PEEP can be guided by measurement of esophageal pressure (and subsequent calculation of transpulmonary distending pressure) with a balloon catheter placed into the esophagus much like a nasogastric tube for enteral access. The investigators believe that the normalization of the disproportionate degree of excursion between the anterior and posterior diaphragm can also be used to identify optimal PEEP, and may be correlated with changes in transpulmonary pressure (the current gold standard).

Inclusion Criteria:

• Adults older than 18 years old who develop ARDS, as defined by the Berlin criteria, within 72 hours of ICU admission.

Exclusion Criteria:

• Any contraindication for nasogastric tube placement including recent injury or pathologic condition of the esophagus.
• Major bronchopleural fistula.
• Solid organ transplant recipient.
• History or current diagnosis of diaphragmatic paralysis.
• Non-conventional mechanical ventilation strategy including high frequency oscillation, airway pressure release ventilation, prone ventilation and extra- corporeal membrane oxygenation.
• Hemodynamic instability defined as MAP<65 with multiple vasopressors.
• Declining to sign consent form.