Diaphragm Ultrasound Vs Transpulmonary Pressure To Set PEEP in ARDS

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).