Failure to recruit/enroll

The use of esophageal balloon catheters, which use esophageal pressure as a surrogate measurement for transpleural pressure, shows promise in improving outcomes of patients with severe acute respiratory distress syndrome (ARDS) requiring mechanical ventilator. The investigators hope to measure changes in in transpleural pressures in patients undergoing treatment with mechanical ventilation while switching from the supine, upright (head of bed >30 degrees), and prone positions. The goal will be to measure the changes in chest wall and lung compliance in mechanically ventilated patients with changes in position.

The use of esophageal balloon catheters shows promise in improving outcomes of patients with severe ARDS. The estimation of pleural pressure (Ppl) has been validated in the upright position in humans with few studies commenting on the changes in supine positioning and almost none on prone positioning aside from radiographic analysis. An abstract presented at the annual American Thoracic Society meeting recently reported a series of 18 patients undergoing spinal surgery. In this population of patients without ARDS, esophageal pressure (Pes) decreased when shifting from supine to prone positioning. This suggests that transpleural pressure (PtmL) would be increased at a given airway pressure.

The investigators hope to measure changes in PtmL in patients undergoing treatment with mechanical ventilation while switching from the supine, upright (head of bed >30 degrees), and prone positions. The esophageal balloon catheter will be placed using standard techniques and secured with tape to the patients' nares during changes in positioning. The goal will be to measure the changes in chest wall and lung compliance in mechanically ventilated patients with changes in position.

Inclusion Criteria:

• age at least 18 years old, need for mechanical ventilation, moderate or severe ARDS, and the ability to tolerate prone positioning.

Exclusion Criteria will be patients who:

• are deemed too ill by their clinicians to be included in the study
• have esophageal or nasopharyngeal pathology (such as tumors, sinusitis, epistaxis, ulcerations, recent surgery or bleeding varices) preventing insertion of the esophageal balloon catheter
• diverticulitis
• bronchopulmonary fistula
• solid-organ transplantation
• history of difficult intubation or airway management
• or contraindications to prone positioning (per the UNC Medical Intensive Care Unit Pronation Therapy protocol, attached)

The study population will be patients admitted to the medical intensive care unit with moderate to severe ARDS who are candidates for prone ventilation therapy and have no contraindications for the placement of an esophageal balloon catheter.