CT evaluation of infradiaphragmatic air in patients treated with mechanically assisted ventilation: a potential source of error.

OBJECTIVE: The purpose of this study was to describe the CT features of infradiaphragmatic air that may develop in patients after mechanically assisted ventilation, its location, its pathway of transdiaphragmatic dissection, and its extension into the abdomen.

MATERIALS AND METHODS: We retrospectively evaluated six consecutive adult patients with pneumomediastinum associated with positive end-expiratory pressure therapy who developed intraabdominal air and were imaged with CT in our institution-between 1993 and 1995. Abdominal CT examinations were reviewed and correlated with the clinical findings, follow-up examinations, and exploratory laparotomies in four patients.

RESULTS: In four patients, air present in the anterior mediastinum (endothoracic fascia) was seen to extend into the anterior abdominal wall within the extraperitoneal space. In a fifth patient, the air was located extraperitoneally and intraperitoneally. In the remaining patient, air was present exclusively in the peritoneal cavity. In only two patients did we detect small amounts of air in the posterior retroperitoneum. In five patients, we also detected subcutaneous emphysema and/or air dissection into the muscle planes of the anterolateral abdominal wall.

CONCLUSION: In patients on mechanically assisted ventilation, anterior mediastinal air can dissect through the diaphragm into the anterior abdominal extraperitoneal space. This anterior pathway of infradiaphragmatic extension of air can be erroneously diagnosed as intraperitoneal air, which may lead to unnecessary exploratory laparotomies. Also, anterior mediastinal air can enter the peritoneal cavity, particularly in patients with a history of median sternotomy.