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 o f infradiaphragmatic air that may develop in patients after mechanical ly assisted ventilation, its location, its pathway of transdiaphragmat ic dissection, and its extension into the abdomen. MATERIALS AND METHO DS. We retrospectively evaluated six consecutive adult patients with p neumomediastinum associated with positive end-expiratory pressure ther apy who developed intraabdominal air and were imaged with CT in our in stitution between 1993 and 1995. Abdominal CT examinations were review ed and correlated with the clinical findings, follow-up examinations, and exploratory laparotomies in four patients. RESULTS. In four patien ts, air present in the anterior mediastinum (endothoracic fascia) was seen to extend into the anterior abdominal wall within the extraperito neal space. In a fifth patient, the air was located extraperitoneally and intraperitoneally. In the remaining patient, air was present exclu sively in the peritoneal cavity. In only two patients did we detect sm all amounts of air in the posterior retroperitoneum. In five patients, we also detected subcutaneous emphysema and/or air dissection into th e muscle planes of the anterolateral abdominal wall. CONCLUSION. In pa tients on mechanically assisted ventilation, anterior mediastinal air can dissect through the diaphragm into the anterior abdominal extraper itoneal space. This anterior pathway of infradiaphragmatic extension o f air can be erroneously diagnosed as intraperitoneal air, which may l ead to unnecessary exploratory laparotomies. Also, anterior mediastina l air can enter the peritoneal cavity, particularly in patients with a history of median sternotomy.