The prone position improves gas exchange in many patients with ARDS. Animal studies have indicated that turning prone restores ventilation to dorsal lung regions without markedly compromising ventral regions. To investigate a potential mechanism by which this might occur, the relative volume of lung located directly under the heart was measured in the supine and prone positions in seven patients. Four axial tomographic sections between the carina and the diaphragm were analyzed (Sections 1 through 4). When supine, the percent of the total lung volume located under the heart increased from 7 +/- 4% to 42 +/- 8%, and from 11 +/- 4% to 16 +/- 4% in Sections 1 through 4, in the left and right lungs, respectively. When prone, the percent of left and right lung volume located under the heart was </= 1 and </= 4 %, respectively, in all four sections (p < 0.05 for each section, supine versus prone). Although a large fraction of the lung, particularly on the left, is located directly under the heart in supine patients, and would be subject to the compressive force resulting from heart weight, almost no lung is located under the heart when patients are prone and the compressive force of the heart is directed towards the sternum.