The prone position eliminates compression of the lungs by the heart

The prone position improves gas exchange in many patients with ARDS. Animal studies have indicated that turning prone restores ventilation to dorsal l ung 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 posi tions in seven patients. Four axial tomographic sections between the carina and the diaphragm were analyzed (Sections 1 through 4). When supine, the p ercent 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 less than or equal to 1 a nd less than or equal to 4 % respectively, in ail four sections (p < 0.05 f or each section, supine versus prone). Although a large fraction of the lun g, particularly on the left, is located directly under the heart in supine patients, and would be subject to the compressive force resulting from hear t 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.