The Prone Position Eliminates Compression of the Lungs by the Heart

Abstract

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 un- der the heart was measured in the supine and prone positions in seven patients. Four axial tomographic sections between the car- ina 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 6 4% to 42 6 8%, and from 11 6 4% to 16 6 4% in Sections 1 through 4, in the left and right lungs, re- spectively. 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). Al- though a large fraction of the lung, particularly on the left, is lo- cated directly under the heart in supine patients, and would be subject to the compressive force resulting from heart weight, al- most no lung is located under the heart when patients are prone and the compressive force of the heart is directed towards the sternum. The beneficial effects of the prone position on lung function were first postulated in 1974 (1), demonstrated in patients with the acute respiratory distress syndrome (ARDS) a few years later (2, 3), and confirmed more recently in a number of institu- tions throughout the world (4-8). We investigated the mecha- nism by which this improvement might occur and found that turning prone had limited effects on the distribution of regional perfusion (9) but markedly improved dorsal lung ventilation and, accordingly, also improved dorsal lung ventilation-perfu- sion relationships, with minimal if any compromise of ventral lung ventilation or ventral ventilation-perfusion relationships (10). The implications of these findings are that reversible air- space closure occurs in dorsal lung regions when patients with ARDS are supine, and that turning prone sufficiently alters dor- sal lung transpulmonary pressures to reverse this closure with- out shifting the air-space closure to the ventral regions. A number of factors could contribute to this differential ability of the prone position to alter dorsal lung transpulmo- nary pressures, including, among others, the compressive ef- fects of consolidated lung (11, 12), direct transmission of the weight of abdominal contents to caudal regions of the dorsal lung (1, 13, 14), and direct transmission of the weight of the heart to the regions lung located beneath it (15-22). The pur- pose of this study was to determine the fraction of lung that might be subjected to the weight of the heart when patients are in the supine versus the prone position.