Effects of Prone Positioning on Venous Return in Patients With Acute Respiratory Distress Syndrome*

To examine the effects of prone positioning on venous return and its determinants such as mean systemic pressure and venous return resistance in patients with acute respiratory distress syndrome.Prospective monocentric study.A 25-bed medical ICU.About 22 patients with mild-to-severe acute respiratory distress syndrome in whom prone positioning was decided.We obtained cardiac index, mean systemic pressure, and venous return resistance (the latter two estimated through the heart-lung interactions method) before and during prone positioning. Preload responsiveness was assessed at baseline using an end-expiratory occlusion test.Prone positioning significantly increased mean systemic pressure (from 24 mm Hg [19-34 mm Hg] to 35 mm Hg [32-46 mm Hg]). This was partly due to the trunk lowering performed before prone positioning. In seven patients, prone positioning increased cardiac index greater than or equal to 15%. All were preload responsive. In these patients, prone positioning increased mean systemic pressure by 82% (76-95%), central venous pressure by 33% (21-59%), (mean systemic pressure - central venous pressure) gradient by 144% (83-215)%, while it increased venous return resistance by 71% (60-154%). In 15 patients, prone positioning did not increase cardiac index greater than or equal to 15%. In these patients, prone positioning increased mean systemic pressure by 28% (18-56%) (p < 0.05 vs. patients with significant increase in cardiac index), central venous pressure by 21% (7-54%), (mean systemic pressure - central venous pressure) gradient by 28% (23-86%), and venous return resistance by 37% (17-77%). Eleven of these 15 patients were preload unresponsive.Prone positioning increased mean systemic pressure in all patients. The resulting change in cardiac index depended on the extent of increase in (mean systemic pressure - central venous pressure) gradient, of preload responsiveness, and of the increase in venous return resistance. Cardiac index increased only in preload-responsive patients if the increase in venous return resistance was lower than the increase in the (mean systemic pressure -central venous pressure) gradient.