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

Abstract

OBJECTIVES: 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. DESIGN: Prospective monocentric study. SETTINGS: A 25-bed medical ICU. PATIENTS: About 22 patients with mild-to-severe acute respiratory distress syndrome in whom prone positioning was decided. INTERVENTIONS: 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. MEASUREMENTS AND MAIN RESULTS: 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. CONCLUSIONS: 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.

Prone positioning during veno-venous or veno-arterial extracorporeal membrane oxygenation: feasibility and complications after cardiothoracic surgery

“…We noted that the flow provided by the ECMO device remained constant during PP. As previously reported, cardiac output can increase, decrease or remain unchanged, depending on preload [ 4 ]. Finally, the beneficial effect of PP on the lung parenchyma outweighs the systemic hemodynamic effect even when cardiac output decreases.…”

A Complex Hemodynamic Response After Prone Positioning in Acute Respiratory Distress Syndrome

“…Inferior vena cava pressure usually rises simultaneously and in parallels with the rise in IAP and may lead to unpredictable changes in venous resistance and venous return (5). A higher baseline IAP in the study by Lai et al (1) seemed to arise from positive end-expiratory pressure (PEEP). PEEP appeared to increase the complexity in hemodynamic changes further by inducing unpredictable changes in intrathoracic pressure and CVP.…”

“…In the study by Lai et al (1), the authors reported an increase in venous return and CI after prone positioning in patients with ARDS, which is now commonly employed in moderate-to-severe ARDS. The only difference from the previous studies (3) was the no use of bolsters in the study by Lai et al (1). Bolsters have been used to support the pelvis and chest to avoid direct compression of the abdominal wall to prevent rise in intra-abdominal pressure (IAP).…”

The authors reply

“…Fourth, looking at the mean arterial pressure alone, the authors assume that arterial resistance was dropping in our study (2), and they are surprised that this drop was dissociated from the increase in venous resistance that we observed. First, although it is true that sympathetic regulation operates in parallel on veins and arteries, venous resistance can vary independently of that of the arteries.…”