Right atrial pressure and venous return during cardiopulmonary bypass.

The relevance of right atrial pressure (RAP) as the back-pressure for venous return (QVR) and mean systemic filling pressure (MSFP) as up-stream pressure is controversial during dynamic changes of circulation. To examine the immediate response of QVR (sum of caval vein flows) to changes in RAP and pump function we used a closed chest, central cannulation, heart bypass porcine preparation (n=10) with veno-arterial extracorporeal membrane oxygenation (ECMO). MSFP was determined by clamping ECMO tubing with open or closed arterio-venous shunt at Euvolemia, Volume Expansion (9.75 mL/kg hydroxyethyl starch) and Hypovolemia (bleeding 19.5 mL/kg after Volume Expansion). The response of RAP and QVR were studied using variable pump speed at constant airway pressure (PAW) and constant pump speed at variable PAW Within each volume state, the immediate changes in QVR and RAP could be described with a single linear regression, regardless of whether RAP was altered by pump speed or PAW (r2 0.586-0.984). RAP was inversely proportional to pump speed from zero to maximum flow (r2 0.859-0.999). Changing PAW caused immediate, transient, directionally opposite changes in RAP and QVR (RAP p ≤ 0.002; QVR p ≤ 0.001), where the initial response was proportional to the change in QVR driving pressure. Changes in PAW generated volume shifts into and out of the right atrium, but their effect on up-stream pressure was negligible. Our findings support the concept that RAP acts as back-pressure to QVR and that Guyton's model of circulatory equilibrium qualitatively predicts the dynamic response from changing RAP