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The relevance of right atrial pressure (RAP) as the backpressure for venous return (Q VR ) and mean systemic filling pressure as upstream pressure is controversial during dynamic changes of circulation. To examine the immediate response of Q VR (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 venoarterial extracorporeal membrane oxygenation. Mean systemic filling pressure was determined by clamping extracorporeal membrane oxygenation tubing with open or closed arteriovenous shunt at euvolemia, volume expansion (9.75 ml/kg hydroxyethyl starch), and hypovolemia (bleeding 19.5 ml/kg after volume expansion). The responses of RAP and Q VR were studied using variable pump speed at constant airway pressure (P AW ) and constant pump speed at variable P AW Within each volume state, the immediate changes in Q VR and RAP could be described with a single linear regression, regardless of whether RAP was altered by pump speed or P AW ( r  2 = 0.586-0.984). RAP was inversely proportional to pump speed from zero to maximum flow ( r  2 = 0.859-0.999). Changing P AW caused immediate, transient, directionally opposite changes in RAP and Q VR (RAP: P ≤ 0.002 and Q VR : P ≤ 0.001), where the initial response was proportional to the change in Q VR driving pressure. Changes in P AW generated volume shifts into and out of the right atrium, but their effect on upstream pressure was negligible. Our findings support the concept that RAP acts as backpressure to Q VR and that Guyton's model of circulatory equilibrium qualitatively predicts the dynamic response from changing RAP. NEW &amp; NOTEWORTHY Venous return responds immediately to changes in right atrial pressure. Concomitant volume shifts within the systemic circulation due to an imbalance between cardiac output and venous return have negligible effects on mean systemic filling pressure. Guyton's model of circulatory equilibrium can qualitatively predict the resulting changes in dynamic conditions with right atrial pressure as backpressure to venous return.

Right atrial pressure and venous return during cardiopulmonary bypass