P108 Right atrial pressure as back-pressure to venous return

Introduction: Guyton’s graphic analysis integrates vascular and cardiac function. Questions remain whether right atrial pressure (RAP) acts as the back-pressure for venous return (VR) with mean systemic filling pressure (MSFP) as up-stream pressure, and if the model, formulated for steady states, can be applied in dynamic settings [1]. Methods: To examine the immediate response of VR (Q[sub] Methods: To examine the immediate response of VR (Q[subVR[/sub]; sum of caval vein flows) to changes in RAP and pump function we used a porcine closed chest, central cannulation, heart bypass preparation (n = 10) with veno-arterial extracorporeal membrane oxygenation (ECMO). MSFP was determined by clamping ECMO tubing with an opened arterio-venous shunt at [i]Reference[/i], [i]Volume Expansion[/i] (after infusing 9.75 mL/kg of hydroxyethyl starch) and [i]Hypovolemia[/i] (after bleeding 19.5 mL/kg). We analyzed the behavior of RAP and Q[sub]VR[/sub] in maneuvers with variable pump speed at constant airway pressure (P[sub]AW[/sub]) and constant pump speed at variable P[sub]AW[/sub]. Results: Data is mean (SD) or range, [i]p[/i]-values reflect one-way repeated measures ANOVA. RAP was inversely proportional to pump speed (linear regression: [i]r[/i]2 0.859–0.999). Q[sub]VR[/sub] was inversely proportional to RAP ([i]r[/i]2 0.594–0.991). Within a volume state, regardless of whether RAP was altered by pump speed or P[sub]AW[/sub], the immediate hemodynamic response operated on a single VR line ([i]r[/i]2 0.594–0.991). Open shunt MSFP (n = 9) was 10.3 (0.5), 12.3 (1.2) and 8.7 (1.0) in [i]Reference, Volume Expansion[/i] and [i]Hypovolemia[/i] ([i]p[/i] <0.0005). Changing RAP via P[sub]AW[/sub] caused immediate, transient, directionally opposite changes in Q[sub]VR[/sub] (n = 9, [i]p[/i] < = 0.002 for RAP, [i]p[/i] < = 0.001 for Q[sub]VR[/sub]), proportional to the change in VR driving pressure as predicted from a stable MSFP. Conclusions: We conclude from these findings that RAP acts as back-pressure to VR and that Guyton’s model can be applied in dynamic conditions. References 1. Brengelmann GL. Letter to the editor: Why persist in the fallacy that mean systemic pressure drives venous return? [i]American Journal of Physiology - Heart and Circulatory Physiology[/i] 311: H1333-H1335, 2016