Instantaneous venous return curves in an intact canine preparation.

The relationship between right atrial pressure (Pra) and venous return describes a venous return curve. Because changes in venous return and right ventricular stroke volume (SVRV) are similar during small tidal volume (VT) breathing, we compared the relationship of SVRV and Pra during positive-pressure ventilation (VT less than 10 ml/kg) in 17 pentobarbital-anesthetized, closed-chest, canine preparations. The SVRV-Pra relationship describes a straight line with a negative slope and a positive mean systemic pressure (Pms)-zero flow intercept (instantaneous Pms). Instantaneous Pms is similar to ventricular fibrillation-induced (stop-flow) Pms (8.1 +/- 0.8 vs. 8.4 +/- 0.7 mmHg, mean +/- SE). With volume infusion, both instantaneous and stop-flow Pms increase to a similar degree (R = 0.9014, P less than 0.001). The effect of opening an arteriovenous fistula is time dependent and variable between dogs, but by 30 s it is associated with an increase in instantaneous Pms (5.2 +/- 3.2 mmHg). Vascular compliance determined by adding and removing blood from the intravascular space displays a curvilinear hysteresis. The instantaneous venous return curve is affected by intravascular blood volume, vasomotor tone, and resistance to venous return. The relationship between SVRV and Pra during small VT breathing define instantaneous venous return curves similar to those described using right-heart bypass preparations.