Tone-dependent coronary arterial-venous pressure differences at the cessation of venous outflow during long diastoles.

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The origin and magnitude of the back pressure opposing diastolic coronary inflow remain controversial. The arterial pressure at which coronary inflow stops during a prolonged diastole, ie, "zero-flow pressure," is higher than coronary venous pressure. However, because of capacitive discharge as distending pressure falls, flow at the microcirculatory level exceeds inflow, and coronary outflow ceases later than inflow. If coronary arterial pressure continues to exceed venous pressure at the point of venous flow cessation, zero-flow pressure cannot be an artifact of capacitive discharge. Coronary inflow and outflow, arterial pressure, and right atrial pressure have been measured during long diastoles in closed-chest dogs chronically instrumented with volumetric flow probes on the great cardiac vein or coronary sinus as well as the circumflex artery. Although venous outflow continued for 1 to 4 seconds after arterial inflow ceased, coronary artery pressure at the point of venous flow cessation (Pfv = 0) always exceeded right atrial pressure (13 +/- 1.3 mm Hg [SEM] vs 6 +/- 0.7 mm Hg, P < .001). When vasomotor tone was augmented using vasopressin, the diastolic pressure-flow relation shifted to the right, with Pfv = 0 increasing to 21 +/- 2.4 mm Hg despite an unchanged right atrial pressure (6 +/- 0.5 mm Hg). Transcoronary pressure differences persist when venous outflow stops and are larger when vasomotor tone is augmented. Measurements of zero-flow pressure that exceed venous pressure cannot be considered an artifact of continuing capacitive discharge after the cessation of arterial inflow. Diastolic coronary back pressure exceeds right atrial pressure and is tone dependent.