Coronary venous outflow persists after cessation of coronary arterial inflow.

During single long diastoles we simultaneously measured coronary arterial blood flow and coronary venous outflow. When vasomotor tone was intact, approximately 20% of the total volume of diastolic coronary venous outflow appeared after cessation of inflow. During maximal coronary dilation, approximately 40% of the total volume of diastolic coronary venous outflow appeared after cessation of inflow. Theoretically, coronary capacitance could enable persistence of diastolic coronary venous outflow during zero coronary artery inflow. Intramyocardial coronary capacitance calculated from these data was 0.10 ml X mmHg-1 X 100 g-1 with vasomotor tone intact and 0.21 during maximal coronary dilation. These results indicate that the diastolic coronary arterial stop-flow pressure cannot be equated with cessation of flow throughout the coronary vasculature due to the significant contribution of intramyocardial capacitance. We also found that the arterial pressure at which coronary outflow stopped (10-14 mmHg) was substantially less than that at which coronary artery inflow ceased (19-31 mmHg), yet slightly above right atrial pressure (5-9 mmHg). These studies indicate that coronary venous outflow stops at a lower pressure than arterial inflow. Furthermore, the cessation of venous blood flow in the coronary system during long diastoles occurs at pressures only a few mmHg above right atrial pressure.