Phasic Coronary Blood Flow Velocity in Intramural and Epicardial Coronary Arteries

Knowledge concerning phasic coronary blood flow is based primarily on measurements obtained from epicardial coronary arteries, which, in part, function as capacitors. If present, epicardial capacitance effects could obscure the dynamic nature of phasic intramyocardial perfusion. To analyze this effect of epicardial capacitance, we simultaneously measured coronary blood flow velocity in an epicardial artery (left anterior descending) and an intramural artery (septal) in open- chest, anesthetized dogs. During control conditions, the percentage of total coronary blood flow velocity occurring during diastole per cardiac cycle was significantly greater (P < 0.05) in the septal artery (92%) than in the left anterior descending artery (75%). Furthermore, blood flow velocity during mid-systole in the septal artery was retrograde (−7.2%), whereas blood flow velocity at this time was antegrade in the left anterior descending artery (+3.5%). Blood flow velocity measurements from small epicardial arteries just before they penetrated into the myocardium revealed a phasic pattern similar to that of the septal artery. This suggests that the phasic blood velocity pattern in penetrating coronary arteries, in general, is different than that in large epicardial arteries. During vasodilation following nitroglycerin, dipyridamole, or a 20-second occlusion of the left main coronary artery, the retrograde component of mid-systolic blood velocity persisted in the septal artery, despite large increases (300–400%) in the mid-systolic antegrade component of blood flow velocity in the left anterior descending artery. These qualitative and quantitative differences in phasic blood flow velocity between intramural and large epicardial arteries are best reconciled by postulating the existence of a significant coronary capacitor.