Characteristics and Possible Origins of Blood Velocity Waveforms of the Epicardial and Intramyocardial Coronary Circulation in the Ventricles and the Atria

Abstract

We measured blood velocities in small epicardial coronary arteries and veins of the ventricles and the left atrium, and intramyocardial arteries and veins using our optical-fiber laser Doppler velocimeter which provides excellent access to vessels. The phase opposition of velocity waveforms between coronary arteries and veins was consistent for both left and right ventricles when velocity measurements were performed in a small artery just before its penetration into myocardium, and in a small vein just after its emergence. The phase opposition was more marked in intramyocardial vessels. Diastolic displacement of blood from superficial veins to deeper portions was frequently observed. Atrial contraction caused a transient sharp decrease in arterial flow of the left atrial coronary arteries (systolic dip), and a prominent systolic flow in atrial veins. Thus, the effect of muscle contraction and relaxation on coronary arterial and venous flows may be fundamentally similar in the left and right ventricles, and in the left atrium. The phase opposition indicates the importance of intramyocardial capacitance vessels as a determinant of phasic coronary arterial and venous flows. To investigate the functional characteristics of the intramyocardial capacitance vessels, we analyzed the change in venous flow following changes in coronary arterial inflow. It was shown that during diastole the intramyocardial capacitance vessels have two functional components, unstressed volume and ordinary capacitance. Unstressed volume is defined as the volume of blood in a vessel at zero transmural pressure, and it was approximately 5% of the volume of the myocardium. When the unstressed volume was saturated, the coronary inflow was decreased significantly, compared with that for the unsaturated condition. The systolic coronary venous outflow showed a significant, positive correlation with the total displaceable blood volume stored in the intramyocardial capacitance vessels. Thus, the increase in intramyocardial blood volume decreases the coronary artery inflow, whereas it enhances coronary venous outflow.