PROPERTIES OF THE MYOCARDIUM AFFECTING THE CORONARY CIRCULATION

The mean coronary bloodflow increases in response to an increase in my ocardial oxygen consumption. Conversely, an increase in coronary perfu sion is itself reported to induce an increase in myocardial oxygen con sumption. Such an effect can be explained by stretching of the myocard ial fibers surrounding the vessels, which become more distended with a n increase in per fusion. The flow in the left descending and circumfl ex coronary arteries is reduced in systole because of the compression exerted by the contracting myocardium on the intramyocardial vessels. Due to the thinner wall of the right ventricle, this reduction is not obvious in the right coronary artery. The intramyocardial pump model p rovides a satisfactory explanation of the mechanism by which contracti on reduces the flow. It also explains the attenuation of the diastolic -systolic oscillations of flow which occurs in the presence of a steno sis of a large epicardial artery. The varying elastance model shows th e dependence of the extent of the reduction of the flow in systole on myocardial contractile force rather than on the pressure developed in the ventricle by the contraction. However, although the ventricular sy stolic pressure does not affect the flow in hearts With a relatively t hick wall, it contributes to the systolic reduction of flow in hearts With a relatively thin wall. Owing to a mechanism involving the corona ry capacitance, contraction is also responsible for the level of coron ary flow in diastole.