How does coronary blood flow vary with the cardiac cycle?

The coronary blood flow in the different phases of cardiac cycle depends on the pressure gradient between the aortic pressure, which tends to perfuse the coronary capillaries, and the ventricular pressure, which tends to prevent perfusion by compressing the coronary capillaries.

In diastole, the intraventricular pressure is zero in both the ventricles and hence the coronary perfusion pressure is approximately equal to the aortic pressure. The intraventricular pressure however raises high during systole, compressing the coronary vessels.

The compression is most intense in the subendocardial layers of the left ventricle because the left ventricular pressure (121mmHg) slightly exceeds the aortic pressure (120mm Hg) during systole.

Hence, the subendocardial portion of the left ventricle is not perfused at all during systole. The compressive effect of the left ventricular pressure decreases towards the subepicardial layers, permitting some amount of blood flow does during systole.

However, in the right ventricle, the perfusion pressure remains adequate throughout systole because the right ventricular pressure rises to a maximum of 25 mm Hg, which is 95 mm Hg lower than the peak aortic pressure.