Q. What are Diastolic Compressive Forces?

Coronary perfusion pressure equal to pressure gradient between the coronary arteries and the pressure in LV in diastole. When coronary perfusion pressure is lowered, diastolic blood flow decreases.

Extravascular compressive forces are greater in subendocardial than in subepicardial zones. So systolic flow is more reduced in the subendocardium than subepicardium. But there is preferential dilatation of the subendocardial vessels causing a large increase in diastolic flow in the subendocardium. This is due to increased wall stress which is 20 per cent greater than that of the subepicardial muscle. Average endocardial to epicardial flow ratio is 1.25:1 throughout the cardiac cycle.

A low subendocardial to epicardial flow occurs in epicardial coronary stenosis and heart failure. A low endocardial to epicardial flow ratio can be increased by elevation of aortic pressure