Whatever the mechanism of coupling between perfusion pressure and coronary blood flow may be, the importance of this interaction becomes most prominent when the perfusion pressure exceeds 160 or falls below 60 mm Hg. Under the latter situation, coronary arteries become maximally dilated and totally dependent on perfusion pressure. In the presence of significant coronary artery disease, the perfusion pressure distal to an atherosclerotic plaque may fall below this critical threshold, thereby rendering the distal vascular bed maximally dilated and dependent on the perfusion pressure.
Extravascular compressive forces: This term refers to purely mechanical forces that compress coronary vasculature. It includes the intrapericardial, intra-myocardial, and intraventricular pressures during the cardiac cycle. The contribution of the intrapericardial forces to extravascular compressive forces is minimal in most clinical circumstances. The intramyocardial pressure refers to the twisting motion associated with myocardial contraction. As expected, intramyocardial forces exert their maximal effect during systole and the magnitude of these compressive forces decrease from endocardium to epicardium. The intraventricular pressure reaches a peak during systole and has its maximal compressive effect on the subendocardium, as do intramyocardial forces. Hence, the composite effect of intramyocardial and intraventricular forces during systole is to maximally compress the subendocardial coronary vasculature. flovent inhaler