Researchers are studying the coronary circulation in various healthy and diseased states. They measure blood flow and pressure (blue circle) in one of the epicardial coronary arteries of an experimental animal with cardiovascular physiology similar to that of humans. Then they clamp the artery to simulate 60% atherosclerotic stenosis and remeasure the flow and pressure distal to the lesion. Which of the following changes are most likely to be observed once steady state is achieved?
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Coronary autoregulation helps maintain relatively constant coronary blood flow despite changes in perfusion pressure. A significant stenosis (eg, >50%) causes reduced distal perfusion pressure, resulting in an initial decrease in distal blood flow with corresponding myocardial ischemia. In response, the ischemic myocardium triggers the release of vasodilators (eg, nitric oxide, adenosine), to facilitate arteriolar vasodilation and reduce downstream vascular resistance. The reduced vascular resistance allows for a corrective increase in blood flow at the new, reduced perfusion pressure (blood flow = perfusion pressure / vascular resistance), keeping blood flow nearly unchanged.
The effects of coronary autoregulation are limited in the setting of extreme changes in perfusion pressure. Once arterioles reach the point of maximal vasodilation, vascular resistance cannot be further reduced, and a further decrease in perfusion pressure results in a precipitous drop in blood flow (as seen on the far-left side of the coronary autoregulation curve).
(Choices A, B, and C) Perfusion pressure decreases (rather than increases or remains the same) distal to a significant stenosis. Coronary autoregulation allows blood flow to remain nearly unchanged distal to the stenosis.
Educational objective:
Perfusion pressure decreases distal to a significant coronary artery stenosis, resulting in an initial decrease in blood flow. Coronary autoregulation stimulates arteriolar vasodilation, which reduces downstream vascular resistance and helps return blood flow to near-normal levels to prevent myocardial ischemia.