Transmural Distribution and Heterogeneity of Coronary Flow and Flow Reserve

Summary

Left ventricular subendocardial vulnerability to ischemia occurs in almost all forms of heart disease. The underlying mechanism of this vulnerability is due to the variation of intramyocardial pressures, which are highest under the endocardial surface of the left ventricle and lowest below the epicardium. At the onset of systole, pressures in the subendocardial vessels exceed those in the extramural coronary arteries. Consequently, flow from subendocardial arteries and arterioles is retrograde as well as anterograde. This retrograde flow accounts for retrograde systolic flow in the extramural arteries and also for forward systolic flow in the subepicardial vessels; little or no blood enters the myocardium from the extramural arteries in systole. At the end of systole, the subendocardial arteries and arterioles are narrowed, and therefore have a high resistance to refill in the next diastole. If diastolic time or perfusion pressure are decreased, then subendocardial flow will be compromised. Even in the subendocardium, however, there is variability of coronary flow reserve, so that decreased perfusion pressure affects first those small pieces of muscle that have the lowest reserve. Then additional pieces are recruited until, eventually, almost the whole subendocardium is ischemic. This variability, of unknown origin, probably accounts for the patchy necrosis seen in chronic ischemia.