Pressure-Flow Relationships of the Coronary Arteries

Summary

During a long diastole, coronary flow decreases as pressure decreases, and zero flow may be about 40–50 mmHg during autoregulation and about 10–20 mmHg when vessels are maximally dilated. These high zero-flow pressures have been used as evidence for diastolic intramyocardial waterfalls, but other explanations are possible. The high zero-flow pressures are partly due to the capacitance of the extramural coronary arteries. When capacitive effects are eliminated by steady-state, partial coronary occlusions, high zero-flow pressures may be due in part to collateral flow from other branches. Even in the absence of such causes of high zero-flow pressures, those pressures reflect only the last portion of muscle to be perfused, and zero-flow pressures are probably higher elsewhere in the myocardium. Alternative explanations for the pressure-flow behavior emphasize the large intramyocardial blood volume with its long time constants; according to this hypothesis, the zero-flow pressures in the extramural arteries represent the input pressures to the intramyocardial compartment. In the beating heart, it is not possible to reach equilibrium in all the vessels in one cycle, and not yet possible to determine if indeed there are intramyocardial waterfalls in diastole. In systole, recent studies have shown that blood is pumped retrogradely from the subendocardium to the subepicardium, which thus manifests forward systolic flow, even though little or no blood enters the myocardium in systole from the extramural coronary arteries. Therefore, in systole there is no evidence for typical waterfalls, but more evidence in favor of regional changes in intramyocardial compliance to explain coronary pressure-flow relationships.