Coronary Flow Mechanics of the Hypertrophied Heart

Summary

The main determinants of coronary flow are coronary artery pressure and coronary vascular resistance. The extent of the latter depends on the contractile state of the blood vessel wall, and on the amount of compression of the vessels through intramyocardial-intravascular pressure differences.

In the autoregulated hypertrophied hearty under resting conditions the resistance vessels are more dilated at any given arterial pressure than in the normal heart, because the increased muscle mass requires a larger resting flow. As a result, vasodilatory reserve is diminished in the hypertrophied heart. If hypertrophy is caused by hypertension or supravalvular constriction of the aorta, loss of vascular reserve can in part be compensated by the high coronary perfusion pressure. This is because maximal flow increases more steeply with aortic pressure than resting flow, which increases as oxygen demand increases with aortic pressure. In aortic valvular stenosis, compression of the intramyocardial blood vessels may have additional unfavorable effects on coronary vascular reserve. Increased intramyocardial pressure not balanced by an increased perfusion pressure may be expected to increase minimal vascular resistance especially in the subendocardium and thereby reduce the flow reserve.

Coronary pressure and flow patterns can be analyzed by assuming a hemodynamic model of the coronary circulation comprising intramyocardial compliance, loaded by intramyocardial pressure, and inflow and outflow resistances. The model has been validated in open-chest experiments in which the coronary arterial inflow and coronary venous outflow are measured simultaneously. Variations of intramyocardial blood volume can be calculated from these measurements. From these studies it follows that the use of coronary vascular resistance values calculated from end-dia-stolic pressures and flows is questionable. It can be demonstrated that resistances calculated in this way partly depend upon the shape of the arterial pressure pulse.

The typical reversal of the coronary flow pattern found in severe aortic insufficiency is in keeping with the myocardial compression model. It can be predicted that systolic flow will exceed diastolic flow if the arterial pulse pressure becomes larger than half the intraventricular systolicdiastolic pressure difference. Low diastolic flow in aortic insufficiency does not imply that coronary vascular resistance is raised.