Effects of Myocardial Strains on Coronary Blood Flow

Systole causes a redistribution of coronary blood flow away from the subendocardium. In the present study the relative contribution of shortening of the myocardial fibers (wall strains) and of pressure development in the ventricular lumen to redistribution were determined. The distribution of coronary blood flow during systole in hearts ejecting into a severed aorta (large wall strains and near-zero afterload) was compared with that in isovolumetrically contracting hearts (reduced wall strains and significant afterload). A quantitative index of the distribution of coronary blood flow during systole was provided by the myocardial uptake of a bolus of 86 Rb or 42 K injected when constant-pressure perfusion of the left coronary artery was restricted with a solenoid-controlled pump to the period of systole. The coronary blood flow during systole in the subendocardium of the ejecting heart was 62% of that in the subepicardium. A similar gradient in the systolic flow with the endocardium receiving 37% of the blood flow to the epicardium was observed in the hearts contracting isovolumetrically. Removing the afterload by severing the aorta abolished the transmural differences in coronary blood flow. In the present experiment cardiac strains per se did not redistribute coronary blood flow through shear or traction forces on the coronary vasculature, but rather, coronary blood flow was affected only by compressive stresses in the myocardium. Contraction in the absence of afterload influenced overall coronary resistance, however; 18% of the resting coronary resistance was associated with shortening alone. An additional 11% of the resting coronary resistance appeared when pressure development accompanied shortening. Coronary blood flow patterns indicated two separate compressive stresses in the left ventricle. The first stress was associated with pressure development and increased with myocardial depth. The second stress was smaller; it was associated with shortening and uniformly distributed.