Effects of preload on the transmural distribution of perfusion and pressure-flow relationships in the canine coronary vascular bed.

Abstract

Effects of preload on transmural myocardial perfusion and coronary pressure-flow relationships were studied in mongrel dogs. The left circumflex (LC) artery was cannulated and perfused through an external circuit of rigid tubing originating in the left subclavin artery and containing an electromagnetic flowmeter (EMF), a solenoid and a short segment of soft tubing which could be occluded. The solenoid, triggered by the R-wave of the ECG, was adjusted so that perfusion occurred only during diastole. Autoregulation was abolished by maximal vasodilation with carbocromen. In 16 dogs overall radioactive microsphere flow in the perfused LC segment was 1.67 .+-. 0.17 (SEM [standard error of the mean]) ml/min per g at normal preload and 1.54 .+-. 0.19 ml/min per g at elevated preload. The endocardial: epicardial flow ratio decreased from 0.76 .+-. 0.03 at normal preload to 0.55 .+-. 0.04 at elevated preload (P < 0.01). EMF pressure-flow curves, determined by stepwise constriction of the perfusion circuit in the same dogs, showed a rightward shift with increased preload and an increase in PZF, the pressure at which inflow became zero (12 .+-. 0.9 mm Hg at normal preload vs. 19 .+-. 1.0 mm Hg at elevated preload, P < 0.01). Microsphere pressure-flow curves in 8 additional dogs suggested that a small endocardial: epicardial difference in diastolic PZF at normal preload is accentuated at elevated preload. Effects of increased preload produced by volume expansion are 2-fold: a selective reduction of flow in the inner layers of the heart, and a shift to the right of the diastolic pressure-flow relationship. The former seems reasonably related to direct effects of preload on intramyocardial tissue pressure; the latter suggests additional effects of local reflex, myogenic or mechanical phenomena.