Effect of Left Ventricular Hypertension, Ischemia and Vasoactive Drugs on the Myocardial Distribution of Coronary Flow

Distribution of coronary flow to the inner and outer layers of the left ventricle of the anesthetized, open chest dog was estimated by the myocardial uptake of 86 Rb chloride infused into the cannulated common left coronary artery. With normal relationship of coronary perfusion pressure and left intra-ventricular pressure, there was no significant underperfusion of the endocardium. When left intraventricular pressure was raised and coronary perfusion pressure was held at levels sufficient to provide normal coronary flow, endocardial distribution remained equal to or slightly greater than that to the epicardium. However, when coronary perfusion pressure was lowered, particularly to levels causing obvious signs of myocardial hypoxia, maintenance of a normal left intraventricular pressure resulted in marked underperfusion of the endocardium. The coronary vasoactive drugs, dipyridamole, norepinephrine, vasopressin, and the β-adrenergic receptor blocking agent, propranolol, increased the flow distribution to the endocardium of both the normotensive and hypertensive left ventricle. It is concluded that the systolic tissue pressure which increases from epicardium to endocardium does not cause significant underperfusion of the endocardium in either the normotensive or hypertensive left ventricle as long as normal coronary perfusion pressure and flow are maintained.