Effect of heart rate on coronary blood flow distribution in dogs

Abstract

We investigated the hypothesis that tachycardia augments the time of systolic compression of coronary blood vessels and shifts coronary blood flow from the subendocardial (inner) to the subepicardial (outer) layer of the left ventricle. Coronary blood flow distribution was determined in tranquilized dogs by the radioactive microsphere technique, with injection of approximately 200,000 spheres, 15 , into the left ventricular chamber. During the control period (mean heart rate 73 beats/min), there were consistent differences in the sphere concentrations (blood flow) in different regions of the heart, and the inner layer of the left ventricle always received more spheres than its outer layer. The mean inner/outer sphere ratios were 1.26 for the free wall and 1.39 for the septal wall. Tachycardia (heart rates of 151 and 193 beats/min) induced by atrial pacing or administration of atropine increased total coronary blood flow but systematically altered its distribution: The increase in blood flow to the atria was much greater than that to the ventricles, and the inner/outer sphere ratio of the left ventricle decreased in proportion to the increase in heart rate. Although the redistribution of coronary blood flow was not accompanied by chemical signs of myocardial hypoxia in these normal dogs, we propose that the shift in blood flow away from the left ventricular inner layer is exaggerated in patients with coronary artery disease, resulting in subendocardial ischemia and hypoxia during tachycardia. We investigated the hypothesis that tachycardia augments the time of systolic compression of coronary blood vessels and shifts coronary blood flow from the subendocardial (inner) to the subepicardial (outer) layer of the left ventricle. Coronary blood flow distribution was determined in tranquilized dogs by the radioactive microsphere technique, with injection of approximately 200,000 spheres, 15 , into the left ventricular chamber. During the control period (mean heart rate 73 beats/min), there were consistent differences in the sphere concentrations (blood flow) in different regions of the heart, and the inner layer of the left ventricle always received more spheres than its outer layer. The mean inner/outer sphere ratios were 1.26 for the free wall and 1.39 for the septal wall. Tachycardia (heart rates of 151 and 193 beats/min) induced by atrial pacing or administration of atropine increased total coronary blood flow but systematically altered its distribution: The increase in blood flow to the atria was much greater than that to the ventricles, and the inner/outer sphere ratio of the left ventricle decreased in proportion to the increase in heart rate. Although the redistribution of coronary blood flow was not accompanied by chemical signs of myocardial hypoxia in these normal dogs, we propose that the shift in blood flow away from the left ventricular inner layer is exaggerated in patients with coronary artery disease, resulting in subendocardial ischemia and hypoxia during tachycardia.