Effect of Heart Rate on Cardiac Work, Myocardial Oxygen Consumption and Coronary Blood Flow in the Dog

Summary. 1. The effects of variations in heart rate on ventricular function, myocardial oxygen consumption, and coronary blood flow were studied in anesthetized, open‐chest dogs. The heart rate was held at desired rates by electrical stimulation. 2. The following changes occurred when the rate was increased in steps from 33—80 per min. to 200—280 per min.: cardiac output increased 0 to 73 per cent and reached a maximum at rates varying from 90 to 180 per min.; above these rates there was usually a decrease. Stroke volume decreased progressively. Mean aortic pressure rose slightly, and the pulse pressure was diminished. Myocardial oxygen consumption and left coronary flow increased, even at the high rates when ventricular work was constant or decreasing; there was a marked fall in coronary resistance. 3. Ventricular function curves, i. e. ventricular stroke work plotted against the respective mean atrial pressure, were obtained at the various rates. The curves were always lower at higher than at lower rates. 4. Myocardial oxygen consumption per unit of mechanical work was greater at higher rates, probably because of the larger amount of “isometric work” per min. Coronary blood flow per unit of work increased similarly with rate. Coronary vascular resistance was always much lower at high heart rates than at low, indicating coronary vasodilation. 5. It is concluded that during normal circumstances myocardial oxygen consumption is a major determinant of coronary blood flow. 6. Effects of heart rate during abnormal circulatory conditions were illustrated and discussed.