Intrinsic effects of heart rate on left ventricular performance

The effects of heart rate on left-ventricular performance were studied in an areflexic dog right-heart bypass preparation which allowed independent control of aortic pressure, cardiac output, and heart rate. When the heart rate was increased while stroke volume and mean aortic pressure were maintained constant the left-ventricular mean rate of pressure rise during isovolumic systole, the maximal rate of pressure rise during isovolumic systole, and the mean rate of ejection were all increased without any change in left-ventricular end-diastolic pressure. Further, it was shown that the left ventricle performed the same amount of stroke work over a wide range of heart rates without an increase in end-diastolic pressure in spite of the markedly shortened time available for performing this work. This was accomplished because of the increase in stroke power. These observations demonstrate that the performance of the left ventricle becomes intrinsically "faster" as the heart rate is increased. When the transient phenomena that occur when the heart rate is increased are considered, the fact that the same stroke work is produced over a wide range of heart rates without an increase in end-diastolic pressure indicates that the left ventricle has also become "stronger" than it would have been if the adaptive change had not occurred.