Increased Myocardial Oxygen Consumption and Contractile State Associated with Increased Heart Rate in Dogs

The effects of increasing the frequency of contraction on myocardial oxygen consumption per minute (MV · o 2 ) were examined in eight dogs using an isovolumic left ventricular preparation. MV · o 2 was determined at two to four levels of heart rate in each animal. Peak wall stress was maintained constant in each animal so that changes in it would not influence the effects of heart rate on oxygen consumption per beat. As heart rate was increased, there was a highly significant linear increase in MV · o 2 . Oxygen consumption per beat was shown to be a negative linear function of the reciprocal of heart rate. Thus, as heart rate increased there was a significant increase in oxygen consumption per beat; when basal oxygen consumption was subtracted from total oxygen consumption, there was a much larger increase in oxygen consumption per beat. Myocardial contractile state, defined as the maximum observed contractile element velocity at the lowest common level of wall stress, was significantly increased by increasing heart rate. The data suggest that the increased M · Vo 2 associated with augmented heart rate is secondary to augmentation of contractile state, as well as to the increase in stress development per minute.