Determinants of myocardial oxygen consumption in fibrillating dog hearts. Comparison between normothermia and hypothermia.

The purpose of the present study was to elucidate the mechanism of the difference in myocardial oxygen consumption between heating and fibrillating states during normothermia and hypothermia. In five isolated cross-circulated dog hearts, we measured left ventricular pressure at several ventricular volumes and myocardial oxygen consumption at V0 and V100, at which peak isovolumic pressures were zero and approximately 100 mm Hg, respectively, in beating and fibrillating states during normothermia and hypothermia (29 degrees C). As a measure of the total mechanical energy at V100, we obtained pressure-volume area in the beating state and equivalent pressure-volume area for fibrillation. We calculated equivalent heart rate as an estimate of the contraction frequency of individual myocytes in a fibrillating ventricle from myocardial oxygen consumption at V0 in the beating and fibrillating states. During normothermia, myocardial oxygen consumption per minute at V0 and V100 and myocardial oxygen consumption for mechanical purposes at V100 (myocardial oxygen consumption at V100-myocardial oxygen consumption at V0) were significantly higher during fibrillation than in the beating state. Equivalent pressure-volume area during fibrillation and pressure-volume area in the beating state at V100 were comparable, whereas equivalent heart rate during fibrillation was significantly higher than heart rate in the beating state. During hypothermia, myocardial oxygen consumption was comparable between beating and fibrillating states at V0, although myocardial oxygen consumption at V100 was slightly lower during fibrillation than in the beating state. Myocardial oxygen consumption for mechanical purposes during fibrillation was half of that in the beating state. Equivalent pressure-volume area was significantly smaller than pressure-volume area, whereas equivalent heart rate and heart rate were comparable. We conclude that during normothermia, higher myocardial oxygen consumption during fibrillation than in the beating state at V0 and V100 is attributable to the higher contraction frequency. During hypothermia the comparable myocardial oxygen consumption values at V0 are attributable to the comparable contraction frequencies, whereas slightly lower myocardial oxygen consumption during fibrillation at V100 is ascribed to the lower total mechanical energy.