Basal metabolism adds a significant offset to unloaded myocardial oxygen consumption per minute

Abstract

Myocardial oxygen consumption (MV̇O2) includes components for 1) mechanical energy generation, 2) activation, and 3) basal metabolism. Whereas the first two components are expected to increase in proportion with heart rate, a significant basal level of metabolism would consume oxygen even ,f the heart rate were zero. Contrary to this expectation, however, a previous study reported that, during unloaded beats, MV̇O2 per beat (which includes basal metabolism) was independent of heart rate. Accordingly, unloaded MV̇O2 per minute would extrapolate to zero at zero heart rate; this result is unexpected considering basal metabolism. To resolve this inconsistency, we varied heart rate over a wide range after inducing atrioventricular block in eight isolated cross-circulated canine hearts that contracted isovolumically. We examined whether a term representing rate-independent basal metabolism was needed to describe MV̇O2 per minute. Mechanical energy generated by the left ventricle was evaluated from the pressure-volume area, which was altered by changing isovolumic ventricular volume over at least five levels at each heart rate. Contractility, evaluated by the slope of the end-systolic pressure-volume relation, did not vary significantly with heart rate in this study. In contrast to the previous report, unloaded MV̇O2 per beat (i.e., MV̇O2 extrapolated to a pressure-volume area of zero) was not constant but fell monotonically with increases in heart rate in every heart. We considered that this trend was caused by a significant rate-independent basal level of MV̇O2 per minute. Multiple linear regression analysis confirmed that this rate-independent basal term differed significantly from zero in seven of the eight hearts studied. The average basal metabolism was 2.00±0.99 (mean±SD) ml O2 · min-1 · 100 g left ventricle-1; at low heart rate (≈50 beats per minute), this represented 35-60% of the total unloaded MV̇O2 per beat. We conclude that a rate-independent basal metabolic component is needed to describe MV̇O2 per minute and that unloaded MV̇O2 per beat is a decreasing function of heart rate when rate-dependent contractility is not large.