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

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 if the heart rate were zero. Contrary to this expectation, however, a previous study reported that, during unloaded beats, MVE~₂ 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.