Heart rate affects the dependency of myocardial oxygen consumption on flow in goats

The effect of flow steps in coronary arterial flow (Q_a) on myocardial oxygen consumption (MVo₂) was investigated at different heart rates (HR) to further elucidate the dependency of myocardial oxygen consumption on perfusion. In six anesthetized goats the left main coronary artery and the great cardiac vein were cannulated. The hearts were paced alternately at 60 and 130 beats per min. Flow steps were applied at both HR during control and maximal vasodilation by adenosine. MVo₂, in steady state before and after the flow step, was calculated by multiplication of Q_a and arterio-venous oxygen content difference (Fick's law). Heart rate affected the MVo₂ dependency on flow during control as well as during maximal vasodilation. With vascular tone present, the MVo₂ dependency on flow (ΔMVo₂/ΔQ_a), in μl O₂/ml, was 16.0 ± 3.6 at HR 60 and 21.7 ± 3.9 at HR 130. During maximal vasodilation, these values were 9.5 ± 2.9 and 17.0 ± 5.3 at HR 60 and 130, respectively. The higher MVo₂ dependency on flow at high HR may be explained via a dependency of MVo₂ on microvascular pressure. The pressure change in the microvessels induced by a flow step is probably larger at high HR than at low HR because of increased venous resistance at high HR, due to increased compression by the heart contraction.