Effect of Alterations of Coronary Blood Flow on the Oxygen Consumption of the Nonworking Heart

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Abstract
The effect of varying CBF and myocardial O2 delivery on MVOO2 was studied in 23 experiments. In 14 of the experiments an isolated dog heart was perfused, while in the others the heart of a dog, whose systemic circulation was maintained on cardiopulmonary bypass, was studied. The ventricles were kept empty, developed no pressure and performed no external work, while their temperature was held constant. CBF and myocardial O2 delivery were controlled by pumping blood into the coronary arteries, total coronary venous return was collected from the right side of the heart and MVOO2, was calculated during a steady state by the Fick principle. Myocardial anoxia was avoided by maintaining the coronary venous O2 content above 4 vol % and myocardial O2 extraction below 78%. A comparison of MVOO2 at two levels of CBF (and O2 delivery) was made in 42 instances, and in 32 of them 11 MV increased substantially as CBF was elevated, or vice versa. The ten exceptions all occurred when O2 delivery greatly exceeded MVOO2 with O2 extraction ratios below 35%. The fundamental mechanisms responsible for these findings are not clear, but a number of possible explanations are discussed.