Contractility to minimize oxygen consumption for constant work in dog left ventricle

Summary We studied whether an optimal E_max (contractility index) to minimize myocardial O₂ consumption (MVo₂) for a constant external work (EW) and, hence, maximize mechanical work efficiency exists as a nadir of MVo₂ within the physiological range of E_max. MVo₂ and E_max were measured in the left ventricles (LV) of 10 dogs. In each experiment, heart rate, cardiac output, and mean aortic pressure were kept constant by pacing, bypassing the right heart, and adjusting the inflation of an intra-aortic balloon, respectively, to maintain a constant EW. MVo₂ was determined as the product of coronary arteriovenous O₂ difference, and coronary blood flow drained from the right heart. E_max was obtained by an abrupt aortic occlusion method. We changed E_max over 1.4–16.1 mmHg·ml^–1·100 g) by dobutamine, propranolol, and sodium pentobarbital. The obtained MVo₂-E_max relationship showed that MVo₂ correlated positively with E_max in 9 of the 10 dogs (correlation coefficients=0.68–0.96). In one of the 9 dogs and the remaining dog, MVo₂ slightly increased with decreases in E_max below 7–8 mmHg·ml^–1·100 g. We conclude that the optimal E_max to minimize MVo₂ for a constant EW does not generally exist as a nadir of MVo₂ in the physiological range of E_max in dog hearts.