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

We studied whether an optimal Emax (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 Emax. MVo2 and Emax 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. Emax was obtained by an abrupt aortic occlusion method. We changed Emax over 1.4-16.1 mmHg.ml-1.100 g) by dobutamine, propranolol, and sodium pentobarbital. The obtained MVo2-Emax relationship showed that MVo2 correlated positively with Emax 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 Emax below 7-8 mmHg.ml-1.100 g. We conclude that the optimal Emax to minimize MVo2 for a constant EW does not generally exist as a nadir of MVo2 in the physiological range of Emax in dog hearts.