Myocardial Oxygen Consumption in Acute Experimental Cardiac Depression

Myocardial oxygen consumption (MV · o 2 ) during drug-induced cardiac depression was measured in 8 anesthetized, open chest dogs in which myocardial wall tension was controlled. The right side of the heart was bypassed and myocardial contractility was reduced with procaine HCl, propranolol, or pronethalol. MV · o 2 consistently fell during cardiac depression (avg = 1.52 ml/min per 100 g left ventricle or - 11.6%). These reductions occurred despite small increases in developed tension. Changes in the tension-time index, contractile element work, and contractile element power did not correlate invariably with ΔMV · o 2 , while reductions in velocity of the contractile elements at isotension, maximum left ventricular dp/dt, and the extent of shortening of the contractile elements and circumferential fibers were associated in every experiment with the reductions in MV · o 2 . The finding that negative inotropic influences are associated with a reduction in myocardial energy utilization, when considered with earlier observations showing that positive inotropic influences induce an augmentation of MV · o 2 , provides evidence that the inotropic state and its mechanical correlates are coupled with myocardial energy utilization by a mechanism that is independent of tension development.