Effect of positive inotropic agents on the relation between oxygen consumption and systolic pressure volume area in canine left ventricle.

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Abstract
The effect of positive inotropic agents on the relation between left ventricular O12 consumption and the systolic pressure-volume area was analyzed. Pressure-volume area is a measure of total mechanical energy for ventricular contraction, and is a specific area in the ventricular pressure-volume diagram circumscribed by the end-systolic and end-diastolic pressure-volume relation curves and the systolic segment of the pressure-volume trajectory. Either epinephrine (1 .mu.g/kg per min i.v.) or Ca ion (0.03 meq/kg per min i.v.) was administered to canine excised cross-circulated hearts. These agents increased an index of ventricular contractility, Emax, or the slope of the end-systolic pressure-volume line, by 70%. The regression lines of ventricular O2 consumption on pressure-volume area in control and in enhanced contractile states were of the same formula: ventricular O2 consumption (ml O2/beat per 100 g) equals A times pressure-volume area (mm Hg ml/beat per 100 g) plus a constant B. Coefficient A remained unchanged at 1.8 .times. 10-5 ml .times. O2/(mm Hg ml), but constant B increased from 0.03 ml oxygen/beat per 100 g by more than 50% with either agent. The reciprocal of A reflects the energy conversion efficiency for the total mechanical energy, and this efficiency remained near 36%. The increase in B was equal to the directly measured increment in ventricular O2 consumption for mechanically unloaded contraction. The basal metabolism remained unchanged. The augmented O2 consumption under the acutely enhanced contractile state with either epinephrine or Ca apparently was caused primarily by an increased energy utilization associated with the excitation-contraction coupling.