Constant mechanical efficiency of contractile machinery of canine left ventricle under different loading and inotropic conditions.

Abstract

The total mechanical energy generated in each cardiac contraction can be quantified by the systolic pressure-volume area (PVA). PVA is the area in the pressure-volume(P-V) diagram that is circumscribed by the end-systolic and end-diastolic P-V relation curves and the systolic segment of the P-V trajectory. This area had dimensions of energy and comprises the external mechanical work and the elastic potential energy. In the left ventricle of cross-circulated canine hearts, the relation between PVA and O2 consumption per beat (VO2), above VO2 for mechanically unloaded contraction was studied. This excess VO2 was assumed to be utilized for mechanical contraction by the contractile machinery. The percentage of PVA in the excess of VO2, both in the same unit of energy, J, would then represent the efficiency of energy conversion from the excess VO2 to the total mechanical energy in the contractile machinery. This efficiency was observed in variously loaded contractions in both control and enhanced contractile states with epinephrine and Ca. The efficiency was constant at 30-50 (mean 40%) regardless of the changes in both loading conditions and contractile states. By this constant efficiency and a variable fraction of external work in PVA, the load- and contractility-dependent variability of the conventional mechanical efficiency (0-30%) of the heart was accounted for.