Efficiency of energy transfer from pressure-volume area to external mechanical work increases with contractile state and decreases with afterload in the left ventricle of the anesthetized closed-chest dog.

We studied the effects of ventricular end-systolic elastance (Ees) and effective arterial elastance (Ea) on the efficiency of energy transfer from pressure-volume area (PVA) to external mechanical work (EW) in the left ventricle of anesthetized closed-chest dogs. PVA represents the total mechanical energy generated by ventricular contraction, which is an intermediate form of energy between oxygen consumption, the total energy input, and EW, the effective energy output. PVA and EW were determined from ventricular pressure and volume, which were continuously measured with a volumetric conductance catheter. Measurements of Ees were obtained by transiently increasing afterload by an inflation of a Fogarty catheter in the thoracic descending aorta. Ea was determined as the ratio of end-systolic pressure to stroke volume. The EW/PVA efficiency of a steady-state contraction increased from 55% to 64%, with a 58% increase in Ees after dobutamine. Ees, which was smaller than Ea before dobutamine, became nearly equal to Ea after dobutamine, maximizing EW for a given end-diastolic volume. EW/PVA efficiency decreased with an abrupt increase in afterload before and after dobutamine. The sensitivity of the decrease in the EW/PVA efficiency to an increase in end-systolic pressure was significantly less after than before dobutamine. We could account for all these changes in EW/PVA efficiency by the relative changes in Ees and Ea in the pressure-volume diagram.