Ventricular perspective on efficiency.

The heart has many efficiencies of different definitions, of which mechanical work efficiency is the most popular and conventional. We have proposed a method to quantify the total mechanical energy generated by ventricular contraction. This energy can be quantified as a specific area called "systolic pressure-volume area" or "PVA" in the ventricular pressure-volume diagram. In the left ventricle of excised, cross-circulated dog heart preparations, we found a closely linear relation between PVA and oxygen consumption (VO2) under various loading conditions in a stable contractile state (Emax). An enhanced contractility was accompanied by an elevation of the load-independent VO2-PVA relation in a parallel manner, where the elevation was proportional to Emax. The slope of the VO2-PVA relation represents the "oxygen cost of mechanical energy (or PVA)" and its reciprocal indicates the "contractile efficiency", i.e., the energy conversion efficiency from PVA-dependent VO2 to PVA. This efficiency was 40% on the average, independent of various inotropic interventions. The slope of the PVA-independent VO2-Emax relation represents the "oxygen cost of contractility (or Emax)". This cost was relatively constant for different inotropic interventions except for myocardial cooling and stunning. We considered the discrepancy between the stable contractile efficiency and the variable thermal economy of force generation and maintenance.