Optimal contractility and minimal oxygen consumption for constant external work of heart.

We theoretically considered the relation between left ventricular O2 consumption (MVO2) and a contractility index (Emax), searching for an optimal Emax that minimizes MVO2 for a given external mechanical work. We used one equation relating Emax with ventricular pressure and volume and another equation relating MVO2 with pressure-volume area (PVA). PVA is theoretically and experimentally a good predictor of MVO2 with Emax as a parameter. Then we could theoretically show the existence of the optimal Emax. For example, MVO2 was minimized to 8.9 ml O2.min-1.100 g-1 at an Emax of 6.3 mmHg.ml-1.100 g when cardiac output was 1.2 l/min, afterload pressure was 100 mmHg, and heart rate was 150 beats/min. These values can be observed experimentally in a 10- to 15-kg dog. Optimal Emax values for a wide range of external work fall in the middle of the normal working range of Emax. Thus our MVO2-PVA-Emax relationship supports the contemporary concept of the optimal contractility that MVO2 for a given cardiac external work is minimum at a middle level of ventricular contractility.