Force-time integral decreases with ejection despite constant oxygen consumption and pressure-volume area in dog left ventricle.

We have shown that systolic pressure-volume area (PVA), which is equivalent to the total mechanical energy generated by ventricular contraction, correlates linearly with myocardial oxygen consumption, VO2, in canine left ventricle. Systolic force-time integral, FTI, also correlates with VO2. In this study, stroke volume was increased from 0 (isovolumic) in isolated cross-circulated canine left ventricle in a stable contractile state while keeping PVA constant with a servo pump. Ventricular total force was calculated from ventricular pressure and volume by the force-equilibrium equation and was integrated from the end of diastole to the end of systole as identified with the time of Emax to yield FTI. Under the conditions of constant PVA, FTI significantly (p less than 0.001) decreased by 10 +/- 7% and 25 +/- 8% with increases in stroke volume from 0 to 8 +/- 4 ml and 17 +/- 3 ml and in ejection fraction from 0 to 0.24 +/- 0.10 and 0.61 +/- 0.05, respectively, while VO2 and Emax remained constant. Therefore, we conclude that in a stable contractile state, it is possible to keep PVA constant even when stroke volume and ejection fraction are varied and that under such conditions, FTI no longer predicts VO2. PVA remains a reliable predictor of VO2 regardless of ventricular loading conditions.