External mechanical work from relaxing ventricle.

The possibility has been proposed earlier that the specific pressure-volume (P-V) area bounded by the left ventricular end-systolic and end-diastolic P-V curves and the isovolumic relaxation part of the P-V loop represents mechanical potential energy that has been built during systole and is stored at end systole in the wall of the ventricle. In the present study on canine left ventricles, as much as 70% of the P-V area was actually converted into external mechanical work when ventricular volume was allowed to decrease at an appropriate speed (about 55 ml/s in 70 g left ventricle) during relaxation. Less external work was extracted from the same P-V area when the speed of volume reduction was either higher or lower than that speed. These results indicate that the P-V area is equivalent to a form of potential energy, which is wasted with isovolumic relaxation but most of which is convertible to external mechanical work if the ventricle is allowed to eject against an appropriately decreasing afterload during relaxation.