Contractility-dependent curvilinearity of end-systolic pressure-volume relations

The shape of the end-systolic tension-length relationship (ESTLR) changes when contractile state is changed, whereas the end-systolic pressure-volume relationship (ESPVR) remains linear despite changes in contractility. To investigate this disparity, the ESPVR was determined with contractility altered extensively by dobutamine, BAY K 8644, nifedipine, lowering coronary blood flow, and the introduction of extrasystolic and postextrasystolic stimulations. The ESPVRs were fitted by nonlinear regression analysis to the parabolic equation P(es) = aV(es)² + bV(es) + c, where P(es) is end-systolic pressure, V(es) is end-systolic volume, and a, b, and c are parameters. There was a negative, statistically significant correlation between a, which serves as a shape index of the ESPVR, and E'(max), the slope of the ESPVR in a low volume range. When E'(max) was large a was negative, indicating increasing concavity of the ESPVR to the volume axis at high contractility. When E'(max) was small a was positive, indicating convexity of the ESPVRs to the volume axis at low contractility. Within the average range of E'(max) between 3.4 and 7.8 mmHg/ml, however, the parabolic fit to the data was not statistically better than a linear fit over the range of volumes testable in the isolated heart. We conclude that the shape of the ESPVR measured in the isolated canine heart changes with contractile state. In accordance with previous interpretations of shape changes in the muscle ESTLR, these results are consistent with the existence of length-dependent activation of cardiac muscle in the intact heart.