The end-systolic pressure-diameter relationship is independent of preload but sensitive to changes in contractility, serving as a specific indicator of inotropic state.
Pressure--diameter loops were generated in normal, conscious dogs during left ventricular contraction by means of endocardial ultrasonic gauges and micromanometers. Studies were made in the control state, during volume and pressure loading, and during enhancement of contractility. The diameter at the end of ejection was linearly related to systolic pressure, and the pressure--diameter relation at end-ejection was not affected by changes in preload. However, this relation was substantially displaced by changes in inotropic state, suggesting that the instantaneous pressure--length plane produced by pressure loading is unique to a given level of contractility.
Mahler et al. (1975) studied this question.