Abstract
If characteristic muscle properties such as myocardial viscosity and/or shortening deactivation influence left ventricular ejection in the whole heart, the slope of the left ventricular end-systolic pressure-volume relation should be a function of both the contractile state and the loading mode. Thus, the load dependence of the end-systolic pressure-volume relation was examined using isolated, perfused canine hearts ejecting saline into a hydraulic loading system. The instantaneous left ventricular volume was measured with a plethysmograph. Under constant coronary flow and heart rate, two regression lines for end-systolic pressure-volume relations in two sets of loading modes were obtained: (1) Preload (left ventricular end-diastolic pressure; 4-15mmHg) changes under fixed afterload impedance (preload changes); (2) Afterload impedance (peripheral resistance; 1.9-9.6×103dyn sec cm-5) changes under fixed left ventricular end-diastolic volume (afterload changes).
The slope of the end-systolic pressure-volume relation with afterload changes was steeper than that with preload changes (6.3±0.7 vs 4.8±0.6mmHg/ml, p<0.05). Accordingly, under constant coronary flow, the slope of the end-systolic pressure-volume relation depended on load-ing conditions within the physiological range of afterload impedance and preload. This finding supports our hypothesis and implies that the slope change is of limited value as a contractile index in the ejecting heart
