End-systolic pressure determines stroke volume from fixed end-diastolic volume in the isolated canine left ventricle under a constant contractile state.

We studied the effect of systolic pressure and volume changes on the end-systolic pressure at a fixed end-systolic volume in the left ventricle of excised, cross-circulated canine hearts. Instantaneous ventricular volume was controlled and both end-diastolic and end-systolic volumes were clamped, as preprogrammed by a volume servo pump system. Ventricular ejection was completed at the end of natural systole. When the onset and velocity of ejection were widely varied during contractions with a given set of end-diastolic and end-systolic volumes, the end-systolic pressure was little affected by the changes in the systolic pressure and volume under a stable contractile state. When the end-diastolic volume was increased from the isovolumic condition, the end-systolic pressure at the same end-systolic volume decreased (P < 0.05) from the peak isovolumic pressure by 5-14%, for an ejection fraction of 40-70%. When the end-systolic volume was decreased while the end-diastolic volume was fixed, the end-systolic pressure decreased in proportion to end-systolic volume. These results were interpreted to indicate that, when ejection ends at the end of systole, stroke volume of the ventricle with a given end-diastolic volume is determined predominantly by the end-systolic pressure rather than by the entire systolic courses of the pressure and volume, drc Res 44; 238-249, 1979