Hemodynamic determinants of the time-course of fall in canine left ventricular pressure

time-course of fall in isovolumic left ventricular pres-sure were assessed in isolated canine left ventricular preparations. Pressure fall was studied in isovolumic beats or during prolonged isovolumic diastole after ejection. Pressure fall from the time of maximum nega-tive dP/dt was found to be exponential during isovol-umic relaxation for isovolumic and ejecting beats (r> 0.98) and was therefore characterized by a time con-stant, T. Higher heart rates shortened T slightly from 52.6 ±4.5 ms at 110/min to 48.2±6.0 ms at 160/min (P < 0.01,) = 8): Higher ventricular volumes under iso-volumic conditions resulted in higher peak left ven-tricular pressure but no significant change in T. T did shorten from 67.1+5.0 ms in isovolumic beats to 45.8+ 2.9 ms in the ejecting beats (P < 0.001, n = 14). In the ejecting beats, peak systolic pressure was lower, and end-systolic volume smaller. To differentiate the effects of sy-stolic shortening during ejection from those of lower systolic pressure and smaller end-systolic volume, beats with large end-diastolic volumes were compared-to beats with smaller end-diastolic volumes. The beats with smaller end-diastolic volumes exhibited less short-ening but similar end-systolic volumes and peak systolic pressure. T again shortened to a greater extent in the beats with greater systolic shortening. Calcium chloride and acetylstrophanthidin resulted in no significant change in T, but norepinephrine, which accelerates active relaxation, resulted in a significant shortening of T (65.6±13.4 vs. 46.3+7.0 ms, P < 0.02)