Left ventricular relaxation in the filling and nonfilling intact canine heart

We studied left ventricular relaxation in the filling and transiently nonfilling working hearts of seven open-chest pentobarbital-anesthetized dogs by totally occluding the mitral annulus during one systole. In the completely isovolumic nonfilling cycle, the ventricle relaxes to a lower pressure minimum (usually negative) than in the normal filling cycle. By clamping the ventricle at end systole, we determined the pressure asymptote (Poo) under dynamic conditions. With this information, we evaluated the validity of a monoexponential characterization of relaxation. P = (P0 - Poo) exp(-t/T) + Poo (T, time constant, P0, pressure at t = 0). Plots of In(P-Poo) versus t are nonlinear and concave to the origin, thereby revealing that late relaxation is more rapid than predicted by a monoexponential relation. Nevertheless, the monoexponential T remains a useful index of relaxation and correlates well with other temporal indexes (isovolumic relaxation time and relaxation half-time). When T is calculated from a filling cycle by assuming a zero pressure asymptote, i.e., the conventional way, there is no significant difference with the true value based on the nonfilling cycle.