Global analysis of myocardial isotonic shortening: comparison with isometric dynamics

Although potentially analytically useful, a global empirical model of the myocardial isotonic curve, L(t), has not been described. We propose the following relation: L(t) = C(t/A)B-1e-(t/A)B, where A, B, and C are global parameters, L is length, and t is time. We evaluated this model in nine in situ canine papillary muscles studied with a servo-system to produce isotonic twitches at different isotonic forces (F). For each twitch, the parameters were determined by nonlinear curve fitting. The model fit the observed curves of L(t) closely, with the coefficient of determination being 0.995 +/- 0.002. C changed with F, but A and B varied little with F, averaging 0.262 +/- 0.021 s and 2.76 +/- 0.17, respectively. Our predictions that A reflects chronotropic, B reflects lusitropic, and C reflects heterotonic (different afterloads) and inotropic states were supported. Comparison done in five muscles showed that A was the same but B was higher for isotonic than for isometric twitches: 0.270 +/- 0.020 vs. 0.264 +/- 0.038 s (P = not significant) for A and 2.79 +/- 0.18 vs. 2.39 +/- 0.05 (P less than 0.008) for B. Dobutamine increased A but not B in isotonic twitches. Thus shortening is lusitropic but leaves no lusitropic reserve to be mobilized by dobutamine. The relation L(t) = C(t/A)B-1e-(t/A)B provides a framework that undergirds global analysis of myocardial shortening and enables comparison with isometric dynamics.