Double-exponential curve fitting of isometric relaxation: a new measure for myocardial lusitropism.

New indexes for evaluation of isometric myocardial relaxation were proposed. In fully isometric and physiologically sequenced twitches, the time course of isometric force decline fitted well with Gompertz's double-exponential curve (r > or = 0.9995). We conformed the original equation to suit myocardial mechanics, i.e., F(t) = gamma 0 - gamma.exp [-alpha.exp (-beta t)] (t = 1, 2, ..., n), where F(t) denotes force as a function of time t. The gamma 0 and gamma relate to upper asymptote and force amplitude, respectively. Phase-plane analysis of F(t) revealed that alpha [3.56 +/- 0.67 (SD)] related to the phasic delay of relaxation onset but did not affect the F(t) vs. dF(t)/dt relation. The beta (0.127 +/- 0.021) and gamma were linearly related to negative dF(t)/dtmax; however, the terminal slope of the phase-plane diagram was governed by beta alone. The tau beta (0.081 +/- 0.017 s), a reciprocal of beta multiplied by sampling time, was practically independent of preload, total load, and muscle shortening. In isometric twitches, tau beta was substantially decreased by global ischemia, isoproterenol, and CaCl2 but increased by reperfusion. The alpha was independent of inotropic interventions but fell significantly during ischemia and was increased by reperfusion.