AJP - Heart and Circulatory Physiology, Vol 269, Issue 2 393-H406, Copyright © 1995 by American Physiological Society
Double-exponential curve fitting of isometric relaxation: a new measure for myocardial lusitropism
K. Tamiya, T. Beppu and K. Ishihara
Department of Surgical Science, Tokyo Women's Medical College, Japan.
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.
