Multivariate regression analysis of the influence of aortic pressure, end-diastolic pressure, and heart rate on left ventricular relaxation in isolated ejecting rat and guinea pig hearts.

The effect of moderate changes of peak aortic and end-diastolic pressure and of heart rate on the left ventricular relaxation of isolated working rat and guinea pig hearts was investigated by multivariate regression analysis. Each of these three independent variables was set to three different levels, yielding 27 sets of data by combination in each experiment. Relaxation was quantified by the maximum pressure fall velocity (min LVdP/dt) and by the time constant tau of left ventricular isovolumic pressure fall. tau was obtained by fitting that pressure curve to the three-parametric exponential regression model p(t) = P infinity + (P0-P infinity) exp (-t/tau) and to an extended four-parametric model p(t) = P infinity + (P0-P infinity) exp [(-t/(tau 0 + r tau t)]. The influence of the three independent variables on min LVdP/dt, tau, and r tau was checked by analysis of variance and quantified by standardized regression coefficients obtained by trivariate regression analysis. A positive dependence of min LVdP/dt on precedent maximum pressure and of the three-dimensionally (but not four-dimensionally) estimated tau on beat interval length are the only unequivocally significant effects of the investigated hemodynamic changes on the lusitropic parameters min LVdP/dt and tau. Principal species differences do not occur. It is concluded that considerable lusitropic effects seen especially in pharmacologic studies must be attributed to intrinsic effects of the substance rather than to hemodynamic changes caused by the substance if the former remain moderate. The four-dimensionally calculated tau is slightly more often found to be independent of the investigated hemodynamic parameters than the usual three-dimensional estimate. This may indicate higher reliability of the four-dimensional regression model of isovolumic pressure fall.