Half-logistic time constant: A more reliable lusitropic index than monoexponential time constant regardless of temperature in canine left ventricle

Temperature changes influence cardiac diastolic function. The monoexponential time constant (τ_E), which is a conventional lusitropic index of the rate of left ventricular (LV) pressure fall, increases with cooling and decreases with warming. We have proposed that a half-logistic time constant (τ_L) is a better lusitropic index than τ_E at normothermia. In the present study, we investigated whether τ_L can remain a superior measure as temperature varies. The isovolumic relaxation LV pressure curves from the minimum of the first time derivative of LV pressure (dP/dt_min) to the LV end-diastolic pressure were analyzed at 30, 33, 36, 38, and 40 °C in excised, cross-circulated canine hearts. τ_L and τ_E were evaluated by curve-fitting using the least squares method and applying the half-logistic equation, P(t) = PA/[1 + exp(t/τ_L)] + P_B, and the monoexponential equation, P(t) = P0exp(-t/τ_E) + P∞. Both τ_L and τ_E increased significantly with decreasing temperature and decreased with increasing temperature. The half-logistic correlation coefficient (r) values were significantly higher than the monoexponential r values at the 5 above-mentioned temperatures. This implies that the superiority of the goodness of the half-logistic fit is not temperature dependent. The half-logistic model characterizes the amplitude and time course of LV pressure fall more reliably than the monoexponential model. Hence, we concluded that τ_L is a more useful lusitropic index regardless of temperature.