LOGISTIC TIME CONSTANT OF ISOVOLUMIC RELAXATION PRESSURE-TIME CURVE IN THE CANINE LEFT-VENTRICLE - BETTER ALTERNATIVE TO EXPONENTIAL TIME CONSTANT

Background The time constant of left ventricular (LV) relaxation derived from a monoexponential model has been widely used as an index of LV relaxation rate or lusitropism, although this model has several well-recognized problems. In the present study, we proposed a logistic model and derived a ''logistic'' time constant (T-L) as a better alternative to the conventional ''exponential'' time constant (T-E). Methods and Results A total of 189 beats (147 isovolumic and 42 ejecting beats) were investigated in seven canine excised cross-circulated heart preparations. We found that the logistic model fitted much more precisely all the observed LV isovolumic relaxation pressure-time [P(t)] curves than the monoexponential model (P<.05). The logistic model also fitted well both the time curve of the first derivative of the observed P(t) (dP/dt) and the dP/dt-P(t) phase-plane curve. Like T-E, T-L indicated that volume loading depressed LV lusitropism and that increasing heart rate and ejection fraction augmented it. T-L was independent of the choice of cutoff point defining the end of isovolumic relaxation; T-E was dependent on that choice. Conclusions We conclude that the logistic model better fits LV isovolumic relaxation P(t) than the monoexponential model in the present heart preparation. We therefore propose T-L as a better alternative to T-E for evaluating LV lusitropism.