Logistic Time Constant of Isovolumic Relaxation Pressure-Time Curve in the Canine Left Ventricle

Better Alternative to Exponential Time Constant

Matsubara, Hiromi MD
Takaki, Miyako PhD
Yasuhara, Shingo MD
Araki, Junichi MD
Suga, Hiroyuki MD, DMSc.

Received February 7, 1995; revision received April 19, 1995; accepted May 22, 1995.
From the Department of Physiology II, Okayama University Medical School, Okayama, Japan.
Correspondence to Hiromi Matsubara, MD, Department of Physiology II, Okayama University Medical School, 2-5-1 Shikatacho, Okayama City, Okayama 700, Japan.

Circulation 92(8):p 2318-2326, October 15, 1995.

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. (Circulation. 1995;92:2318-2326.)

Key Words

ventricles, mechanics, diastole, diagnosis.