Starling-Effect-Independent Lusitropism Index in Canine Left Ventricle: Logistic Time Constant

The logistic time constant (τL) has been proposed as a better index of the rate of left ventricular (LV) relaxation or lusitropism than the conventional monoexponential time constant (τE). However, whether and how the Frank-Starling effect influences τL remains to be elucidated. We compared the effect of LV volume (LVV) loading on both logistic and monoexponential fittings. The isovolumic LV relaxation pressure curves from the maximum negative time derivative of pressure (-dP/dtmax) were analyzed at 3 different end-points at 4 LVVs of 10, 12, 14, and 16 mL in 8 excised, cross-circulated canine hearts. We found that the logistic fitting was superior to the monoexponential fitting at all LVVs and end-points. LVV loading did not affect τL but affected τE slightly. Although the advancing end-point increased both τL and τE, the increases were significantly smaller for τL than for τE at all LVVs. Moreover, the changes in both the amplitude constants and nonzero asymptotes with the advancing end-point were significantly smaller for the logistic fitting than for the monoexponential fitting. We conclude that τL served as a more reliable index of lusitropism that is independent of the change in LVV loading or the Frank-Starling effect.