Ability of the Lapicque and Blair strength-duration curves to fit experimentally obtained data from the dog heart

The ability of the empirical Lapicque and theoretically derived Blair expressions for excitation to fit experimentally obtained threshold current values to evoke a ventricular extrasystole using rectangular-wave stimuli applied to the dog heart is determined. The data points were fitted to both expressions, and the ability of each to predict the measured values were determined. The Levenberg-Marquardt (L-M) algorithm was used to fit the Lapicque and Blair expressions. The Lapicque data were also fitted to the linear charge-duration expression of Weiss (W). The ratio of the predicted to measured current was 0.95 (L-M) and 1.06 (W) for the Lapicque and 0.92 (L-M) for the Blair expression. Thus, there appears to be little difference between the ability of the expressions to fit the same experimentally obtained data. The L-M/Lapicque fit is best for the short-duration range; the W/Lapicque fit overestimates in the short duration range and underestimates near chronaxie. The L-M/Blair fit is best for the short-duration range and poor for durations near the membrane time constant.