Chronaxie calculated from current-duration and voltage-duration data

To determine the rheobase and the chronaxie of excitable cells from strengt h-duration curves both constant-current pulses and constant-voltage pulses are applied. Since the complex impedance of the electrode-tissue interface varies with both the pulsewidth and the stimulation voltage, chronaxie valu es estimated from voltage-duration measurements will differ from the proper values as determined from current-duration measurements. To allow a compar ison of chronaxie values obtained by the two stimulation methods, voltage-d uration curves were measured in human subjects with a deep brain stimulatio n electrode implanted, while the current and the load impedance of the stim ulation circuit were determined in vitro as a function of both stimulation voltage and pulsewidth. Chronaxie values calculated from voltage-duration d ata were shown to be 30-40% below those estimated from current-duration dat a. It was also shown that in the normal range of stimulation amplitudes (up to 7 V) the load impedance increases almost linearly with the pulsewidth. This result led us to present a simple method to convert voltage-duration d ata into current-duration data, thereby reducing the error in the calculate d chronaxie values to approximate to 6%. For this purpose voltage-duration data have to be measured for pulses up to 10-20 times the expected chronaxi e. (C) 2000 Elsevier Science B.V. All rights reserved.