Chronaxie calculated from current-duration and voltage-duration data.
To determine the rheobase and the chronaxie of excitable cells from strength-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 values estimated from voltage-duration measurements will differ from the proper values as determined from current-duration measurements. To allow a comparison of chronaxie values obtained by the two stimulation methods, voltage-duration curves were measured in human subjects with a deep brain stimulation electrode implanted, while the current and the load impedance of the stimulation circuit were determined in vitro as a function of both stimulation voltage and pulsewidth. Chronaxie values calculated from voltage-duration data were shown to be 30-40% below those estimated from current-duration data. 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 data into current-duration data, thereby reducing the error in the calculated chronaxie values to approximately 6%. For this purpose voltage-duration data have to be measured for pulses up to 10-20 times the expected chronaxie.
Holsheimer
J
J
Department of Electrical Engineering, Institute for Biomedical Technology, University of Twente, PO Box 217, NL-7500 AE, Enschede, The Netherlands. j.holsheimer@el.utwente.nl
Dijkstra
E A
EA
Demeulemeester
H
H
Nuttin
B
B
D001921:Brain / Q000166:cytology / Q000502:physiology*; D002907:Chronaxy; D017097:Electric Impedance; D004558:Electric Stimulation / Q000295:instrumentation / Q000379:methods*; D004599:Electric Stimulation Therapy / Q000379:methods*; D004567:Electrodes, Implanted; D006801:Humans; D009474:Neurons / Q000502:physiology; D010300:Parkinson Disease / Q000150:complications / Q000628:therapy; D013997:Time Factors; D014202:Tremor / Q000209:etiology / Q000628:therapy