the resulting impedance was actually reduced, according to equation 3. Z =  R 2 + X C 2 (3) where X C is the capacitive reactance and is defined by: X = 1 2fC (4) where f is the frequency of the applied waveform. Dependent on the magnitude of the capacitive and resistive changes, the overall imped- ance decreased dramatically. This finding of Tsui et al., although it does not demonstrate in- creased impedance on intraneural needle tip placement, is interesting, because it shows that the complex system time constant changes on close approach to neural tissue when using externally applied electri- cal fields, an observation I have also made. 5 Also, individual time con- stants, equation 1, may be derived from the observed charging/decay voltage curve through additional mathematical methods (logarithmic peel- ing), and the time constants contributed by the nerve, the insulated needle, or the remaining tissue electrical path determined separately. 6 Philip C. Cory, M.D., St. James Healthcare, Butte, Montana. pcory@littleappletech.com References 1. Tsui BC, Pillay JJ, Chu KT, Dillane D: Electrical impedance to distinguish intraneural from extraneural needle placement in porcine nerves during direct exposure and ultrasound guidance. ANESTHESIOLOGY 2008; 109:479–83 2. Cooper MS, Miller JP, Fraser SE: Electrophoretic repatterning of charged cytoplasmic molecules within tissues coupled by gap junctions by externally applied electric fields. Dev Biol 1989; 132:179–88 3. Cory PC: inventor; Nervonix, Inc., assignee: Non-Invasive, Peripheral Nerve Mapping Device and Method of Use. U.S. patent 5 560 372. October 1, 1996 4. Prokhorov E, Llamas F, Morales-Sa ´nchez E, Gonza ´lez-Herna ´ndez J, Prokhorov A: In vivo impedance measurements on nerves and surrounding skeletal muscles in rats and human body. Med Biol Eng Comput 2002; 40:323–6 5. Cory PC: inventor; Nervonix, Inc., assignee: Nerve Stimulator and Method. U.S. patent 7 212 865. May 1, 2007 6. Rall W: Membrane potential transients and membrane time constant of motoneurons. Exp Neurol 1960; 2:503–32 (Accepted for publication January 15, 2009.) Anesthesiology 2009; 110:1194–5 Copyright © 2009, the American Society of Anesthesiologists, Inc. Lippincott Williams & Wilkins, Inc. In Reply:—We are very pleased that our report has stimulated the important comments made by Dr. Cory. We would like to take this opportunity to address the correspondent’s concerns and to clarify the simplified electrical circuit depicted in the original manuscript. 1 Using a clinically relevant low frequency (2 Hz) stimulation, we had hoped to detect any possible warning signs of intraneural needle placement by understanding how to interpret the displayed impedance from one of the common commercial stimulators. We were most encouraged to note the distinct impedance change displayed on the stimulator upon the needle entering the intraneural compartment. The specific concern expressed in the above letter relates to the proposed inaccuracy in the interpretation of the displayed impedance. From observations based on human data, Dr. Cory posits that the maximum voltage may not have been reached within such short pulse duration (0.1 ms). First of all, it is important to clarify that the simplified circuit of the original manuscript represents only the resistive portions of the circuit. Strictly speaking, an accurately depicted circuit would be much more complex and include the capacitance and inductance of the many tissue types (fig. 1). However, we believed such complex electrical circuitry may have distracted readers from the primary goal of the research. Despite this, there was no intention on our part to under- mine the research methodology. Specifically, the effect of capacitance in the porcine model on the dynamic time-dependent component of impedance is negligible when compared with that in humans. This is based on the observed rapid rise in the voltage–time curve, which has a maximum voltage plateau phase near 0.1 ms in a porcine model (fig. 2). 2 Therefore, the displayed impedance from the stimulator is less affected by an increase in pulse duration and is a reasonable approximation. This is in contrast with our unpublished human volunteer data (fig. 3). In humans, the voltage–time response curve takes longer to reach the maximum voltage plateau phase (2–2.5 ms). Therefore, the displayed impedance will change substantially, along with the prolonged pulse duration for extended periods. This is why we clearly pointed out the limitations of our investigation in the manuscript as “we anticipate that there may be substantial interspecies differences in EI... alter- natively a percentage change in EI from the extraneural compart- ment in humans indicative of intraneural placement would be of high clinical value.” This may rectify the confusion to which Dr. Cory refers, as he may have missed or was unaware of such interspecies differences. We thank the correspondent for his helpful comments, and we are grateful for the opportunity to clarify our results. We must also em- phasize that it was never our intention to relate the absolute mecha- nism of the complex circuit. Instead, our intent was to examine the Fig. 1. Schematic complex impedance resistance– capacitance equivalent circuit model. Fig. 2. Voltage–time curve in a porcine model. Example of the voltage response after applying 0.5 mA with a 0.2 ms pulse width via an 18-gauge insulated needle placed extraneurally. Adapted from Tsui et al., 2 with permission from Elsevier. 1194 CORRESPONDENCE Anesthesiology, V 110, No 5, May 2009 Downloaded from anesthesiology.pubs.asahq.org by guest on 05/23/2020