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 undermine 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
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 This is in contrast with our unpublished human volunteer data (fig. 3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... alternatively a percentage change in EI from the extraneural compartment 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 emphasize that it was never our intention to relate the absolute mechanism of the complex circuit. Instead, our intent was to examine the practical interpretation of impedance changes in a commercially available nerve stimulator in a pilot study carried out in a porcine model. The true value of this study lies in the detection of impedance changes that signify intraneural needle placement. Depending on the species, the noted change in impedance may be an increase or a decrease, and it may even be transient in nature. As it is said in a famous Chinese proverb attributed to Deng Xiaoping for his pragmatic policies, “It doesn’t matter if a cat is black or white; as long as it can catch mice, it’s a good cat.”
Ban C. H. Tsui, M.D., M.S.C., F.R.C.P.(C.), *
Derek Dillane, M.B., B.Ch.
*University of Alberta Hospital, Edmonton, Alberta, Canada. email@example.com
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. Tsui BC, Wagner A, Finucane B: Electrophysiologic effect of injectates on peripheral nerve stimulation. Reg Anesth Pain Med 2004; 29:189–93
© 2009 American Society of Anesthesiologists, Inc.