Letters to the Editor: Letters & Announcements
We appreciate this letter for it gives us an opportunity to clarify the goals and results of our study.
Our work simply intended to determine whether a supraclavicular block could be injected at the initial “seeking current” without having a negative impact on success rate. As we mentioned in our study, it was not our intention to compare 0.9 mA with 0.5 mA as “minimum stimulating currents.” That work, interesting as it might be, has not been performed yet.
The authors think that our results are “intriguing since they disagree with those found by other investigators,” giving two references to back such a statement. However, those two papers never investigated whether a supraclavicular block, or any other block for that matter, can or cannot be injected at a higher current than other techniques.
It has never been our practice to determine the current at which our elicited response disappears. Instead, we are content that for most of our techniques, our sought response is still present at 0.5 mA. One of the exceptions is the supraclavicular block that we now inject routinely at around 0.85 mA, the same current at which we initiate it. When we used to inject the supraclavicular block at 0.5 mA, we did not know exactly where the tip of the needle was at the moment of injection nor do we know it now when we inject it at a higher current. To avoid intraneural injection we still rely on light sedation, small injection pressure, and meticulous technique. As we also mentioned in our study, we do not advocate raising the output at which the injection is initiated with any other technique, at least not at the present time.
We argue that single injection plexus anesthesia (the anesthesia of more than one nerve by a single injectate) involves more than just proximity to a nerve structure, but also the identification of a point from where the solution is able to evenly reach the plexus. We believe that every plexus technique has such a point from where the spread is more satisfactory. Our experience with the supraclavicular block clearly demonstrates that an injection performed in front of the lower trunk at 0.9 mA is definitely better than a similar injection performed in front of the upper trunk at 0.5 mA. Having now performed close to 1,000 supraclavicular blocks injected immediately after finding a response from the lower trunk has convinced us that this is a safe and practical approach that saves time and needle manipulations, and, from our perspective, this is the main value of our work.
In terms of stimulus duration, it is known that the Stimuplex DIG provides a fixed pulse width of 0.1 ms. A short pulse like this is considered the most suitable for nerve localization, and the clinical utility of changing the pulse width is not yet well defined (1).
The magnitude of the response is indeed inversely proportional to the needle tip-nerve distance, thus there is not a “spelling error” as suggested.
Finally, we believe that the techniques of regional anesthesia performed with a nerve stimulator are still evolving, and those who practice them are still discovering the specifics that differentiate one technique from another.
Carlo D. Franco, MD
Department of Anesthesiology and Pain Management
John H. Stroger Jr. Hospital of Cook County
1. Hadzic A. Peripheral nerve stimulators and nerve stimulation. In: Hadzic A, Vloka JD, eds. Peripheral nerve blocks principles and practice. New York: McGraw-Hill, 2004:43–49.