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LETTERS TO THE EDITOR

Excessive Voltage Output?

University of California, San Francisco, San Francisco General Hospital, San Francisco, CA

To the Editor:

I read with interest the recent letter of Hadzic and Vloka regarding the voltage output of peripheral nerve stimulators (1). They express concern over the excessive voltage output of the specific model they studied, the Stimuplex^®-DIG (B. Braun Medical Inc., Bethlehem, PA). However, their information is at times misleading, even incorrect. First, their measurements of voltage output may be in error. In my own tests of new, used, and older models of the Stimuplex^®-DIG, I have found that the maximum voltage output under large loads (>6 kohms) is limited to 32 volts. This observation is consistent with the specifications given by the manufacturer. Therefore, if the voltage output of their unit can indeed reach 70 volts, it should be returned immediately to the manufacturer for inspection. Furthermore, when Stimuplex^®-DIG units reach maximum voltage output, the user is made aware by flashing of the current display. This property makes it possible to estimate the net impedance when it exceeds 6 kohms. Ironically, the maximum voltage output of the Stimuplex^®-DIG is less than that of other commercially available peripheral nerve stimulators.

Second, the authors postulate that excessive voltage output by peripheral nerve stimulators can result in burning pain from local heat production. Aside from injection of acidic local anesthetic solutions, I have not observed patients complain of burning pain during electrolocation procedures. Even at maximum output (32 volts and 5 milliamperes), the power of the Stimuplex^®-DIG 100-µs impulse (160 milliwatts) is quite small. Although potentially capable of activating nociceptive nerve fibers, it should not result in tissue injury by thermal or electrochemical means. The greatest risk of low-voltage direct current burns relates to prolonged and continuous alkali production at a fixed cathode terminal (2–4). Long interpulse intervals of 500–1000 ms will allow dissipation of heat and voltage between pulses. Finally, the authors refer to the anode terminal as ground. This is incorrect, as care is taken to isolate patients from ground during surgery to avoid electrical hazards.

References

1. Hadzic A, Vloka JD. Peripheral nerve stimulators for regional anesthesia can generate excessive voltage output with poor ground connection. Anesth Analg 2000; 91: 1306.[Free Full Text]
2. Leeming MN, Ray C Jr, Howland WS. Low-voltage, direct-current burns. JAMA 1970; 214: 1681–4.[Abstract/Free Full Text]
3. Etchin A, Mamet Y. Electrochemical skin burn after transcutaneous electronerve analgesia. Anesth Analg 1982; 61: 801–2.
4. Cooper JB, DeCesare R, D’Ambra MN. An engineering critical incident: direct current burn from a neuromuscular stimulator. Anesthesiology 1990; 73: 168–72.[Medline]

Admir Hadi, MD, PhD, and Jerry D. Vloka, MD, PhD

Department of Anesthesiology, St. Luke’s-Roosevelt Hospital Center, New York, NY

In Response:

We thank Dr. Gray for his reply to our report and for raising some important questions regarding nerve stimulators for regional anesthesia.

Methods we used to measure voltage outputs are described in our letter (1) and have been previously used to successfully alert our colleagues and manufacturers to problems with nerve stimulators (2). The LCD current display flashes when the set current is not delivered, not when the maximum voltage output is reached. This is logical, as clinicians monitor the current (mA) rather than the voltage (volts) output during nerve stimulator-assisted nerve blockade.

In our letter we attempted to explain the etiology of the discomfort occasionally seen with nerve stimulation; the actual energy delivered by nerve stimulators for regional anesthesia is small and should not result in tissue injury. This discomfort occurs primarily with deeper nerve blocks where a needle traverses multiple tissue layers (infraclavicular, lumbar plexus, sciatic, and lateral popliteal blocks). Because many anesthesiologists perform only superficial blocks (interscalene, axillary, or femoral), it is possible that the described phenomenon is infrequently encountered by majority of our colleagues (3).

We agree with Dr. Gray that the term "grounding electrode" is not correct. Nevertheless, this term remains commonly used in the literature (4) and it is better understood by many clinicians than "anode" or "return" electrode. Finally, Stimuplex-Dig was taken as an unfortunate example in our letter, as this is an ergonomic and accurate unit that is being used in many centers worldwide. The title of our letter, however, clearly implies that most peripheral nerve stimulators for regional anesthesia are capable of generating unnecessarily high voltages. An indicator of abnormally high impedance would be clinically more useful instead.

References

1. Hadi A, Vloka JD. Peripheral nerve stimulators for regional anesthesia can generate excessive voltage output with poor ground connection. Anest Analg 2000; 91: 1306.
2. Hadi A, Vloka JD, Koorn R. Effects of the auditory volume control knob on the stimulus amplitude display of the DualStim/Deluxe model NS-2CA/DX peripheral nerve stimulator. Anesthesiology 1997; 87: 714–5.[Medline]
3. Hadi A, Vloka JD, Kuroda MM, et al. The practice of peripheral nerve blocks in the United States: a national survey. Reg Anesth Pain Med 1998; 23: 241–6.[Web of Science][Medline]
4. Urmey WE. Femoral nerve block for the management of postoperative pain: techniques in regional anesthesia and pain management. 1997; 1: 88–92.

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