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Department of Anesthesiology; New York Medical College; Valhalla, NY; akopman{at}nyc.rr.com; akopman{at}gmail.com
To the Editor:
I read with interest the recent paper by Nepveu et al. (1). In their study the authors show rather convincingly that, when using surface electrodes, conventional peripheral nerve stimulators (currents of 
60 mA; pulse durations of 0.20 ms) are unlikely to produce direct muscle stimulation. The authors suggest that selective indirect muscle stimulation is the rule even when the stimulating electrodes directly overlay the muscle because a "high voltage or current is required to stimulate muscle directly." They then state that "direct stimulation of the adductor pollicis may require as much as 1,000 volts."
Although the authors' data are persuasive, their accompanying explanation is less than complete. First, delivered voltage may have little to do with the efficacy of a depolarizing stimulus. Although a minimum current is required to produce depolarization (the rheobase), it is actually the product of pulse duration and current (electrical charge expressed in micro-coulombs) that is the important parameter. For any given nerve or muscle, a strength-duration curve can be constructed that demonstrates this relationship (Fig. 1, adapted from Mortimer) (2).
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It is true (Fig. 1) that direct muscle stimulation may require a stimulus on an order of magnitude greater than that which will initiate nerve depolarization. However, there appears to be an additional and crucial difference in the stimuli required to initiate nerve versus direct muscle depolarization. As illustrated in Figure 1 (if data from the cat can be applied to humans), a pulse duration of <0.30 msec is unlikely to directly initiate muscle depolarization even at rather high stimulating currents.
The Train-of-Four-Watch stimulator employed in the authors' study was used in the 0.20 ms mode. However, this unit has the capability of delivering pulses of 0.30 ms. Thus it is possible that the authors' observations might have been different if a longer pulse duration had been utilized. This possibility is worthy of further study.
References
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