Can Conventional Peripheral Nerve Stimulators Induce Direct Muscle Depolarization?

In Response: We thank Dr. Kopman for his thoughtful comments on our article. He expresses quite nicely the differences between the excitability of nerve and muscle by comparing their strength-duration curves. These curves show the current (y-axis) that must be applied for a given stimulus duration (x-axis) to depolarize either nerve or muscle. The figure shows that, irrespective of duration of stimulus applied, muscle requires a larger current than nerve to be depolarized, and the difference is approximately one order of magnitude. However, this difference between nerve and muscle is greater for short stimuli. In other words, the chronaxie, which is the stimulus duration required to elicit a response at twice the rheobase, is greater for muscle than nerve. Dr. Kopman suggests correctly that increasing stimulus duration could decrease the difference in excitability between nerve and muscle (1). However, we think that an increase in stimulus duration from 0.2 to 0.3 ms is unlikely to affect the likelihood of direct muscle stimulation. Dr. Kopman did not accurately reproduce Mortimer's (2) Figure. The figure should have been drawn with a logarithmic x-axis. As shown in the redrawn figure (Fig. 1), the 0.2-ms and 0.3-ms durations are both in the range where the excitability gap between nerve and muscle is wide. The chronaxie is at least 10 times less in nerve (1 ms) than in muscle (>10 ms).Figure 1.: Redrawn figure from Mortimer 2). The difference between currents required to stimulate nerve and muscle with pulse durations of 0.2 and 0.3 ms is indicated.There are few data in the peer-reviewed literature about strength-duration curves in paralyzed muscle. In 1974, paralyzed muscle of cats was stimulated at supramaximal current for nerve stimulation. The resulting contraction was 3.5%–7% of that obtained in non paralyzed muscle; the smaller number was obtained with shorter duration stimulation (3). These data were obtained with needle electrodes, and the use of surface electrodes might produce different results. Recently, the differential response to short and long pulses was used in the diagnosis of denervation, as denervated muscle responds only to long (1 ms to >20 ms) pulses (4). However, the difference in excitability between nerve and muscle is not infinite, and direct muscle stimulation may occur in practice with stimulation at the wrist or at the hand. In our study, (5) we had evidence of direct muscle stimulation with stimulation at the wrist in two patients. In practice, such an occurrence can be detected by 1) absence of fade, 2) small amplitude of the twitch, and 3) disappearance of the response when the location of stimulating electrodes is changed (Fig. 2).Figure 2.: Original tracings from a patient who showed direct muscle stimulation with electrodes at the wrist but not at the hand. The dose of rocuronium was 0.6 mg/kg. Direct muscle stimulation in the presence of a neuromuscular blocking drug can be recognized by the small amplitude of response (in this case 3%), and lack of fade. The response disappeared when stimulating electrodes were applied elsewhere. Only two such occurrences were observed in our study, with wrist stimulation in both cases.Marie-Eve Nepveu, MD François Donati, PhD, MD, FRCPC Louis-Philippe Fortier, MSc, MD, FRCPC Department of Anesthesia Université de Montréal Hôpital Maisonneuve-Rosemont Montréal, Québec, Canada [email protected]

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