Effects of succinylcholine at the masseter and adductor pollicis muscles in adults

The effect of succinylcholine on baseline tension and evoked twitch height was measured at the masseter and the adductor pollicis muscles in ten adults undergoing elective surgery. During thiopental-nitrous oxide-enflurane (end-tidal concentration less than 0.25%) anesthesia, supramaximal stimulation was applied to both the ulnar nerve and the nerve to the masseter. Baseline tension and the isometric force of contraction were measured at the jaw and the thumb. Cumulative dose-response relationships were obtained by giving succinylcholine in incremental doses (initial dose, 0.10 mg/kg, followed by 0.05- or 0.1-mg/kg increments, depending on response). An infusion was started after the maximum effect of the first dose, at a rate adjusted to compensate for the rapid metabolism of the drug. There was no difference between the sensitivity of the masseter and the adductor pollicis. The ED50 values were (mean +/- SEM) 0.11 +/- 0.01 mg/kg at both muscles. The ED90 values were 0.17 +/- 0.02 mg/kg at the masseter and 0.16 +/- 0.01 mg/kg at the adductor pollicis. Onset of action was more rapid at the masseter. In six of the ten patients, baseline tension at the jaw was found to increase by a mean of 80 +/- 24 g (range 25-188 g). It is concluded that in adults, masseter neuromuscular blockade can be achieved with succinylcholine doses approximately equal to those required to block the adductor pollicis. In addition, the drug may cause increased tension in the muscles of the jaw.