Increase in twitch force of the adductor pollicis muscle with stabilized preload at constant thumb abduction before and after administration of muscle relaxant

Objective. To determine whether the twitch force of the adductor pollicis remains stable when 0.1 Hz single twitch stimulation is started after stabilization of the thumb preload at a constant degree of thumb abduction; also to study any possible increase in twitch force before the onset of and after the recovery from neuromuscular block. Methods. Measurements were performed in thirty patients under general anaesthesia. Twitch forces were first allowed to stabilize after allowing the preload to drift to its resting tension at a constant degree of thumb abduction. Three groups of ten patients then each received either vecuronium (2, 4, 8, 16 and 32 mu g/kg(-1), successively at intervals of 2 min), d-tubocurarine (5, 10, 20, 40 and 80 mu g/kg(-1), successively at intervals of 2 min), or suxamethonium (0.025, 0.05, 0.1, 0.2 and 0.4 mg/kg(-1), successively at intervals of 2 min). Measurements were continued until twitch forces had recovered from neuromuscular block and were stable. Results. Twitch forces stabilized at 114% (sd = 8.9) of the initial value after 10.9 (6.1) min of stimulation. Increase in twitch force before the onset of neuromuscular block was seen in two patients receiving vecuronium and in two patients receiving d-tubocurarine. Increase in twitch force after recovery from neuromuscular block was seen in all patients receiving suxamethonium. Conclusions. Twitch forces may increase when stimulation is started after stabilization of thumb preload at a constant degree of thumb abduction. In some patients twitch forces may increase before the onset of neuromuscular block with vecuronium or d-tubocurarine; twitch forces increase after recovery from suxamethonium.