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 r emains stable when 0.1 Hz single twitch stimulation is started after stabil ization 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 afte r the recovery from neuromuscular block. Methods. Measurements were perform ed in thirty patients under general anaesthesia. Twitch forces were first a llowed to stabilize after allowing the preload to drift to its resting tens ion at a constant degree of thumb abduction. Three groups of ten patients t hen each received either vecuronium (2, 4, 8, 16 and 32 mu g/kg(-1), succes sively 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). Measurement s were continued until twitch forces had recovered from neuromuscular block and were stable. Results. Twitch forces stabilized at 114% (sd = 8.9) of t he initial value after 10.9 (6.1) min of stimulation. Increase in twitch fo rce before the onset of neuromuscular block was seen in two patients receiv ing vecuronium and in two patients receiving d-tubocurarine. Increase in tw itch force after recovery from neuromuscular block was seen in all patients receiving suxamethonium. Conclusions. Twitch forces may increase when stim ulation is started after stabilization of thumb preload at a constant degre e of thumb abduction. In some patients twitch forces may increase before th e onset of neuromuscular block with vecuronium or d-tubocurarine; twitch fo rces increase after recovery from suxamethonium.