Re-establishment of paralysis using mivacurium following apparent full recovery from mivacurium-induced neuromuscular block.

Abstract

Recent published data suggest that despite apparently satisfactory recovery from nondepolarising block (train-of-four ratios in excess of 0.90), even very small doses of additional relaxant may re-establish significant paralysis. We sought to verify this observation and quantify its magnitude. Twelve adult patients were studied under nitrous oxide-propofol-opioid anaesthesia and neuromuscular block was monitored electromyographically. Train-of-four stimuli were delivered to the ulnar nerve every 20 s throughout the period of observation. After baseline stabilisation, an initial bolus of mivacurium 25 micrograms.kg-1 was administered and the twitch depression noted. When the twitch was stable for two consecutive stimuli, a second bolus, calculated to produce approximately 90% twitch depression, was administered. Recovery was then allowed to proceed spontaneously until the train-of-four ratio reached 0.95. At that time a second 25 micrograms.kg-1 dose was administered and the effect on twitch height recorded. Using the slope for the log-dose/logit dose-response relationship of mivacurium (5.5), it was possible to estimate any change in the ED50 of mivacurium. The control ED50 of mivacurium (calculated from the initial dose of mivacurium) averaged 43 micrograms.kg-1. When the same dose of drug was given at 95% recovery of the train-of-four ratio, the ED50 was reduced to 19 micrograms.kg-1 (p < 0.0001). Hence, there remains a considerable reduction in the neuromuscular margin of safety even at a train-of-four ratio of 0.95.