Dose-response studies of the interaction between mivacurium and suxamethonium

We have determined the effect of pretreatment with mivacurium on the potency of suxamethonium and the effect of prior administration of suxamethonium on the potency of mivacurium. We studied 100 ASA I or II patients during thiopentone-fentanyl-nitrous oxide-isoflurane anaesthesia. Neuromuscular block was recorded as the evoked thenar mechanomyographic response to train-of-four stimulation of the ulnar nerve (2 Hz at 12-s intervals). Single dose-response curves were determined by probit analysis. Pretreatment with mivacurium had a marked antagonistic effect on the development of subsequent depolarizing block produced by suxamethonium. The dose-response curves for suxamethonium alone and after pretreatment with mivacurium did not deviate from parallelism, but those constructed after mivacurium were shifted significantly to the right (P < 0.0001). The calculated doses producing 50% depression of T1 (ED50) were 86 (95% confidence intervals 83-88) and 217 (208-225) micrograms kg-1 for suxamethonium alone and after mivacurium, respectively. This study also demonstrated that prior administration of suxamethonium did not appear to influence either the slope of the regression lines or the potency of mivacurium. Combining the results of this study with a previous study (mivacurium ED50 = 20.8 (20.3-21.3) micrograms kg-1 during isoflurane-nitrous oxide anaesthesia), we suggest that the potency of mivacurium did not differ from that observed after suxamethonium (17.4 (16.9-17.9) micrograms kg-1).