Dose-response studies of the interaction between mivacurium and suxamethonium

We have determined the effect of pretreatment with mivacuriumon the potency of suxamethonium and the effect of prior administrationof suxamethonium on the potency of mivacurium. We studied 100ASA I or II patients during thiopentone-fentanyl-nitrous oxide-isofluraneanaesthesia. Neuro-muscular block was recorded as the evokedthenar mechanomyographic response to train-of-four stimulationof the ulnar nerve (2 Hz at 12-s intervals). Single dose-responsecurves were determined by probit analysis. Pretreatment withmivacurium had a marked antagonistic effect on the developmentof subsequent depolarizing block produced by suxamethonium.The dose-response curves for suxamethonium alone and after pretreatmentwith mivacurium did not deviate from parallelism, but thoseconstructed after mivacurium were shifted significantly to theright (P < 0.0001). The calculated doses producing 50% depressionof T1 (ED₅₀) were 86 (95% confidence intervals 83–88)and 217 (208–225) µg kg^–1 for suxamethoniumalone and after mivacurium, respectively. This study also demonstratedthat prior administration of suxamethonium did not appear toinfluence either the slope of the regression lines or the potencyof mivacurium. Combining the results of this study with a previousstudy (mivacurium ED₅₀ = 20.8 (20.3–21.3) µg kg^–1during isoflurane-nitrous oxide anaesthesia), we suggest thatthe potency of mivacurium did not differ from that observedafter suxamethonium (17.4 (16.9–17.9) µg kg^–1).(Br.J. Anaesth. 1995; 74: 26–30) ^*Present address: Department of Anaesthesia, King Faisal University,Saudi Arabia