The influence of the duration of isoflurane anaesthesia on neuromuscular effects of mivacurium

BACKGROUND: The pharmacodynamic profile of muscle relaxants is usually changed by volatile anaesthetics. These changes seem to be time-dependent, even though few data are available to substantiate this. METHODS: We studied neuromuscular effects of a single dose of mivacurium (0.2 mg.kg-1) during short and intermediate duration of isoflurane anaesthesia. Forty-five children 1-10 years of age were randomized to receive 1.5% end-tidal concentration of isoflurane in N2O/O2 for 10 or 30 min (groups Iso-10 and Iso-30, respectively) or to receive nitrous oxide in oxygen for 10 min (Group N2O) before 0.2 mg.kg-1 of mivacurium was given. Neuromuscular response was recorded by adductor pollicis electromyogram. RESULTS: The onset time of mivacurium was shorter in Group Iso-30, 1.7 (1.0-2.3) min than in Group Iso-10, 2.3 (1.7-3.3) min or Group N2O, 2.3 (1.7-3.3) min (median with 10-90% percentiles) (P < 0.05). In Group Iso-30 the recovery time of the first EMG response was significantly longer than in groups Iso-10 and N2O (P < 0.0001). Groups Iso-10 and N2O did not differ from each other. CONCLUSION: Our results indicate that the duration of a constant concentration of isoflurane anaesthesia influences significantly the pharmacodynamics of mivacurium. The duration of a volatile anaesthesia is critical when potentiation of NMB is evaluated or compared in neuromuscular studies.