THE INFLUENCE OF THE DURATION OF ISOFLURANE ANESTHESIA ON NEUROMUSCULAR EFFECTS OF MIVACURIUM

Background: The pharmacodynamic profile of muscle relaxants is usually changed by volatile anaesthetics. These changes seem to be time-depen dent, 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 ana esthesia. Forty-five children 1-10 years of age were randomized to rec eive 1.5% end-tidal concentration of isoflurane in N2O/O-2 for 10 or 3 0 min (groups Iso-10 and Iso-30, respectively) or to receive nitrous o xide in oxygen for 10 min (Group N2O) before 0.2 mg . kg(-1) of mivacu rium was given. Neuromuscular response was recorded by adductor pollic is electromyogram. Results: The onset time of mivacurium was shorter i n 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 w as significantly longer than in groups Iso-10 and N2O (P<0.0001). Grou ps Iso-10 and N2O did not differ from each other. Conclusion: Our resu lts indicate that the duration of a constant concentration of isoflura ne anaesthesia influences significantly the pharmacodynamics of mivacu rium. The duration of a volatile anaesthesia is critical when potentia tion of NMB is evaluated or compared in neuromuscular studies. (C) Act a Anaesthesiologica Scandinavica 41 (1997).