Synergism between mivacurium and pancuronium in adults

Mivacurium could be a useful agent as a final dose of a muscle relaxant following pancuronium if only additivity exists between these agents. We examined the interaction between mivacurium and pancuronium in 70 patients (ASA I-II) during propofol-alfentanil-N2O-O2 anaesthesia. Neuromuscular function was monitored by adductor pollicis EMG. Firstly we established dose-response curves for mivacurium and pancuronium. Thereafter, 20 patients received a combination of 0.5 times the ED50 doses of mivacurium and pancuronium (cMP) determined in the first part of this study. Patients were randomized to receive the cMP to the same IV-line (n = 10) or to two separate IV-lines in opposite hands (n = 10). ED50 values for mivacurium and pancuronium were 57.7 and 37.1 micrograms kg-1, respectively. Maximal neuromuscular block following the cMP was 91.8 +/- 5.0% (mean +/- SD). This was highly significantly different from the estimated 50% NMB if only additivity exists between mivacurium and pancuronium (P = 0.0001). After the cMP, the 25-75% recovery time was 9.4 +/- 1.3 min and the time to train-of-four ratio of 0.70 was 35.8 +/- 5.4 min. There was no statistical difference in any recorded neuromuscular parameter between the two subgroups receiving mivacurium and pancuronium to the same or to opposite hands (P > 0.40). We conclude that a significant synergism exists between mivacurium and pancuronium which may indicate that mivacurium does not produce a short-acting NMB if given after pancuronium. We do not recommend using mivacurium together with pancuronium.