Interaction between mivacurium and pancuronium: Impact of the order of administration

Introduction: Potentiation of mivacurium by low-dose pancuronium is mostly due to an inhibition of plasma butyryl cholinesterase (BchE) resulting in a decreased rate of hydrolysis of mivacurium. Nevertheless, an interaction at the receptor site could not be ruled out. By changing the order of the muscle relaxant injections, we may lessen the pharmacokinetic interaction and assess the impact at the acetylcholine receptor level. Methods: Twenty patients scheduled for general anesthesia with propofol and fentanyl, and isoflurane were randomized into two groups receiving, mivacurium 100 μg kg^-1 followed by pancuronium 15 μg kg^-1 (group 1) or pancuronium 15 μg kg^-1 followed by mivacurium 100 μg kg^-1 (group 2). BchE before and after injection of each relaxant was measured. Neuromuscular block was assessed with a force transducer at the adductor pollicis measuring the elicited twitch to ulnar nerve stimulation. Results: The neuromuscular block was greater when pancuronium was administered before mivacurium (100% versus 96±3%; P<0.05). Times to recovery of the elicited twitch response to 25% and 75% of control value were increased by 100% (P<0.05). After pancuronium, decreases in BchE of 11% and 14% in groups 1 and 2 were observed, respectively. Conclusion: Interaction between mivacurium and low dose pancuronium is significant only when mivacurium is injected after pancuronium.