Pharmacodynamics and the plasma concentration of mivacurium during spontaneous recovery and neostigmine-facilitated recovery

Background: The authors examined the plasma concentrations of the isomers o f mivacurium and its pharmacodynamics during spontaneous and neostigmine-fa cilitated recovery after a mivacurium infusion. Methods: Sixteen patients receiving nitrous oxide-opioid anesthesia receive d 0.25 mg/kg mivacurium. Patient response to neuromuscular stimulation was determined using a mechanomyograph. Once T1 had recovered to 25% of its bas eline height, a mivacurium infusion was begun and adjusted to maintain 95-9 9% neuromuscular block. The infusion was discontinued after 90 min and musc le strength allowed to recover either spontaneously or after neostigmine/gl ycopyrrolate (0.05/0.01 mg/kg). Plasma concentrations of the isomers of miv acurium after discontinuation of the infusion were determined using an HPLC assay. Differences between the groups were determined using a one-way anal ysis of variance with a Bonferroni-corrected t test or Student t test as ap propriate. P less than or equal to 0.05 was considered significant. Results: Differences in the times for recovery to a train-of-four ratio of 70% did not achieve statistical significance (mean +/- SD, 13.3 +/- 6.0 us. 16.3 +/- 2.5 min for the neostigmine and spontaneous groups, respectively) . Plasma cholinesterase activity decreased significantly from baseline valu es after administration of neostigmine (5.88 +/- 0.21 us. 0.43 +/- 0.04 U/m l plasma). Plasma concentrations of the trans-trans isomer were significant ly greater in the neostigmine group than in the spontaneous recovery group 5, 6, 8, and 10 min after discontinuation of the infusion. Differences in t he plasma concentration of the cis-trans isomer did not achieve statistical significance. Conclusions: Although administration of neostigmine decreased plasma cholin esterase activity and caused the trans-trans isomer to remain in the plasma at higher concentration, it did not delay recovery from mivacurium-induced block.