The infusion rate of mivacurium or atracurium for cesarean section compared with gynecological procedures.

Mivacurium is mainly metabolized by plasma cholinesterase, whereas atracurium is removed by Hofman elimination. The purpose of this study was to compare the infusion rate of atracurium and mivacurium in maintaining surgical relaxation, and to compare their recovery indices between parturients and non-pregnant women. Muscle relaxation was maintained by the continuous infusion of relaxants to retain the first response of train-of-four (TOF) at 5% of control. When mivacurium was used, Bolus-T5 (duration from the end of mivacurium bolus injection to 5% single twitch recovery) was measured. After discontinuing the infusion, the recovery index was measured. The infusion rate of mivacurium, not atracurium, was significantly lower in parturients and Bolus-T5 of parturients was significantly longer than that of non-pregnant women. There was no significant difference in the recovery indices of both relaxants. The authors concluded that the infusion rate of mivacurium in maintaining muscle relaxation in parturients should be reduced compared to the rate in non-pregnant women and measuring Bolus-T5 may be helpful in determining the infusion rate to maintain muscle relaxation.

The infusion rate of mivacurium and its spontaneous neuromuscular recovery in magnesium-treated parturients.

UNLABELLED: Magnesium (Mg) enhances the activity of nondepolarizing neuromuscular blocking drugs. However, no interaction between mivacurium and magnesium has been described. Therefore, we sought to determine the effect of the influence of Mg on the infusion rate of mivacurium and its spontaneous recovery. We studied 24 parturients who had undergone cesarean section under general anesthesia. Those who had been given MgSO4 for the treatment of preeclampsia were assigned to the Mg group (n = 12), and the other normal parturients were assigned to the NonMg group (n = 12). In both groups, the train-of-four (TOF) response to stimuli of the ulnar nerve was measured at intervals of 15 s. Anesthesia was induced with thiopental and succinylcholine. In both groups, a bolus dose of mivacurium 0.06 mg/kg was administered when the first twitch of TOF (T1) reached 100% after the succinylcholine injection. When the electromyographic response after mivacurium had recovered to approximately 5%-10% of the baseline, a continuous infusion of mivacurium was given to maintain 93%-97% neuromuscular blockade. The plasma concentration of Mg in blood of the Mg group was 4.0 1.0 mEq/L, higher than that (1.4 mEq/L) of the NonMg group (P < 0.01). The infusion rates of mivacurium of Mg and NonMg groups were 1.6 and 5.4 mEq x kg(-1) x min(-1), respectively. In addition, the recovery indexes of the Mg and NonMg groups were 12.9 and 4.3 min, respectively. We conclude that a smaller dose of mivacurium should be infused to patients receiving Mg. IMPLICATIONS: Magnesium, used as a standard therapy for severe toxemia, may act to enhance muscle relaxants such as mivacurium, a short-acting drug used in general anesthesia. Among women undergoing a cesarean section who were given a magnesium pretreatment, the infusion rate of mivacurium required to obtain relaxation was lower than that among women who did not receive pretreatment.