A comparison of mivacurium dosage requirements during isoflurane and desflurane anesthesia

Study Objective: To evaluate the dose requirement and recovery characteristics of mivacurium infusions during anesthesia with equipotent concentrations of either desflurane or isoflurane. Design: Randomized, open-study comparing the effects of desflurane and isoflurane on mivacurium-induced neuromuscular blockade. Setting: Operating suite of a university-affiliated medical center Patients: 41 ASA status I, II, and III adult patients, requiring more than 45 minutes of neuromuscular blockade for surgery. Interventions: Following a standardized induction sequence and established steady desflurane-nitrous oxide (DES group) or isoflurane-nitrous oxide (ISO group) anesthesia at 1 minimum alveolar concentration (MAC), an intubating dose of 0.2 mg/kg of mivacurium chloride was administered. Ventilation was maintained with a face mask until the first twitch (T1) of the evoked train-of-four (TOF) reached 10% or less of control when tracheal intubation was performed. T1 was allowed to return to 10% of its control value. An infusion of mivarcurium at the initial rate of 5 mu g/kg/min was then started and adjusted to maintain T1 at 10% +/- 2% of control. Within 20 minutes of completion of surgery, the mivacurium infusion was stopped and the time for the evoked electromyograph (EMG) to return to 25% and 75% of the original baseline was noted. Measurements and Main Results: Neuromuscular function was monitored continuously by an evoked EMG. The average infusion rate was 5.7 +/- 2.4 mu g/kg/min (mean +/- SD) for DES group (n = 20) and 6.6 +/- 2.7 mu g/kg/min for ISO group (n = 21) (p = NS). There was no change In the infusion rate of mivacurium over time for both groups. However, there was an inverse relationship in both groups between the time to recovery following a bolus dose and the subsequent mean infusion rate of mivacurium (correlation coefficient = -5.0; p < 0.005). The spontaneous recovery index (T25-75) for the two groups was identical, 11.5 +/- 4.9 min (mean +/- SD) for DES group and 11.5 +/- 7.9 min for ISO group ip = NS). Conclusion: There were no differences in the dose requirement and recovery indices of mivacurium during either desflurane or isoflurane-based anesthesia. Patients who took longer to recover from the bolus dose in both groups showed a subsequent reduction in dose requirements of mivacurium.