Sevoflurane and isoflurane, but not propofol, decrease mivacurium requirements over time

Volatile anesthetic agents potentiate neuromuscular blockade, but the magnitude of potentiation appears to be time dependent. The time course of this interaction was studied by measuring mivacurium infusion rates during sevoflurane, isoflurane and propofol anesthesia.After informed consent, anesthesia was induced in 48 ASA physical status I-II adults with propofol, fentanyl and mivacurium 0.25 mg.kg(-1) and maintained with N(2)O (60%) and one of the three agents chosen at random: sevoflurane 1.9%; isoflurane 1.2%; or propofol 100-150 microgram.kg(-1).min(-1). Train-of-four stimulation was applied every 15 sec to the ulnar nerve. Neuromuscular blockade was monitored with accelerometry. At 5% recovery of the first twitch (T1), a mivacurium infusion was started and adjusted every five minutes to maintain 90-95% T1 depression.The time to 5% T1 recovery after the initial dose was similar in all groups (13-15 min). Fifteen minutes after the start of the infusion mivacurium requirements were greater (P0.05) in the propofol group (7.5 +/- 1.7 microgram.kg(-1).min(-1); mean +/- SD) than in either isoflurane (4.7 +/- 1.6 microgram.kg(-1).min(-1)) or sevoflurane (4.5 +/- 1.5 microgram.kg(-1).min(-1)) group. Then, the rate remained stable for propofol (6.2 +/- 1.4 microgram.kg(-1).min(-1) after 90 min of infusion) while it decreased with isoflurane to 2.9 +/- 1.6 microgram.kg(-1).min(-1) at 90 min (P0.05 vs propofol) and to 1.4 +/- 1.0 microgram.kg(-1).min(-1) in the sevoflurane group (P0.05 vs propofol and isoflurane).Sevoflurane and isoflurane do not prolong the effect of a bolus dose of mivacurium, but potentiation increases with time from 30-105 min of exposure. This interaction is greater with sevoflurane than isoflurane.