Quantifying the effect of isoflurane on mivacurium infusion requirements.

We evaluated the effect of different concentrations of isoflurane in a nitrous oxide/oxygen mixture on the infusion requirements of mivacurium in 60 adult surgical patients. Anaesthesia was induced with thiopentone and fentanyl, and intubation was facilitated with mivacurium 0.15 mg.kg-1. The patients were randomly assigned to one of four study groups. The control group received nitrous oxide in oxygen (2:1) anaesthesia supplemented with fentanyl. In the other groups, isoflurane was administered at different end-tidal concentrations: 0.29%, 0.58% and 1.15%, corresponding to 0.25, 0.5 and 1.0 MAC of isoflurane, respectively. Neuromuscular block was maintained at 95% with a computer-controlled infusion of mivacurium and monitored with electromyography. The mean (SD) steady-state infusion requirements of mivacurium in patients receiving nitrous oxide-fentanyl anaesthesia or isoflurane 0.25-0.5 MAC were similar, ranging from 6.1 (2.2) to 5.1 (2.1) micrograms.kg-1.min-1. Isoflurane 1.0 MAC reduced mivacurium infusion requirements by 32% (p < 0.01). Interindividual differences in mivacurium infusion requirements were large.