The effect of halothane on mivacurium infusion requirements in adult surgical patients.

BACKGROUND

The extent of interaction between volatile anaesthetics and neuromuscular blocking agents depends both on the inhalational anaesthetic and the muscle relaxant. Halothane has the weakest potentiating effect on neuromuscular blocking drugs and previous studies of the interaction between halothane and mivacurium have been contradictory. We were interested in determining the effect of different levels of halothane-nitrous oxide anaesthesia on infusion requirements of mivacurium.

METHODS

Sixty adult surgical patients were studied. Anaesthesia was induced with thiopentone and fentanyl and intubation 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) supplemented with fentanyl, while in the other groups halothane was administered at different end-tidal concentrations: 0.19% (group 2), 0.37% (group 3), 0.74% (Group 4), corresponding to 0.25, 0.5 and 1.0 MAC of halothane. Neuromuscular block was kept at 95% with a closed-loop feedback infusion of mivacurium and monitored with electromyography. Plasma cholinesterase concentrations and dibucaine numbers were determined.

RESULTS

Mivacurium infusion requirements (mean +/- SD) were 7.5 +/- 3.1 micrograms.kg-1.min-1 with nitrous oxide-fentanyl anaesthesia. In the groups receiving 0.25, 0.5 or 1.0 MAC of halothane the steady-state infusion rates of mivacurium were reduced to 6.3 +/- 2.8, 5.6 +/- 1.4 and 5.7 +/- 2.5 micrograms.kg-1.min-1 (P < 0.05), respectively. There was a linear relationship between mivacurium infusion requirements and plasma cholinesterase activity.

CONCLUSIONS

Halothane anaesthesia reduces mivacurium infusion requirements by 15-25% compared to nitrous oxide-fentanyl anaesthesia. Interindividual differences in the extent of this interaction are great.