EFFECTS OF ISOFLURANE OR HALOTHANE ON MOTOR EVOKED-POTENTIALS IN THE RABBIT

The effects of isoflurane or halothane on motor evoked potentials (MEP s) were assessed and compared in the rabbit, in order to contribute to devise a rigorous human anaesthetic protocol to be used for monitorin g of MEPs in corrective spinal surgery. Ten black adult New Zealand ra bbits were anaesthetised twice at a month interval, once with isoflura ne and once with halothane. Once a control cortical stimulation had be en carried out after the animal had breathed pure oxygen for three min utes, the following concentrations of anaesthetic agent were given for 3 min each : respectively 0.3 vol %, 0.5 vol % and 1 vol % of isoflur ane, and 0.5 vol % and 1 vol % of halothane. Cortical stimulation was carried out every minute. The signs of anaesthesia (diameter decrease of the pupil, eye covered by the nictating membrane), muscle relaxatio n (ears drop) and breathing rate were recorded. MEPs were recorded 1. 2, 3 and 5 minutes after the end of anaesthesia. Isoflurane had a stro nger effect on MEPs than halothane. The effect was more pronounced on amplitude than on latency. MEPs remained present whatever the concentr ation of halothane. In 70 % of cases, MEPs, discontinued with isoflura ne, more rapidly, more deeply, and for a longer time. Mean latency was more constantly increased in the isoflurane than in the halothane gro up. The effect of volatile halogenated anaesthetics on mean latency of MEPs seemed to be more delayed than that on amplitude. One should als o take into account an individual sensitivity. it is concluded that th e interpretation of MEPs during anaesthesia with volatile halogenated agents should be carried out with caution.