Depressive effect of isoflurane on motor evoked potentials in the Nubian goat

Purpose: To determine the effect of isoflurane on motor evoked potentials ( MEP) in a new animal model designed to verify the applicability of MEPs in brachial plexus surgery, and to compare the results with previous reports i n other animals. Methods: In seven goats, anesthesia was induced with 3 mg.kg(-1) ketamine i v and maintained with nitrous oxide 40% in oxygen and 2 mu g.kg(-1).hr(-1) fentanyl iv. The MEP were performed with two subcutaneous needle electrodes placed over the occiput (cathode) and the nasion (anode), with their plugs connected to the power output of a Digitimer D 180 electrical stimulator, connected to the trigger input of an electromyograph (model 8400, Cadwell L aboratories, Inc., Kennwick, Washington). Activation of the Digitimer cause d central stimulation of the motor cortex, evoking baseline compound muscle action potentials (CMAPs) which were recorded from the left triceps muscle , Subsequently, isoflurane 2% was administered together with repeated centr al stimulation at 30 sec intervals. Results: Onset of I- (indirect) waves increased from median 15,8 msec to me dian 26,8 msec P = 0,018 (latency increase ranged from: 9 to 11.5 msec), wh ile peak-to-peak amplitudes decreased and subsequently disappeared, D- (dir ect) waves showed no latency increase, and finally disappeared as well. Aft er disappearance of CMAPs, isoflurane administration was stopped and MEP re peated. The CMAPs reappeared (range: 210-360 sec) and regained initial peak -to-peak amplitudes and latencies. Conclusion: These animal studies suggest that isoflurane should not be used during the recording of MEPs.