ISOFLURANE COMPARED WITH NITROUS-OXIDE ANESTHESIA FOR INTRAOPERATIVE MONITORING OF SOMATOSENSORY-EVOKED POTENTIALS

Intraoperative monitoring of somatosensory-evoked potentials is a rout ine procedure. To determine the depressant effect of nitrous oxide rel ative to isoflurane, the authors recorded the scalp, cervical and brac hial plexus-evoked responses to stimulation of the median nerve under different anaesthetic conditions. Eight subjects, age 35 +/- 6 (SD) yr , weight 68 +/- 12 kg, were studied. Following recording of awake cont rol responses, anaesthesia was induced with thiopentone 5 mg . kg-1 an d fentanyl 3 mug . kg-1 and was followed by succinylcholine 1 mg . kg- 1. During normocapnia and normothermia, and with a maintenance infusio n of fentanyl 3 mug . kg-1. hr-1, evoked potential recording was repea ted under three different anaesthetic conditions; 0.6 MAC nitrous oxid e, 0.6 MAC nitrous oxide +/- 0.6 MAC isoflurane, and 0.6 MAC isofluran e. Among the anaesthetic conditions, the combination of nitrous oxide- isoflurane had the most depressant effect on the cortical amplitude (6 7 +/- 4% reduction, P < 0.05). Nitrous oxide decreased the cortical am plitude more than an equipotent dose of isoflurane (60 +/- 4% vs 48 +/ - 7%, P < 0.05). The latency was unchanged by nitrous oxide, but incre ased slightly by isoflurane and isoflurane-nitrous oxide anaesthesia ( 1.0 and 0.9 msec respectively, P < 0.05). We conclude that somatosenso ry-evoked potential monitoring is feasible both during nitrous oxide a naesthesia and isoflurane anaesthesia, but the cortical amplitude is b etter preserved during 0.6 MAC of isoflurane alone relative to 0.6 MAC of nitrous oxide alone. The depressant effect is maximal during nitro us oxide-isoflurane anaesthesia but less than the predicted additive e ffect.