COMPARISON OF THE EFFECTS OF KETAMINE-MIDAZOLAM WITH THOSE OF FENTANYL-MIDAZOLAM ON CORTICAL SOMATOSENSORY-EVOKED POTENTIALS DURING MAJOR SPINE SURGERY

Cortical somatosensory evoked potentials (CSEP) allow monitoring of sp inal cord function during surgery. Ketamine has been shown to enhance CSEP amplitude, but there is no previous study comparing its effects w ith those of other anaesthetic regimens. Therefore, we have compared t he effects of ketamine with those of fentanyl, both combined with mida zolam, on CSEP monitoring during major spine surgery. Twenty patients with normal preoperative CSEP were allocated randomly to a ketamine or fentanyl group. Anaesthesia was induced with ketamine 3 mg kg(-1) or fentanyl 6 mu g kg(-1) i.v., and midazolam 0.3 mg kg(-1) i.v in both g roups, and maintained with continuous i.v infusion of ketamine 2 mg kg (-1) h(-1) or fentanyl 3 mu g kg(-1) h(-1), combined in both groups wi th midazolam 0.15 mg kg(-1) h(-1) and 60% nitrous oxide in oxygen. CSE P were elicited by tibial posterior nerve stimulation and measured P1 and N1 latencies, and P1-N1 amplitude. CSEP were recorded before and a fter induction, at 15 min, 1 and 2 h after induction, during skin clos ure and after removal of nitrous oxide. Both groups were comparable in characteristics, duration of surgery, mean arterial pressure and temp erature. CSEP latencies were not significantly affected in either grou p. CSEP amplitude decreased significantly overtime in the fentanyl gro up (from mean 2.02 (SEM 0.41) to 0.95 (0.17) mu V, P < 0.05), but not in the ketamine group (from 1.33 (0.36) to 1.05 (0.31) mu V, ns). Neve rtheless, we did not observe any significant differences in amplitudes or latencies between the two groups. The delay in obtaining the first voluntary postoperative motor response was significantly greater in t he ketamine group (170 (54) vs 55 (17) min, P < 0.01). Both ketamine a nd fentanyl allowed us to obtain reliable CSEP during major spine surg ery, and there were no significant difference between these two anaest hetic regimens for CSEP monitoring, but a longer delay for voluntary p ostoperative motor assessment was observed in the ketamine group.