CLONIDINE - AN ADJUNCT IN ISOFLURANE N2O O-2 RELAXANT ANESTHESIA - EFFECTS ON EEG POWER SPECTRA, SOMATOSENSORY AND AUDITORY-EVOKED POTENTIALS/

We investigated the effect of clonidine in 19 patients having lumbar d isc surgery under nitrous oxide/isoflurane/relaxant anaesthesia. The E EG, spinal and cortical somatosensory evoked potentials, and the brain stem auditory evoked potentials were recorded. After equilibration of the general anaesthetic, two successive infusions of clonidine (5 mu g .kg(-1)) were given. After the second infusion of clonidine, the plasm a concentration increased from 0.2(SEM 0.05) to 6.4(SEM 0.06)ng.ml(-1) (p < 0.05). In the EEG, delta-activity was maintained, but the beta-f raction, and the 95% and 50% spectral frequencies were reduced. Total EEG power progressively decreased from 296(152-397)mu V-2 to 108(51-24 0)mu V-2. The somatosensory evoked spinal potential (N13) decreased in amplitude, (1.77(SEM 0.35)mu V to 1.59(SEM 0.35)mu V, p < 0.05) and i ncreased in latency (14.37(SEM 0.29)ms to 14.69(SEM 0.31)ms, p < 0.05) . The central conduction time increased from 6.47(SEM 0.16)ms to 6.92( SEM 0.25)ms, ns. There was no effect on the cortical somatosensory pot entials, or the brainstem auditory evoked potentials. According to the EEG spectral indices, anaesthesia appealed to deepen despite a reduct ion in the end expiratory isoflurane concentration from 0.53(SEM 0.07) to 0.28(SEM 0.06)vol%, which indicated a reduction in anaesthetic req uirements following clonidine.