Clonidine: an adjunct in isoflurane N2O/O2 relaxant anaesthesia. Effects on EEG power spectra, somatosensory and auditory evoked potentials.

We investigated the effect of clonidine in 19 patients having lumbar disc surgery under nitrous oxide/isoflurane/relaxant anaesthesia. The EEG, spinal and cortical somatosensory evoked potentials, and the brainstem auditory evoked potentials were recorded. After equilibration of the general anaesthetic, two successive infusions of clonidine (5 micrograms.kg-1) were given. After the second infusion of clonidine, the plasma 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-fraction, and the 95% and 50% spectral frequencies were reduced. Total EEG power progressively decreased from 296(152-397) microV2 to 108(51-240) microV2. The somatosensory evoked spinal potential (N13) decreased in amplitude, (1.77(SEM 0.35) microV to 1.59(SEM 0.35) microV, p < 0.05) and increased 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 potentials, or the brainstem auditory evoked potentials. According to the EEG spectral indices, anaesthesia appeared to deepen despite a reduction 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 requirements following clonidine.