CHANGES IN THE EEG POWER SPECTRUM AFTER MIDAZOLAM ANESTHESIA COMBINEDWITH RACEMIC OR S-(+)KETAMINE

Changes in the EEG power spectrum were studied in 50 patients (ASA sta tus I or II), receiving either 2 mg . kg(-1) of racemic ketamine or 1 mg . kg(-1) of S-(+) ketamine in a randomized and double-blind manner after prior administration of 0.1 mg . kg(-1) of midazolam. The patien ts receiving intramuscular premedication with midazolam about 45 minut es prior to induction of anaesthesia showed, in a deliberately quiet e nvironment and mostly in the early morning, a delta dominated EEG (56% delta power) with a reduced alpha peak (17% alpha power) and an avera ge median of 4 Hz as the baseline findings of the EEG power spectrum. The intravenous administration of midazolam led to activation of the l ower beta range (13-18 Hz) and the subsequent injection of ketamine ca used an increase in activity in the fast beta range (21-30 Hz), both b eing accompanied by a reduction of delta power from 56% to 40%. Corres pondingly, an increase in the median frequency was noted. Causing near ly the same changes in EEG, S-(+) ketamine was confirmed to be twice a s potent as racemic ketamine.