Changes in the EEG power spectrum after midazolam anaesthesia combined with racemic or S – (+) ketamine

Changes in the EEG power spectrum were studied in 50 patients (ASA status 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 patients receiving intramuscular premedication with midazolam about 45 minutes prior to induction of anaesthesia showed, in a deliberately quiet environment and mostly in the early morning, a delta dominated EEG (56% delta power) with a reduced alpha peak (17% alpha power) and an average median of 4 Hz as the baseline findings of the EEG power spectrum. The intravenous administration of midazolam led to activation of the lower beta range (13–18 Hz) and the subsequent injection of ketamine caused an increase in activity in the fast beta range (21–30 Hz), both being accompanied by a reduction of delta power from 56% to 40%. Correspondingly, an increase in the median frequency was noted. Causing nearly the same changes in EEG, S – (+) ketamine was confirmed to be twice as potent as racemic ketamine.