Quantitative electroencephalographic analysis of the biphasic concentration-effect relationship of propofol in surgical patients during extradural analgesia.

We studied effects on the EEG of propofol infused at a rate of 0.5 mg kg-1 min-1 for 10 min in 10 healthy male surgical patients under extradural analgesia. The EEG amplitude in six frequency bands was related to arterial blood propofol concentrations and responsiveness to verbal commands. The EEG amplitude showed a characteristic biphasic response to increasing blood propofol concentrations in all frequency bands. During the infusion, patients lost responsiveness when EEG amplitudes in the high frequency bands were decreasing after having reached a maximum. EEG changes were different during infusion and emergence. Pharmacodynamic modelling, using two effect compartments with dissimilar equilibration constants, resulted in satisfactory fits. We conclude that propofol exerts a biphasic effect on the EEG amplitude in all frequency bands. The dissimilarity of EEG changes during infusion and during emergence suggests that two effect compartments with different equilibration constants exert opposing effects on the EEG.