• A Maksimow, M Särkelä, J W Långsjö, E Salmi, K K Kaisti, A Yli-Hankala, S Hinkka-Yli-Salomäki, H Scheinin, and S K Jääskeläinen.
• Turku PET Centre, University of Turku, P.O. Box 52, FIN-20521 Turku, Finland. anu.maksimow@utu.fi
• Clin Neurophysiol. 2006 Aug 1;117(8):1660-8.

ObjectiveTo study the effects of S-ketamine on the EEG and to investigate whether spectral entropy of the EEG can be used to assess the depth of hypnosis during S-ketamine anesthesia.MethodsThe effects of sub-anesthetic (159 (21); mean (SD) ng/ml) and anesthetic (1,959 (442) ng/ml) serum concentrations of S-ketamine on state entropy (SE), response entropy (RE) and classical EEG spectral power variables (recorded using the Entropy Module, GE Healthcare, Helsinki, Finland) were studied in 8 healthy males. These EEG data were compared with EEG recordings from 6 matching subjects anesthetized with propofol.ResultsThe entropy values decreased from the baseline SE 85 (3) and RE 96 (3) to SE 55 (18) and RE 72 (17) during S-ketamine anesthesia but both inter- and intra-individual variation of entropy indices was wide and their specificity to indicate unconsciousness was poor. Propofol induced more pronounced increase in delta power (P<0.02) than S-ketamine, whereas anesthetic S-ketamine induced more high frequency EEG activity in the gamma band (P<0.001). Relative power of 20-70 Hz EEG activity was associated with high SE (P=0.02) and RE (P=0.03) values during S-ketamine anesthesia.ConclusionsThese differences in low and high frequency EEG power bands probably explain why entropy monitor, while adequate for propofol, is not suitable for assessing the depth of S-ketamine anesthesia.SignificanceThe entropy monitor is not adequate for monitoring S-ketamine-induced hypnosis.

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