Increase in high frequency EEG activity explains the poor performance of EEG spectral entropy monitor during S-ketamine anesthesia

Clin Neurophysiol. 2006 Aug;117(8):1660-8. doi: 10.1016/j.clinph.2006.05.011. Epub 2006 Jun 27.


Objective: To 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.

Methods: The 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.

Results: The 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.

Conclusions: These 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.

Significance: The entropy monitor is not adequate for monitoring S-ketamine-induced hypnosis.

Publication types

  • Comparative Study
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adult
  • Anesthetics, Dissociative / pharmacology*
  • Anesthetics, Intravenous / pharmacology
  • Brain / drug effects
  • Electroencephalography / drug effects*
  • Electromyography / drug effects
  • Humans
  • Ketamine / pharmacology*
  • Male
  • Monitoring, Intraoperative / methods*
  • Propofol / pharmacology
  • Sensitivity and Specificity


  • Anesthetics, Dissociative
  • Anesthetics, Intravenous
  • Ketamine
  • Propofol