Ketamine increases the frequency of electroencephalographic bicoherence peak on the α spindle area induced with propofol

Abstract

Background. The reticular and thalamocortical system is known to play a prominent role in spindle wave activity, and the spindle wave is related to the sedative effects of anaesthetics. Recently, bispectral analysis of the EEG has been developed as a better method to indicate nonlinear regulation including the thalamocortical system linking to the cortical area. In the present study, in order to explore the interference of ketamine with the nonlinear regulation of the sub-cortical system, we examined the effect of ketamine on spindle α waves through the bispectral analysis. Methods. The study included 21 patients. Anaesthesia was induced and maintained using a propofol-TCI system (target-controlled infusion, with target concentration 3.5 μ ml-1). An A-2000 BIS monitor was used and the raw EEG signals were collected via an RS232 interface on a personal computer. Bicoherence, the normalized bispectrum, and power spectrum were analysed before and after i.v. administration of 1 mg kg-1 racemic ketamine. Results. Propofol caused α peaks in both power and bicoherence spectra, with average frequencies of 10.6 (sd 0.9) Hz and 10.7 (1.0) Hz, respectively. The addition of ketamine significantly shifted each peak to frequencies of 14.4 (1.4) Hz and 13.6 (1.5) Hz, respectively [P<0.05, mean (SD)]. Conclusions. Ketamine shifted the α peaks of bicoherence induced by propofol to higher frequencies. This suggests that ketamine changes the α spindle rhythms through the modulation of the nonlinear sub-cortical reverberating network. © The Board of Management and Trustees of the British Journal of Anaesthesia 2007. All rights reserved.