We investigate several quantifiers of the electroencephalogram (EEG) signalwith respect to their ability to indicate depth of anesthesia. For 17 patientsanesthetized with Sevoflurane, three established measures (two spectral and onebased on the bispectrum), as well as a phase space based nonlinear correlationindex were computed from consecutive EEG epochs. In absence of an independentway to determine anesthesia depth, the standard was derived from measured bloodplasma concentrations of the anesthetic via a pharmacokinetic/pharmacodynamicmodel for the estimated effective brain concentration of Sevoflurane. In mostpatients, the highest correlation is observed for the nonlinear correlationindex D*. In contrast to spectral measures, D* is found to decreasemonotonically with increasing (estimated) depth of anesthesia, even when a"burst-suppression" pattern occurs in the EEG. The findings show the potentialfor applications of concepts derived from the theory of nonlinear dynamics,even if little can be assumed about the process under investigation.
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