Effect of volatile anesthetics on interhemispheric EEG cross-approximate entropy in the rat

Abstract

Dissimilarity of the electroencephalogram (EEG) between the two hemispheres was characterized by cross-approximate entropy (C-ApEn), an information statistical parameter applicable to nonlinear, aperiodic signals. EEGs were recorded bipolarly with pairs of epidural electrodes in the left and right frontal cortices. The signals were filtered for 1–100 Hz and digitized at 200 Hz. Inhaled anesthetic concentration was varied between 0.3 and 2.1% with 45-min equilibration periods while the rats were breathing spontaneously. Anesthetics produced concentration-dependent changes in C-ApEn. A greater dynamic range of C-ApEn was obtained by reducing the epoch length from 2 s to 100 ms. At a 0.4% inspired agent concentration halothane caused an increase of C-ApEn, whereas isoflurane did not. When the inspired concentrations of both agents were greater than 0.4%, C-ApEn dose-dependently decreased as agent concentrations increased. Isoflurane depressed C-ApEn more than did halothane at all equivalent minimum alveolar concentration levels, but the two agents became equipotent at 1.5% inspired concentration. C-ApEn fell below the awake baseline at 0.8% anesthetic concentration that also abolished the righting reflex. C-ApEn increased after high-pass filtering (>20 Hz) and decreased after-low pass filtering (<20 Hz) of the digitized EEG; the anesthetic dependence of C-ApEn was diminished by both filters. The results suggest that C-ApEn of bihemispheric EEG is a sensitive, agent-specific correlate of anesthetics’ central effect.