The application of a non-linear analysis technique to the monitoring of anesthetic effects in the rat

To find a new measure from the EEG that quantifies the effects of anesthetics during surgery, the correlation dimension (CD) of the EEG of eight rats was estimated. To get informed about the anesthetic state, the noxious induced withdrawal reflex (NIWR) was measured, i.e. the force elicited by transcutaneous electrical stimulation of the hind paw. The algorithm of Grassberger and Procaccia (1983) to compute the CD was slightly modified and labeled as "dimensional complexity" (DC). The DC was estimated by applying a fit procedure to find the scaling region in the correlation integral. Experiments were performed under propofol infusion (30 à 40 mg.kg-1.h-1), with repeated bolus injections (3 mg) to induce periods of more profound anesthesia. The DC was compared to the Spectral Edge Frequency (SEF), a linear measure extracted from the power spectrum. The DC and SEF were correlated with the NIWR responses. The DC showed higher and more consistent correlations with the NIWR than the SEF. Correlation coefficients of NIWR and DC mainly varied between 0.6 and 0.8. Conclusion: New parameters from the field of non-linear dynamics can be an aiding tool in detecting effectual changes induced by anesthetics.