Monitoring of the EEG using the bispectrum analysis on the anesthetic depth

In this study, a distribute characteristics on the bispectral coupling of EEG were observed according to depth of anesthesia using bispectrum analysis method in a type of nonlinear signal processing. 25 patients of the general anesthesia were studied. In the preprocess, base line correction and linear detrend was used. In awakening state of patient in pre-operation and post operation, appearance rate of bispectral coupling was observed that a strong appearance rate was represented in major frequency zone such as (15,15) Hz but weak appearance rate investigated in the other minor frequency zone. During the anesthesia of patient, a strong appearance rate revealed the low frequency range below (10, 10) Hz and a weak appearance rate viewed in the other frequency range. Also the result was compared with the delta ratio and spectrum edge frequency (SEF). Delta ratio shows significant lower values at induction (3.60-0.98) and maintenance (5.0-4.74) of anesthesia than preoperation (10.30-2.44) and postoperation (13.03-2.29) (P<0.05). These result was consistent with a trend of delta ratio that the index of evaluation for depth of anesthesia known as a reflex of the conscious level of patient´s during anesthesia. While these results were not consistent with the analysis result of SEF that the index of evaluation for suppression level of patient´s during recovery stage of anesthesia. SEF shows lower values at maintenance (17.00-1.22) and postoperation (12.20-1.53) than preoperation (22.50-1.75) and induction (22.40-1.18) of anesthesia (P<0.05). The analysis method of bispectrum was useful information about the index of evaluation according to the level of unconsciousness and suppression among other evaluation factor on depth of anesthesia.