In this study, a distribute characteristics on the bispectral coupling of EEG were examined according to the depth of anesthesia by using bispectum analysis a type of nonlinear signal processing.

25 patients during the general anesthesia were studied. In the preprocess, base line correction and linear detrend was used. During the awakening state, preoperatively and postoperatively, 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. Whilst anesthetized, a strong appearance rate revealed the low frequency range below (10, 10) Hz and a weak appearance rate viewed in the other frequency range. As such was compared with the delta ratio and spectrum edge frequency (SEF).

The delta ratio showed significantly lower values at induction (3.60 ± 0.98) and during the maintenance (5.0±1.74) of anaesthesia than preoperatively (10.30 ± 2.44) and postoperatively (13.03 ± 2.29)(P < 0.05). These results were consistent with the trend of delta ratio, namely that the index of evaluation for the depth of anesthesia, known as the reflex of the conscious level of patient's during anesthesia. While these results were not consistent with SEF analysis results 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 postoperatively (12.20 ± 1.53) than preoperatively (22.50 ± 1.75) and at the induction (22.40 ± 1.18) of anesthesia(P < 0.05).

The bispectrum analysis method provided useful information about the index of evaluation according to the level of unconsciousness and suppression among other evaluation factor on depth of anesthesia.