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In this article we investigate on the evolution of EEG spectra over different depth of anesthesia from deep to very light anesthesia where immediately followed by waking. Low frequency components of EEG spectra (delta band) are examined using two different methods. One is based on Fourier transform of the low pass filtered EEG and the other is based on extraction of a kind of negative peak slow wave activity (SWA) and estimating its Fourier transform. Results show that in transition from deep to light anesthesia, not all energies of delta frequencies are decreased. There are some particular frequencies that (e.g. ∼0.7 Hz) their power may even be increased by reduction of anesthetic drug concentration. Deep anesthesia is concurrent with more regular periodic EEG signals that have less SWA. Consideration of this phenomenon can improve the efficiency of the methods of determining depth of anesthesia.

In order to estimate the change of brain activity under general anesthesia, the Lempel-Ziv complexity (C(n)) of electroencephalogram (EEG) of SD rat was studied in this paper. The C(n)s of EEG from different channels under different depth of anesthesia were measured and the relationship between C(n) and the depth of anesthesia (DOA) was analyzed. The result shows that the C(n) variations of EEG of different channels with DOA are similar, and predicates that the activities of every part of brain change similarly. Therefore, it is enough to detect DOA by only one channel EEG. The C(n) of EEG will decrease while the depth of anesthesia increasing and vice versa. Two thresholds of C(n) are defined, one distinguishes awake and light anesthesia state, the other distinguishes light and deep anesthesia state. Besides EEG complexity analysis, the complexity variations of four rhythms of EEG (delta, theta, alpha and beta) are also analyzed. The study shows the dynamic change of complexity of delta rhythm leads to that of EEG, so the delta rhythm is the dominant rhythm during anesthesia for rat.

Clinically and encephalographically distinct, definable stages are passed through on the way from induction to progressive deepening of anesthesia, and again in reverse order during termination of anesthesia. These stages were defined by Guedel in 1920 (according to clinical parameters, giving stages I–IV) and by Kugler in 1966 (according to EEG changes, giving stages A–F). The stages of anesthesia include all levels of consciousness from alertness through analgesia to deep coma with collapse of all vegetative functions. The sequence is analgesia, excitation, light, moderate, and deep anesthesia. At this point the tolerance limit of the administered substance has been reached. Additional application of anesthetic will induce intoxication with total cerebral depression. The correlation between clinical stages of anesthesia and EEG findings will be demonstrated by tables and selected examples.

The need for monitoring depth of anesthesia was emphasized by Hamerhoff and Grantham in their recent review of this topic [25]: it results in prevention of pain perception, awareness, and recall; reduction of untoward autonomic effects of excessively light or deep anesthesia; minimization of stress and its manifestations; and facilitation of prompt recovery.