Abstract
Under general anesthesia (GA), advanced analysis methods enhance the awareness of the electroencephalography (EEG) signature of transitions from consciousness to unconsciousness. For nonlinear and nonstationary signals, empirical mode decomposition (EMD) works as a dyadic filter bank to reserve local dynamical properties in decomposed components. Moreover, cross-frequency phase-amplitude coupling analysis illustrates that the coupling between the phase of low-frequency components and the amplitude of high-frequency components is correlated with the brain functions of sensory detection, working memory, consciousness, and attentional selection. To improve the functions of phase-amplitude coupling analysis, we utilized a multi-timescale approach based on EMD to assess changes in brain functions in anesthetic-induced unconsciousness using a measure of phase-amplitude coupling. Two groups of patients received two different anesthetic recipes (with or without ketamine) during the induction period of GA. Long-term (low-frequency) coupling represented a common transitional process of brain functions from consciousness to unconsciousness with a decay trend in both groups. By contrast, short-term coupling reflected a reverse trend to long-term coupling. However, the measures of short-term coupling also reflected a higher degree of coupling for the group with ketamine compared with that without ketamine. In addition, the coupling phase is a factor of interest. The phases for different combinations of coupling components showed significant changes in anesthetic-induced unconsciousness. The coupling between the delta-band phase and the theta-band amplitude changed from in-phase to out-phase coupling during the induction process from consciousness to unconsciousness. The changes in the coupling phase in EEG signals were abrupt and sensitive in anesthetic-induced unconsciousness.
Highlights
Under general anesthesia (GA), understanding changes occurring in the dynamical properties of electroencephalography (EEG) from consciousness to unconsciousness is a crucial topic in the field of anesthesiology
To map the intrinsic frequency bands of intrinsic mode functions (IMFs) decomposed from EEG signals onto the traditional frequency bands in spectral analysis, we plotted the distributions of instantaneous frequency (Fig. 1) for IMFs decomposed from EEG recordings, which were recorded at a sampling rate of 128 Hz
The ranges of intrinsic frequency bands were determined using 70% of the logarithmic distributions of instantaneous frequency (IF), which represents an appropriate boundary between the IF distributions of two successive IMFs
Summary
Under general anesthesia (GA), understanding changes occurring in the dynamical properties of electroencephalography (EEG) from consciousness to unconsciousness is a crucial topic in the field of anesthesiology. A decrease in low-frequency CFC is considered an anesthetic-induced change in EEG3–5. Coupling changes occurred between the phase of low-frequency rhythms and the amplitude of alpha-band oscillations (8–14 Hz) in EEG recordings during sevoflurane infusion[16]. Multi-timescale PAC analysis can be conducted on multiple pairs of phases of low-frequency IMFs and amplitudes of high-frequency IMFs systematically and automatically. We hypothesized that the results of multi-timescale PAC analysis would sufficiently represent the common characteristics in coupling for anesthetic-induced unconsciousness as well as the particular changes caused by ketamine. The brain state during the first 20 minutes of GA can be roughly divided into three states of awareness: baseline, sevoflurane-induced unconsciousness, and anesthetic-induced unconsciousness
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