Changes in the electroencephalogram during anaesthesia and their physiological basis

Area of Science:

• Anesthesiology
• Neuroscience
• Signal Processing

Background:

• Electroencephalogram (EEG) monitoring is widely used in anesthesia.
• Clinicians often rely on processed EEG indices like Bispectral Index (BIS™), not raw EEG data.
• Understanding raw EEG changes is vital for appreciating the reliability of these indices.

Purpose of the Study:

• To investigate how raw EEG waveforms change during volatile and propofol anesthesia.
• To correlate EEG changes with anesthetic depth and noxious stimuli.
• To propose an optimal anesthesia strategy based on EEG patterns.

Main Methods:

• Analysis of hemi-frontal lead EEG data from patients under volatile or propofol anesthesia.
• Observation of EEG frequency, amplitude, and waveform patterns at different anesthetic depths.
• Evaluation of EEG response to noxious stimuli like surgical skin incision.

Main Results:

• Anesthetic agents decrease EEG frequency and increase amplitude as concentration rises.
• EEG patterns progress from alpha/spindle waves to theta/delta waves, then burst suppression, and finally flatness with increasing anesthetic depth.
• Raw EEG, particularly spindle waves, shows sensitivity to noxious stimuli, which processed indices may miss.

Conclusions:

• Raw EEG provides a more comprehensive picture of anesthetic depth than processed indices alone.
• Spindle wave activity is a sensitive indicator of both anesthetic depth and noxious stimuli.
• Maintaining anesthesia to favor spindle wave predominance may be an effective strategy to prevent intraoperative awareness.