Depth of anesthesia: EEG findings

A full 16-lead, 8-channel electroencephalogram (EEG) is a recording of the electrical activity/potentials occurring in the cerebral cortex and can be used to monitor depth of anesthesia. EEG activity occurs mostly at frequencies between 1-30 Hz and can be broadly categorized into 4 wave patterns: alpha, beta, theta, and delta. Alpha waves, with a frequency of 8-14 Hz, are found in a relaxed but alert patient (i.e. resting with eyes closed). Beta waves, with a frequency of 14-30 Hz, are found in a highly alert and focused patient. Theta waves, with a frequency of 4-8 Hz, are found in the first stage of sleep or during “light” anesthesia. Delta waves, with a frequency of 0.5-4 Hz, are found in a deep sleep or “deep” anesthesia.

A BIS monitor processes two-channel EEG signals and examines 4 components associated with the anesthetic state: high frequency beta activation found during light anesthesia, low frequency delta waves found during deep anesthesia, suppressed EEG waves, and burst suppression.

EEG and BIS findings (~with example reading) in different patient states are summarized below.

Awake: EEG shows very high frequency, very low amplitude beta > alpha waves. BIS reveals high beta ratio (~96).

Sedated: EEG shows high frequency, low amplitude alpha/theta > delta waves. BIS reveals low beta ratio (~78).

Unresponsive: EEG shows spindles, K complexes, and alpha/theta/delta waves. BIS reveals bispectral coherence (~52).

Surgically anesthetized: EEG shows slow delta wave predominance. BID reveals bispectral coherence (~42).

Deeply anesthetized: EEG shows burst suppression and isoelectricity. BIS reveals high burst suppression ratio (~8).