EEG/BIS Monitoring Tips

1) Place the EEG and obtain a waveform prior to induction.

2) Phillips Monitor Setup:

a. Display at least 2 channels dedicated to the EEG so that the waveform can be displayed at 2 speeds:

i. 6.25 or 12.5 mm/sec are good for visualization of slow waves

ii. 25 or 50 mm/sec are good for visualization of [theta, alpha, and sigma] spindles

b. Set the EEG amplitude scale to 50 μV (or sometimes 100 μV)

c. Turn EEG filter OFF to prevent low frequency filtering (i.e., delta wave filtering).

d. EEG alarms: BIS alarm range is set to low/high limits of 40/60 by default.

e. Turn ON relevant parameters: The BIS (bispectral index), SQI (signal quality index), SR (suppression

ratio), EMG (electromyographic strength), and SEF (spectral edge frequency 95%).

* EEG waveform reflects changes in patients’ hypnotic state much more rapidly that processed EEG parameters*

3) EEG waveform nomenclature and frequency ranges:

 Delta: 1 - 4 or 0 - 4 Hz; Slow delta: <1 Hz

 Theta: 4 - 8 Hz

 Alpha: 8 - 12 Hz (Mu)

 Beta: 12 - 30 Hz (or 14 - 30 Hz); Low beta:

<20 Hz; High beta: 20 - 30 Hz

 Gamma: >30 Hz (to 100s Hz)

 Sigma: 12 - 14 Hz; frequency band for sleep

spindles during physiological sleep

4) EEG of the awake patient: Dominated by high frequency (i.e., high beta and gamma) activity, usually of low

amplitude, producing a fuzzy-appearing wave on the faster 25-50 mm/s tracing. High-frequency, high-amplitude

activity (EMG) and periodic high-amplitude deviations (from blinking) may be observed.

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5) EEG during general anesthesia: A pattern of slow delta waves (<1 Hz) coupled with alpha spindles (8-12 Hz) is

often desired. There should be an absence of high beta (20-30 Hz) waves and absence of any periods of burst

suppression.

 Waves in the alpha (8-12 Hz), theta (4-8 Hz) or low beta (12-20 Hz) frequency ranges, often termed

spindles, may not be prominent in older patients with cognitive impairment

EEG waveform demonstrating slow waves in the delta range (~1 Hz) and spindle pattern consistent with general

anesthesia:

Same image from above zoomed in: dotted white trace demonstrates waves in the slow delta range (~1Hz):

6) EEG suppression: Any flattened interval on the EEG tracing (EEG suppression) indicates excessive anesthetic

depth or suggests the presence of other suppressive stimuli (e.g., cerebral ischemia).

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EEG Suppression

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Very slow high amplitude delta waves

Persistent alpha (~10Hz) spindle pattern

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