Bispectral index

Spectral edge frequency

The frequency (Hz) below which 95% of the EEG power resides

usually indicates GA if SEF <= 12Hz (alpha range)

Disadv

artefact prone
inaccurate w/ EEG suppression

Lower SEF values indicate a deeper level of anesthesia.

However, it’s pointed out that the SEF is heavily dependent on filters applied to the raw EEG signal and it poorly reflects the center of the power spectrum distribution and the activities in the low frequency bands. It also compresses the EEG waveform into a single number, losing information on slower EEG frequencies.

Studies have shown that SEF can correlate closely with serum concentrations of certain anesthetics like thiopental, etomidate, and fentanyl, but it can also hide shifts in EEG activity from alpha band to low frequency bands.

Spectrogram signatures

Propofol

High power exhibited:

0 - 4 Hz → Slow / delta oscillation
8 - 12 Hz → Alpha oscillation

Alpha rhythms (8-12Hz) strongly couple the thalamus & cortex restricting communication

slow wave (<1Hz) rhythms create local islands preventing communication w/i the cortex

Anteriorisation is a mechanism for uncoupling the frontal & parietal parts of the brain

Induction

flowchart LR
	A("awake
	low amplitude γ & ß") 
	-->|"LOC
	atonia"| B["↑ power over 0-5Hz
	(slow δ)"]
	B --> B1("slow δ + α")

Emergence

flowchart LR
	A("combined slow δ + α") 
	-->|"unzippering"| B["low power ß & γ oscillation"]

Volatiles

similar to propofol: slow delta & alpha oscillations

When approaching 1 MAC:

alpha oscillation ↓ frequency
theta oscillation appears
→ reflecting non-GABAergic mechanism?

Ketamine

oscillation over high beta / low gamma frequency range 25-30Hz
also slow delta oscillation

less regular than that of propofol / volatile

Dexmedetomidine

lower doses

slow delta oscillations
spindles
- 9 - 15Hz oscillation occurring in bursts
- closely resembles Stage 2 non-REM sleep
  
  higher doses
↑ slow delta oscillations
↓ spindles
→ resemble slow wave sleep (NREM Stage 3)

Effect of NMBA on BIS number

B-AWAKE trial: unreliable indicator of awareness if given NMBA

References

2024-07-11_EEG Monitoring in Anesthesia_CKChan.mp4 (sharepoint.com)

EEG for Anesthesiology - Part 3: Propofol (youtube.com)

Emery Brown (Harvard Med., MIT) 1: Unconsciousness Under General Anesthetic is a Dynamic State (youtube.com)

EEG Derived Monitors of General Anesthesia (youtube.com)

Interpretation of pEEG (youtube.com)

Response of Bispectral Index to Neuromuscular Block in Awake Volunteers† - B-AWAKE