Processed EEG Interpretation

Processed EEG monitors are:

Commonly used in general anaesthesia
Particularly TIVA.
May reduce incidence of anaesthetic awareness
Particularly in high-risk patients.
Provide both a:
- Set of numbers crudely indicating anaesthetic depth
- Raw EEG waveform
A poor predictor of responsiveness to nociceptive stimulation

Like any monitor, outputs may be misleading:

Artifacts are common
May be:
- Extracranial
  - Electrical equipment
  - Pacemakers
  - EEG
  - Movement
- Extra-cerebral
  - Muscular activity
- Pathologic cranial activity
  - Seizures
  - Normal low-amplitude EEG
    Present in 5-10% of the population.
Need to be interpreted in the broader clinical context

Principles

Frontal cortical activity is ↑ by anaesthetic agents
Forehead allows this to be measured.
Electrodes detect potential difference from multiple current sources
- Frontalis
- Extrocular muscles
- Heart

Common EEG Patterns

EEG has no fixed repeating pattern, and will change randomly over time

In steady levels of anaesthesia or wakefulness, it will have some constant statistical features

Awake:

Small amplitude, high frequency
“Fuzzy”
Blinks and eye movements
May be high amplitude and high frequency EMG activity
Alpha oscillations if relaxed and eyes closed

Drowsy:

Loss of EMG, eye movements, and blinks
↑ amplitude

Light anaesthesia:

Bigger, slower waves that wax and wane
Sleep spindles
Slow delta waves

Deep anaesthesia:

Longer lasting, slower spindles
Burst suppression
Isoelectricity

Drug Effects on the EEG

Effect of anaesthetic adjuncts:

Ketamine
- ↑ high frequency EEG activity
  ↑ BIS and M-entropy values.
Nitrous oxide
Generally minimal effect:
- Unconsciousness occurring predominantly from nitrous oxide results in a high BIS value
- Addition of nitrous oxide results in minimal EEG change
- Has been associated with a paradoxical ↓ in BIS values 5-10 minutes after discontinuation
Opioids
Variable effect:
- Small doses typically minimal effect
- Larger doses may produce slowing
- Do reduce dose requirement of GABAergic agents
α2-agonists
Similar to GABAergic drugs.

References

Bennett C, Voss LJ, Barnard JPM, Sleigh JW. Practical Use of the Raw Electroencephalogram Waveform During General Anesthesia: The Art and Science: Anesthesia & Analgesia. 2009 Aug;109(2):539–50.