What are the considerations for performing an electroencephalogram (EEG) under anesthesia in patients who cannot undergo the procedure while awake due to age, severe neurological conditions, or other factors?

EEG Under Anesthesia

EEG can be safely and effectively performed under anesthesia when patients cannot cooperate due to age, severe neurological conditions, or other factors, but anesthetic agents significantly alter EEG patterns and must be carefully selected and monitored to ensure interpretable recordings.

Key Considerations for Patient Selection

Patients who require EEG under anesthesia include:

• Young children who cannot remain still during the procedure 1
• Patients with severe neurological conditions preventing cooperation 1
• Uncooperative patients where standard EEG recording is not feasible 2
• Neonates and infants requiring brain death determination (37 weeks gestation to 30 days of age) 1

Anesthetic Agent Selection and EEG Impact

Critical principle: Different anesthetics produce distinct EEG signatures that must be understood for proper interpretation 3.

Preferred Anesthetic Agents:

• Propofol: Produces predictable alpha-spindles (around 10 Hz) phase-amplitude coupled with slow delta waves (around 1 Hz), making EEG interpretation more reliable 4, 3
• Volatile anesthetics (sevoflurane, isoflurane, desflurane): Produce characteristic dose-dependent EEG changes with alpha-spindle patterns at appropriate depths 3

Agents to Avoid or Use with Caution:

• Ketamine: Does NOT produce the typical alpha-spindle coupled with delta wave pattern, making standard EEG interpretation difficult 4, 3
• Nitrous oxide: Similarly lacks predictable alpha-spindle patterns 4, 3
• High-dose benzodiazepines: Produce anterior fast (beta range) activities that can obscure underlying pathology 1

Monitoring Requirements During Anesthetized EEG

Essential monitoring includes:

• Processed EEG (pEEG) monitoring such as BIS when using total intravenous anesthesia (TIVA) with neuromuscular blockade 1, 5, 6
• Standard ASA monitoring: ECG, SpO2, blood pressure, capnography 6
• Core body temperature maintained ≥35°C (95°F) 1
• End-tidal anesthetic concentration (ETAC) monitoring when using volatile agents, with alarm set for <0.7 age-adjusted MAC 1

Anesthetic Depth Optimization for EEG Recording

Target lighter levels of anesthesia to preserve interpretable EEG patterns:

• For patients >60 years: Target BIS approximately 50 rather than deeper levels (BIS 35) 5
• Avoid burst suppression patterns which indicate excessive anesthetic depth 5
• Maintain ETAC >0.7 age-adjusted MAC during volatile anesthetic maintenance 1
• Use slow induction rates (approximately 20 mg propofol every 10 seconds) rather than rapid bolus to prevent excessive depth 7

Special Population Considerations

Pediatric Patients:

• Neonates (37 weeks to 30 days): EEG is less sensitive in this age group; consider that ancillary testing may have limitations 1
• Children >30 days: EEG and cerebral blood flow studies have equal sensitivity 1
• Higher maintenance infusion rates typically needed in younger children (200-300 mcg/kg/min propofol initially, then 125-150 mcg/kg/min) 7

Elderly and Debilitated Patients:

• Avoid rapid bolus administration which increases cardiorespiratory depression 7
• Older adults may not generate strong alpha-spindle patterns during anesthesia, limiting EEG interpretation 4
• Target lighter anesthesia depth (BIS ~50) to reduce postoperative delirium risk 5

Patients with Neurodegenerative Disorders:

• Less likely to generate predictable alpha-spindle patterns during general anesthesia 4
• May require alternative interpretation strategies or ancillary testing 4

Critical Pitfalls and How to Avoid Them

Environmental interference:

• The ICU and operating room are electrically contaminated environments that can obscure EEG signals 1, 8
• Evoked potentials are more resistant to electrical noise and anesthetic effects than raw EEG 1
• Ensure proper electrode placement and artifact reduction techniques 8

Anesthetic-related confounders:

• EEG patterns suggesting adequate anesthesia do not guarantee absence of awareness in rare cases 4
• Some patients may follow commands despite frontal EEG showing apparent general anesthesia patterns, though typically without distress or memory formation 4
• Abrupt discontinuation of propofol can cause rapid awakening with anxiety and agitation 7

Interpretation limitations:

• BIS and other processed EEG indices are NOT validated for brain-injured patients in the ICU 1
• Large intra- and inter-individual variability exists with BIS measurements 1
• The frontal EEG alone is "an imperfect but clinically useful mirror" of brain function 4

Procedural Algorithm for Anesthetized EEG

1. Pre-procedure assessment:

   • Identify reason patient cannot undergo awake EEG (age, cooperation, neurological condition) 1, 2
   • Review medications that may affect EEG interpretation 1
   • Ensure core temperature will be maintained ≥35°C 1

2. Anesthetic selection:

   • Choose propofol or volatile agents for predictable EEG patterns 4, 3
   • Avoid ketamine and nitrous oxide if possible 4, 3
   • Plan for lighter anesthetic depth to preserve EEG interpretability 5, 4

3. Induction and maintenance:

   • Use slow induction (20 mg propofol every 10 seconds) 7
   • Target maintenance with propofol 100-200 mcg/kg/min or ETAC >0.7 MAC 1, 7
   • Monitor with pEEG (BIS) if using TIVA with neuromuscular blockade 1, 5, 6

4. EEG recording:

   • Begin EEG recording after stable anesthetic state achieved 9
   • Document anesthetic agents, doses, and timing relative to EEG changes 9
   • Minimize electrical interference and optimize electrode placement 8

5. Emergence:

   • Avoid abrupt discontinuation of propofol 7
   • Maintain light sedation through emergence if needed 7
   • Document recovery and any neurological changes 1

Documentation Requirements

Essential documentation includes:

• Time of second examination and completion of testing (if for brain death determination) 1
• All anesthetic agents, doses, and timing 9
• Core temperature throughout procedure 1
• Any technical difficulties or artifacts encountered 8
• Correlation of EEG findings with clinical examination 1