Bispectral Index

  • BIS is a numerical EEG-derived index (range 0–100) estimating level of consciousness.

  • Primarily monitors hypnotic depth, not analgesia or neuromuscular blockade.

BIS Value

Interpretation

100

Awake

80–100

Light sedation

60–80

Moderate sedation

40–60

General anaesthesia (target range)

<40

Deep anaesthesia / burst suppression

0

Isoelectric EEG

Indications for Use

  • High-risk of awareness (e.g., TIVA, paralysed patients)

  • Optimise anaesthetic delivery in elderly or fragile patients

  • Short cases where rapid emergence is beneficial

Benefits & Safety

  • May reduce awareness risk under GA (especially with TIVA + NMBD)

  • Decreases anaesthetic use and improves titration

  • Shortens recovery times and PACU duration

Limitations & Pitfalls

  1. Time lag: BIS value reflects EEG from 15–30 seconds ago.

  2. Not analgesia: BIS does not detect pain or nociception.

  3. Interference: Poor electrode contact, EMG (muscle activity), diathermy, pacemaker noise.

  4. Drug-specific limitations:

    • Ketamine: dissociation with high BIS despite unconsciousness.

    • Dexmedetomidine/N₂O: unreliable BIS suppression.

  5. Neurologic variability: Baseline altered in elderly, stroke, seizure disorders.

Advanced BIS Metrics

Suppression Ratio (SR)

  • Percentage of time EEG is isoelectric over the last 63 seconds.

  • SR > 10 → excessive anaesthesia (deep burst suppression)

  • Use in neuro cases or to detect excessively deep anaesthesia

  • Clinical tip: SR >10 with BIS <40 = excessive depth → reduce agent

EMG (Electromyography)

  • Reflects frontalis muscle activity (measured 0–100 scale)

  • EMG >30–40 → indicates increased muscle activity or tension

    • Can falsely elevate BIS

    • Common in light anaesthesia, pain, or inadequate NMBD

  • Clinical tip: Sudden BIS rise with high EMG = probably artefact or inadequate block → Confirm paralysis, deepen anaesthesia if needed

Clinical Application Tips

  • Target BIS 40–60 for GA (with SR <10, EMG <30)

  • Combine with:

    • Clinical signs (HR, BP, lacrimation, movement)

    • Neuromuscular monitoring

  • If BIS rises:

    • Check IV line

    • Confirm EMG and SR readings

    • Assess for inadequate anaesthesia or equipment issue

Key BIS Safety Strategy

  1. Confirm signal integrity (Signal Quality Index >50)

  2. Monitor trends, not just single values

  3. Investigate any BIS rise >60 or SR >10

  4. Don’t rely on BIS alone—correlate with full clinical picture

  5. If paralysed: use BIS + SR to ensure adequate depth

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