The Bispectral Index (BIS) monitor displays a value of 40 during general anaesthesia. What does this indicate, and what BIS range is targeted for anaesthetic maintenance?

• A Awake state; target is 60–100
• B Burst suppression; target for maintenance is 20–40
• C REM sleep; target for maintenance is 70–80
• D General anaesthesia with deep hypnosis; target for maintenance is 40–60 ✓

Explanation

The BIS is derived from the processed EEG and correlates with depth of hypnosis: 100 = fully awake, 80–100 = light sedation, 60–80 = moderate sedation, 40–60 = general anaesthesia (target for maintenance to prevent both awareness and excessive depth), 20–40 = deep anaesthesia/burst suppression, <20 = isoelectric EEG. A BIS of 40 indicates adequate but deep general anaesthesia. Targeting BIS 40–60 balances awareness prevention against haemodynamic stability. The B-Aware and B-Unaware trials showed BIS-guided anaesthesia reduces intraoperative awareness compared to routine practice, though not uniformly across all studies.

Reference: Morgan & Mikhail's Clinical Anesthesiology, 6th ed.

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Written and medically reviewed by the StethoPrep medical team.