The Bispectral Index (BIS) monitor value of 40 in an anaesthetised patient indicates:

• A An inadequate depth of anaesthesia with high risk of awareness
• B An isoelectric EEG consistent with cerebral ischaemia
• C A deep hypnotic state with low probability of consciousness and acceptable risk of awareness ✓
• D Optimal depth for endotracheal intubation without additional analgesia

Explanation

BIS values range from 100 (fully awake) to 0 (isoelectric EEG). A BIS of 40–60 represents adequate general anaesthesia, with values of 40–60 corresponding to a low probability of explicit recall and conscious awareness. A BIS of 40 is within this target range, indicating a deep hypnotic state. An isoelectric EEG corresponds to a BIS near 0. Values above 60 increase the risk of awareness, while values below 40 may represent unnecessarily deep anaesthesia. BIS monitoring is one of several monitors used to guide anaesthetic depth and reduce the risk of intraoperative awareness.

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

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Written and medically reviewed by the StethoPrep medical team.