Bispectral Index (BIS) monitoring is used intraoperatively. A BIS value of 40 is recorded during general anaesthesia. What does this indicate?

• A The patient is in light sleep and may be aware
• B Complete cerebral electrical silence (burst suppression equivalent to iso-electric EEG)
• C Deep hypnotic state appropriate for general anaesthesia with very low probability of consciousness ✓
• D Adequate analgesia confirmed — no additional opioid required

Explanation

BIS is derived from processed frontal EEG and ranges from 0 (isoelectric/burst suppression) to 100 (fully awake). Target range for general anaesthesia is 40–60: BIS 40 is at the lower end of this range, indicating a deep hypnotic state with very low probability of intraoperative awareness. BIS 60–80 indicates sedation; BIS >80 indicates light or no sedation; BIS <40 may indicate excessively deep anaesthesia with burst suppression (BIS 0–20 = isoelectric). BIS monitoring is recommended for patients at high risk of awareness (cardiac surgery, TIVA, difficult intubation with delayed intubating agents). BIS does NOT measure analgesia.

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

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Written and medically reviewed by the StethoPrep medical team.