The Bispectral Index (BIS) monitor provides a dimensionless number from 0–100 reflecting depth of anaesthesia. A BIS value of 40–60 corresponds to:
- A Awake and alert state
- B Light sedation with preserved response to commands
- C General anaesthesia suitable for surgery with low probability of explicit recall ✓
- D Isoelectric (burst suppression) EEG pattern associated with excessive depth
Correct answer: C. General anaesthesia suitable for surgery with low probability of explicit recall
Explanation
BIS scale interpretation: 100 = fully awake; 80–100 = sedation/light sleep; 60–80 = moderate sedation; 40–60 = general anaesthesia (target for surgery — adequate depth, low probability of explicit awareness); 20–40 = deep anaesthesia/burst suppression; 0–20 = isoelectric EEG. A BIS of 40–60 is the standard intraoperative target during general anaesthesia. The B-Unaware and BAG-RECALL trials showed BIS guidance reduces intraoperative awareness in high-risk patients compared to standard monitoring alone.
Reference: Morgan & Mikhail's Clinical Anesthesiology, 6th ed.
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Written and medically reviewed by the StethoPrep medical team.