The Bispectral Index (BIS) monitor processes the EEG to generate a dimensionless number (0–100). A BIS value of 40–60 corresponds to:

• A Deep hypnosis with burst suppression — appropriate for brain protection in neurosurgery
• B Sedation level equivalent to midazolam 2 mg IV — patient arousable to verbal stimulus
• C Awake but calm baseline — equivalent to light premedication
• D General anaesthesia depth appropriate for surgical anaesthesia with low awareness risk ✓

Explanation

The BIS scale ranges from 0 (isoelectric EEG) to 100 (fully awake). Clinical target ranges are: >80 awake/light sedation; 60–80 moderate sedation; 40–60 general anaesthesia for surgery (optimal range to minimise awareness while avoiding overly deep anaesthesia); <40 deep anaesthesia with increasing burst suppression risk; and <20 burst suppression. Maintaining BIS 40–60 during surgery is associated with reduced intraoperative awareness, less inhalational agent use, and faster recovery compared to unmonitored anaesthesia, although BIS does not eliminate awareness entirely.

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

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Written and medically reviewed by the StethoPrep medical team.