The bispectral index (BIS) monitor displays a value of 75 in a patient receiving total intravenous anaesthesia with propofol and remifentanil. The anaesthetist increases the propofol infusion. The BIS is a processed EEG parameter. Which EEG feature drives a HIGH BIS value (indicating wakefulness)?

• A High-amplitude delta waves (0.5–4 Hz) with burst suppression
• B K-complexes and sleep spindles indicating NREM sleep
• C Flat isoelectric EEG indicating cerebral inactivity
• D High-frequency beta activity (13–30 Hz) indicating cortical arousal ✓

Explanation

The BIS algorithm incorporates multiple EEG features including power spectrum analysis, bispectral analysis (non-linear coupling of frequency components), burst suppression ratio, and near-suppression percentage. During wakefulness, the EEG shows high-frequency low-amplitude beta activity (13–30 Hz), generating a high BIS value (85–100). As anaesthesia deepens, the EEG shifts to lower frequencies and higher amplitude (theta, delta); burst suppression emerges at BIS 20–40. A flat isoelectric EEG gives BIS near 0. A BIS of 75 is in the lightly sedated/light anaesthesia range, indicating insufficient depth for general anaesthesia (target 40–60).

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

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Written and medically reviewed by the StethoPrep medical team.