Anaes · Depth of anaesthesia & awareness
Depth of anaesthesia: processed EEG, BIS and entropy
Also known as Depth of anaesthesia monitoring · Processed EEG · Bispectral index · BIS monitoring · Entropy monitoring · Anaesthetic depth
Processed electroencephalogram monitors — the bispectral index (BIS), spectral entropy and related indices — estimate the depth of anaesthesia by quantifying the shift of the frontal EEG from fast, low-amplitude awake activity to slow, high-amplitude, synchronised anaesthetic activity. They reduce the risk both of awareness and of overdose, but only when the dimensionless index is read alongside the raw trace and the clinical signs, because each is confounded by age, electromyographic activity, electrocautery and paradoxical excitation.
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Red flags
- A processed-EEG index in the surgical range does not guarantee unconsciousness — awareness has occurred with a 'normal' index; always corroborate with the raw trace and clinical signs.
- Electromyographic activity from the frontalis, electrocautery, a pacemaker or a displaced sensor can push the index falsely high or low — check signal quality before acting on the number.
- Paradoxical excitation can raise the index during deepening anaesthesia; an index rising at induction does not always mean the patient is awake.
- Nitrous oxide, ketamine and high-dose opioid can deepen anaesthesia without a proportionate fall in the index — the index under-reads with these agents.
- Burst suppression and an isoelectric EEG signal overdose, not adequate depth; a very low index warrants reducing the anaesthetic.
- Age shifts the bispectral index for a given depth — the elderly may show a lower index at equivalent anaesthetic concentration; interpret relative to the individual baseline.
Meet the patient
A seventy-year-old is sedated with target-controlled propofol for an ERCP. The entropy monitor reads a reassuring fifty-two, square in the surgical range — yet he reaches up and grabs the scope. The number looked perfect; the patient was light.[4]
This is the central lesson of processed-EEG monitoring: the index is a guide, never a guarantee, and it lies whenever a confounder shifts it. Hold habit whole topic builds toward — read the raw trace, not just the number — and the indices become a genuine aid rather than a false sense of security.[2]
References7ShowHide
- [1]Yu H, et al. Depth of anesthesia monitoring: Current evidence, clinical impact, and future directions J Int Med Res, 2026.PMID 42333677
- [2]Berger-Estilita J, et al. Making brain monitoring routine: why processed EEG monitoring should be standard practice J Clin Monit Comput, 2026.PMID 42247119
- [3]Aasheim A, et al. Association of Age, Sex and ASA Physical Status With Bispectral Index Values During General Anaesthesia: A Large Observational Cohort Study Acta Anaesthesiol Scand, 2026.PMID 42289314
- [4]Popovici SE, et al. EEG-Derived Entropy Monitoring During Propofol Sedation for ERCP: Sedation Profiles, Age-Related Effects, and Implications for Procedure-Specific Target Ranges Medicina (Kaunas), 2026.PMID 42356061
- [5]Taplin CF, et al. The quest for real time propofol monitoring: a review Anaesth Intensive Care, 2026.PMID 42290030
- [6]Vakitbilir N, et al. Comparative analysis of processed EEG indices and entropy-based metrics for assessing anesthetic depth: a scoping review - PRISMA-ScR BMC Biomed Eng, 2026.PMID 42098835
- [7]Avidan MS, et al. Anesthesia awareness and the bispectral index N Engl J Med, 2008.PMID 18337600