Intraoperative awareness and postoperative cognitive dysfunction (POCD) are among the most feared complications of anesthesia, particularly in the elderly. With the advent of processed EEG technologies, anesthesiologists gained tools to monitor hypnotic depth beyond hemodynamic surrogates. Among these, the Bispectral Index (BIS) remains the most clinically validated.
This article dissects BIS findings from a 76-year-old, 50 kg female undergoing open supraumbilical hernioplasty under general anesthesia with sevoflurane and atracurium, where BIS stabilized around 49. This value lies within the target range for surgical anesthesia (40–60), but its interpretation requires integration with molecular pharmacology, physiology of aging, and clinical data.
References – Introduction
Rampil IJ. A primer for EEG signal processing in anesthesia. Anesthesiology. 1998;89(4):980–1002.
Punjasawadwong Y, et al. Bispectral index for improving anaesthetic delivery and postoperative recovery. Cochrane Database Syst Rev. 2014;(6):CD003843.
BIS is derived from frontal EEG using algorithms combining:
Power spectral analysis (relative beta, alpha, delta power).
Bispectral analysis (phase coupling between frequency bands).
Time domain features (burst suppression detection).
These are condensed into a dimensionless scale (0–100). Cortical synchronization increases with anesthetic depth, producing slower oscillations (delta/theta) and reduced BIS.
Sevoflurane: enhances GABA-A receptor Cl⁻ currents, hyperpolarizing cortical neurons → increased slow-delta oscillations → BIS falls.
Propofol (if given): similar GABA-A enhancement, with prominent alpha-delta EEG signatures.
Opioids: act at μ-opioid receptors; minimal direct cortical EEG change (hence BIS less sensitive).
Atracurium: nicotinic ACh receptor antagonist at NMJ → muscle paralysis → reduces EMG interference but does not affect cortical EEG.
Awake: beta (13–30 Hz) dominance.
Light sedation: alpha (8–12 Hz) increases.
Surgical anesthesia: slow-delta (0.1–4 Hz) with reduced beta → BIS 40–60.
Burst suppression: intermittent flatline + spikes → BIS <20.
References – BIS Physiology
Brown EN, Purdon PL, Van Dort CJ. General anesthesia and altered states of arousal: a systems neuroscience analysis. Annu Rev Neurosci. 2011;34:601–28.
Sleigh JW, et al. EEG signatures of anesthetic drugs. Anesth Analg. 2011;113(3):539–51.
MAC decreases ~6% per decade after 40.
At 76 years: MAC ~0.7 of young adult.
Causes: reduced synaptic density, altered lipid membrane composition, decreased NMDA receptor expression.
Aging → cortical thinning, reduced white matter connectivity, increased baseline EEG slowing.
Elderly brains produce slower...
