Monitoring anesthetic depth ensures both safety and neuroprotection. The Bispectral Index (BIS) converts raw electroencephalography (EEG) data into a numerical measure (0–100) of cortical hypnosis.
This chapter tracks BIS, Total Power (TP), and Spectral Edge Frequency (SEF) across three stages in a 78-year-old female undergoing completion thyroidectomy using oxygen–nitrous oxide–sevoflurane anesthesia.
Understanding the physiologic meaning and normal ranges of TP, SEF, and SR allows clinicians to distinguish genuine cortical suppression from drug synergy or artifact. The chapter helps residents view BIS as the language of the brain, not just a number.
Imagine the cortex as an orchestra:
Awake: many instruments play fast, complex rhythms (beta 20–30 Hz).
Light anesthesia: tempo slows; alpha (8–13 Hz) dominates.
Deep hypnosis: only slow drums (delta 0.5–4 Hz) remain.
Burst suppression: brief music, then silence.
The BIS monitor listens and translates this into a 0–100 scale—anesthetic depth in real time.
Elderly brains have:
Fewer cortical neurons
Reduced beta activity and metabolic rate
Increased vulnerability to prolonged suppression
Thus, a BIS ≈ 25 in an 80-year-old can represent deeper anesthesia than the same BIS in a 40-year-old. Monitoring helps avoid postoperative delirium and cognitive decline.
Key insight:
TP reflects how loud the orchestra plays, SEF how fast the rhythm is, and SR how often it stops playing.
During thyroidectomy, avoidance of residual paralysis is critical for recurrent laryngeal nerve monitoring. With a NIM tube in place and no muscle relaxant, BIS offers a unique real-time assessment of hypnotic depth unaffected by neuromuscular block.
Teaching Pearls
BIS < 40 for > 30 min in elderly → avoid (risk of postoperative delirium).
TP < 50 µV² = excessive suppression; re-evaluate anesthetic.
SEF < 10 Hz = very slow EEG; lighten anesthesia.
SR > 10 % = burst suppression; prevent cerebral hypoperfusion.
Always interpret BIS together with TP, SEF, SR, EMG, and hemodynamics.
References
Rampil IJ. A primer for EEG signal processing in anesthesia. Anesthesiology. 1998;89(4):980-1002.
Purdon PL et al. Clinical electroencephalography for anesthesiologists I–III. Anesthesiology. 2015;123:937-85.
Brown EN, Lydic R, Schiff ND. General anesthesia, sleep, and coma. N Engl J Med. 2010;363:2638-50.
Age/Sex: 78-year-old female (ASA II)
Procedure: Completion thyroidectomy
Previous Surgery: Right hemithyroidectomy → right vocal-cord palsy
Airway Device: 7.0 mm NIM ET tube for RLN monitoring
Special Requirement: No long-acting relaxant after
