Case Summary

A 23-year-old ASA I male is undergoing inferior parathyroid adenoma excision. Intubation was performed with a NIM (nerve integrity monitoring) endotracheal tube, and only 25 mg of atracurium was administered 90 minutes earlier. No further neuromuscular blockade was used to preserve nerve monitoring.

References

Randolph GW, Dralle H, Abdullah H, et al. Electrophysiologic recurrent laryngeal nerve monitoring during thyroid and parathyroid surgery: international standards guideline statement. Laryngoscope. 2011;121 Suppl 1:S1–16.

BIS Monitor Values

• BIS: 61
• Signal Quality Index (SQI): 97
• Electromyographic (EMG) activity: 28
• Suppression Ratio (SR): 0

References

Johansen JW, Sebel PS. Development and clinical application of electroencephalographic bispectrum monitoring. Anesthesiology. 2000;93(5):1336–44.

Physiology of BIS and EMG Interaction

The Bispectral Index (BIS) is derived from processed frontal EEG signals:

• Low-frequency EEG (delta, theta, alpha): sedation, unconsciousness.
• High-frequency EEG (beta, gamma): arousal, wakefulness.

Problem: EMG contamination

• Frontal muscle activity produces signals in the 30–47 Hz range, overlapping with EEG beta/gamma frequencies.
• This overlap falsely elevates BIS, suggesting lighter anesthesia than reality.
• Thyroid/parathyroid surgery (with no relaxant) often shows high EMG interference.

References

Rampil IJ. A primer for EEG signal processing in anesthesia. Anesthesiology. 1998;89(4):980–1002.

Dahaba AA. Different conditions that could result in the bispectral index indicating an incorrect hypnotic state. Anesth Analg. 2005;101(3):765–73.

BIS Parameters: Normal Ranges and Significance

ParameterNormal RangeAbnormal Value & Clinical SignificanceBIS40–60 (surgical anesthesia)>65 = light anesthesia, awareness risk; <40 = excessive anesthesia, delayed recoverySQI>90%<80 = poor signal quality; values unreliableEMG<20>30 = contamination of BIS (falsely high readings)SR (Suppression Ratio)0–2%>10% = burst suppression, very deep anesthesia; >40% = excessive depth, brain risk

References

Myles PS, Leslie K, McNeil J, Forbes A, Chan MT. Bispectral index monitoring to prevent awareness during anaesthesia: the B-Aware trial. Lancet. 2004;363(9423):1757–63.

Pilge S, Zanner R, Schneider G, Blum J, Kreuzer M, Kochs EF. Time delay of electroencephalogram index calculation: analysis of cerebral state, bispectral, and narcotrend indices. Anesthesiology. 2006;104(3):488–94.

Interpretation in This Case

• BIS: 61 → Upper end of surgical range; likely artifactually high due to EMG.
• SQI: 97 → Reliable data.
• EMG: 28 → Elevated due to absence of relaxant; artificially raises BIS.
• SR: 0 → No burst suppression; not excessively deep.

Integrated Clinical Meaning:

• Patient is deeper than BIS suggests (because EMG is elevating BIS).
• Sevoflurane 1.1 MAC ensures adequate hypnosis.
• Clinical parameters (HR 66, MAP 57) confirm stability.

References

Sleigh JW, Leslie K, Voss L. The bispectral index: a measure of depth of sleep or sedation? Best Pract Res Clin Anaesthesiol. 2008;22(1):81–93.

Awareness vs Depth Clarification

• A BIS of 40–60 reduces the probability of awareness, but does not guarantee unconsciousness.
• BIS is a probabilistic tool, not an absolute marker.
• BIS 60–65 may be acceptable in short procedures if MAC and hemodynamics are reassuring.

References

Mashour GA, Shanks A, Tremper KK, et al. Prevention of intraoperative awareness with explicit recall in an unselected surgical population: a randomized trial. Anesthesiology. 2012;117(4):717–25.

Anesthetic Drug Implications

• Sevoflurane 1.1 MAC → reliably slows EEG into delta/theta range. If BIS remains high, EMG interference is most likely.
• Opioid sparing → may allow BIS elevation despite adequate volatile depth.
• Ketamine/N₂O → unreliable with BIS, as they elevate EEG frequency despite unconsciousness.

References

Aime I, Verdonck O, Ben Abdelaziz R, et al. Effect of nitrous oxide on bispectral index during sevoflurane anesthesia. Anesthesiology. 2006;104(3):488–94.

Hans P, Dewandre PY, Brichant JF, Bonhomme V. Comparative effects of ketamine on BIS and spectral entropy. Br J Anaesth. 2005;94(3):336–40.

Hemodynamic Correlation

• MAP 57, HR 66 bpm → borderline hypotension but acceptable in a young, fit patient.
• Would be concerning in elderly, carotid stenosis, or cerebrovascular disease.
• Emphasizes: BIS must always be cross-checked with MAP/HR.

References

Monk TG, Saini V, Weldon BC, Sigl JC. Anesthetic management and one-year mortality after noncardiac surgery. Anesth Analg. 2005;100(1):4–10.

Evidence Base and Critical Commentary

• B-Aware (2004): BIS reduced awareness in high-risk TIVA. Limitation: benefit not shown in general population.
• B-Unaware (2008): No difference between BIS and MAC monitoring. Implication: end-tidal monitoring equally effective.
• Cochrane 2019: BIS reduces anesthetic dose and recovery time, but awareness prevention inconsistent.
• Guidelines: ASA/NICE recommend BIS for high-risk awareness cases, not universally.

References

Avidan MS, Zhang L, Burnside BA, et al. Anesthesia awareness and the bispectral index. N Engl J Med. 2008;358(11):1097–108.

Punjasawadwong Y, Phongchiewboon A, Bunchungmongkol N. BIS for improving anaesthetic delivery and postoperative recovery. Cochrane Database Syst Rev. 2019;6:CD003843.

Decision-Making Algorithm

• BIS >65 + High EMG → Artifact → Check relaxant status, analgesia, electrode placement.
• BIS >65 + Low EMG → True light anesthesia → Increase volatile or opioid.
• BIS 40–60 → Adequate anesthesia → Maintain.
• BIS <40 + Low EMG → Excessive depth → Reduce anesthetic dose, support BP.
• BIS <40 + High EMG → Rare → Treat as overdose with artifact contribution.

References

Kertai MD, Whitlock EL, Avidan MS. Brain monitoring with EEG and BIS during cardiac surgery. Anesth Analg. 2012;114(3):533–46.

Clinical Decision Box

• Red Flag: BIS >70 + Low MAC + Hypotension = High awareness risk.
• Green Flag: BIS 50–60 + MAC ≥1 + Stable vitals = Safe anesthetic depth.

Teaching Points for Residents

• BIS is probabilistic, not absolute.
• Always cross-check BIS with EMG, MAC, and hemodynamics.
• Common pitfalls in thyroid/parathyroid surgery (NIM tube, no relaxant).
• Do’s: Use BIS as adjunct in TIVA/high-risk cases.
• Don’ts: Never interpret BIS in isolation, or rely on it in ketamine/N₂O anesthesia.