Tuning Into BIS Truth: Unraveling the Static in Anesthesia Monitoring

Ink & Air

Patient Profile

• 51-year-old female undergoing surgery.
• Anesthesia maintained at MAC 1.2, corresponding to 1.2 times the alveolar concentration of an inhalational anesthetic such as sevoflurane.
• Intended depth: sufficient to prevent movement and maintain unconsciousness.

Monitoring Data (Philips IntelliVue System)

• BIS: 95 (expected 40–60 under general anesthesia).
• SQI: 56 (normal >70).
• EMG: 37 (normal <30).
• Heart rate: 58 bpm (normal 60–100 bpm).
• Blood pressure: 72/45 mmHg (normal 90/60–120/80 mmHg).
• SpO₂: 100% (normal >95%).
• Arterial pressure: 72/43 mmHg.

Contextual Factors

• Date/Time: August 1, 2025, at 11:54 AM IST.
• Electrode Placement: Around the articular (temporomandibular joint) area, increasing risk of artifact contamination.
• Observation: Significant discrepancy between BIS (95) and hemodynamic stability, suggesting artifacts rather than inadequate anesthesia.

Artifacts Affecting BIS Readings

Electromyographic (EMG) Interference

• EMG 37 exceeded normal range (<30), indicating substantial muscle activity.
• Muscle activity contaminates EEG signals, falsely elevating BIS values.
• Articular electrode placement increases exposure to muscular and joint movement artifacts.
• Literature shows EMG >30 µV can increase BIS by 10–20 units depending on contraction intensity.

Signal Quality Index (SQI)

• SQI 56 was below the acceptable range (>70).
• Low SQI suggests poor electrode contact or noise contamination.
• Articular proximity increases susceptibility to motion artifacts.
• SQI <70 is associated with 15–25% error rates in BIS interpretation.

Anesthetic Context

• At MAC 1.2, expected BIS range is 40–60.
• A BIS of 95 strongly indicates artifact rather than insufficient anesthetic depth.
• Non-standard electrode placement (articular site) increases artifact susceptibility by 30–40% compared to frontal Fp1–Fp2 placement.

Clinical Implications

Artifact Impact

• BIS 95 may falsely suggest intraoperative awareness.
• True incidence of awareness: approximately 0.1–0.2%.
• Stable hemodynamics (bradycardia, hypotension, normoxia) are inconsistent with awareness, which is often accompanied by tachycardia and hypertension.

Risks of Misinterpretation

• Over-reliance on BIS alone may result in unnecessary deepening of anesthesia.
• Consequences of excessive anesthetic dosing include hypotension, prolonged emergence, and postoperative cognitive dysfunction.
• Conversely, failure to address artifact-related discrepancies risks inadequate monitoring fidelity.

Electrode Placement Challenges

• Articular region placement amplifies EMG and motion artifacts.
• Proper frontal electrode placement remains critical for reliable BIS data.

Recommendations for Anesthesia Practice

Artifact Recognition

• Continuously monitor EMG, SQI, and BIS values.
• Recognize thresholds: EMG <30, SQI >70, BIS 40–60.
• Use automated alerts where available.

Electrode Optimization

• Reassess electrode positioning when BIS values deviate from expected ranges.
• Avoid articular sites; preferentially use frontal placements (Fp1–Fp2).
• Proper placement reduces artifact incidence by 20–30%.

Muscle Activity Management

• Consider neuromuscular blocking agents if excessive EMG persists.
• Reposition electrodes away from active muscle regions.
• These interventions reduce EMG-related interference by 15–20%.

Corroborative Clinical Assessment

• Always integrate BIS values with clinical signs, hemodynamic parameters, and anesthetic concentration.
• Cross-checking minimizes false positives and improves anesthetic titration accuracy.

Documentation

• Record electrode location, BIS artifacts, and corrective actions.
• Documentation supports postoperative review and quality assurance.

Conclusion

• A 51-year-old female patient demonstrated a BIS of 95 despite MAC 1.2 and stable hemodynamics.
• Artifact sources included elevated EMG (37), low SQI (56), and articular electrode placement.
• These factors likely explain the elevated BIS, rather than true light anesthesia or awareness.
• Optimal practice involves proper electrode placement, vigilant artifact recognition, and correlation with clinical parameters.
• Integration of BIS with clinical judgment ensures safe and effective anesthesia management.