BIS vs Entropy Monitoring
Neither BIS nor entropy monitoring should be preferentially selected over the other based on current evidence, as major guidelines do not distinguish between specific processed EEG (pEEG) monitoring technologies—both are considered equivalent for clinical decision-making regarding depth of anesthesia. 1
Guideline Recommendations
The 2021 Association of Anaesthetists guidelines on monitoring standards make no distinction between BIS, entropy, or other pEEG monitoring systems when recommending their use 1. The guidelines specify that:
- Processed EEG monitoring should be used when TIVA is administered together with neuromuscular blockade 1
- pEEG monitoring should be considered when TIVA is used alone 1
- There may be a role for pEEG monitoring even during inhalational anesthesia to reduce risk of excessive anesthesia 1
Critically, anaesthetists should not rely solely on index values from any pEEG monitor—they must develop understanding of raw EEG waveforms and power spectral analysis 1.
Clinical Performance Differences
While guidelines treat these monitors equivalently, research reveals important practical differences:
Entropy Shows More Discordant Elevations
Entropy demonstrates significantly higher rates of falsely elevated readings during adequate anesthesia. In a large cohort study, state entropy (SE) values >70 occurred at sevoflurane concentrations >1.5% (well above awareness threshold) in 3.6% of cases versus only 0.24% for BIS—a 17-fold difference 2. This means entropy is more likely to falsely suggest inadequate anesthesia when the patient is actually adequately anesthetized, potentially leading to unnecessary deepening of anesthesia.
Correlation and Agreement
Studies show moderate correlation between BIS and entropy (Kappa ~0.54, bias of -2 with wide limits of agreement) 3. While completely contradictory decisions would be rare, there is substantial risk of disagreement about whether to adjust anesthetic depth 3.
Response Characteristics
Entropy (particularly response entropy) provides earlier warning of lightening anesthesia compared to BIS 4. However, both monitors show delays in detecting loss of consciousness during induction, particularly with certain agents like xenon 5.
Artifact Susceptibility
Entropy indices are more susceptible to electromyographic (EMG) contamination than BIS 6, which can falsely elevate readings with muscle activity. Conversely, entropy may be more resistant to certain other artifacts 6.
Practical Algorithm for Selection
Use either monitor based on institutional availability and clinician familiarity, with these caveats:
If using entropy: Be aware that SE values >70 may occur frequently even at adequate anesthetic depth with sevoflurane 2. Do not reflexively deepen anesthesia based on elevated entropy alone—verify with clinical signs and end-tidal anesthetic concentration.
If using BIS: Recognize it may lag slightly behind entropy in detecting lightening anesthesia 4, though this difference is clinically modest.
For both monitors:
- Target range 40-60 during maintenance 7
- Always correlate with clinical signs and end-tidal anesthetic concentration
- Review raw EEG waveforms, not just index numbers 1
- Be especially cautious in elderly patients and with different anesthetic agents, as age and drug type significantly influence monitor performance 6
Key Pitfalls to Avoid
- Do not assume elevated entropy values always indicate inadequate anesthesia—this is the most common error given entropy's propensity for discordant elevations 2
- Do not use pEEG monitoring as the sole determinant of anesthetic depth—integrate with clinical assessment and, for inhalational agents, end-tidal concentration monitoring 1
- Do not expect perfect agreement between monitors—they measure overlapping but distinct aspects of cortical activity 6
- Do not forget that most awareness events occur before surgery starts or after it ends, periods where pEEG monitoring may be less protective 1