Entropy Monitoring in General Anaesthesia
Entropy monitoring uses processed electroencephalogram (EEG) analysis to assess depth of hypnosis during general anaesthesia by calculating two numerical indices—State Entropy (SE) and Response Entropy (RE)—that reflect brain electrical activity in response to anaesthetic drugs. 1, 2
What Entropy Measures
Entropy monitoring quantifies the complexity and irregularity of brain electrical signals to provide real-time assessment of anaesthetic depth 3, 4:
- State Entropy (SE): Calculated primarily from EEG signals (0.8-32 Hz frequency range), reflecting the hypnotic component of anaesthesia 2
- Response Entropy (RE): Calculated from both EEG and frontal electromyogram (FEMG) signals (0.8-47 Hz), incorporating muscle activity that may indicate inadequate anaesthesia 5, 2
- Both indices range from 0 (deep hypnosis) to 100 (awake state) 2
The difference between RE and SE increases with nociceptive stimulation, as facial muscle activity contributes to the RE value 5. However, this difference does not always reliably indicate inadequate analgesia and must be interpreted carefully 5.
Clinical Application During General Anaesthesia
The Society for Neuroscience in Anesthesiology and Critical Care recommends that anaesthesiologists should not rely solely on index values displayed by processed EEG monitors, but rather develop a basic understanding of EEG waveforms and spectral power analysis 6. This is critical because:
- Entropy indices show high sensitivity and specificity for distinguishing conscious from unconscious states during propofol, sevoflurane, and thiopental anaesthesia 2
- During deep hypnosis with burst suppression, entropy parameters (RE and SE) decrease monotonously, unlike BIS which shows biphasic behavior 2
- RE typically indicates emergence from anaesthesia approximately 11 seconds earlier than SE and 12.4 seconds earlier than BIS 2
Target Values and Interpretation
Based on validated research correlating entropy with depth of anaesthesia 2, 7:
- Awake state: SE and RE values near baseline (typically 80-100)
- Light sedation: SE 60-80, RE 60-85
- General anaesthesia: SE 40-60, RE 40-65
- Deep hypnosis: SE and RE < 40
An overall classification accuracy of 88% has been demonstrated for distinguishing between awake, light, general, and deep anaesthetized states during sevoflurane anaesthesia 4.
When to Use Entropy Monitoring
Processed EEG monitoring should be used when total intravenous anaesthesia (TIVA) is administered, particularly when neuromuscular blockade is involved 6, 8. The Association of Anaesthetists recommends that processed EEG monitoring may:
- Reduce the risk of accidental awareness during general anaesthesia 1
- Improve early recovery times 1
- Reduce the incidence of postoperative delirium and postoperative cognitive dysfunction 1
Critical Limitations and Pitfalls
Entropy monitoring cannot accurately differentiate the intensity of nociceptive stimulation despite showing statistically significant changes with increasing stimulus intensity (prediction probability values < 0.8) 5. Key caveats include:
- The increase in RE-SE difference with noxious stimulation does not reliably indicate inadequate analgesia 5
- Overlap exists in entropy values between different sedation stages in real-time monitoring 7
- Clinical signs alone are inadequate to assess depth of sedation, making quantitative monitoring essential 8
- Entropy values must be interpreted in conjunction with other clinical parameters including end-tidal anaesthetic concentration, haemodynamic responses, and patient movement 1, 5
Integration with Other Monitoring
When using inhalational anaesthetics, maintain end-tidal anaesthetic concentration > 0.7 age-adjusted MAC with an audible alarm enabled to reduce awareness risk 1. Entropy monitoring complements but does not replace this standard, as it provides additional information about individual patient response to anaesthetic drugs 3, 2.