Evidence‑Based Recommendations for Entropy Monitoring in General Anaesthesia

Definition and Indices

• Entropy monitoring processes the electroencephalogram (EEG) to generate two numerical indices—State Entropy (SE) and Response Entropy (RE)—that reflect the brain’s electrical activity in response to anaesthetic drugs, providing a quantitative measure of hypnotic depth. 1

Target Entropy Values for Different Depths of Anaesthesia

• The ranges above are derived from validated research correlating entropy indices with anaesthetic depth, supporting their use for intra‑operative monitoring. 1

Clinical Benefits of Using Entropy Monitoring

• The Association of Anaesthetists recommends that processed EEG (entropy) monitoring can reduce the risk of accidental awareness during general anaesthesia. 1
• Use of entropy monitoring is associated with shorter emergence and earlier recovery times after surgery. 1
• Incorporating entropy monitoring may lower the incidence of postoperative delirium and postoperative cognitive dysfunction in the postoperative period. 1

Interpretation and Integration with Other Parameters

• Entropy values should be interpreted in conjunction with additional clinical parameters, such as end‑tidal anaesthetic concentration, haemodynamic responses, and patient movement, to ensure accurate assessment of anaesthetic depth. 1

Implementation Recommendations

• When total intravenous anaesthesia (TIVA) is employed—especially with neuromuscular blockade—processed EEG (entropy) monitoring is advised to optimise drug dosing and avoid awareness. 1
• For inhalational anaesthesia, maintain end‑tidal anaesthetic concentration greater than 0.7 age‑adjusted MAC and enable an audible alarm; this practice, combined with entropy monitoring, helps further reduce the risk of intra‑operative awareness. 1

Anesthetic Management for Intraoperative Neuromonitoring

Introduction to Anesthetic Techniques

• The American Society of Anesthesiologists recommends using total intravenous anesthesia (TIVA) with propofol and short-acting opioids (fentanyl, remifentanil) for intraoperative neuromonitoring, as it allows for rapid recovery with minimal residual effects 2

Anesthetic Effects on Evoked Potentials

• The amplitude of evoked responses is proportional to the number of stimulated motor neurons, making them highly sensitive to neuromuscular blocking agents and anesthetics 3
• The use of sevoflurane and isoflurane can potentiate partial neuromuscular blockade, reducing the train-of-four ratio 3

Clinical Implications for Anesthetic Management

• The American College of Cardiology recommends avoiding nitrous oxide due to its effects on intestinal function and increased risk of postoperative nausea and vomiting 2
• Nitrous oxide can expand gas bubbles, resulting in potential adverse neurological effects through expansion of gas emboli 4

Considerations for Neuromonitoring

• The International Society for the Advancement of Spine Surgery recommends using transcranial motor-evoked potentials (TcMEPs) in addition to somatosensory-evoked potentials (SSEPs) for intraoperative monitoring, as TcMEPs are more sensitive for detecting spinal cord ischemia 3
• The Society for Neuroscience in Anesthesiology and Critical Care recommends that anesthesiologists 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 5, 6, 7

Decision-Making Algorithm for Anesthetic Technique

• When planning intraoperative neuromonitoring, the first-line technique should be TIVA with propofol and short-acting opioids, with quantitative neuromuscular monitoring when using neuromuscular blockers 5, 6