Monitoring the brain: numbers or waves? 

Intraoperative neurophysiological monitoring (IONM) is used to assess the function of the brain, brainstem, spinal cord, cranial nerves, and peripheral nerves during anaesthesia. Intraoperative neurophysiological monitoring aims to reduce nerve damage while preserving the integrity of the nervous system. It can help prevent intraoperative awareness, avoid excessive anaesthetic depth and improve postoperative neurological outcomes (e.g. delirium). (1)(2) Intraoperative monitoring is now becoming part of standard medical practice. (3) Guidelines from the Association of Anaesthetists (4), the European Society of Anaesthesiology and Intensive Care (ESAIC), (5) and the WHO–WFSA (6) recommend depth-of-anaesthesia monitoring during total intravenous anaesthesia (TIVA) with neuromuscular blockade, and in patients at risk of awareness or postoperative delirium. Yet there is growing debate about whether the BIS and similar indices (Entropy, Narcotrend, qCON), produced by pEEG monitors are sufficient, as the pEEG index value may not always define the same level of unconsciousness for all anaesthetics. (7) Are pEEG indices sufficient, or could raw EEG data better guide anaesthesia depth? (8)(9) 

In our Pro-Con Debate “Monitoring the brain: numbers or waves?” our speakers will debate both sides of this argument.  

Prof. Basak Ceyda Meco is a Professor of Anaesthesiology and Intensive Care at Ankara University Faculty of Medicine (İbn-i Sina Hospital), Turkey. She is also affiliated with the Ankara University Brain Research Center (AÜBAUM). She will present on the side of pEEG-guided anaesthesia in her presentation “Quantifying the mind: the processed EEG approach”. Her research focuses extensively on perioperative brain health, postoperative delirium (POD), postoperative neurocognitive disorders (NCD), and the clinical implementation of EEG monitoring as part of structured care. She is a core member of the ESAIC Safe Brain Initiative (SBI) research group. The SBI is a multi-component, evidence-based programme designed to reduce perioperative neurocognitive complications. EEG-guided anaesthesia, specifically the use of pEEG to avoid excessively deep anaesthesia and burst suppression, is one of its 18 core recommendations.  

The initiative focuses on monitoring and preventing POD and NCD, providing effective anaesthesia care, assessing patient and team satisfaction, and evaluating environmental sustainability impact. Preliminary results showed a notable reduction in POD and increased awareness among anaesthesia team members regarding patient-reported outcome measures. The SBI interest not only highlights using processed EEG but in teaching clinicians to use it correctly. The initiative developed an EEG bootcamp for standardised training in perioperative EEG use. (10)(11)(12) 

Speaking for the other side is Prof. Dr. Anthony Absalom is Professor of Anaesthesiology at the University Medical Centre Groningen (UMCG), University of Groningen, the Netherlands. He is one of Europe’s foremost authorities on the pharmacokinetics and pharmacodynamics (PK/PD) of intravenous anaesthetic agents and on target-controlled infusion (TCI) systems. (13) He will present “Interpreting the signal: raw EEG as a window into brain function”.  

Prof. Dr. Absaolom has led or co-authored many of the landmark papers establishing how propofol behaves in the body and brain. Prof. Absalom is a named co-author on the joint Association of Anaesthetists / Society for Intravenous Anaesthesia guidelines for safe TIVA practice, which recommend pEEG monitoring during TIVA with neuromuscular blockade but also note its limitations. (14) As most commercial pEEG monitors also display the raw EEG, these guidelines recommend anaesthetists observe the raw EEG signal alongside the index. However, interpreting the raw EEG requires extra training, and published resources exist to support this. (15)  

pEEG monitors were developed to display an index that correlates with the anaesthetic effect on the electrical activity of the brain, usually using proprietary algorithms to analyse the EEG from the frontal cortex, an area of the brain not critical to consciousness. For commonly used drugs such as propofol and sevoflurane, the indices give a broad indication of the probability of consciousness for these drugs, but they cannot reliably indicate the presence or absence of consciousness.  

Different anaesthetics act at different molecular targets and neural circuits to produce distinct brain states that are readily visible in raw EEG data. pEEG indices commonly show paradoxical results when using drugs such as ketamine or N2O. This shows that a universal index cannot be completely reliable. (16) When anaesthetists focus on a single index, rather than on the raw EEG, or better still the full frequency spectrogram (a display showing the changes in power in all the underlying frequencies of the EEG), much useful information about the underlying rhythms in the brain, is lost. 

He will argue that raw EEG data, alongside pEEG data, would be the safest approach. However, training pathways for raw EEG competency, common pEEG pitfalls, and integration with advanced monitoring would be necessary. This would lead to improved safety in complex cases, better avoidance of awareness or neurocognitive risks through pattern-based titration, and why raw signal interpretation ultimately enhances rather than complicates anaesthetic decision-making when supported by appropriate education. 

The pro-con debate “Monitoring the brain: numbers or waves?” will take place on Monday, June 8, at 15:00–16:00 CEST in room DELTA A. 

References 

1. Myles PS, Leslie K, McNeil J, Forbes A, Chan MT. Bispectral index monitoring to prevent awareness during anaesthesia: the B-Aware randomised controlled trial. Lancet. 2004 May 29;363(9423):1757-63. doi: 10.1016/S0140-6736(04)16300-9. PMID: 15172773. https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(04)16300-9/abstract 

2. Luo C, Zou W. Cerebral monitoring of anaesthesia on reducing cognitive dysfunction and postoperative delirium: a systematic review. J Int Med Res. 2018;46(10):4100-4110. doi:10.1177/0300060518786406 https://journals.sagepub.com/doi/10.1177/0300060518786406 

3. Ghatol D, Widrich J. Intraoperative Neurophysiological Monitoring. [Updated 2023 Jul 24]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2026 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK563203/  

4. Yentis, S.M., Lucas, D.N., Brigante, L., Collis, R., Cowley, P., Denning, S., Fawcett, W.J. and Gibson, A. (2020), Safety guideline: neurological monitoring associated with obstetric neuraxial block 2020. Anaesthesia, 75: 913-919. https://doi.org/10.1111/anae.14993  

5. Fuchs-Buder, Thomas; Romero, Carolina S.; Lewald, Heidrun; Lamperti, Massimo; Afshari, Arash; Hristovska, Ana-Marjia; Schmartz, Denis; Hinkelbein, Jochen; Longrois, Dan; Popp, Maria; de Boer, Hans D.; Sorbello, Massimiliano; Jankovic, Radmilo; Kranke, Peter. Peri-operative management of neuromuscular blockade: A guideline from the European Society of Anaesthesiology and Intensive Care. European Journal of Anaesthesiology 40(2):p 82-94, February 2023. | DOI: 10.1097/EJA.0000000000001769 https://journals.lww.com/ejanaesthesiology/Fulltext/2023/02000/Peri_operative_management_of_neuromuscular.3.aspx   

6. https://resources.wfsahq.org/wp-content/uploads/397_english.pdf  

7. Purdon, Patrick L. et al. “Clinical Electroencephalography for Anesthesiologists: Part I Background and Basic Signatures.” Anesthesiology 123 (2015): 937–960. https://journals.lww.com/anesthesiology/fulltext/2015/10000/clinical_electroencephalography_for.34.aspx 

8. Introna M, Gemma M, Carozzi C. Improving the benefit of processed EEG monitors: it’s not about the car but the driver. J Clin Monit Comput. 2023;37(3):723-725. doi:10.1007/s10877-023-01004-6 https://link.springer.com/article/10.1007/s10877-023-01004-6  

9. Lee KH, Egan TD, Johnson KB. Raw and processed electroencephalography in modern anesthesia practice: a brief primer on select clinical applications. Korean J Anesthesiol. 2021;74(6):465-477. doi:10.4097/kja.21349 https://ekja.org/journal/view.php?doi=10.4097/kja.21349  

10. Meco BC, Jakobsen K, De Robertis E, Buhre W, Alkış N, Kirkegaard PR, Hägi-Pedersen D, Bubser F, Koch S, Evered LA, Saunders SJ, Caterino M, Paolini F, Berger-Estilita J, Radtke FM. A first assessment of the safe brain initiative care bundle for addressing postoperative delirium in the postanesthesia care unit. J Clin Anesth. 2024 Oct;97:111506. doi: 10.1016/j.jclinane.2024.111506. Epub 2024 Jul 6. PMID: 38972091. https://www.sciencedirect.com/science/article/pii/S0952818024001351  

11. Meço BC, de Agua Reis AB, Berger-Estilita J, Jakobsen K, Alkış N, Radtke FM. Precision Anaesthesia: Advancing Patient-Centered Precision Care Through Repetitive Assessment of PROMs with the Safe Brain Initiative Approach. Turk J Anaesthesiol Reanim. 2023;51(5):374-379. doi:10.4274/TJAR.2023.231420 https://turkjanaesthesiolreanim.org/articles/doi/TJAR.2023.231420 

12. von Dincklage F, Helfrich J, Koch S, et al. Introducing the Safe Brain Initiative’s EEG boot camp for anaesthesia for standardised training on how to use the electroencephalogram for perioperative care. BMC Anesthesiol. 2025;25(1):449. Published 2025 Sep 20. doi:10.1186/s12871-025-03276-8 https://link.springer.com/article/10.1186/s12871-025-03276-8 

13. Nimmo, A.F., Absalom, A.R., Bagshaw, O., Biswas, A., Cook, T.M., Costello, A., Grimes, S., Mulvey, D., Shinde, S., Whitehouse, T. and Wiles, M.D. (2019), Guidelines for the safe practice of total intravenous anaesthesia (TIVA). Anaesthesia, 74: 211-224. https://doi.org/10.1111/anae.14428  

14. Eleveld DJ, Colin P, Absalom AR, Struys MMRF. Pharmacokinetic-pharmacodynamic model for propofol for broad application in anaesthesia and sedation. Br J Anaesth. 2018 May;120(5):942-959. doi: 10.1016/j.bja.2018.01.018. Epub 2018 Mar 12. Erratum in: Br J Anaesth. 2018 Aug;121(2):519. doi: 10.1016/j.bja.2018.05.045. PMID: 29661412. https://www.bjanaesthesia.org/article/S0007-0912(18)30051-5/fulltext 

15. 15. Berger-Estilita J, Baron Shahaf D, Barreto Chang OL, Bonhomme V, Crisan I, Fernández-Candil JL, Garcia PS, Gisselbaek M, Glezerson BA, Hight D, Jakobsen K, Rimstad IJ, Kaiser HA, Kratzer S, Koch S, Dilmen OK, Kreuzer M, Helfrich J, Lersch F, Lobo FA, Marty AP, Marcolino I, Meco BC, Palanca B, Pilge S, Radtke FM, Rasulo F, Raz A, Romagnoli S, Romero CS, Schneider G, Sepúlveda PO, Shmukler D, Sinner B, Soehle M, Thyrso de Lara FS, Vacas S, Vide S, von Dincklage F, Saxena S, Absalom AR. Consensus document on electroencephalography education in anaesthesiology: defining learning outcomes: A modified four-round Delphi study. Eur J Anaesthesiol. 2026 Feb 19. doi: 10.1097/EJA.0000000000002346. Epub ahead of print. PMID: 41709666.];  https://academy.esaic.org/esaic/2025/learnpath/4158861/Purdon, Patrick L. Ph.D.;  

16. McCulloch TJ, Sanders RD. Depth of anaesthesia monitoring: time to reject the index? Br J Anaesth. 2023 Aug;131(2):196-199. doi: 10.1016/j.bja.2023.04.016. Epub 2023 May 15. PMID: 37198033.)