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Current state of intra operative cerebral monitors

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Electroencephalography (EEG) and Anaesthesia.
Current depth of anaesthesia monitors.
Brain Anaesthesia Response Monitor (BAR Monitor) which is a physiologically inspired method of EEG analysis that allows more accurate monitoring during anaesthesia.

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Current state of intra operative cerebral monitors

  1. 1. Current State of Intra-operative Cerebral Monitors Mehrnaz Shoushtarian David T.J. Liley & Louis Delacretaz Cortical Dynamics Ltd. 1
  2. 2. • Electroencephalography (EEG) and Anaesthesia • Current depth of anaesthesia monitors • Brain Anaesthesia Response Monitor (BAR Monitor) Overview 2
  3. 3. EEG & Anaesthesia • Anesthetic agents target a host of synaptic sites, at differing levels of the CNS. • Synaptic (micro) modifications produce observable (macro) changes in human EEG. Kuizenga et al. (1998), Br. J. Anaesth.3
  4. 4. • EEG-based depth of anaesthesia monitors are now commonly used due to: ▪ Sensitivity of the EEG to anaesthetic agents ▪ Advances in computer power and miniaturisation EEG-based Cerebral Monitors 4
  5. 5. EEG-based Cerebral Monitors NEUROsense IoC-view aepEX BIS SEDline SNAP E-ENTROPY Narcotrend 5
  6. 6. EEG-based Cerebral Monitors • All current electrophysiological measures heuristically derived • All current depth-of-anaesthesia monitors use QEEG, MLAEP, or combination ▪ QEEG - median frequency, spectral edge frequency (e.g. SEF95), bispectrum, BSR, relative and absolute band powers ▪ Middle latency auditory evoked potential (MLAEP) • EEG waveforms are analysed to produce a dimensionless number between 0 and 100. 6
  7. 7. EEG Processing Methods: BIS Freye et al. (2005), J Clin Monitor Comp EEG Artefact rejection Feature extraction Bispectrum Power spectrum Select features which correlate best with clinical endpoints Combine features to produce BIS index 7
  8. 8. • To generate the BIS index, several descriptors of the EEG are integrated into a single index • Descriptors are: o power spectrum o degree of synchronisation o the amount of burst suppression • This index correlates with behavioural assessments of sedation and hypnosis EEG Processing Methods: BIS 8
  9. 9. EEG Processing Methods: Narcotrend • Based on the visual assessment of EEG sub- stages during anaesthesia • Originally performed for sleep classification, where changes in EEG during human sleep were classified to five stages 9
  10. 10. • The entropy is a measure of the irregularity, complexity or unpredictability of a signal • The Entropy module generates two numbers: o State Entropy: calculated over 0.8-32 Hz, mainly consists of EEG activity. o Response Entropy: calculated over 0.8 to 47 Hz, includes EMG activity and is used to indicate movement due to impending arousal EEG Processing Methods: Entropy 10
  11. 11. EEG-based Cerebral Monitors NEUROsense (wavelet) IoC-view (symbolic dynamics) aepEX (auditory EP) BIS (bispectrum) SEDline (qEEG) SNAP (qEEG) E-ENTROPY (spectral entropy) Narcotrend (visual pattern recog.) 11
  12. 12. • Statistical process used to calculate indices which are of no physiological significance • Subcortical mechanisms not considered in most algorithms. • Algorithms are based on effects of commonly used anaesthetic drugs on the EEG and are blind to a range of common adjuvant anaesthetic agents - nitrous oxide, ketamine, opioids • Shifts in levels of consciousness are not instantly reflected (processing time) Limitation of Current Depth of Anaesthesia Monitors 12
  13. 13. How are these limitations addressed by the BAR Monitor? BAR Monitor 13
  14. 14. BAR Monitor Development of physiological approaches • Improved monitoring approaches will result if we better understand the links between the targets of drug action and their clinical effects. • Neural Field models seek to link micro and macro scales through meso-scale modeling. site of drug action level of clinical measurement explanatory bridge 14
  15. 15. BAR Monitor Development of physiological approaches cortex is horizonally “parcellated” into columns • NFT models populations of interacting excitatory and inhibitory neurons. • Typically scaling to a cortical macrocolumn 15
  16. 16. BAR Monitor Liley et al. NF model Passive electrical properties and reversal potentials Transmitter kinetics, intra-cortical connectivity and cable delays Conduction delays and cortico-cortical connectivity Liley, Dafilis & Cadusch. (2002), Network: Comp Neural Sys.16
  17. 17. BAR Monitor Liley et al. NF model Liley, Dafilis & Cadusch. (2002), Network: Comp Neural Sys.17
  18. 18. BAR Monitor EEG Analysis from Theory 18
  19. 19. Predict new value from K previous values y [Auto-Regressive] and from L previous plus the current Gaussian white noise process values w [Moving Average] BAR Monitor: Time Series Modelling Electro-cortical filter Subcortical input is filtered to give rise to the surface recordable EEG. 19
  20. 20. BAR Monitor Cortical State and Cortical Input • Cortical State (CS): scalar representation of state of cortical filter estimated using fixed order ARMA modeling • Cortical Input (CI): RMS amplitude of estimated noise driving the filter 20
  21. 21. •Resonant state of cortex (cortical state, CS) = bell’s natural frequency •Input to cortex (cortical input, CI) = the force of the hammer striking the bell BAR Monitor Cortical State and Cortical Input 21
  22. 22. D. Liley BAR Monitor Typical Intravenous Induction 22
  23. 23. BAR Studies Liley et al. 2010, Anesthesiology CS correlated with hypnosis CI not correlated with hypnosis Increasing level of unconsciousness (hypnosis) 23
  24. 24. D. Liley increasing analgesic action BAR Studies Nitrous oxide alters CI Liley et al. (2008), Computers in Biology and Medicine • LOC in the presence of N2O associated with a statistically significant reduction in the CI. • In contrast BIS responded only to low, but not to high, concentrations of N2O 24
  25. 25. D. Liley CVI = weighted combination of BIS, sBIS (standard deviation of over previous 3 minutes) and sEMG (standard deviation of EMG over previous 3 minutes) 120 patients, (BIS = 70,50,30)x(TCI remi 0,3,4, 6 ng/ml) + noxious stimulus MCC = 0.55 MCC = 0.27 Mathews Correlation Coefficient (MCC) = balanced measure of the quality of a binary classifier, ranges from -1 (total disagreement) to 1 (perfect agreement) BAR Studies Analgesia measurement? 25
  26. 26. Monitoring limitations addressed by the BAR Monitor: • A physiologically inspired method of EEG analysis allows more accurate monitoring during anaesthesia • BAR indices provide measures of cortical as well as sub- cortical mechanisms • Wider range of anaesthetic agents can be monitored • Shifts in levels of consciousness reflected with only a 2- second delay BAR Monitor 26
  27. 27. BAR Monitor Validation Trial • BAR validation trial at St. Vincent’s Hospital Validation of the BAR Monitoring System during Anaesthesia for Cardiac Surgery: a Double-Blinded, Randomised, Controlled Trial using Two Different Doses of Fentanyl 27
  28. 28. Acknowledgements David Scott Desmond McGlade Alison Graham Simone Said Babak Shoghi Simon Tang And all staff at St.Vincent’s who assisted with the BAR Study. 28
  29. 29. Modeling Empirical Brain Activity •plot height/color = EEG activity (red = high, blue = low) •60 x 60 cm square cortex •equal ‘induction’ and ‘recovery’ phases 29 http://corticaldynamics.com/content/isofluranemodel

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