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.

1. Current State of Intra-operative Cerebral
Monitors
Mehrnaz Shoushtarian
David T.J. Liley & Louis Delacretaz
Cortical Dynamics Ltd.
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2. • Electroencephalography (EEG) and
Anaesthesia
• Current depth of anaesthesia monitors
• Brain Anaesthesia Response Monitor (BAR
Monitor)
Overview
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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. • 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
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5. EEG-based Cerebral Monitors
NEUROsense IoC-view aepEX
BIS SEDline
SNAP
E-ENTROPY
Narcotrend
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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.
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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
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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
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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
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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
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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.)
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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
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13. How are these limitations addressed by the BAR
Monitor?
BAR Monitor
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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
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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
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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. BAR Monitor
Liley et al. NF model
Liley, Dafilis & Cadusch. (2002), Network: Comp Neural Sys.17

18. BAR Monitor
EEG Analysis from Theory
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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.
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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
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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
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22. D. Liley
BAR Monitor
Typical Intravenous Induction
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23. BAR Studies
Liley et al. 2010, Anesthesiology
CS correlated with
hypnosis
CI not correlated
with hypnosis
Increasing level of unconsciousness (hypnosis)
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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
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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?
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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
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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
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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.
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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
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http://corticaldynamics.com/content/isofluranemodel