Intraoperative EEG

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Intraoperative EEG

• What is it?
• How to use it?
• Why use it?
• What are its limitations?

Every major surgical monitor manufacturer offers
a 1-4 channel EEG module to be used by anesthesia
provider.
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EEG Neurophysiologic Basis
Cortical Column
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Feedback circuits control EEG rhythms.
modulator
pacemaker
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EEG requires amplification of tiny scalp
potentials.
.00005 Volts
5 Volts
100,000x EEG differential amplifier
Large signal amplification increases the
susceptibility to artifactual contamination.
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Voltage change measured from scalp electrodes.
Standardized electrode interconnections (montages)
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Detection of focal ischemia may require many
electrodes.
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Needle electrodes are easiest to place, but
discomfort limits awake recording.
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Electrode caps allow non-invasive recording of
entire 10-20 array.
electrode gel
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Disk electrodes use conductive paste. Self-adhesiv
e disks only outside hairline.
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EEG montage describes the electrode
configuration.
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EEG Amplitude Patterns Describe Synaptic Activity
Awake
Drowsy
Light sleep
Deep sleep
Burst-suppression
Electrocortical silence
Hartikainen, K. Dissertation, U. Tampere, Ser A,
Vol. 498, 1996
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EEG Frequency Bands Measure Synaptic Activity
Highest activity Lowest activity
Hess, R., EEG Handbook, 1966
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EEG Signs of Synaptic Depression

• Minor
• loss of high (beta) frequency power
• Intermediate
• increase in low (delta) frequency power
• Major
  decreased power in all frequencies
• burst-suppression
• flat line

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Computer-Processed EEGFourier Spectral Analysis
Spectrum
Light
Lens
Prism
Analyzer
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Power Spectral Analysis
Power (nanoWatts)

• Power Absolute Relative
• Delta 15 nW 45
• Theta 13 39
• Alpha 4 11
• Beta 2 5 fast/slow ratio
  0.18
• Univariate Frequency Descriptors
• Peak power 2.1 Hz
• Median 3.1
• Mean 4.6
• 98 Spectral edge 15.8

?V2
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Compressed Spectral ArrayDensity Spectral Array
oldest newest
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EEG depends on cerebral blood flow
Jones, T.H. et al. J Neurosurg 54773, 1981
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Cerebral Blood Flow Distribution
Cerebral Blood Flow, James H. Woods, McGraw-Hill,
Inc., 1987. (Chapter 2, Arterial Distributions
and Variants, Arthur L. Day, M.D.)
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Hypotension may cause regional EEG slowing.
Asymmetry suggests intracranial vascular disease.
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EEG Suppression for Neuroprotection Maximal
Cooling Benefit With Total Synaptic Suppression.
More cooling increases chance of coagulopathy.
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EEG Documents Hypnotic Effect estimates
probability of unconsciousness
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Univariate EEG Descriptors ofHypnotic Effect
Conscious Unconscious
Spectral Edge 95
Peak Power
Median
Mean
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BISpectral Index (BIS)

• Adaptive multivariate discriminant BIS A B
  C D
• suppression ratio, EMG, frequency descriptors
• Includes phase information
• Empirical weighting coefficients BIS k1A
  k2B k3C k4D
• movement, response, recall
• State-dependent differential weighting
• Validated only for fronto-temporal derivation
• Not for opiates, midazolam, ketamine

Rampil IJ Anesthesiology 89(4)980, 1998
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Glucose metabolism ( baseline)
EEG Bispectral Index (BIS)
Conscious
95
Isoflurane Anesthesia
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Alkire M Anesthesiology 199889323
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BIS Detects Inadequate Hypnosis Preceding Recall
CPB
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BIS Increased by High Frequency High Amplitude
Seizure Activity
6 mo AV canal
BIS 28 BIS 50
021021CC
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Thiopental EEG Suppression
Comparison of multivariate (BIS) and univariate
(SEF) measures.
Comparison of multivariate (BIS) and univariate
(SEF) measures.
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BIS Detects Stroke in Progress
BIS
cardiopulmonary bypass
X-clamp off
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Loss of consciousness corresponds to a
thermodynamic phase transition.
Bührer et al., Anaesthesiology (1992) MacIver et
al., ibid. (1996) Kuizenga et al., Br. J.
Anaesthesia (1998)
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Entropy disorder
Spectral entropy disorder in frequency
space few relevant frequencies gt low
entropy wide spectrum of frequencies gt high
entropy Rezek et al., IEEE Trans Biomed Eng
(1998) Approximate Kolmogorov-Sinai entropy loss
of information over time Pincus et al., J Clin
Monit (1991) Bruhn et al., Anesthesiology
(2000) Lempel-Ziv Complexity encoding
algorithm equal to source entropy Zhang Roy,
Med Biol Eng Comput (1999)
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Quantifying Hypnosis
Entropy BIS Sensitivity Conscious
78 70 Unconscious 93 90 Specificity
Conscious 87 81 Unconscious
88 88 Average 86 82
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Intraoperative EEG

• Prevent cerebral ischemia
• Guide neuroprotective techniques
• Detect and treat seizures
• Avoid inadequate or excess hypnosis
• Low specificity may require additional monitoring
  .