2017B04 Describe the generation and features of a normal awake EEG (60%). Briefly discuss the
processing performed by quantitative EEG monitors (such as BIS or entropy) to produce a single
number from the EEG (40%).

 

List:

·     EEG principles

·     EEG waves

·     Physiological basis

·     Monitoring device

·     Monitoring principles

·     Specific devices

 

EEG principles:

Definition

·         Recording of the brain’s electrical activity

Aetiology

·         From pyramidal cells with dendrites radiating toward cortical surface

·         Summation of excitatory and inhibitory post-synaptic potentials

Magnitude

·         Usually 0-200 microvolts or amplitude

·         Usually 0-50 Hz frequency

Waves

·         γ: 30-80Hz

·         β: 12-30Hz

·         α: 8-12Hz

·         Θ: 4-8Hz

·         δ: 0-4Hz

 

Physiological basis:

Awake

·         Complex activity -> chaotic pattern

·         High frequency (predominantly β, γ)

·         Low amplitude (↓synchrony = ↓ summation)

Anaesthetised

·         Suppression of cortex and brainstem reticular activating system

·         Disinhibition of thalamic pacemaker nuclei -> synchronised pattern

·         Low frequency (predominantly α, Θ, δ)

·         High amplitude (↑synchrony = ↑summation)

 

Monitoring device:

Components

·     Electrodes

1.   Exploratory

2.   Reference

3.   EMG

4.   Virtual ground

·     Cabling

·     Processor

·     Display

Function

·     Acquisition

·     Amplification

·     Filtration (removal of artifact)

·     Processing

·     Display

 

Monitoring principles:

Algorithms

·     Proprietary

·     Empirical – i.e. observe EEG response to variable drug concentration

·     Derived from statistical analysis, not biological models

·     Designed for GABA-ergic anaesthetics

Fourier transformation

·     EEG taken in short epochs

·     Separation into component sine waves

·     Represented as a spectral array (frequency, power (=amplitude2), ± time)

·     Precursor to many types of analysis

Types of analysis

·     Time domain analysis: e.g. burst suppressions

·     Frequency domain analysis: e.g. beta ratio, spectral edge frequency 95%

·     Both: degree of entropy / phase coupling / synchrony / symmetry

 

Bispectral index (BIS):

Algorithm

(1) Burst-suppression ratio (correlates with BIS 0-30)

·     % per 63 seconds spent in burst-suppression pattern (not isoelectricity)

·     Associated with depletion of intracellular calcium

(2) QUAZI suppression index

·     Detect burst suppression if wandering baseline

(3) SynchFastSlow index (correlates with BIS 40-60)

·     Index of relative synchrony of fast and slow waves

·     Derived from bispectral analysis

·     Processed to yield bicoherence number (0-1 scale of phase coupling)

·     N.B. PowerFastSlow yields similar results (? bispectral analysis unimportant)

(4) Beta ratio (correlates BIS 60-100)

·     Ratio of higher to lower frequency beta waves

·     Important due to paradoxical increase in SFS/SEF at low drug concentration

(5) EMG

·     Frontalis muscle activity

·     High frequency (70-110Hz)

·     ? Indicator of nociception or arousal

Readouts

·     BIS: 40-60 general anaesthesia, 100 awake

·     Suppression ratio (SR) – ideally zero

·     Spectral edge frequency 95% - not included in published algorithm

·     Electromyography (EMG) – 0-100 but no recommendation

·     Signal quality index (SQI) – 0-100 but no recommendation

 

Entropy:

Definition

·     Degree of disorder in a system

·     In EEG: ↓Chaos = ↑synchrony = ↓entropy = ↓number

Readouts

(1) State entropy

·     Scale 0-91

·     Frequency 0.8-32Hz

·     Longer epochs -> transient fluctuations eliminated

·     Index of hypnotic component of anaesthesia

(2) Response entropy

·     Scale 0-100

·     Frequency 0.8-47Hz

·     Includes EMG data

·     Shorter epochs -> rapid response

·     Sudden increase may indicate nociception or arousal

 

SedLine:

Algorithm

·     Four EEG channels

·     Linear and non-linear analysis

·     Temporal and spatial frequency gradients

·     Symmetry and synchronization

·     Arousal observer sub-algorithm

Readouts

·     Patient status index (0-100, aim 25-50)

·     Four EEG waveforms

·     Bilateral density spectral array

·     Suppression ratio

·     Spectral edge frequency

·     Asymmetry graph

·     Artifact

·     EMG

 

 

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