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

·      Basis of EEG depth monitoring

·      BIS

·      Entropy

 

EEG principles:

Definition

·  Recording of the brain’s electrical activity

Aetiology

·  Dendrites perpendicular to the cortical surface belonging to the superficial layer of pyramidal cells

·  Detect changes in post-synaptic potentials (summation of EPSP and IPSP)

Magnitude

·  Usually 0-200 microvolts or amplitude

·  Usually 0-50 Hz frequency

 

EEG waves:

Name

Frequency

Occurrence

β

12-30Hz

Awake, alert

α

8-12Hz

Awake, relaxed with eyes closed

Θ

4-8Hz

Non-rapid eye movement (NREM) 1-2

General anaesthesia

δ

1-4Hz

NREM 3-4

General anaesthesia

 

Basis of EEG monitoring under GA:

Awake

·  Cortex active

·  Complex activity appears chaotic

Anaesthetised

·  Suppression of ascending reticular activating system (ARAS)

·  Disinhibition of thalamic nuclei, resulting in synchronised rhythmic discharge

·  Low frequency

·  Synchronised hence additive hence high amplitude

·  Several patented devices exist for analysis of EEG under GA: e.g. BIS, entropy

·  Exact nature of algorithms is unknown

 

Bispectral index (BIS):

Components

(1) Bicoherence:

o A measure of how related cortical signals are to one another, scale of 0 to 1

(2) Compressed spectral array:

o =Frequency below which 50% of activity occurs

o Is a form of frequency domain analysis

o EEG signal taken in five second epochs

o Fourier analysis performed

o % each frequency displayed using a histogram

o Axes: frequency, power, time.

 (3)Burst suppression

o Indicator of depletion of intracellular

o Is a form of time domain analysis

(4)Frontalis EMG

o Measures ~70-110Hz

o Indicator of nociception

Readouts

·  BIS: 40-60 anaesthesia, 60-85 sedation, 100 awake

·  Suppression ratio (SR) - % BS in past 63 seconds, ideally zero

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

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

 

Entropy:

Definition

·  0-100 score of how chaotic the EEG signal

·  Less chaotic, more synchronised = less entropy = lower score.

Readouts

·  State entropy (SE): 0-91, analyses lower frequencies only

·  Response entropy (RE): 0-100, includes high frequencies signal such as EMG

·  RE > SE may indicate nociception

 

 

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