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5
Principle of Operation
High Quality Amplifier
5.1
An instrumentation amplifier collects ongoing EEG with a high Common Mode Rejection Ratio
ensuring a high-quality EEG acquisition. Special artifact detection algorithms are used to eliminate
their effects on subsequent CSI calculations.
Measurement Principle
5.2
The performance of the CSM is based on the analysis of the frequency content of the EEG signal.
The energy of the EEG is evaluated in specific frequency bands. These are used to define two energy
ratios called alpha (α) and beta (β). Both of these show a shift in energy content from the higher to
the lower frequencies during anesthesia. The relationship between these quantities is also analysed
as a separate parameter (β-α).
E
α
=
−
30
ln
ratio
E
−
6
The monitor also on-line evaluates the amount of instantaneous burst suppression (BS) in each
thirty-second period of the EEG. This measurement quantifies the amount of "silent" or "flat" EEG
periods characteristic of the deepest levels of hypnosis.
These four parameters are used as input to a fuzzy logic classifier system that calculates the
Cerebral State Index.
α
ratio
β
EEG
ratio
β-α
BS%
Cerebral State Monitor - CSM DFU
42
5 .
Hz
12
Hz
FUZZY LOGIC
CLASSIFIER
E
β
=
−
30
42
5 .
Hz
ln
ratio
E
−
11
21
Hz
CSI
Page 14
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