GE CARESCAPE, User Manual

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EEG and auditory evoked potentials
EEG practicalities
EEG measurement description
The EEG signal is continuously measured from up to four channels. This supports
the international 10-20 system.
Measurement can be referential or bipolar. On the signal, a spectral analysis is
performed using Fast Fourier Transform (FFT). The result is a spectrum, which indicates
what frequencies are present in the EEG signal. EEG is traditionally divided into four
frequency bands: delta, theta, alpha and beta.
In addition to the continuous EEG measurement, it is also possible to measure the
response of the brain to external auditory stimulation by measuring evoked potentials,
EP. When measuring auditory evoked potentials, AEP, auditory stimuli are delivered to
the patient’s ears by earphones or headphones, and the resulting signal is measured
from above the cortex. During auditory evoked potential measurement, EEG is also
simultaneously measured from the same electrodes. Frontal muscular activity is also
assessed by electromyography, EMG, from one of the EEG channels.
EEG frequency bands
To characterize the spectral content of the signal, the following parameters are
calculated:
1. Delta
2. Theta
3. Alpha
4. Beta
5. Peak frequency
6. 50% of power
7. Median frequency
8. 95% of power
9. Spectral edge frequency, SEF
Power in delta range
Delta% =
Total power
x 100%
Parameters:
●
Spectral edge frequency, SEF: The frequency below which nearly all of the power is
present.
●
Median frequency, MF: Divides the spectrum in half: 50% of the power is below
and 50% above this frequency.
●
Frequency band ratios (Delta%, Theta%, Alpha%, Beta%) define what portion of
the power is in a certain frequency band. For example, Delta% = power in delta
band / Total power. The spectrum is displayed graphically as a single, continuously
updating curve, or compressed spectral array (CSA), where spectra are stacked one
on top of the other to form a trend display.
382 CARESCAPE Modular Monitors 2062971-001