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Compressed spectral analysis

Created: 20/7/2005
The compressed spectral array (CSA) is obtained by superimposing linear plots of successive epochs of time on each other, generating a three-dimensional ‘hill and valley’ display of the power amplitude vertically (y-axis), frequency horizontally (x-axis) and time (z-axis). However, as successive epochs are added to the display, information can become hidden behind ‘hills’ of increased power at particular frequencies. In order to reduce this problem, some displays arbitrarily truncate the peaks of high-amplitude activity, consequently affecting the legibility of the trace.

This reflects cerebral electrical activity rather than peripheral muscular or autonomic changes. Anaesthesia causes a reduction in high-frequency and an increase in low-frequency amplitudes, which is easier to interpret than raw EEG. However, there remain the problems of great patient- and agent variability and the confounding effects of other pathophysiological processes such as hypoxia, hypotension and hypercarbia. It is not a reliable monitor of the depth of anaesthesia, but can provide a trend for use in conjunction with clinical observations.

EEG with compressed spectral analysis

Philips Electroencephalography (EEG) measurement module produces real-time waveforms from two channels. Software algorithms filter typical artifacts from eye movement and pulse, among others.

The EEG module is designed for continuous, real-time monitoring of adult, paediatric and neonatal patients in anaesthesia and intermediate/critical care environments. The module provides the following measurements:

• Two channels of raw real-time EEG waves
• CSA for each channel of EEG
• Total power (TP)
• % TP in each frequency band (d, ?, a, ß)
• Spectral edge frequency
• Mean dominant frequency
• Peak power frequency
• Continuous impedance for each electrode

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