GE HEALTHCARE E-PRESTN, Technical Manual

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Module Frames and Modules
13 - 2
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1.2 Measurement principle
The BIS measurement is based on EEG signals, these are processed as the BIS index. The BIS
sensor is placed on the patient’s forehead to acquire the high-resolution signals required.
These EEG signals are transferred to the BISx Digital Signal Processing Unit that amplifies and
digitizes the EEG signal. The BISx unit calculates the BIS index and sends the signal and the
index to the module. Then the module sends both the signal and the index to the monitor via
MBUS.
1.2.1 BIS measurement on the monitor screen
The waveform field shows the BIS EEG waveform. The following BIS related data appears in
parameter window and graphical trends (except SQI):
BIS number indicates the patient’s level of hypnosis, ranging from 100 for wide awake to 0 in
the absence of brain activity.
Signal Quality Index (SQI) bar graph indicates the quality of the EEG signal in the range of 0 to
100.
Electromyograph (EMG) bar graph represents the absolute power in the 70 to 110 Hz
frequency band and it ranges from 30 to 55 dB. This frequency band contains power from
muscle activity (electromyograph). High frequency artifacts may contribute to the measured
signal.
Suppression ratio (SR) number indicates the percentage of suppressed (flat line) EEG detected
over the last 63 seconds. It ranges from 0 to 100%.
1.2.2 Sensor check
Sensor check is performed automatically at the beginning of each case when the sensor is
attached to the patient interface cable (monitor). An initial ‘Checking sensor’ message is shown
in the parameter window with a picture of the sensor electrodes. The information on the
passed or failed sensor check is shown in the parameter window for each electrode. The BIS
measurement cannot continue if the first sensor check fails. In such a case a message ‘Sensor
check failed’ is shown in the parameter window and waveform field.
Figure 2 BIS sensor check
Continuous checking of the reference and signal electrodes and periodic checking of the
ground electrode are performed by default. The automatic check can be switched off by
selecting OFF in the BIS Setup menu, and the message ‘Automatic check off’ appears on the
screen. Sensor check can be started manually by pushing a module key or selecting the
appropriate command from the menu. Manual sensor check can be useful e.g. when AEP's are
being monitored at the same time, as continuous sensor check might disturb the AEP
measurement.