GE HEALTHCARE E-PSMP, Technical Manual

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Module Frames and Modules
11 - 2
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1.2 Measurement principle
Entropy is a measure of irregularity in any signal. During general anesthesia, EEG changes from
irregular to more regular patterns when anesthesia deepens. Similarly, FEMG quiets down as
the deeper parts of the brain are increasingly saturated with anesthetics. The Entropy Module
measures these changes by quantifying the irregularity of EEG and FEMG signals.
Entropy parameters and BSR are calculated from EEG and FEMG signals acquired with a sensor
which is attached to the patient's forehead. The sensor consists of three electrodes. This
referential measurement yields one channel of raw EntrEEG.
Figure 2 A general view of the cable connections
1.2.1 EntrEEG
EEG is a differential voltage signal measured from electrodes attached to the patient’s skin.
EEG measures the spontaneous electrical activity of the brain. This electrical activity reflects
the state of the brain. In referential measurement, the referential electrode delivers its potential
to every channel’s minus-input. The signal is the potential difference between this common
reference electrode (electrode #3) and the electrode connected to the plus input (electrode #1).
The purpose of the ground electrode (electrode #2) is to reduce common mode noise.
The EntrEEG signal is amplified, antialias filtered, digitized, and software filtered. After that, the
EntrEEG signal is shown on the screen and the RE, SE, and BSR characteristics are calculated
from it.
1.2.2 FEMG
FEMG is an electrical signal originating from facial muscles. The FEMG signal has much broader
spectrum than EEG and it overlaps with EEG at low frequencies. The FEMG signal gives its
contribution to the RE values (see “
1.2.3. RE and SE ”). Mains power frequency and its harmonics
are digitally filtered away to reject interference noise from power lines.
1.2.3 RE and SE
Entropy numbers range from 100 to zero (RE 0-100, SE 0 – 91), correlating to the patient's
anesthetic state. High values of Entropy indicate high irregularity of the signal, signifying that
the patient is awake. There are two Entropy parameters: the fast-reacting
Response Entropy
and the more steady and robust State Entropy . State Entropy consists of the entropy of EEG
signal calculated up to 32 Hz. Response Entropy includes additional high frequencies up to 47
Hz and, consequently, the fast frontalis EMG (FEMG) signals enable a fast response time.
Table 1 Frequency ranges for Entropy calculation
Response Entropy, RE 0 < f < 47 Hz
State Entropy, SE 0 < f < 32 Hz