Entropy monitoring is a method of assessing the effect of certain anaesthetic drugs on the brain's EEG. It was commercially developed by Datex-Ohmeda, which is now part of
GE Healthcare.
Entropy is a quantitative EEG device which captures a single-lead frontal EEG via a 3-electrode sensor applied to the patient's forehead. The system calculates the "spectral entropy" of the
electroencephalogram (EEG) signals, which is a measure of the degree that the power spectrum is uniform. Increasing brain levels of
anaesthetic drugs causes the predominant frequencies in the EEG to be lower than when awake, and this is reflected in a decrease in the spectral entropy.
Entropy monitors generate two numbers that are derived from different frequency bands used. The State Entropy (SE) is calculated from the 0.8 Hz to 32 Hz range, whereas the Response Entropy (RE) uses frequencies up to 47 Hz. Electromyogram activity is more predominant in those higher frequencies, and so the Response Entropy may respond more quickly when muscle activity is present.
Published studies show that Entropy values do relate to clinical levels of anaesthetic depth. Most of the commonly used anaesthetic drugs are detectable by Entropy monitoring, a notable exception being
nitrous oxide, in common with BIS monitoring.
Other
vital signs such as
pulse,
heart rate,
blood pressure
Blood pressure (BP) is the pressure of circulating blood against the walls of blood vessels. Most of this pressure results from the heart pumping blood through the circulatory system. When used without qualification, the term "blood pressure" r ...
, and movement are indirect indicators of
consciousness, but are unreliable. When these are combined with expired gas analysis of
inhalational anaesthetic
An inhalational anesthetic is a chemical compound possessing general anesthetic properties that can be delivered via inhalation. They are administered through a face mask, laryngeal mask airway or tracheal tube connected to an anesthetic vapori ...
agents, an experienced
anaesthetist can be confident a patient is unconscious and not aware of their surroundings. However, the direct measurement of brain activity using a basic
EEG is purported to measure effects of anaesthetics more comprehensively. Unlike the
Bispectral Index monitor, the algorithm of the Entropy monitor has been fully disclosed.
See also
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Bispectral index
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Evoked potentials
References
General references
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{{Anesthesia
Anesthesia
Anesthetic equipment
Medical monitoring