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 relies on a method of assessing the degree of irregularity in electroencephalogram (EEG) signals. The principle is that the irregularity of an EEG signal decreases with increasing brain levels of anaesthetic drugs and that the entropy is a measure of that irregularity.
Entropy monitors generate two numbers that are derived from different to 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, and movement are indirect indicators of consciousness, but are unreliable. When these are combined with expired gas analysis of inhalational anaesthetic 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.