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.
Published studies show that entropy values do relate to clinical levels of anaesthetic depth.
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.