Depth of anaesthesia monitors – Bispectral Index (BIS), E-Entropy and Narcotrend-Compact M

The BIS system uses a disposable 4-electrode sensor on the patient's forehead to measure electrical activity in the brain before using a proprietary algorithm to process the EEG data and calculate a number between 0 (absence of brain electrical activity) and 100 (wide awake). This provides a direct measure of the patient's response to anaesthetic drugs. The target range of BIS values during general anaesthesia is 40 to 60; this range indicates a low probability of awareness with recall. The BIS sensors are only compatible with BIS modules. Other manufacturers have licensed the BIS (or BISx) technology in order to produce BIS modules that are compatible with their own anaesthesia systems. The manufacturer estimates that 100% of all UK operating theatres would be compatible with the BIS system.

The E-Entropy monitor measures irregularity in spontaneous brain and facial muscular activity. It uses a proprietary algorithm to process EEG and frontal electromyography data to produce 2 values that indicate the depth of anaesthesia, response entropy (RE) and state entropy (SE).

Highly irregular signals with variation of wavelength and amplitude over time produce high entropy values and may indicate that the patient is awake. More ordered signals with less variation in wavelength and amplitude over time produce low or zero entropy values, indicating suppression of brain electrical activity and a low probability of recall. The RE scale ranges from 0 (no brain activity) to 100 (fully awake) and the SE scale ranges from 0 (no brain activity) to 91 (fully awake). The target range for entropy values is 40 to 60. RE and SE values near 40 indicate a low probability of awareness with recall.

E-Entropy is a plug-in module that is compatible with the Ohmeda S/5 Anaesthesia monitor and S/5 Compact Anaesthesia monitor using software L-ANE03(A) and L-CANE03(A), and all subsequent software releases since 2003. It is not compatible with other systems. Brain and facial muscular activity is recorded using a disposable sensor with 3 electrodes that are attached to the patient's forehead and a sensor cable that connects the sensor to the Entropy module. The sensors are not compatible with other systems. The manufacturer estimates that 45% of all UK operating theatres would be compatible with the E-Entropy monitor; for the remaining 55%, investment in new monitoring equipment may be needed for compatibility with the Entropy module.

The Narcotrend-Compact M monitor automatically analyses the raw EEG data using spectral analysis to produce a number of parameters. Multivariate statistical methods using proprietary pattern recognition algorithms are then applied to these parameters to provide an automatically classified EEG. The automatic classification functions were developed from visual classification of EEGs. The EEG classification scale is from stage A (awake) to stage F (very deep hypnosis), with stage E indicating the appropriate depth of anaesthesia for surgery. As a refinement to the A to F scale, an EEG index (100=awake, 0=very deep hypnosis) is also calculated. Generic sensors can be used with Narcotrend-Compact M monitors.