Assessing Anesthetic Depth using Complexity

Anesthesia is a state of controlled, temporary loss of sensation or awareness that is induced for medical purposes. General anesthesia suppresses central nervous system activity and results in unconsciousness and total lack of sensation, using either injected or inhaled drugs (e.g. propofol, a hypnotic).

For obvious reasons, it is important to know the depth of anesthesia. One definition of anestheric depth is the probability of non-response to stimulation. In 1920 Guedel introduced his classification based on muscular movements. Today, there exist monitoring devices such as the BIS (Bi-spectral) monitor. “The bispectral index is a statistically based, empirically derived complex parameter. It is a weighted sum of several electroencephalographic subparameters, including a time domain, frequency domain, and high order spectral subparameters. The BIS monitor provides a single dimensionless number, which ranges from 0 (equivalent to EEG silence) to 100. A BIS value between 40 and 60 indicates an appropriate level for general anesthesia, as recommended by the manufacturer. The BIS monitor thus gives the anesthetist an indication of how “deep” under anesthesia the patient is. The essence of BIS is to take a complex signal (the EEG), analyse it, and process the result into a single number. (from Wikipedia).

We have analyzed the complexity of patients sedated with different doses of propofol, using an algorithm which is a variation of a technique known as Complexity Profiling and which produces an index we call CI_c. Examples of EEGs corresponding to slow introduction of propofol are shown below (from https://www.frontiersin.org/files/Articles/256368/fnsys-11-00039-HTML/image_m/fnsys-11-00039-g001.jpg)

The actual EEGs that we have analyzed are reported in https://pubs.asahq.org/anesthesiology/article/89/4/980/37138/A-Primer-for-EEG-Signal-Processing-in-Anesthesia and are not reproduced herein.

In the cited cases, the BIS index assumed the following values:

Awake: 93.8

Light sedation: 84.7

Heavy sedation: 59.8

Deep anesthesia: 39.8

The corresponding CI_c indices, as well as the associated Complexity Profiles, are illustrated below.

The CI_c index does not involve weights – it is based on a Complexity Profile and its properties. In other words, it is not “an empirically derived complex parameter”.

The Complexity maps, corresponding to light sedation and deep anesthesia are illustrated below.

The simple EEG analysis described herein shows how complexity-based indices have the potential of providing an novel means of assessing anesthesia depth, in addition to the BIS. “Today, the BIS monitor has become the most controversial medical device in anesthesiology, if not all of surgery.” (from Wikipedia).