An updated introduction to electroencephalogram-based brain monitoring during intended general Anesthesia

An updated introduction to electroencephalogram-based brain monitoring during intended general anesthesia

Considering that the brain is humankind’s most distinctive organ, it is surprising that it is the only vital organ whose function we do not routinely monitor in the operating room or the intensive care unit. The most plausible explanation is that, unlike with the kidney, heart, lungs or liver, we have not had the ability to easily monitor aspects of brain function during pharmacologically induced unconsciousness. In clinical medicine, the only way we can reliably monitor certain brain functions is when people are conscious; through their interactions with the environment, we can generally ascertain how well their brains are working. Given that consciousness is dependent on neuronal activity, and that general anesthetics have their disruptive effect at the neuronal level, a measure of neuronal activity when responsiveness and memory are disrupted should be of deep interest to every anesthesiologist. The electroencephalogram (EEG) is an easily obtained measure of neural activity, quantifying non-invasively from the scalp the summed, synchronous electrical activity of millions of neurons in the cerebral cortex.

The purpose of this review is to summarize the current state of knowledge regarding the use of the EEG during intended general anesthesia. The first section will briefly focus on the well-known archetypal changes in the frontal EEG with increasing anesthetic concentration, whereas the second section will provide scenarios where the EEG may not always change in a typical manner with increasing or decreasing anesthetic concentration. A final section will cover unsolved problems and future directions for EEG monitoring during general anesthesia. Note that this review will not address in any detail the processed, proprietary, EEG indices, but rather focus on the unprocessed EEG signal displayed by all monitors.

Objectives of the Continuing Professional Development module:

After reading this module, the reader should be able to:

1. Formulate basic concepts of the typical electroencephalogram (EEG) changes accompanying general anesthesia with volatile anesthetics or propofol.

2. Differentiate excessive, adequate, and possibly insufficient “hypnosis” during intended general anesthesia, based on the EEG.

3. Apply information gained from the raw EEG or its density spectral array/spectrogram to guide administration of volatile or intravenous anesthetic agents.

4. Recognize some less common situations in which frontal EEG features appear to be discordant with brain state, based on patient behaviour.

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