Impact of Anesthetics on Brain Electrical Activity and Principles of pEEG-Based Monitoring During General Anesthesia

Abstract

Starting from the first half of the nineteenth century, investigations proved that anesthetics are able to induce significant modifications in the brain's electrical activity and, in turn, in the recorded electroencephalogram (EEG). The anesthesia-related electrical activity involves different EEG correlates. Stage of anesthesia and type of anesthetics used influence EEG expression. Nevertheless, the complex waves of unprocessed EEG may not be easily interpreted. Moreover, several issues obstacle the utilization of standard EEG in anesthesia. These limitations stimulated the search for easy and solid techniques and tools. The EEG raw signal has been sectioned to extract its core element and to simplify the interpretation of the huge amount of data that it contains. Technically, the increased flexibility, speed, and economy of digital circuits, as well as progress in computer hardware and signal-processing algorithms, induced radical changes in the field of signal processing. This evolutionary process involved the application of mathematical models such as the Fourier analysis and its improvement by the bispectral (BIS) analysis. Technical advances and algorithms allowed to process the EEG raw (processing EEG, pEEG) and extrapolate values (indices) which express the depth of anesthesia (DoA) status and other features (e.g., response to noxious stimuli). Apart from the pEEG-based brain monitoring devices, other instruments work on acoustic evoked potentials. Because many mathematical models are proprietary algorithms, it is usually problematic to precisely interpret mechanisms underlying of DoA monitors. After its commercialization, in 1994, the BIS analysis-based monitor was intended as the best instrument to follow the cerebral activity during general anesthesia. The availability of this technology was enthusiastically considered as the discovery of the “Holy Grail” of anesthesia monitoring and the definitive solution of one among the major concerns of anesthesia: the awareness phenomenon. After two decades of debates and investigations in the field, the scientific community has understood that this is not completely true. The aim of this chapter is to dissect the technology and functionality behind these monitors. Explanation of functionality assumes the recognition of the functional mechanisms of anesthetics, specifically their mechanisms of action, from the molecular level to neural correlates, responsible for the anesthetic-induced unconsciousness, maintenance, and recovery of consciousness. Furthermore, concepts of neurophysiology and anesthesia-related electrical activity are offered because they are the functional basis of the brain monitoring devices. The chapter also addresses the limitations of DoA devices and perspectives in brain monitoring.