Quantitative Variables Derived from the Electroencephalographic Signal to Assess Depth of Anaesthesia in Animals: A Narrative Review

Abstract

Accurately assessing the depth of anaesthesia in animals remains a challenge, as traditional monitoring methods fail to capture subtle changes in brain activity. This review aimed to systematically map and critically evaluate the range of quantitative variables derived from electroencephalography (EEG) used to monitor sedation or anaesthesia in live animals, excluding laboratory rodents, over the past 35 years. Studies were identified through comprehensive searches in major biomedical databases (PubMed, Embase, CAB Abstract). To be included, studies had to report EEG use in relation to anaesthesia or sedation in living animals. A total of 169 studies were selected after screening and data extraction. Information was charted by animal species and reported EEG-derived variables. The most frequently reported variables were spectral edge frequencies, spectral power metrics, suppression ratio, and proprietary indices, such as the Bispectral Index. Methodological variability was high, and no consensus emerged on optimal EEG measures across species. While EEG-derived quantitative variables provide valuable insights, their interpretation remains highly context-dependent. Further research is necessary to refine these methods, explore variable combinations, and improve their clinical relevance in veterinary medicine.

Keywords: animal anaesthesia; depth of anaesthesia; electroencephalography; nociception; quantitative EEG variables; veterinary monitoring.

Figure 1

Flow diagram of the systematic selection process for references reporting the use of EEG-derived quantitative variables to estimate depth of anaesthesia in animals.

Figure 2

Distribution per animal species of research articles investigating the use of EEG-derived quantitative variables to evaluate depth of anaesthesia over the years (publication date).

Figure 3

Distribution per categories of research articles investigating the use of EEG-derived quantitative variables to evaluate depth of anaesthesia over the years (publication date). Edge frequency: Articles reporting on the use of the dominant EEG frequency to evaluate DoA; Index: Articles reporting on the use of an EEG Index of DoA; Power: Articles reporting on the use of the EEG power to evaluate DoA; SR: Articles reporting on the use of the EEG suppression ratio to evaluate DoA.