EEG-based Depth of Anesthesia-monitoring, Effects on Dosage and Cognition

It has been over 80 years since Gibbs et al showed how the electroencephalogram (EEG) systematically changed in concurrence with increasing doses of hypnotic drugs such as penthobarbital and Ether. The study concluded that "Electroencephalography may therefore be of value in controlling depth of anesthesia and sedation". In spite of a solid documentation of the systematic connection between dosing of anesthetic drugs, EEG-patterns and level of sedation/anesthesia , EEG-based DoA has not become a part of standard of care in anesthetic management. There is abundant evidence of how different anesthetic drugs leads to characteristic fluctuations in human brain electrical activity, relating to depth of anesthesia, anesthetic drug of choice, and age . These anesthetic induced fluctuations are readily visible as changes in the patients EEG.

Anesthetic drugs are usually administered in pharmacological models based on a population taking into account their age, weight and height. However, there is a significant difference in how patients respond to these models. In adults there is evidence that the doses needed to achieve consciousness varies with a factor of 2 above and below suggested doses. In under-dosing of anesthetics there is a risk of peroperative awareness . On the other hand there is also evidence that overdosing of anesthetics has harmful effects; children receiving more than 4% Sevoflurane can demonstrate epileptiform activity , and adults overdosing into "burst suppression" during anesthesia has a higher risk of postoperative delirium (POD) and increased occurrence of postoperative cognitive dysfunction (POCD) .

Bispectral Index (BIS) is an algorithm developed by Aspect Medical Systems in 1994, which is based on weighted sums of EEG subparameters to present an index from 0 to 100 for depth of anesthesia, where 100 is wide awake, and 0 is an isoelectric EEG. The BIS target for a deep enough anesthesia is set to be between 40 and 60. The BIS number is often in concurrence with other clinical observations related to anesthetic depth, however there is also an experience of divergence. BIS and other EEG-based indices are programmed from adult cohorts, and cannot be directly trusted in children, or the elderly . There is also an incapability in these preprogrammed indices (BIS and other) to integrate how specific anesthetic drugs affect the EEG, and thenceforth the BIS value. An example of this is how the drug Ketamine induces a specific gamma-frequency in the EEG, which the BIS-index translate as a lighter anesthesia, even though the drug is administered "on top of" an already deep level of anesthesia.