Bispectral Index - BIS Relevance

BIS Relevance

The BIS is an electroencephalogram-derived multivariant scale that correlates with the metabolic ratio of glucose (Akire M., Anesthesiology 1998). From this metabolic activity the brain obtains its functionality, the ability to capture information from outside and inside the body and integrate that information into conscious perception, with the ability to remember it later. Both loss of consciousness and awakening from anesthesia are correlated with this scale (Flashion R, et al. Anesthesiology 97). The efficacy of BIS index monitoring is not without controversy. Some controlled studies have found that using the BIS reduced the incidence of memory but this was not confirmed in several very large multicenter studies on awareness. The Sociedad de Anestesiología Reanimación y Terapéutica del Dolor de Madrid recommends monitoring of anesthetic depth in accordance with literature-based evidence. BIS, however, is not explicitly endorsed. In fact, they cite an American Society of Anesthesiologists (ASA) statement saying that the decision for cerebral function monitoring should be made on an individual basis. The bispectral index has not been proven to measure the level of consciousness, independently of the cause of reduced consciousness (whether this be drugs, metabolic disease, hypothermia, head trauma, hypovolemia, natural sleep and so on). Not all unconscious patients will have a low BIS value, although the general clinical state may be very different from one to the other, and the prognosis may also differ. The bispectral index is prone to artifacts. Its numbers cannot be relied upon in all situations, including brain death, circulatory arrest or hypothermia. A monitor of the Autonomic Nervous System (such as the ANSiscope) may be more appropriate for purposely assessing the reaction to noxious stimuli during surgery. However, a monitor of the central nervous system may be more appropriate for monitoring consciousness. After the publication of the B-Aware Trial (P. Myles, K. Leslie et al. Lancet 2004) BIS is suggested as a parameter that allows the anesthetist to reduce the risk of anesthesia awareness during surgery for a 'high risk' group. However, this result was not reproduced by a recently published randomized control trial, the "B-Unaware Trial". In it, the use of BIS monitoring was not associated with a lower incidence of anesthesia awareness. In some cases, the BIS may underestimate the depth of anesthesia, leading the anesthetist to administer a higher than necessary dose of anesthetic agent(s). In such cases, the patient may be anesthetized to a lower BIS level than is necessary for the surgery or procedure—this is called "treating the BIS," and may result in a deeper level of anesthesia than required). The monitoring of EEG in ICU patients has been employed in one form or other for more than two decades. BIS monitoring is also being used during transport of critically ill patients in ambulances, helicopters and other vehicles. Some studies show a greater incidence of intraoperative awareness in children, when compared to adults. The correlation between bispectral index in children over one year and state of consciousness has already been proven, although in younger patients the monitor is unreliable because of the differences between immature infant EEG patterns and the adult EEG patterns that the BIS algorithm utilises.