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Bispectral index (BIS) is one of several technologies used to monitor depth of anesthesia. BIS monitors are intended to replace or supplement Guedel's classification system for determining depth of anesthesia. Titrating anesthetic agents to a specific bispectral index during general anesthesia in adults (and children over 1 year old) allows the anesthetist to adjust the amount of anesthetic agent to the needs of the patient, possibly resulting in a more rapid emergence from anesthesia. Use of the BIS monitor could reduce the incidence of intraoperative awareness during anaesthesia. The exact details of the algorithm used to create the BIS index have not been disclosed by the company that developed it.
The reliability of BIS has been questioned, in part because its calculation does not rely on any underlying physiological model of how the brain functions, nor how awareness is generated. In addition, the BIS number is insensitive to several commonly used anaesthetic agents such as ketamine and nitrous oxide, which have different mechanisms of pharmacological action to cause the loss of awareness required in anaesthesia. On that basis, it is not so much an "awareness" monitor, as a monitor which indicates the effects of certain types of drugs.
The BIS was introduced by Aspect Medical Systems, Inc. in 1994 as a novel measure of the level of consciousness by algorithmic analysis of a patient's electroencephalogram during general anesthesia. This is used in conjunction with other physiologic monitoring such as electromyography to estimate the depth of anesthesia in order to minimize the possibility of intraoperative awareness. The US Food and Drug Administration (FDA) cleared BIS monitoring in 1996 for assessing the hypnotic effects of general anesthetics and sedatives. The FDA further stated in 2003 that "...A reduction in awareness provides a public health benefit, in that BIS technology can now provide anesthestists with a way to reduce this often debilitating, yet preventable medical error". Aspect Medical was acquired by Covidien in 2009.
Calculation of BIS
The bispectral index is a statistically based, empirically derived complex parameter. It is a weighted sum of several electroencephalographic subparameters, including a time domain, frequency domain, and high order spectral subparameters. The BIS monitor provides a single dimensionless number, which ranges from 0 (equivalent to EEG silence) to 100. A BIS value between 40 and 60 indicates an appropriate level for general anesthesia, as recommended by the manufacturer. The BIS monitor thus gives the anesthetist an indication of how "deep" under anesthesia the patient is. The essence of BIS is to take a complex signal (the EEG), analyse it, and process the result into a single number. Several other systems claim to be able to perform the same thing. This calculation is very computer-intensive. The recent availability of cheap, fast computer processors has enabled great advances in this field. When a subject is awake, the cerebral cortex is very active, and the EEG reflects vigorous activity. When asleep or under general anesthesia, the pattern of activity changes. Overall, there is a change from higher-frequency signals to lower-frequency signals (which can be shown by Fourier analysis), and there is a tendency for signal correlation from different parts of the cortex to become more random.
The developers of the BIS monitor collected many (around 1000) EEG records from healthy adult volunteers at specific clinically important end points and hypnotic drug concentrations. They then fitted bispectral and power spectral variables in a multivariate statistical model to produce the BIS index. As with other types of EEG analysis, the calculation algorithm that the BIS monitor uses is proprietary. Therefore, although the principles of BIS and other monitors are well known, the exact method used in each case is not.
The BIS is an electroencephalogram-derived multivariant scale that correlates with the metabolic rate of glucose. Both loss of consciousness and awakening from anesthesia are correlated with this scale. 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. A Cochrane review in 2014 found that "Four studies (7761 patients) that used clinical signs as a guide to anaesthetic administration in standard practice, as the control group, demonstrated a significant reduction in the risk of awareness with BIS monitoring. Four studies (26,530 patients) compared BIS monitoring with end tidal anaesthetic gas (ETAG) monitoring as a guide to management of anaesthesia and they did not demonstrate any difference in terms of intraoperative 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. Furthermore, not all conscious patients will have a high BIS value.
The bispectral index is prone to artifacts. Some conscious patients who are administered neuromuscular blocking agents such as succinylcholine may have low bispectral index scores; thus, the BIS may fail to detect consciousness in such patients. Its numbers cannot be relied upon in all situations, including brain death, circulatory arrest or hypothermia. A monitor of the Autonomic Nervous System (the first commercial monitor was the ANEMON-I monitor developed by former Swiss company Medical System SA based on the patent WO1997037586) 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 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 intra-operative 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.
- Anaesthetic machine
- Entropy monitoring
- Evoked potentials
- Spectral edge frequency
- Respiratory monitoring
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