Bispectral index
A Bispectral index (BIS) monitor is a modern neurophysiological monitoring device which continually analyses a patient's electroencephalograms during general anaesthesia to assess the level of consciousness during anaesthesia. The "depth of anaesthesia" is commonly used as a surrogate for "the likelihood of forming experiences or memory."  The use of BIS monitors is increasing.
Some professional bodies have announced recommendations that BIS be used routinely. The Sociedad de Anestesiologia Reanimacion y Terapeutica del Dolor de Madrid recommends monitoring of anaesthetic depth in accordance with literature-based evidence. BIS, however, is not explicitly endorsed. In fact, they cite an ASA statement saying that the decision for cerebral function monitoring should be made on an individual basis. 
The efficacy of BIS index monitoring is not without controversy. The reference cited below (2)(Rosow, Manberg) is not from a peer reviewed article. Some controlled studies have found that using the BIS reduced the incidence of memory but this was not confirmed in a very large study on awareness.  See: 

# Explanation
The BIS monitor provides a single dimensionless number, the BIS value, which ranges from 0 to 99.  A BIS value of 0 equals EEG silence, near 100 is the expected value in a fully awake adult, and between 40 and 60 indicates a level for general anaesthesia recommended by the manufacturer.
The BIS monitor thus gives the anaesthetist an indication of how "deep" under anaesthesia the patient is. BIS is one of several techniques which purport to monitor depth of anaesthesia. The classical way to judge depth of anesthesia by Guedel's classification is now obsolete.
Titrating anaesthetic agents to a specific bispectral index during general anaesthesia in adults (and children over 1 year old) allows the anaesthetist to adjust the amount of anaesthetic agent to the need of the patient, possibly resulting in faster recovery from anaesthesia.
The BIS monitor may reduce the incidence of intraoperative awareness in high risk procedures or patients (see discussion ) and may have an additional role in predicting recovery from severe brain injury although this is far from clear at present.
The introduction of BIS to the intensive care environment allows physicians to titrate sedative drugs safely, and to monitor the patient (together with measurement of intracranial pressure) during therapeutic burst suppression. 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.

# Calculation of BIS
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 anaesthesia, the pattern of activity changes. Overall, there is less activity (the "power") is less, 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 bispectral index of an electroencephalogram is a weighted sum of electroencephalographic subparameters including a time domain, a frequency domain, and higher order spectral information (Bispectral Analysis). 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 a BIS number.
As with other types of EEG analysis, the calculation algorithm that the BIS monitor uses is patented, and the company refuses to make the algorithm public. Therefore, although the principles of BIS and other monitors are well known, the exact method in each case is not.

# Anaesthesia depth in infants
Some studies show a greater incidence of anaesthesia 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.

# FDA approval
The US Food and Drug Administration approved BIS monitoring in 1996 for assessing the hypnotic effects of general anaesthetics and sedatives. BIS was approved by the FDA in 2003 as a parameter that could help to control awareness incidence in the patient, in this statement: "...A reduction in awareness provides a public health benefit, in that BIS technology can now provide anesthesiologists with a way to reduce this often debilitating, yet preventable medical error."

# 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 anaesthesia are correlated with this scale (Flashion R, et al. Anesthesiology 97).
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: Anesth Analg. 2004 Mar;98(3):706-7. Eur J Anaesthesiol. 2006 Jul;23(7):618-9. Resuscitation. 2006 May;69(2):207-12. Intensive Care Med. 2007 Jan;33(1):133-6
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 anaesthetist to reduce the risk of anaesthesia awareness during surgery for a 'high risk' group.