BIS MonitorBispectral index (BIS) monitoring is one of several technologies which monitor depth of anaesthesia.

BIS monitors are intended to replace or supplement Guedel's classification system for determining depth of anaesthesia. 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 needs of the patient, possibly resulting in a more rapid emergence from anaesthesia.

Use of the BIS monitor is thought to reduce the incidence of intraoperative awareness in surgeries. The algorithm is proprietary information, which means that it is kept secret by the company that developed it - Aspect Medical Systems.

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 anaesthesia. This is used in conjunction with other physiologic monitoring such as electromyography to estimate the depth of anaesthesia 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 anaesthetics 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 anaesthesiologists 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 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 (equivalent to fully awake and alert). A BIS value between 40 and 60 indicates an appropriate level for general anaesthesia, as recommended by the manufacturer. The BIS monitor thus gives the anaesthetist an indication of how "deep" under anaesthesia 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 anaesthesia, 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 a BIS number. 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 in each case is not.

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 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 anaesthetic depth in accordance with literature-based evidence. BIS, however, is not explicitly endorsed. In fact, they cite an American Society of Anaesthesiologists (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 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. However, this result was not reproduced by a recently published randomised control trial, the "B-Unaware Trial". In it, the use of BIS monitoring was not associated with a lower incidence of anaesthesia awareness. In some cases, the BIS may underestimate the depth of anaesthesia, leading the anaesthetist to administer a higher than necessary dose of anaesthetic 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 anaesthesia 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.

New insight into brain responses during anaesthesia and surgery

  • BIS™ technology can help clarify the distinction between brain and spinal cord responses, enabling you to manage anaesthetic goals of hypnosis, analgesia and immobility.
  • Because BIS™ technology measures electrical activity in the brain, it provides a direct correlation with level of consciousness (hypnosis).
  • Responses to surgical stimulation are frequently indicators of the need for additional analgesia. These responses are often mediated at the spinal cord.
  • BIS™ technology enables you to assess consciousness and sedation separately from cardiovascular reactivity.

BIS™ monitoring and reduced risk of awareness in adults

  • Research demonstrates that awareness with recall occurs in one to two patients per 1,000 receiving general anesthesia.
  • Awareness is a leading cause of patient dissatisfaction with anesthesia.
  • BIS™ monitoring has been shown in rigorous prospective clinical studies to help clinicians reduce the incidence of awareness with recall in adults by approximately 80 percent.

Improving care and contributing to cost savings in Operating Theatres

  • BIS™ monitoring and faster wake ups
  • BIS™ monitoring and shorter PACU stays
  • BIS™ monitoring and drug savings
  • BIS™ monitoring and reduced risk of awareness in adults


BIS Benefit

Sedation (Critical Care)

A tool for objective assessment of sedation in the ICU
Until now, sedation assessment has been primarily guided by vital signs or subjective sedation assessment scales.
These approaches may not be sufficient to achieve optimal patient assessment.

Complications of Over-sedation

  • Increased time on mechanical ventilation
  • Increased length of stay in ICU and/or hospital
  • Additional cost of care
  • Increased risk of delirium
  • Need for additional diagnostic testing
  • Decreased wound healing and GI motility
  • Impaired reliability of neurological examinations

Complications of Under-sedation

  • Patient fear, anxiety and agitation
  • Increased nursing time
  • Additional costs
  • Medical device removal
  • Unpleasant recall


Now, with an objective way to assess sedation, the BIS™ monitor can help determine the patient's level of sedation and allow informed decisions about titration of sedative drugs. Especially useful for sedation assessment during:

  • Mechanical ventilation
  • Neuromuscular blockade
  • Barbiturate coma
  • Bedside procedures

Improving care and contributing to cost savings in the ICU

  • Spectrum Health reduced average monthly costs of sedatives
  • Medical College of Pennsylvania Hospital decreased its cost of sedative drugs
  • Medical College of Pennsylvania Hospital reduced recall of unpleasant experiences
  • Duke University, Durham, NC study: BIS use reduces drug costs while maintaining adequate sedation levels

Licensed for integration into the patient monitoring systems of leading manufacturers:

  • Datascope
  • Dixtal Medical
  • Dräger Medical GmbH
  • GE Healthcare
  • Mennen Medical
  • Mindray
  • Nihon Kohden
  • Philips
  • Spacelabs Healthcare




Edited by: John Sandham



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