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Bispectral index (BIS) is one of several technologies used to
monitor Monitor or monitor may refer to: Places * Monitor, Alberta * Monitor, Indiana, town in the United States * Monitor, Kentucky * Monitor, Oregon, unincorporated community in the United States * Monitor, Washington * Monitor, Logan County, West ...
depth of
anesthesia Anesthesia is a state of controlled, temporary loss of sensation or awareness that is induced for medical or veterinary purposes. It may include some or all of analgesia (relief from or prevention of pain), paralysis (muscle relaxation), ...
. BIS monitors are used to supplement Guedel's classification system for determining depth of anesthesia. Titrating
anesthetic An anesthetic (American English) or anaesthetic (British English; see spelling differences) is a drug used to induce anesthesia ⁠— ⁠in other words, to result in a temporary loss of sensation or awareness. They may be divided into two ...
agents to a specific bispectral index during general anesthesia in adults (and children over 1 year old) allows the
anesthetist Anesthesiology, anaesthesiology, or anaesthesia is the medical specialty concerned with the total perioperative care of patients before, during and after surgery. It encompasses anesthesia, intensive care medicine, critical emergency medicin ...
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 Awareness under anesthesia, also referred to as intraoperative awareness or accidental awareness during general anesthesia (AAGA), is a rare complication of general anesthesia where patients regain varying levels of consciousness during their su ...
during anaesthesia. The exact details of the algorithm used to create the BIS index have not been disclosed by the company that developed it. BIS cannot be used as the sole monitor of anaesthesia, as it is affected by several other factors, including the anaesthetic drugs used (BIS is relatively insensitive to agents such as
ketamine Ketamine is a dissociative anesthetic used medically for induction and maintenance of anesthesia. It is also used as a recreational drug. It is one of the safest anesthetics, as, in contrast with opiates, ether, and propofol, it suppress ...
and
nitrous oxide Nitrous oxide (dinitrogen oxide or dinitrogen monoxide), commonly known as laughing gas, nitrous, or nos, is a chemical compound, an oxide of nitrogen with the formula . At room temperature, it is a colourless non-flammable gas, and ha ...
), and muscle movement or artefact from surgical equipment. BIS is used as an adjunct to monitoring under anaesthesia - its use has been shown to reduce overall dose of anaesthetic agent used and therefore may improve recovery time from anaesthesia.


History

The BIS was introduced by Aspect Medical Systems, Inc. in 1994 as a novel measure of the level of
consciousness Consciousness, at its simplest, is sentience and awareness of internal and external existence. However, the lack of definitions has led to millennia of analyses, explanations and debates by philosophers, theologians, linguisticians, and scien ...
by algorithmic analysis of a patient's
electroencephalogram Electroencephalography (EEG) is a method to record an electrogram of the spontaneous electrical activity of the brain. The biosignals detected by EEG have been shown to represent the postsynaptic potentials of pyramidal neurons in the neocortex ...
during
general anesthesia General anaesthesia (UK) or general anesthesia (US) is a medically induced loss of consciousness that renders the patient unarousable even with painful stimuli. This effect is achieved by administering either intravenous or inhalational general ...
. This is used in conjunction with other physiologic monitoring such as
electromyography Electromyography (EMG) is a technique for evaluating and recording the electrical activity produced by skeletal muscles. EMG is performed using an instrument called an electromyograph to produce a record called an electromyogram. An electromyo ...
to estimate the depth of anesthesia in order to minimize the possibility of intraoperative awareness. The US
Food and Drug Administration The United States Food and Drug Administration (FDA or US FDA) is a federal agency of the Department of Health and Human Services. The FDA is responsible for protecting and promoting public health through the control and supervision of food ...
(FDA) cleared BIS monitoring in 1996 for assessing the hypnotic effects of general anesthetics and
sedative A sedative or tranquilliser is a substance that induces sedation by reducing irritability or excitement. They are CNS depressants and interact with brain activity causing its deceleration. Various kinds of sedatives can be distinguished, but ...
s. 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

The bispectral index is a statistically based, empirically derived complex parameter. It is a weighted sum of several electroencephalographic subparameters, including a
time domain Time domain refers to the analysis of mathematical functions, physical signals or time series of economic or environmental data, with respect to time. In the time domain, the signal or function's value is known for all real numbers, for the c ...
,
frequency domain In physics, electronics, control systems engineering, and statistics, the frequency domain refers to the analysis of mathematical functions or signals with respect to frequency, rather than time. Put simply, a time-domain graph shows how a s ...
, 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 General anaesthesia (UK) or general anesthesia (US) is a medically induced loss of consciousness that renders the patient unarousable even with painful stimuli. This effect is achieved by administering either intravenous or inhalational general ...
, as recommended by the manufacturer. The BIS monitor thus gives the anesthetist an indication of how "deep" under anesthesia the patient is. Additionally, BIS values less than 40 for greater than 5 minutes have been associated with increased risk of stroke ( hazard ratio of 3.23), MI (1.94) and death (1.41). 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. The recent availability of cheap, fast
computer processor In computing and computer science, a processor or processing unit is an electrical component (digital circuit) that performs operations on an external data source, usually memory or some other data stream. It typically takes the form of a micropr ...
s 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 In mathematics, Fourier analysis () is the study of the way general functions may be represented or approximated by sums of simpler trigonometric functions. Fourier analysis grew from the study of Fourier series, and is named after Joseph ...
), and there is a tendency for signal correlation from different parts of the cortex to become more random. As with other types of EEG analysis, the calculation algorithm that the BIS monitor uses is proprietary, although it has been successfully reverse engineered.


Limitations

Some of the limitations of this form of processed EEG monitoring are: * Unreliability with certain drugs, such as ketamine, nitrous oxide and ephedrine. Ketamine may produce a paradoxical transient increase in BIS values as this increase is associated with deepening of the hypnotic state. * Difficulty titrating anesthetics in patients 6 months or younger. * Interference by other medical devices such as electrosurgical cautery. * Unreliable in cases of hypothermia or neurological impairment. * Events unrelated to titration of anesthetic drugs can produce rapid changes in BIS values, such as cerebral hypoperfusion, gas embolism, and hemorrhage. * The BIS value may lag behind the observed clinical state by 5 to 15 seconds related to EEG processing.


Benefits

BIS monitoring is widely used in operating rooms and ICUs, as well as in emergency departments. Some of the benefits that have been attributed to it include: * Reduced time to extubation and discharge from the operating room and PACU * Reduced risk of postoperative nausea and vomiting * Reducing the use of propofol * Reduced risk of awareness, unless this claim is questioned by some studies.


Relevance

The BIS is an electroencephalogram-derived multivariant scale that, when a drug such a propofol is used, 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 The American Society of Anesthesiologists (ASA) is an educational, research and scientific association of physicians organized to raise the standards of the medical practice of anesthesiology and to improve patient care. As of 2021, the organizat ...
(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 Hypothermia is defined as a body core temperature below in humans. Symptoms depend on the temperature. In mild hypothermia, there is shivering and mental confusion. In moderate hypothermia, shivering stops and confusion increases. In severe ...
, head
trauma Trauma most often refers to: * Major trauma, in physical medicine, severe physical injury caused by an external source * Psychological trauma, a type of damage to the psyche that occurs as a result of a severely distressing event *Traumatic i ...
,
hypovolemia Hypovolemia, also known as volume depletion or volume contraction, is a state of abnormally low extracellular fluid in the body. This may be due to either a loss of both salt and water or a decrease in blood volume. Hypovolemia refers to the los ...
, natural
sleep Sleep is a sedentary state of mind and body. It is characterized by altered consciousness, relatively inhibited sensory activity, reduced muscle activity and reduced interactions with surroundings. It is distinguished from wakefulness by a de ...
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 Suxamethonium chloride, also known as suxamethonium or succinylcholine, or simply sux by medical abbreviation, is a medication used to cause short-term paralysis as part of general anesthesia. This is done to help with tracheal intubation or ele ...
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 autonomic nervous system (ANS), formerly referred to as the vegetative nervous system, is a division of the peripheral nervous system that supplies viscera, internal organs, smooth muscle and glands. The autonomic nervous system is a control ...
(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 The central nervous system (CNS) is the part of the nervous system consisting primarily of the brain and spinal cord. The CNS is so named because the brain integrates the received information and coordinates and influences the activity of all par ...
may be more appropriate for monitoring consciousness. After the publication of the ''B-Aware Trial'' ensuring that the BIS index was below 60 was suggested to reduce the risk of
anesthesia awareness Awareness under anesthesia, also referred to as intraoperative awareness or accidental awareness during general anesthesia (AAGA), is a rare complication of general anesthesia where patients regain varying levels of consciousness during their sur ...
during surgery for a 'high risk' group, when using a conventional anaesthetic technique. 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
ambulance An ambulance is a medically equipped vehicle which transports patients to treatment facilities, such as hospitals. Typically, out-of-hospital medical care is provided to the patient during the transport. Ambulances are used to respond to medi ...
s,
helicopter A helicopter is a type of rotorcraft in which lift and thrust are supplied by horizontally spinning rotors. This allows the helicopter to take off and land vertically, to hover, and to fly forward, backward and laterally. These attributes ...
s 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. Specifically for intraoperative awareness, according to a 2013 article in ''
The Atlantic ''The Atlantic'' is an American magazine and multi-platform publisher. It features articles in the fields of politics, foreign affairs, business and the economy, culture and the arts, technology, and science. It was founded in 1857 in Boston, ...
'', "Today, the BIS monitor has become the most controversial medical device in anesthesiology, if not all of surgery."


See also

* Anaesthetic machine *
Entropy monitoring Entropy monitoring is a method of assessing the effect of certain anaesthetic drugs on the brain's EEG. It was commercially developed by Datex-Ohmeda, which is now part of GE Healthcare. Entropy is a quantitative EEG device which captures a singl ...
*
Evoked potentials An evoked potential or evoked response is an electrical potential in a specific pattern recorded from a specific part of the nervous system, especially the brain, of a human or other animals following presentation of a stimulus such as a light ...
*
Respiratory monitoring Mechanical ventilation, assisted ventilation or intermittent mandatory ventilation (IMV), is the medical term for using a machine called a ventilator to fully or partially provide artificial ventilation. Mechanical ventilation helps move air i ...
* Spectral edge frequency


References


Further reading

*


External links

(Commercial interest publications)
BIS Education
BIS Education {{DEFAULTSORT:Bispectral Index Anesthesia Anesthetic equipment