Bispectral index
Encyclopedia
Guedel's classification
Guedel's classification is a means of assessing of depth of general anesthesia introduced by Arthur Ernest Guedel in 1937.-History:...
system for determining depth of anesthesia. Titrating anesthetic
General anaesthetic
A general anaesthetic is a drug that brings about a reversible loss of consciousness. These drugs are generally administered by an anaesthesia provider to induce or maintain general anaesthesia to facilitate surgery...
agents to a specific bispectral index during general anesthesia in adults (and children over 1 year old) allows the anesthetist
Anesthesiologist
An anesthesiologist or anaesthetist is a physician trained in anesthesia and peri-operative medicine....
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 is thought to reduce the incidence of intraoperative awareness
Anesthesia awareness
Anesthesia awareness, or unintended intra-operative awareness occurs during general anesthesia, on the operating table, when the patient has not been given enough of the general anesthetic or analgesic to render the patient unconscious during general anesthesia...
in surgeries. The algorithm is proprietary information, which means that it is kept secret by the company that developed it.
History
Bispectral index (BIS) was introduced by Aspect Medical Systems, Inc. in 1994 as a novel measure of the level of consciousnessConsciousness
Consciousness is a term that refers to the relationship between the mind and the world with which it interacts. It has been defined as: subjectivity, awareness, the ability to experience or to feel, wakefulness, having a sense of selfhood, and the executive control system of the mind...
by algorithmic analysis of a patient's electroencephalogram
Electroencephalography
Electroencephalography is the recording of electrical activity along the scalp. EEG measures voltage fluctuations resulting from ionic current flows within the neurons of the brain...
during general anesthesia. This is used in conjunction with other physiologic monitoring such as electromyography
Electromyography
Electromyography 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 electromyograph detects the electrical potential generated by muscle...
to estimate the depth of anesthesia in order to minimize the possibility of intraoperative awareness. The US Food and Drug Administration
Food and Drug Administration
The Food and Drug Administration is an agency of the United States Department of Health and Human Services, one of the United States federal executive departments...
(FDA) cleared BIS monitoring in 1996 for assessing the hypnotic effects of general anesthetics and sedative
Sedative
A sedative or tranquilizer is a substance that induces sedation by reducing irritability or excitement....
s. The FDA further stated in 2003 that "...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". 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 domainTime domain
Time domain is a term used to describe 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 case of continuous time, or at various...
, frequency domain
Frequency domain
In electronics, control systems engineering, and statistics, frequency domain is a term used to describe the domain for analysis of mathematical functions or signals with respect to frequency, rather than time....
, and high order spectral subparameters. The BIS monitor provides a single dimensionless number, the , 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 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
Hypnotic
Hypnotic drugs are a class of psychoactives whose primary function is to induce sleep and to be used in the treatment of insomnia and in surgical anesthesia...
drug
Drug
A drug, broadly speaking, is any substance that, when absorbed into the body of a living organism, alters normal bodily function. There is no single, precise definition, as there are different meanings in drug control law, government regulations, medicine, and colloquial usage.In pharmacology, a...
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 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 AnesthesiologistsAmerican Society of Anesthesiologists
The American Society of Anesthesiologists is an association of physicians, primarily anesthesiologists, that share a common goal of raising the standard of the medical specialty of anesthesiology and the improvement of patient care by fostering and encouraging education through research and...
(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
Hypothermia is a condition in which core temperature drops below the required temperature for normal metabolism and body functions which is defined as . Body temperature is usually maintained near a constant level of through biologic homeostasis or thermoregulation...
, head trauma
Physical trauma
Trauma refers to "a body wound or shock produced by sudden physical injury, as from violence or accident." It can also be described as "a physical wound or injury, such as a fracture or blow." Major trauma can result in secondary complications such as circulatory shock, respiratory failure and death...
, hypovolemia
Hypovolemia
In physiology and medicine, hypovolemia is a state of decreased blood volume; more specifically, decrease in volume of blood plasma...
, natural sleep
Sleep
Sleep is a naturally recurring state characterized by reduced or absent consciousness, relatively suspended sensory activity, and inactivity of nearly all voluntary muscles. It is distinguished from quiet wakefulness by a decreased ability to react to stimuli, and is more easily reversible than...
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
Autonomic nervous system
The autonomic nervous system is the part of the peripheral nervous system that acts as a control system functioning largely below the level of consciousness, and controls visceral functions. The ANS affects heart rate, digestion, respiration rate, salivation, perspiration, diameter of the pupils,...
(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
Central nervous system
The central nervous system is the part of the nervous system that integrates the information that it receives from, and coordinates the activity of, all parts of the bodies of bilaterian animals—that is, all multicellular animals except sponges and radially symmetric animals such as jellyfish...
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
Anesthesia awareness
Anesthesia awareness, or unintended intra-operative awareness occurs during general anesthesia, on the operating table, when the patient has not been given enough of the general anesthetic or analgesic to render the patient unconscious during general anesthesia...
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 ambulance
Ambulance
An ambulance is a vehicle for transportation of sick or injured people to, from or between places of treatment for an illness or injury, and in some instances will also provide out of hospital medical care to the patient...
s, helicopter
Helicopter
A helicopter is a type of rotorcraft in which lift and thrust are supplied by one or more engine-driven rotors. This allows the helicopter to take off and land vertically, to hover, and to fly forwards, backwards, and laterally...
s 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.
See also
- anaesthetic machineAnaesthetic machineThe anaesthetic machine is used by anaesthesiologists and nurse anaesthetists to support the administration of anaesthesia...
- entropy monitoringEntropy monitoringEntropy monitoring is a relatively new method of assessing anaesthetic depth. It was commercially developed by Datex-Ohmeda, now part of GE Healthcare.It relies on a method of assessing the degree of irregularity in electroencephalogram signals...
- evoked potentials
- spectral edge frequencySpectral edge frequencyThe spectral edge frequency or SEF is a measure used in signal processing. It is usually expressed as "SEF x", which stands for the frequency below which x percent of the total power of a given signal are located...
- Respiratory monitoringRespiratory monitoringMonitoring a patient's respiratory status usually takes place in a hospital setting and may be the primary purpose for a patient being observed or admitted to a medical setting....
External links
(Commercial interest publications)- BIS Education BIS Education
- Monitoring Consciousness Using the Bispectral Index During Anesthesia by Scott D. Kelley M.D. (PDF)