Bispectral index

Bispectral index (BIS) monitors are modern neurophysiologic monitoring devices that continually analyse patients' electroencephalograms during general anaesthesia to assess the level of counciousness during anesthesia. The "depth of anaesthesia", is commonly used as a surrogate for "the likelihood of forming memory". The use of BIS monitors is increasing. Some profesional bodies have anounced their recomendations as the Sociedad Madrid Centro de Anestesiología y Reanimación http://www.sarmadrid.org]]and the Australian & New Zealand College of Anaesthesia.

Explanation

Titrating anaesthetic agents to a specific bispectral index during general anaesthesia in adults and children over 1 year old allows anaesthetists to adjust the amount of anaesthetic agent to the need of the patient resulting in faster recovery from anaesthesia. BIS monitors reduce the incidence of intraoperative awareness in high risk procedures or patients(see discussion [1]) and may have an additional role predicting recovery from severe brain injury though this is far from clear at present. Its introduction in ICU enviroment allows to titrate drugs in order to maintain the patient during barbiturate coma together with the intracraneal preasure measure, to maintain the balance between hypnotics and analgesics drugs to adapt the patient to the ventilator and to avoid counciousness during the neuromuscular blockade prodedure. As well some authors talks about the possibility of using BIS monitor as an alarma monitor in cases of brain damage and rain death. BIS technology is being used aswell in transport of patients, ambulances, helicopters etc....

BIS monitors provide 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 generally accepted level for so called general anaesthesia.

Calculation of BIS

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.

Anaesthesia depth in infants

The relevance of measuring depth of counciousness during anesthesia in children seems to be bigger than in adults, some studies shows a big incidence on awareness than in adults. The correlation between Bispectral index in children over one year and state of counciousness has already been proven although in younger patients should be treated with caution because of the differences between immature infant EEG patterns and the adult EEG patterns that the BIS algorithm utilises.

FDA approval

The federal Food and Drug Administration approved BIS monitoring in 1996 for assessing the hypnotic effects of general anaesthetics and sedatives. And aproved by the FDA in 2003 as a parameter that could help to control awareness incidence in the patient as it ir reproduced below:"...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.Anesthesiolgy 98), from this metabolic activity the brain obtains its fuctionality, the ability of capturing information from outside and inside the body and building up reality and the posibility afterwards to remember it, both loss of consciousness and awakening from anaesthesia are correlated with this scale (Flashion.R et al Anesthesiology 97). BIS parameter has been proven to measure in a direct way the level of counciousness, in a independent way of the factor that cause that uncounsciousness(drugs, metabolic disease,hypothermia,head trauma,hypovolemia, natural sleep....etc...) all are uncouncios patients and wll have a low BIS value,although its general clinical state is very different from one to the other, and the prognosis will differ aswell. In a inderect way BIS in the OR can be use as a lack of analgesia monitor, if a stimulus is added to the patient and the BIS reacts there was a lack of analgesia for that stimulus, in any case a monitor of the Autonomic Nervous System (such as the ANSiscope) seems more appropriate for purposely assessing the level of stimulus.A monitor of the central nervous system seems more appropiate to control counciousness.After the publication of the B-Aware Trial (Ps Myles, K.Leslie et al. Lancet 2004) BIS is recognised as a parameter that allows the anesthesilogists to avoid awareness during anesthesia. BIS is the only parameter that has proven its benefits under a randomised,double blinded, prospective trial, neither pulseoximetry has proven its benefit:“The two groups did not differ significantly in cardiovascular, respiratory, neurologic or �infectious complications.”Randomized Evaluation of Pulse Oximetry in 20,802 Patients(Moller et al. Anesthesiology �1993; 78:445-453)

Footnotes

^2.^ Rosow, C, Manberg, PJ (2001) Bispectral index monitoring. Anesthesiol Clin North America 19(4): 947-66, xi.