Bispectral index
From Free net encyclopedia
Bispectral index (BIS) monitors are modern neurophysiologic monitoring devices that continually analyse patients' electroencephalograms during general anaesthesia to assess the level of anaesthesia. The "depth of anaesthesia", is commonly used as a surrogate for "the likliehood of forming memory". The use of BIS monitors is increasing.
Contents |
Explanation
Titrating anaesthetic agents to a specific bispectral index during general anaesthesia in adults and children over 2 years old allows anaesthetists to potentially use less anaesthetic agent resulting in faster recovery from anaesthesia. BIS monitors may also 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.
BIS monitors provide a single dimensionless number, the BIS value, which ranges from 0 to 100. 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
There is little correlation between bispectral index values and other measures of anaesthesia depth in infants (by definition under 1 year old). This is probably 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 anesthetics and sedatives.
BIS relevance
Though the BIS is an electroencephalogram derived multivariant scale that reflects the level of hypnosis in anesthetized patientsTemplate:Ref, both loss of consciousness and awakening from anaesthesia are only somehow correlated with this scale. A monitor of the Autonomic Nervous System (such as the ANSiscope) seems more appropriate for purposely assessing the depth of anesthesia. However BIS is recognised as a scale related to the probability of formation of explicit memory whilst under anaesthesia.
Footnotes
2.Template:NoteRosow, C, Manberg, PJ (2001) Bispectral index monitoring. Anesthesiol Clin North America 19(4): 947-66, xi.