Bispectral Index - Calculation of BIS

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 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 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.