Bispectral index (BIS) monitors are modern neurophysiologic monitoring devices. They continually analyse patients' electroencephalograms during general anaesthesia to assess the level of anaesthesia. The use of BIS monitors is increasing.

Titrating anaesthetic agents to a specific bispectral index during general anaesthesia in adults and children over 2 years old allows anaesthetists to use less anaesthetic agent resulting in faster recovery from anaesthesia. BIS monitors also reduce the incidence of intraoperative awareness and may have an additional role predicting recovery from severe brain injury.

BIS monitors provide a single dimensionless number, the BIS value, which ranges from 0 to 100. A BIS value of 0 equals EEG silence, 100 is the expected value in a fully awake adult, and between 40 and 60 indicates hypnosis.

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. The developers of the BIS monitor collected many 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.

There is little correlation between bispectral index values and other measures of anaesthesia depth in infants, particularly those under 6 months.

The federal Food and Drug Administration approved BIS monitoring in 1996 for assessing the hypnotic effects of general anesthetics and sedatives.