Bispectral index

Bispectral index (BIS) is one of several technologies used to monitor depth of anesthesia. BIS monitors are used to supplement Guedel's classification system for determining depth of anesthesia. Titrating anesthetic agents to a specific bispectral index during general anesthesia in adults (and children over 1 year old) allows the anesthetist 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 could reduce the incidence of intraoperative awareness during anaesthesia. The exact details of the algorithm used to create the BIS™ index have not been disclosed by the company that developed it.

The BIS™ monitoring system should not be used as the sole basis for diagnosis or therapy and is intended only as an adjunct in patient assessment. Its use has been shown to reduce overall dose of anaesthetic agent used and therefore may improve recovery time from anaesthesia.

The BIS™ brain monitoring system was introduced by Aspect Medical Systems, Inc. in 1994 as a novel measure of the level of consciousness by algorithmic analysis of a patient's electroencephalogram during general anesthesia. This is used in conjunction with other physiologic monitoring such as electromyography to estimate the depth of anesthesia in order to minimize the possibility of intraoperative awareness. The US Food and Drug Administration (FDA) cleared BIS™ monitoring in 1996 for assessing the hypnotic effects of general anesthetics and sedatives. The FDA further stated in 2003 that "...A reduction in awareness provides a public health benefit, in that BIS™ technology can now provide anesthestists with a way to reduce this often debilitating, yet preventable medical error". Aspect Medical was acquired by Covidien in 2009.

The bispectral index is a statistically based, empirically derived complex parameter. It is a weighted sum of several 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. 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. Additionally, BIS values less than 40 for greater than 5 minutes have been associated with increased risk of stroke (hazard ratio of 3.23), MI (1.94) and death (1.41).

The essence of the BIS™ system 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. 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.

As with other types of EEG analysis, the calculation algorithm that the BIS™ monitor uses is proprietary, although it has been successfully reverse engineered.

Some of the limitations of this form of processed EEG monitoring are:

BIS™ monitoring is widely used in operating rooms and ICUs, as well as in emergency departments. Some of the benefits that have been attributed to it include: