EEG monitoring using bispectral analysis

There is a need for an objective monitor of consciousness to prevent inadvertent awareness during surgery and intensive care sedation. The ideal monitor would meet several criteria: (1) ease of use in the operating room or intensive care unit, with automatic rejection of artefacts; (2) readily interpreted by anaesthetists and support staff; (3) accurate reflection of the depth of anaesthesia or sedation with a number of anaesthetic agents; (4) response in real-time, i.e. so that appropriate clinical action may be taken, should the depth of anaesthesia or sedation fall, in order to prevent awareness. Spectral analysis of the raw EEG, usually using fast Fourier techniques, has been used extensively to dissect the information contained within the raw EEG into a form that a clinician can readily interpret. Several statistical derivatives describing the distribution of power within the spectral analysis have been proposed as putative indices of anaesthetic and sedative depth, including median frequency and spectral edge (90% and 95%) frequencies. The Bispectral Index (BIS) is a proprietary, statistically-derived multi parametric function of conventional power spectral derivatives ofthe EEG combined with bispectral analyses. BIS is routinely derived from EEG electrodes placed on each side of the head with a frontomastoid montage (positions Fp1, Fp2, A1 and A2) referred to a common ground. BIS is capable of discriminating subtle changes in the EEG during anaesthesia that are not apparent using simpler EEG analyses