Electromyographic activity falsely elevates the bispectral index.

THE bispectral index (BIS, Aspect Medical Systems, Framingham, MA) is a complex electroencephalographic (EEG) parameter that integrates several disparate descriptors of the EEG into a single variable and correlates behavioral assessments of sedation and hypnosis. We report two cases in which the bispectral index failed to measure depth of anesthesia but instead was a measure of electromyographic (EMG) activity. In the first case bispectral index paradoxically increased after increasing propofol concentration, correlating with increasing EMG activity. In the second case the administration of a nondepolarizing muscle relaxant decreased the bispectral index value at constant anesthetic drug concentrations.