How can doctors measure consciousness during general anesthesia?

An active brain during general anesthesia is particularly difficult to monitor. Doctors might suspect that consciousness isn't fully dampened when they see signs such as an elevated blood pressure or heart rate, or muscle movement. But certain drugs used to achieve anesthesia may mask these changes. Some hospitals use the bispectral index (BIS) brain-sensing monitor, similar to an electroencephalogram (EEG), to measure a patient's level of consciousness. But not all hospitals use this monitoring method and some doctors dispute their value in signaling or preventing anesthesia awareness.