The Composite Variability Index (CVI, Aspect Medical System, Norwood MA) is a new parameter (measurement) that provides information about the amount of painful stimulation during surgery. It is part of the Bispectral Index (BIS, Aspect Medical Systems, Norwood MA), a well-established and widely used parameter that shows how awake/asleep patients are during general anesthesia and may indicate a need for more medication. The CVI component provides specific information about whether the patient needs pain medication (called opioids) and not just medication that makes you sleep (called hypnotics). The CVI is made of the BIS number and a measure of facial muscle movement (EMG). Both these components increase with painful stimulation, and decrease when pain medication is given.