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.

Neuromuscular blocking drugs (NMD) are paralyzing medications that are often used during surgery to relax the body and make it easier for the surgeon to perform the surgery and the anesthesiologist to insert the breathing tube (called intubation). Facial muscles are resistant to NMD effects, but they might affect the EMG reading, making the CVI less useful when a patient needs NMD. The M-entropy monitor (GE Healthcare, Helsinki, Finland) is similar to the BIS monitor with CVI and is affected by large NMD doses. The purpose of this study is to examine the effect of rocuronium, a NMD, on the response of CVI and M-entropy to laryngoscopy under anesthesia. A laryngoscopy is the standard way of examining and opening the patient's airway before inserting the breathing tube. It is fairly stimulating so anesthesiologists sometimes, but not always, administer a NMD (in varying doses) before performing the laryngoscopy in standard care.

Eighty subjects will be recruited from those scheduled for surgery requiring general anesthesia at St Vincent's Hospital. Subjects must have ASA status of 1 or 2 (be fairly healthy), a BMI between 18-35 (reasonably healthy weight), and be between the ages of 18 and 75. They will not be eligible if they take certain medications or are expected to have a difficult intubation.

Subjects will be randomized (assigned by chance) to one of four rocuronium doses of 0, 0.2, 0.4, or 0.6 mg kg-1. All are acceptable clinical doses for performing a laryngoscopy. In the operating room, routine monitors will be applied, including a BIS sensor and an M-Entropy sensor. Subjects will receive 0.025 mg kg-1 midazolam (a standard pre-op dose) and will be put to sleep. Once asleep, the subject will receive a rocuronium dose, followed by laryngoscopy three minutes later. The anesthesiologist performing the laryngoscopy will not know what dose of rocuronium the subject received. CVI, entropy, amount of muscle relaxation, and vital signs will be monitored and recorded throughout the procedure.

Subjects will receive propofol and remifentanil infusions during the case. These are commonly used medications for anesthesia. The subjects will also be randomized (assigned by chance) to two additional groups. One group will receive a remifentanil infusion of 2ng ml-1 and the other group will receive a 8ng ml-1 remifentanil infusion. Both doses are acceptable and often used during standard clinical care. The propofol infusion will be adjusted to keep the BIS number between 45-60. The anesthesiologist will not be able to see the CVI value. The times of certain intraoperative events, such as intubation and incision, will be recorded. All subjects will receive a morphine bolus (0.10-0.15 mg/kg) towards the end to reduce post-operative pain, as per standard clinical care. As the subject wakes up, time to eye opening and orientation will be recorded. The subject will rate their pain on a numerical pain scale and the quality of emergence will be assessed.

Upon arrival in the PACU, subjects will be asked to rate their pain again using the same pain scale. The pain score will be evaluated every 10 minutes for half an hour, then every hour until they are discharged from PACU. The total dose of pain medications administered in PACU will be recorded. Upon PACU discharge, subjects will be asked to rate their overall experience with anesthesia. Demographic data will also be collected. All data will be kept in locked file cabinets and study results will be shared with research personnel at Aspect Medical Systems.