Confirmatory Study of HFVI Guided Analgesic Administration in Surgical Subjects

Inclusion Criteria: * Able to provide informed consent * American Society of Anesthesiologist Physical Status 1 or 2 * Body Mass Index range of 19-35 kg.m-2. * Planned spinal or open abdominal surgery expected to last 1-3 hours utilizing a balanced sevoflurane-fentanyl general anesthetic Exclusion Criteria: * Subjects are unable or unwilling to give informed consent. * Emergency surgery * Women who are currently pregnant or not using a medically acceptable means of birth control * Cardiac morbidity, including non-regular sinus cardiac rhythm or implanted cardiac pacemaker * Concurrent medications with a major effect upon the sinus node including prescribed antimuscarinic agents, α2-adrenergic agonists, β1-adrenergic antagonists, and antiarrhythmic agents * Expected duration of surgery less than 1 hour (60 minutes) or greater than 3 hours (180 minutes) * Pre-operative chronic opioid use or chronic pain, equivalent to requiring oxycodone 20mg per oral, per day for more than 6 weeks * Allergy or intolerance to any of the anticipated study medications, such as history of malignant hyperthermia during anesthesia * Planned use of neuraxial anesthesia * Clinically significant abnormality or clinically significant unstable medical condition, as indicated by medical history, physical examination, ECG results, or clinical laboratory testing, that in the Investigator's judgment might pose a potential safety risk to the subject or limit interpretation of the trial results, e.g., any u…