The current study is intended to be a prospective clinical performance validation study designed to confirm the clinical utility of HFVI-guided fentanyl administration during sevoflurane anesthesia.

The overall objective of this study is to confirm the safety and efficacy of HFVI-guided analgesic administration in comparison to standard clinical practice. The primary efficacy endpoint will be the amount of postoperative pain reported in the PACU, as measured using a nurse administered Numerical Rating Scale (NRS) pain score.

HFVi was designed to provide a standardized measure of the parasympathetic component (p∑) of the Autonomous Nervous System (ANS). It tracts changes of the p∑ tonus induced by each respiratory cycle (spontaneous or artificial) to measure the relative balance of parasympathetic and sympathetic tone as reflected in the EKG signal.

These rapid changes of the p∑ tonus express themselves at the sinus node level by changes of the time interval separating two R waves of the electrocardiogram. The following normal RR intervals constitute the tachogram (displayed as respiratory pattern on the monitor). The p ∑ component is evaluated after filtering, standardization and re-sampling of the R-R series, by measuring the surface generated by respiratory cycles on the tachogram. Higher p∑ tonus is reflected as a larger measured surface area. In contrast, the measured surface decreases when the p∑ decreases.

HFVi, a measure of the surface area, is expressed in the form of a dimensionless index ranging from 0 to 100. This index reflects the relative activity of the parasympathetic nervous system. It expresses the relative quantity of the present p∑ tonus compared to the Para and sympathetic tonus. The measure of HFVi displayed represents the average of a sequence of measurements: each elementary measure is realized on 64 seconds of data, updated each second using a sliding window.

During development, a probabilistic interpretation of the HFVi was used to relate index values to a clinical state in an anesthetized subject. A significant hemodynamic response (increase of the heart rare or blood pressure of 20% compared to the baseline) within 10 minutes was used as an indicator of nociception/inadequate analgesia.

As a measure of parasympathetic tone, HFVi may be influenced by many factors and thus be difficult to interpret in many of the following situations:

• arrhythmia
• no breathing (ex : apnea due to intubation)
• Respiratory rate lower than 9 cycles/min
• Tidal volume variable on the measuring time, thus 64 seconds
• Irregular breathing (when the patient speaks, laugh or cough)
• pace maker (some types)
• heart transplantation
• Drug use having a significant effect on the sinus cardiac activity

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 uncontrolled thyroid disorders, hepatic, cardiac, pulmonary and renal malfunctioning.

Investigators administering anesthesia will be informed as to whether or not HFVI information will be unmasked to help guide analgesic administration. However, subjects who participate in the trial and the PACU assessors of pain levels will be blinded to study group assignment.