N2O has been used during general anesthesia (GA) for more than 100 years. It is known to have anesthetic agents sparing effect. But small is know on his real analgesic effect during GA. So far, the only way to monitor pain during GA was based on vital signs that are not specific and not sensitive. Few devices tried to evaluated pain under GA during the last 2 decades. More recently, better devices were proposed such as the Physiodoloris device and the PMD200 device. The first offers an index called ANI based on heart rate variability (HRV) assessment. The second offers the NoL index based on the analysis of 5 parameters. The aim of the present study is to evaluate quantitatively the analgesic index of N2O during GA using the two indices ANI and NoL.

Title: Assessment of the intraoperative analgesic effect of 50%/50% Nitrous Oxide / Oxygen inhalation by the pain index ANI following a standardized electrical forearm stimulus in patients undergoing laparotomies under general anesthesia and with an intraoperative active epidural analgesia. Comparison to the new NoL analgesic index.

Objectives: To observe the variations of the intraoperative pain indexes ANI and NoL after a classical and standardized electrical stimulation of the forearm (applied with the commonly used nerve stimulator we use for muscle relaxation monitoring) at different concentrations of inhaled Nitrous Oxide (N2O) (from 0% to 50%, concentrations that are commonly used in daily anesthesia). This will allow a quantification of the analgesic effect of N2O.

Study Design: Prospective, open label, observational and descriptive study Subject Population: Adult patients scheduled to undergo abdominal surgery by laparotomy under general anesthesia with an active intraoperative epidural analgesia Sample Size: 40 patients will be evaluated in this study Study Duration: 1 year Study Center: Maisonneuve-Rosemont Hospital, Montreal, Quebec, Canada Adverse Events: None expected

Inclusion Criteria:

• ASA status I, II or III
• Patients aged 18 to 70 years
• Laparotomy for gynecological or bowel surgery
• Duration and type of surgery requiring an epidural analgesia via an epidural catheter placed prior the general anesthesia induction and an arterial line placed after induction of general anesthesia

Exclusion Criteria:

• When 100% O2 ventilation is required during anesthesia (except for the induction phase during which all the patients receive O2 100%).

• Patients having received intraocular injection of gas (such as SF6, C3F8, C2F6) within 3 months prior to the present surgery.

• Any condition where internal entrapped air could expand and become dangerous, such as:

  • head injury within 6 months prior to surgery
  • maxillofacial injuries within 6 months prior to surgery
  • pneumothorax within 6 months prior to surgery
  • gas embolism within 6 months prior to surgery
  • decompression sickness within 6 months prior to surgery
  • bubbles of emphysema known for this patient
  • middle ear, inner ear and sinus surgery
  • major abdominal distension/occlusion and emergent surgery for intestinal obstruction
  • if air has been injected into the epidural space to determine the placement of the needle for epidural anesthesia (only saline is used in our center for the loss of resistance technique to place the catheter into the epidural space)

Any contra-indication or patient's refusal for epidural placement

• Unexpected difficult airway requesting excessive, possibly painful airway manipulations.
• Unexpected surgical complications requiring strong haemodynamic support (transfusions, volume challenges, vasopressors, inotropic drugs).