ANI and NoL Index Variations After Standard Nociceptive Stimulus at 0, 50, 25 % of Inhaled N2O in the Anesthetic Mixture

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 e…