The investigators will include 100 adult patients, American Society of Anesthesiologists (ASA) physical status I-III, who will be scheduled for elective laparotomies or laparoscopic surgeries under sevoflurane anesthesia lasting 120 minutes or longer. After written informed consent participants will be randomized into two groups: GO2 - air in 30% oxygen and GN2O - the same mixture until the last 30 minutes of surgery when 70% N2O in 30% oxygen will be used. General anesthesia will be performed with propofol and fentanyl for anesthesia induction, rocuronium for muscle relaxation and volatile anesthetic sevoflurane for maintenance. The participants will be extubated in the operation room (OR) after they open their eyes and follow commands. Postanesthetic recovery score by Aldrete, Ramsay sedation scale (RSS), and Simplified postoperative nausea and vomiting impact scale score by Myles (PONV ISS) will be used to assess recovery in PACU and surgical ward. Modified Aldrete score assigns a score of 0, 1, or 2 to activity, respiration, circulation, consciousness, and color, giving a maximal score of 10 ( score of 9 indicates recovery sufficient for the patient to be transferred from the PACU). RSS \[score 1-6\] consists of six levels of sedation \[Awake levels: 1, anxious and agitated or restless or both; 2, co-operative, orientated, and tranquil; 3,responds to commands only. Asleep levels are dependent on the response to a light glabellar tap or loud auditory stimulus: 4, a brisk response; 5, a sluggish response; 6, no response\]. PONV ISS is the sum of the numerical responses to questions Q1 and Q2 ( Q1: The vomiting or dry-retching count? 0 - 2 or 3 (three or more times); Q2: Severity of nausea - interference with activities of daily living: score 0-3 \[0 - not at all, 1-sometimes, 2- often or most of the time, 3 - all of the time\] and score ≥5 defines clinically important PONV. Quality of Recovery questionnaire (QoR-40) on the first, second and third postoperative days will be used for assessment of postoperative quality of recovery.The QoR-40 consists of five clinically relevant dimensions: (i) physical comfort (12 items), (ii) emotional state (9 items), (iii) physical independence (5 items), (iv) psychological support (7 items), and (v) pain (7 items). Each item is rated on a five-point Likert scale. The QoR-40 score ranges from 40 (extremely poor quality of recovery) to 200 (excellent quality of recovery). A 100 mm visual analogue scale (VAS) \[0= no pain, 10= maximal pain\] and the use of postoperative analgesics (opioids/non-opioids: yes or no, quantity in milligrams) shall be used for pain assessment during the first 24 postoperative hours.The sample size for early recovery time (primary endpoint) is based on the results of our recent study. The sample size was calculated on the assumption that 15 (SD 7) min would be needed for early recovery for the GO2 patients and that the GN2O patients would require four minutes less time (11 \[SD 5\] min). We would need 38 participants in each group for the primary outcome to have a power of 0.8 and an alpha level of 0.05.
