The recent increase in obesity has led to an increase in the need for bariatric surgery. In this group of patients the postoperative pain management is of vital importance. One strategy to improve postoperative analgesia is the use of intraoperative methadone, specially in those patients which regional anesthesia is not feasible. There is evidence that the use of intraoperative methadone can lead to a analgesia lasting 24 to 36 hours, while not associated with increased side effects when compared to other opioids with short or intermediate duration of action. In this study the investigators will evaluate the efficacy of intraoperative methadone in reducing postoperative pain and opioid consumption.

Despite recent developments in postoperative pain control, many patients still suffer from moderate to severe pain after surgery. It is estimated that postoperative severe pain occurs in 20 to 40% of surgical procedures. With the recent increase in obesity incidence in the last years, the need for bariatric surgical intervention is greater. The management of postoperative pain in obese patients is particularly important, since this population have increased susceptibility to cardiovascular, pulmonary and thromboembolic perioperatively.

One of the strategies to improve pain management in the perioperative period is the intraoperative use of intravenous methadone, given its pharmacokinetic profile, specially in those patients in which regional anesthesia is contraindicated. Methadone is an opioid μ (MOR) receptor agonist, also a glutamate antagonist by blocking the N-methyl-D-aspartate (NMDA) receptor and a reuptake of serotonin and noradrenaline inhibitor. Intraoperative analgesia generated by administration of 20 to 30 mg methadone can last 24 to 36 hours. There is also evidence that methadone at 0.2 to 0.3 mg / kg is not associated with an increased incidence of side effects compared to other opioids with short or intermediate duration of action, such as fentanyl, sufentanyl and morphine.

The aim of this study is to evaluate the efficacy of intraoperative methadone in reducing postoperative pain and opioid consumption in patients undergoing open gastroplasty with or without associated Roux Y. Patients will undergo standardized general anesthesia, and the opioid used in anesthesia induction is methadone 0.15 mg / kg fentanyl or 6 mcg / kg bolus with additional if necessary. After extubation a intravenous morphine patient controlled analgesia device will be already available in the operating room. Groups will be compared regarding opioid consumption, pain scores, side effects, patient satisfaction and development of chronic postoperative pain.

Inclusion Criteria:

• Open bariatric surgery
• No contraindications for any drugs used during the trial
• Read, understood and signed the informed consent

Exclusion Criteria:

• Laparoscopic bariatric surgery
• Known allergy to any drugs used during the trial
• Cardiovascular disease
• Creatinine clearance lower than 60 mL/min/1.73 m2
• Chronic usage of opioids
• History of personality disorder