The introduction of the laparoscopic technique for cholecystectomy significantly reduced the incidence and intensity of postoperative pain, with improvement in other markers, such as patient satisfaction, and reduction in hospital stay. However, pain in the postoperative period of laparoscopic cholecystectomy is still a concern that challenges modern anesthesiology. Ketamine and magnesium sulfate are two blockers of N-methyl-D-aspartate (NMDA) receptors with the ability to reduce postoperative pain and postoperative opioid consumption. A frequent concern among anesthesiologists is the quality and time of awakening in patients receiving these medications The main objective of this trial is to compare the quality and time of awakening in patients receiving magnesium sulfate or ketamine.

The secondary objective is to compare postoperative analgesia during the postoperative hospital stay.

Hypothesis: Our hypothesis is that patients have a faster awakening when they receive magnesium sulfate as an analgesic adjunct, when compared to patients who receive ketamine .

Design: this is a prospective, controlled, covered and randomly distributed trial.

Intervention: the sample of participants will be distributed in 4 groups. The placebo (PG) group will receive 100 ml of saline solution 15 min before anesthetic induction. The ketamine group (KG) will receive 0.3 mg / kg of ketamine in saline (total volume of 100 ml) 15 min before anesthetic induction. The magnesium sulfate group (SG) will receive 40 mg / kg of magnesium sulfate in saline solution (total volume of 100 ml) 15 min before anesthetic induction. The mixed group (MG) will receive 0.15 mg / kg of ketamine + 20 mg / kg of magnesium sulfate in saline solution (total volume of 100 ml) 15 min before anesthetic induction. All participants will be submitted to balanced anesthesia. Anesthetic induction will be started after 5 min of pre-oxygenation, with lidocaine 1.5 mg / kg, fentanyl 3 µg / kg, propofol until clinical hypnosis, cisatracurium 0.1 mg / kg and intubation after adequate neuromuscular relaxation. Maintenance will be with sevoflurane in concentration under clinical demand, repeating boluses of fentanyl 1 µg / kg and cisatracurium 0.03 mg / kg, as needed.

Primary Outcomes: time between the end of anesthetic administration and bispectral index (BIS) > 60, and a battery of neuropsychological tests to assess postoperative cognitive dysfunction tested in the Brazilian population 3 h after the end of anesthetic administration.

Secondary Outcomes: response to the brief pain inventory questionnaire and opioid consumption in the first 2 postoperative days.

Recruitment and timeline : patients will be recruited in the pre-anesthetic evaluation and assessments will be made of covertly in the immediate postoperative period and during the two days after surgery, being registered the pain scores and the consumption of opioids.

Sample calculation: the sample of 120 participants was calculated with a 95% confidence index and 80% statistical power. To compensate for losses, 140 individuals will be recruited.

Inclusion Criteria:

• patients scheduled to undergo laparoscopic cholecystectomy, agreement to voluntarily participate in the trial and sign the free and informed consent form.

Exclusion Criteria:

• Coronary disease, ventricular atrial block II or worse, with renal failure , previous history of brain disease, dementia or other psychiatric diseases, and patients with a body mass index> 35 kg / m² , allergy to any products used in the trial, preoperative use of opioids or corticosteroids.

The person responsible for generating the sequence will use the result of the lottery to number the opaque envelopes and place the intervention corresponding to the group to which that lottery number belongs. This professional will not administer anesthesia to the patients in the study and will not participate in the collection or evaluation of the studied variables.

The envelopes will be opened by professionals who will prepare the corresponding solution, keeping this information covered for the other members of the team and for the patients.

The groups will be covered up for patients and for professionals who carry out the assessments.