Evaluation of the Effect of Ketamine on Remifentanil-induced Hyperalgesia

Inclusion Criteria: * ≥ 18 years old * both sexes * ASA physical status I or II * undergoing laparoscopic cholecystectomy Exclusion Criteria: * chronic users of analgesics or had used opioids within 12 h of surgery * history of drug or alcohol abuse or psychiatric disorder * contraindications to self-administration of opioids (ie, unable to understand the patient-controlled analgesia \[PCA\] device) * contraindication for the use of ketamine, such as a psychiatric disorder, acute cardiovascular disorder, or unstable hypertension