In this prospective double-blinded study, The investigators compared acute postoperative pain and rescue analgesic demand during postoperative period after robot thyroidectomy between ketamine and placebo groups.

Robot-assisted endoscopic thyroidectomy has been popularized due to cosmetic advantages. Despite small incisions, robot thyroidectomy did not offer satisfactory reduction in postoperative pain compared to open thyroidectomy. Ketamine is a non-competitive N-methyl-D-aspartate (NMDA) receptor blocker and takes some attractive advantages in terms of pain control. When low dose ketamine is perioperatively administrated, opioid sparing effect during postoperative period is reported in various surgical procedures such as spine, thoracic, and gynecologic surgery. Ketamine's beneficial effect on postoperative pain has not been investigated in patients undergoing robot thyroidectomy. The investigators hypothesized that perioperative ketamine administration can reduce acute postoperative pain after robot thyroidectomy and the incidence of chronic pain hypoesthesia on anterior chest at 3 months after surgery.

Inclusion Criteria:

• Patients scheduled for robot-assisted thyroidectomy
• ASA I-III

Exclusion Criteria:

• Patients with renal dysfunction
• Patients with hepatic dysfunction
• Patients with neurologic dysfunction
• Patients with the history of drug addiction
• Patients with chronic pain
• Patients who are allergic to ketamine
• Patients with increased ocular or intracranial pressure
• Patients with the risk of aspiration