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Background: Intraoperative low-dose ketamine infusion has been reported to be an effective adjuvant to opioids for postoperative pain control without major side effects, but it has not been tested in videoassisted thoracic surgery (VATS). The aim of this study was to examine the effect of low-dose intraoperative intravenous ketamine infusion on 24-hour morphine requirement and acute postoperative pain following VATS for lung resection.Methods: This study was a single center, randomized, double-blind, placebo-controlled study. Thirtytwo patients undergoing elective VATS for lung resection in a university hospital were included. Patients were randomly allocated (1:1 ratio) to receive either intraoperative low-dose ketamine (0.2 mg/kg/h) or normal saline infusion starting from intubation to the beginning of chest closure. All patients received multilevel thoracic paravertebral block (TPVB) and morphine was administered postoperatively via the patient-controlled analgesia pump using the same protocol. Time to first analgesia, postoperative cumulative morphine doses at 10, 30 minutes, and the consecutive 1, 2, 6, 12, 18, and 24 hours were recorded. Pain intensity during rest and deep breathing were also assessed by numeric rating scale (NRS) score at 1-and 24-hour postoperatively.Results: There was no significant difference in median (P 25 , P 75 ) cumulative 24-hour morphine requirement between the ketamine and the control groups [15 (5.5, 29.5) vs. 22.5 (15.3, 40.8) mg, P=0.090]. Patients in ketamine group had significantly longer median pain free time than the control group (27 vs. 2 minutes, P=0.006). No difference in overall NRS score at rest or during deep breathing at 1-and 24-hour postoperatively was demonstrated (P=0.861).Conclusions: Intraoperative low dose ketamine infusion in addition to TPVB does not reduce postoperative morphine consumption or pain intensity but may prolong pain free time in patients undergoing VATS for lung resection.