Abstract: Objective To investigate the clinical efficacy of and complications arising from low-dose ketamine combined with fentanyl for intravenous postoperative analgesia in comparison with the exclusive use of fentanyl in elderly patients.Methods Eighty elderly patients were randomized into two equal groups following thoracoabdominal surgery,and received intravenous analgesia with the combination of 0.5 mg/ml ketamine,5.0 μg/ml fentanyl and 50 μg/ml midazolam(KF group) and with 7.5 μg/ml fentanyl plus 50 μg/ml midazolam(FT group),respectively.The drugs used were diluted in 200 ml normal saline.For analgesic administration,a loading dose(2-4 ml) was given followed by a background infusion(2.5-3.5 ml),with patient-controlled bolus doses of 2.0-3.0 ml with lock-out time of 20 min via PCA pump(Automedical,Korea).The static pain score(VAS),sedation score,and incidences of nausea,vomiting,pruritus and hallucinations were recorded during the initial 48 h after the surgery.Results The total analgesic dosage and PCA dosage in the two groups were similar(P>0.05).With similar VAS in the two groups(P>0.05),the sedative effect in KF group was much better than that in FT group(P<0.05) during the initial 48 h postoperatively.The incidences of nausea,vomiting and itching were lower in KF group than in FT group(P<0.05),and no illusion was reported in two groups during the initial 48 h.Conclusion For producing comparable postoperative analgesic effect,low-dose ketamine combined with fentanyl can markedly reduce fentanyl requirement in the elderly patients and lowers the incidences of nausea,vomiting and itching in comparison with the exclusive use of fentanyl.