Abstract： Objective To investigate the effect of dexmedetomidine application on tourniquet reaction during upper extremity surgery.Methods Eighty ASA Ⅰ or Ⅱ patients aged 25-65 years,weighting 55-75 kg,height 155～175 cm,scheduled for upper extremity surgery were randomized into 2 groups:dexmedetomidine group (group D,n=40) and control group (group C,n=40).A loading dose of dexmedetomidine 1 μg/kg was administered intravenously 10 min before anaesthesia followed by infusion at 0.5 μg· kg-1 · h-1 until the end of operation in group D.While equal volume of normal saline was given in group C.Brachial plexus block was performed under the guidance of ultrasound.After successful location,30 ml of 0.5％ ropivacaine was injected in each group.The tourniquet reaction,tourniquet reaction occurring time,tourniquet reaction degree,adjuvant drug situation of two groups were recorded.Heart rate (HR),mean arterial pressure (MAP),oxygen saturation (SpO2) were recorded before the surgery (T0),and at 5 min (T1),10 min (T2),15 min (T3),30 min (T4),60 min (Ts) after blocking,as well as adverse reactions were recorded.Results Compared with group C,the cases of tourniquet reaction,tourniquet reaction degree,dosage of adjuvant fentanyl during the operation were significantly lower in group D [cases of tourniquet reaction:8/40 vs 14/40,tourniquet reaction degree:2.4±0.8 vs 60.2±20.4,dosage of adjuvant fentanyl during the operation:(4.2± 1.2)pg vs (130.3±38.6) μg,all P＜0.05].The tourniquet reaction occurring time of group D was later than that of group C [(52.2± 12.5)min vs (30.2±11.8)min,P＜0.05].There was no statistical significance of HR,MAP and SpO2 between group C and group D at T0.The level of HR was lower at T1-T5 in group D than that in group C (P＜0.05).There was no statistical significance of MAP at T1-T3 between the two groups.The MAP at T4 and T5 was significantly lower in group D than that in group C (P＜0.05).There was no statistical significance of SpO2 between two groups.Bradycardia was observed in 4 patients in group D.Hypotension,local anesthetic intoxication,and other adverse reactions were not found in two groups.Conclusion A loading dose of dexmedetomidine 1 μg/kg administered intravenously 10 min before anaesthesia followed by infusion at 0.5 μg· kg-1 · h-1 until the end of operation in upper extremity surgery can significantly reduce the tourniquet reaction cases,relieve tourniquet reaction degree and postpone the occurring time of tourniquet reaction.