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Dexmedetomidine (DEX) can prolong duration of anesthesia and shorten onset of sensory and motor block relative to clonidine. This study attempted to compare the efficacy of intravenous DEX and clonidine for hemodynamic changes and block after spinal anesthesia with ropivacaine in lower limb orthopedic surgery. In a double-blind randomized clinical trial, 120 patients undergoing spinal anesthesia in lower limb orthopedic surgery were recruited and divided into three groups using balanced block randomization: DEX group (n = 40; intravenous DEX 0.2 µg/kg), clonidine group (n = 40; intravenous clonidine 0.4 µg/kg), and placebo group (n = 40; intravenous normal saline 10 mL) in which pain scores were assessed using visual analogue scales (at recovery, and 2, 4, 6, and 12 hours after surgery) and time to achieve and onset of sensory and motor block. Statistically significant differences were found in mean arterial pressure among the groups at all times except baseline (P = 0.001), with a less mean arterial pressure and a prolonged duration of sensory and motor block (P = 0.001) in the DEX group where pain relieved in patients immediately after surgery and at above mentioned time points (P = 0.001). Simultaneous administration of intravenous DEX with ropivacaine for spinal anesthesia prolongs the duration of sensory and motor block and relieves postoperative pain, and however, can decrease blood pressure. Although intravenous DEX as an adjuvant can be helpful during spinal anesthesia with ropivacaine, it should be taken with caution owing to a lowering of mean arterial pressure in patients especially in the older adults. This study was approved by Ethical Committee of Arak University of Medical Sciences (No. IR.Arakmu.Rec.1395.450) in March, 2017, and the trial was registered and approved by the Iranian Registry of Clinical Trials (IRCT No. IRCT2017092020258N60) in 2017. [ABSTRACT FROM AUTHOR]