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Adding dexamethasone to LA would increase the duration of block. 

... The median duration of analgesia was significantly prolonged in the Dexamethasone group (332 mins; interquartile range, 225-448 mins) compared with the Normal Saline group (228 mins; interquartile range, 207-263 mins; P = 0.008). The onset times of sensory and motor block were similar between the groups. Complications were minor and transient and did not differ between groups at 2 weeks postoperatively.

Conclusions: The addition of dexamethasone to mepivacaine prolongs the duration of analgesia but does not reduce the onset of sensory and motor blockade after ultrasound-guided supraclavicular block compared with mepivacaine alone.

Regional Anesthesia & Pain Medicine:September/October 2010 - Volume 35 - Issue 5 - pp 422-426doi: 10.1097/AAP.0b013e3181e85eb9
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