Background: Ultrasound guidance has reduced the amount of local anesthetics to achieve a successful block.\nPrevious studies of the relationship between the volume or concentration of local anesthetics and the effects of the\nblock were based on relatively high doses of local anesthetics. We tested the hypothesis that providing low dose of\nropivacaine at three combinations of volumes and concentrations for ultrasound-guided interscalene brachial\nplexus block would produce different effects in the aspect of onset time, pain control and the incidence of side\neffects.\nMethods: Ninety-nine patients undergoing elective arthroscopic shoulder surgery were randomized to receive an\nultrasound guided combined with nerve stimulator mediated interscalene block with ropivacaine 0.75 % (6.7 ml,\nGroup 0.75), 0.5 % (10 ml, Group 0.5) or 0.25 % (20 ml, Group 0.25). The primary end point was the onset time of\nthe sensory blockade, assessed by using a pinprick in the C5-6 dermatome. The secondary end points included the\nonset time of the motor blockade, block success rate, postoperative pain rating score, rescue analgesics\nrequirement, sleep quality, strength of the hand on the block side,and the incidence of hemi-diaphragmatic paresis\nwhich was evaluated by ultrasonography.\nResults: There was a statistically significant difference of the sensory block median onset times among Group 0.75\n(5 min), Group 0.5 (10 min) and Group 0.25 (20 min). One patient in Group 0.5 and 20 patients in Group 0.25 did\nnot achieve a complete motor block within 30 min, which were also significantly different. No significant difference\nwas observed in postoperative analgesia, decrease of handgrip strength and the incidence of hemi-diaphragmatic\nparesis among the 3 groups.\nConclusions: This study demonstrates that ropivacaine 50 mg as 0.25, 0.5 or 0.75 % solution for interscalene\nbrachial plexus block before arthroscopic shoulder surgery produces comparable blockade with few side effects,\nwhile 0.75 % seems to be more preferable as it is associated with faster onset time.
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