The hypothesis of this study is that a reduced extrafascial volume of local anesthetic for an interscalene brachial plexus block is associated with a lower incidence of diaphragmatic hemiparalysis after an arthroscopic surgery of the shoulder, without any reduction of the analgesic efficacy.~Our prospective randomized controlled trial will include two parallel groups: a group will receive a volume of 20 mls of ropivacaine 0.75%, while the other group will receive a volume of 10 mls.~All participants will have a preoperative ultrasound-guided interscalene brachial plexus with an extrafascial injection of 10 or 20 mls of ropivacaine 0.75%. In both groups, participants will have an examination of the hemidiaphgragm with the ultrasound, before and 30 min after the block. The respiratory function will also be assessed with a bedside spirometer before and after the block, and at 12 and 24 postoperative hours. During surgery all participants will receive multimodal analgesia inclusive of iv dexamethasone 8 mg, iv magnesium sulfate 40 milligram kg^2, iv ketorolac 30 milligrams, and iv acetaminophen 1000 milligrams, according to the current practice in our institution. In the postoperative period, Participants will be prescribed an IV pca of morphine.~Assignment to one of these two groups will be done according to a computer-generated list of random numbers, and the sealed envelopes method will be used.
The hypothesis of this study is that a reduced extrafascial volume of local anesthetic for an interscalene brachial plexus block is associated with a lower incidence of diaphragmatic hemiparalysis after an arthroscopic surgery of the shoulder, without any reduction of the analgesic efficacy.