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.

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.

Inclusion Criteria:

• arthroscopic surgery of the shoulder
• ASA class 1 to 3
• age more than 18 years old

Exclusion Criteria:

• patient refusal or inability to understand and/or sign the inform consent
• contraindication for perineural block (allergy to local anesthetics, infection of puncture site, major coagulopathy, sensitive or motor deficiency on the operative side arm
• chronic alcool abuse
• opioid drug abuse or under substitution treatment
• patients known for allergies to paracetamol, non steroidal anti inflammatory drugs, dexamethasone, sulfate magnesium, ondansetron, droperidol, and omeprazole;
• patients under chronic corticotherapy
• patients known for malignant hyperthermia;
• patients with chronic kidney failure (Glomerular Filtration Rate < 20 ml/min)
• patients with severe pulmonary disease
• patients with history of neck surgery or radiotherapy on the operative side;
• pregnancy