The effect of local anaesthetic volume and concentration on the duration of peripheral nerve blockade in patients having shoulder surgery

An experienced operator will place all interscalene catheters (ISCs). Patients will be premedicated with oral acetaminophen 1 g one hour before surgery. Intravenous midazolam 2 mg, alfentanil 0.5 mg and cephazolin 1 g will be administered 5 min prior to interscalene catheter placement. Catheters will be placed following either the administration of a superficial cervical plexus block (SCPB) or after the induction of general anesthesia depending on patient preference. ISCs will be placed using a combination of ultrasound and nerve stimulation as previously described (Fredrickson et al. Anesthesiology 2010; 112:1374-1381, ANZCTR—12609000347268). General anesthesia will then be administered. Patients will then be randomized (using a computerised random number generator) to one of 5 bolus doses of ropivacaine: 1. 5 mL 0.75% 2. 10 mL 0.75% 3. 20 mL 0.375% 4. 20 mL 0.75% 5. 40 mL 0.375% Due to unanticipated block failure in 2 of the 5 groups (5mL/0.75% and 10mL/0.75%), as of patient I.D 130 onwards, randomisation is now to 1 of 7 groups as follows: 1 = 5mL/0.75% 2 = 5mL/0.75% 3 = 10mL/0.75% 4 = 10mL/0.75% 5 = 20mL/0.375% 6 = 20mL/0.75% 7 = 40mL/0.375% At the end of surgery, an ambulatory infusion of ropivacaine 0.2% at 2 mL/h with PRN “on demand” 5 mL boluses will be connected but the background infusion will remain clamped off. Patients will be instructed to turn it on (and to give the first bolus) as soon as they experience shoulder pain. The infusion will be continued for > 48 hours after surgery (Total duration = 2-5 days).