Postoperative pain relief and bupivacaine plasma levels during continuous interscalene brachial plexus block

Interscalene brachial plexus block was performed on 40 patients for prophylactic pain relief after shoulder surgery. A dose of 1.25 mg/kg of 0.5% bupivacaine was injected for the block (Group 1) and continued with an infusion of 0.25% bupivacaine 0.25 mg/kg/h (Group 2). If the postoperative analgesia was insufficient, the patients received i.m. oxycodone 0.15 mg/kg. In Group 1, one patient managed without oxycodone supplementation during the 24-h observation period compared with eight patients in Group 2 (P less than 0.01). The rest of the patients received 3.8 +/- 1.6 doses (Group 1) and 2.5 +/- 1.2 doses (Group 2) of oxycodone (P less than 0.05). At 30 min, the mean bupivacaine plasma concentration was 1.0 microgram/ml in Group 1 and 0.9 microgram/ml in Group 2. The mean plasma level of bupivacaine increased from 0.7 microgram/ml after 180 min to 1.1 micrograms/ml (P less than 0.01) after 24 h of infusion, providing some evidence of accumulation during infusion. The dizziness and confusion experienced by three patients could be associated with the local anaesthetic, as they obtained relief after the infusion was stopped.