A 42-year-old man undergoes arthroscopic knee surgery under interscalene brachial plexus block. The anesthesiologist administers 45 mL of 0.5% bupivacaine with 1:200,000 epinephrine. Fifteen minutes into the procedure, the patient becomes restless and complains of tinnitus and circumoral numbness. His blood pressure rises to 165/95 mmHg, heart rate is 118/min, and he develops generalized muscle twitching. What is the most appropriate immediate management?