A 42-year-old man undergoes an interscalene brachial plexus block for right shoulder arthroscopy. The anesthesiologist administers 30 mL of 0.75% ropivacaine (225 mg total). Within 2 minutes of injection, the patient develops restlessness, tinnitus, and circumoral numbness. His blood pressure rises to 165/95 mmHg, heart rate 118/min, and he begins to have focal seizure activity in the right arm. What is the most appropriate immediate management?