A 38-year-old woman undergoes interscalene brachial plexus block for shoulder arthroscopy at a tertiary care hospital in Delhi. The anesthesiologist uses 30 mL of 0.5% bupivacaine. Within 3 minutes of injection, the patient develops circumoral numbness, tinnitus, and restlessness. Her blood pressure is 145/92 mmHg and heart rate is 98 bpm. What is the most appropriate immediate next step in management?