A 38-year-old woman with severe preeclampsia is scheduled for emergency cesarean section under epidural anesthesia. After successful epidural catheter placement at L3–L4 and injection of 10 mL of 2% lidocaine with epinephrine 1:200,000, she develops tinnitus, perioral numbness, and tremors within 2 minutes. Her blood pressure is 165/110 mmHg, heart rate 118/min, and she is alert but agitated. What is the most appropriate immediate next step?