A 38-year-old woman undergoes an axillary brachial plexus block for hand surgery. The anesthesiologist uses 40 mL of 1% lidocaine (400 mg total). Thirty minutes into the procedure, the patient becomes hypotensive (BP 78/48 mmHg), bradycardic (HR 42/min), and develops a wide-complex rhythm on the monitor. She is unresponsive. What is the most likely mechanism of toxicity in this patient?