A 28-year-old man is brought to the emergency department 1 hour after ingesting an unknown quantity of tricyclic antidepressant tablets in a suicide attempt. He is drowsy with a heart rate of 120 beats/min and blood pressure 90/60 mmHg. ECG shows QRS duration 140 ms with a terminal R wave in aVR. According to toxicology guidelines, what is the most appropriate immediate specific treatment?