A 52-year-old man with a 10-year history of major depressive disorder is on amitriptyline 75 mg at bedtime. He presents with palpitations, dizziness, and syncope on exertion. Physical examination shows an irregular pulse. An ECG is ordered and shows a prolonged QTc interval (480 ms). Before attributing this to amitriptyline toxicity, which investigation is most appropriate to exclude a secondary cause of QT prolongation?