A 52-year-old man with hypertension presents to the emergency department with palpitations and chest discomfort for 2 hours. His vital signs are stable (BP 148/92 mmHg, HR 102/min, RR 18/min, SpO₂ 98% on room air). A 12-lead ECG shows a prolonged QT interval (QTc 520 ms) with normal sinus rhythm and no ST-segment changes. Serum electrolytes are normal. He is not on any QT-prolonging medications. What is the most appropriate next step in management?