A 52-year-old man with a history of hypertension and type 2 diabetes presents to the emergency department with palpitations and syncope. His ECG shows a prolonged QT interval (QTc = 520 ms) with normal sinus rhythm. Serum electrolytes reveal K⁺ = 3.2 mEq/L, Mg²⁺ = 1.6 mg/dL, and Ca²⁺ = 8.2 mg/dL. He is currently on metoprolol and enalapril. What is the most appropriate immediate next step in management?