A 45-year-old man with schizophrenia presents to the emergency department with palpitations and syncope. ECG shows a polymorphic ventricular tachycardia with a characteristic "twisting of the points" appearance. The interval marked **D** on his baseline ECG (obtained 2 weeks ago) measured 520 ms (corrected QTc 480 ms). He has been on haloperidol for 3 months and his recent serum potassium is 3.2 mEq/L. What is the most appropriate immediate management for the acute arrhythmia?