A Story With an Interesting Twist

An 81-year-old woman with hypertension and heart failure was referred to the emergency department after cardiopulmonary resuscitation with documented polymorphic ventricular tachycardia.She was taking levofloxacin and cimetidine.Her potassium level was low at 3.1 mEq/L (normal range, 3.5-5.5 mEq/L) and her magnesium level was 2.4 mg/dL (normal range, 1.9-2.5 mg/ dL).Echocardiography revealed a normal left ventricular systolic function without ventricular hypertrophy and computed tomography of the brain showed no evidence of intracranial hemorrhage.There was no family history of sudden death.A 12-lead ECG (Figure 1A) and Holter monitoring (Figure 1B) were performed on admission.What are the notable ECG findings?What is the mechanism of the polymorphic ventricular tachycardia?Please turn the page to read the diagnosis.