Dobutamine infusion for unmasking long QT syndrome and torsades de pointes

Prolonged QT interval is usually seen on routine electrocardiogram (ECG), but in some patients it may only be seen immediately before the evolution of torsades de pointes (Tdp). To unmask the potential risk of Tdp in a patient with normal or borderline prolongation of QTc interval and recurrent syncope, dobutamine was given intravenously at a rate of 20 microg/kg/min. Strikingly, QTc prolongation was induced along with syncope after dobutamine infusion. Torsades de pointes occurred 5 d later when the patient received an implantable cardioverter defibrillator (ICD). Genetic testing revealed a mutation of the KNCQ(1)-gene encoding serine instead of glycine. The patient was treated with 75 mg of metoprolol twice daily, and at 12 mo follow-up he had no syncope or chest tightness. The ICD revealed no ventricular tachyarrhythmias or therapy delivered.