Ventricular tachycardia after ondansetron administration in a child with undiagnosed long QT syndrome.

To describe a case of ventricular tachycardia after co-administration of ondansetron and dimenhydrinate to a child with occult congenital QT prolongation. A previously healthy 11-yr-old girl presented for surgical excision of a thyroglossal duct cyst under general anesthesia. Induction and maintenance of anesthesia were unremarkable, and the surgery was carried out without incident. Prior to emergence, ondansetron 0.1 mg.kg(-1) and dimenhydrinate 0.4 mg.kg(-1) were administered. Within approximately two minutes, polymorphic premature ventricular contractions developed and, subsequently, polymorphic ventricular tachycardia ensued. A 12-lead electrocardiogram revealed a profoundly prolonged QT interval that decreased but failed to normalize completely postoperatively. Our patient was diagnosed subsequently with congenital QT prolongation. The QT interval is prolonged by the administration of ondansetron in a manner similar to that seen with droperidol, whereas dimenhydrinate is not considered to exert significant effects on the QT interval. Individuals with occult QT prolongation are at risk of experiencing malignant dysrhythmias when ondansetron is administered, especially in conjunction with anesthetic agents that also prolong the QT. The incidence of congenital QT prolongation in the general population has been estimated to be 1:2,500, and it may be undiagnosed preoperatively, especially in pediatric patients.