Electrophysiologic testing in patients with the long QT syndrome.

Electrophysiologic studies were performed in 15 patients with syncope and/or cardiac arrest who had the long QT syndrome and 11 control subjects who had normal QT intervals. The syndrome was familial in five patients and idiopathic in 10. All patients had a prolonged QT (546 +/- 68 msec, mean +/- SD) and corrected QT (550 +/- 51 msec). Incremental atrial pacing at cycle lengths of 600 to 400 msec resulted in shortening of the QT interval, but there was no significant difference in the magnitude or percent of shortening of the QT interval between patients with the long QT syndrome and control subjects. Intravenous propranolol did not influence the QT interval measured at fixed atrial-paced cycle lengths in patients with either the familial or idiopathic form of the syndrome. Programmed right and left ventricular stimulation with up to three extrastimuli before and during isoproterenol infusion did not induce sustained ventricular tachycardia or ventricular fibrillation in any of the patients. However, rapid polymorphic nonsustained ventricular tachycardia was induced in six of the 15 patients (40%). Neither the inducibility of nonsustained ventricular tachycardia nor the results of electropharmacologic testing with beta-blockers proved to be of any prognostic value during the mean follow-up period of 28 +/- 17 months. Electrophysiologic studies are of limited value in the diagnosis and treatment of patients with the long QT syndrome.