Afterdepolarizations as a mechanism for the long QT syndrome: electrophysiologic studies of a case.

A young woman with palpitation and syncope was found to have ventricular tachyarrhythmia and a congenital long QT interval. The QT interval was shortened and the arrhythmia suppressed by propranolol. Electrograms recorded at various sites in both ventricles revealed a distinct diastolic slow wave that followed the T wave and measured 1.1 mV. Epinephrine infusion and emotion augumented this diastolic wave and induced ventricular ectopic complexes arising from this potential. Similar endocardial recordings in eight patients without a long QT interval showed diastolic slow waves that never exceeded 0.28 mV. In normal canine myocardium, afterdepolarizations can be induced by norepinephrine and blocked by propranolol. These findings suggest that the long QT syndrome is associated with abnormally large afterdepolarizations in ventricular myocardial cells, which are enhanced by beta-adrenergic stimulation to attain threshold and produce firing.