Prolonged QT, atrioventricular block, and sudden death in the newborn: an electrophysiologic evaluation.

An electrophysiologic study was performed in the first month of life in a patient with the congenital long QT syndrome and spontaneous episodes of 2/1 atrioventricular block. The block could be reproduced by incremental atrial pacing, and its infrahisian location was associated with a prolongation of the refractoriness in the ventricular muscle itself. Surprisingly, intravenous propranolol aggravated this phenomenon by further prolonging the QT interval. Sudden death occurred shortly thereafter during Holter monitoring and was due to a sudden resumption of normal AV conduction after an episode of 2:1 block, immediately followed by ventricular fibrillation.