Reciprocating tachycardia due to a right-sided unidirectional retrograde anomalous pathway (URAP).

An 8 year-old boy had extensive electrophysiological evaluation of his recurrent supraventricular tachycardias. His ECG never showed delta waves but intracardiac stimulation and recording disclosed the following (1) eccentric retrograde atrial activation; (2) increased cycle length and retrograde conduction time following the development of right bundle-branch block; (3) constant retrograde conduction time for increasingly premature ventricular stimuli; (4) atrial captures by ventricular stimuli when the atrioventricular-His pathways were refractory; and (5) no delta waves upon stimulation of the atrial input site of the anomalous pathway. A diagnosis of reciprocating tachycardia involving retrograde conduction through an accessory pathway was made. Reciprocating tachycardias involving a unidirectional retrograde anomalous pathway can be easily misdiagnosed as atrioventricular node reentrant tachycardias if no evidence of preexcitation can be found, particularly if the anomalous pathway is on the right side. In order to exclude the participation of a concealed unidirectional anomalous pathway in a patient's reentry tachycardia, a complete map must be made of right and left atrial endocardial activity.