Analysis of re-entry mechanism in a patient with concealed Wolff-Parkinson-White syndrome.

This report deals with a patient with concealed Wolff-Parkinson-White syndrome in whom both narrow and wide QRS tachycardias were observed. The simultaneous occurrence of various QRS morphology during supraventricular tachycardia results in a challenging diagnostic ECG problem. The cycle length during tachycardia with left bundle branch block was longer than the cycle length during narrow QRS supraventricular tachycardia and with functional right bundle-branch block. Electrophysiologic studies revealed an increased V-A conduction time during tachycardia with left bundle branch block. These studies suggested the presence of a concealed left-sided anomalous pathway. Differentiation between intra- and extranodal re-entry and therapeutic modalities are also discussed.