Demonstration of a His-Purkinje fatigue phenomenon with programmed stimulation of the right ventricular outflow tract

This report describes the development of atrioventricular block by programmed stimulation of the right ventricular (RV) outflow tract in a patient with undiagnosed syncope. Burst pacing from the RV apex and outflow tract and programmed stimulation from the RV apex were unsuccessful. The observations were consistent with the fatigue phenomenon of the His-Purkinje system and illustrate the importance of an appropriate stimulation protocol in the electrophysiological evaluation of syncope.