Fatigue of the His-Purkinje system during routine electrophysiologic studies.

In this report we describe fatigue of the His-Purkinje system during retrograde stimulation of the His bundle by ventricular programmed stimulation. The patient underwent electrophysiologic evaluation for syncope. Antegrade conduction and supraventricular studies were normal with the exception of baseline left bundle branch block. During programmed ventricular stimulation, the patient developed intra-Hisian and infra-Hisian block with symptomatic 3:1 atrioventricular heart block requiring insertion of a permanent pacemaker. This case demonstrates the need for careful study of both antegrade and retrograde conduction properties of the His bundle and atrioventricular node when performing standard His bundle studies in evaluation of syncope.