Retrograde (transseptal) activation of right bundle branch during sinus rhythm

A 40-year-old man undergoing electrophysiologic study for recurrent palpitations was found to have a right bundle branch block during sinus rhythm and right bundle potentials occurring after ventricular activation. The sequence of activation of the proximal and distal right bundle potentials was suggestive of a retrograde invasion of the right bundle during sinus rhythm. This transseptal activation of the right bundle via the left bundle exhibited decremental conduction characteristics during atrial as well as ventricular stimulation. Observations made in this case may provide some important information concerning the electrophysiologic behavior of transseptal impulses.