Electrocardiographic and electrophysiologic features of dual pathways within the His bundle

Electrophysiological evidence of dual intrahisian pathways has been shown in a patient with diseased His bundle. The main electrocardiographic features, in this case were: 1). 1:1 A-V conduction with alternating short and long PR intervals; 2). unexpected A-V Wenckebach sequences; and 3). unexpected interruption of Wenckebach sequences by early captures of the ventricles with short and fixed PR intervals. These features were due to the following causes: 1). different E.P. properties of the two pathways producing a shift of conduction from a fast conducting pathway (FCP) to a slow conducting pathway (SCP); 2). a concealed reentry from the SCP into the FCP; and 3). a phase of "unexpected conduction" in the FCP.