Aberrancy of junctional escape beats: Evidence for origin in the fascicles of the left bundle branch

The mechanism of aberrancy of so-called atrioventricular (A-V) Junctional escape beats was investigated in 8 cases, in 5 of which His bundle electrography was performed with both conducted and escape beats recorded. Aberrancy in Junctional beats usually shows features of incomplete or complete right bundle branch block and leftward or rightward shift of the QRS axis in the frontal plane. From the morphologic point of view the aberrancy resembles that seen in conducted beats with the additional feature of bifascicular block. Functional aberrancy based on incomplete recovery of the conduction fibers cannot account for aberrancy in slow Junctional rhythms. Phase 4 depolarization may explain the aberrancy in slow heart rates but cannot readily account for the fusion complexes so frequently seen in these rhythms. His bundle electrography in the present study showed a time relation between His spikes and the onset of the QRS complex which excludes the possibility of antegrade conduction of the aberrant beats along the same pathway used by descending sinus beats as well as true Junctional beats descending through the main His bundle. Specifically, the His spikes were found at or near the onset of the QRS complexes in aberrant escape beats, whereas they were ahead of the QRS complexes by the expected interval of 40 to 50 msec in the conducted beats. This temporal relation favors propagation of escape impulses from their origin bidirectionally, that is, simultaneously toward the ventricular myocardium and retrograde toward the main His bundle. This observation, together with the right bundle branch block and the axis deviation, places the escape focus in 1 of 2 fascicles of the left bundle branch: in the superior division for beats displaying right axis deviation and in the inferior division for beats displaying left axis deviation. With acceptance of a fascicular origin for these beats, a satisfactory explanation becomes readily available for the fusion complexes.