Functional bundle branch block as a delayed manifestation of retrograde concealment in the His-Purkinje system

The mechanism of functional bundle branch block induced at the onset of supraventricular tachycardia (SVT) is well established. However, no data exist to address the underlying mechanism of functional bundle branch block occurring in the second beat of SVT, when the first beat is conducted with a narrow QRS morphology and preceded by ventricular stimulation.

Two patients showing such a phenomenon form the basis of this report. Patient 1 with AV nodal reentrant tachycardia of the common variety persistently demonstrated functional right bundle branch block in the second SVT complex when a short train of ventricular pacing was introduced during SVT. This occurred without any discernible change in the SVT cycle length. Patient 2 had a manifest posteroseptal accessory pathway and inducible orthodromic reentrant tachycardia. Functional bundle branch block during propagation of the second SVT complex invariably occurred either in the left bundle when SVT was induced by a bundle branch reentrant complex during premature ventricular stimulation, or in the right bundle when SVT was induced with a short train of ventricular pacing. The development of functional bundle branch block was preceded by minimal or no cycle length variations in the His-bundle inputs.

These observations suggest that the type of functional bundle branch block occurring in the second SVT complex as a de novo phenomenon may be related to the relative timing of the retrograde penetration of the right versus left bundle during ventricular pacing or bundle branch reentrant complex. Therefore, due to its longest cycle length of activation and refractoriness, the earliest site of retrograde penetration is the most likely site of functional block during propagation of the second SVT complex. This delayed manifestation of retrograde concealment may provide new information regarding the electrophysiologic behavior of the His-Purkinje system.