Wit et al. classified 3 types of AV conduction patterns in human [2]. In type 1 response, conduction delay and block were limited in the AV node while type 2 response was characterized by progressive conduction delay in both the AV node and specialized conduction system with block occurring in several instances in the latter. Type 3 response, the least common pattern, was characterized by a progressive delay in the AV node and a sudden marked delay in HPS. The manifestation of type 3 response is linked to a longer refractory period of HPS comparing to the AV node and/or relatively rapid conduction over the AV node. In this case, atrial premature beats with short coupling interval conducted through the AV node and encounters relative refractory period (RRP) of right bundle branch (RBB) while found ERP of left bundle branch (LBB) resulted in pronounced HV prolongation with LBBB pattern (Fig. 2b upper right panel). An alternative potential explanation might be LBB was blocked by the concealed retrograde activation from RBB (Fig. 2b lower right panel). The abrupt increase in AH interval shortened HV interval due to important delay of conduction in the AV node allowed for recovery from the refractoriness in distal elements in the HPS (Fig. 2c). Although the HV interval was shortened, LBB aberrancy was remained suggesting the mechanism of asynchronous recovery from refractoriness in HPS [3].
An alternating pattern of bundle branch block straddling a normal QRS complex with a resumption of 1:1 AV conduction after infra-nodal Wenckebach block can be explained by functional bundle branch block due to a long-short cycle sequence of the HPS and gap phenomenon (Fig. 1b). In 1947, Gouaux and Ashman described the aberrancy of conduction usually follows a long-short ventricular cycle sequence [4]. In Fig. 1, the 3rd paced beat was blocked at infra-nodal level proximal to the bundle branches and produced a long cycle sequence in the bundle branches for the next beat. Following a preceding long cycle, the 5th paced beat was conducted with pronounced prolongation of HV interval with LBBB because the impulse encountered RRP of RBB while was blocked at the proximal LBB which warranted concealed trans-septal retrograde activation from the RBB. This concealed retrograde invasion the so-called linking phenomenon shortens refractoriness in the LBB for the subsequent beat [5]. As a result of the linking phenomenon, the 8th paced beat was able to conduct normally through the LBB while the block occurred in the RBB which remained in the refractory period resulted in RBB aberrancy with normalization of HV interval. Note that from the 8th atrial paced beat showed decremental conduction through the AV node which facilitated the conduction through the distal His bundle. This kind of gap phenomenon produced a resumption of 1:1 AV conduction (Fig. 1b). The maintenance of RBB aberrancy in the following beats can be repetitive concealment by trans-septal retrograde activation from the LBB. Furthermore, there was no typical pattern of long-short R-R sequence on the surface ECG in the alternating bundle branch block because the short cycle sequence produced a pronounced HV interval prolongation.