Discussion
Baseline measurements revealed sinus rhythm with narrow QRS and short HV interval of 25ms. VA conduction was concentric and decremental with VA block at 290ms indicative of retrograde AV nodal conduction.
Incremental atrial pacing reproducibly induced wide complex tachycardia of LBBB morphology following AH prolongation and HV shortening (Figure 2A), suggestive of right sided decremental accessory pathway (AP).1
Atrial extra-stimuli administered at the timing of His refractoriness advanced the next V and the next A of the tachycardia (Figure 2B) which is consistent with antidromic tachycardia. The differential diagnosis includes ”long” Mahaim atrio-fascicular AP and ”short” decremental atrio-ventricular AP. Activation mapping demonstrated earliest ventricular activation at the antero-lateral aspect of the Tricuspid annulus implying of ”short” decremental AP rather than the apical portion of the right ventricle as seen in atrio-fascicular accessory pathway.2 Unexpectedly, a discrete AP potential (Mahaim-like) was noted at the antero-lateral aspect of the Tricuspid annulus, at sites of early ventricular activation during antidromic tachycardia (Figure 3A). The AP was successfully ablated via internal jugular approach at 10 o’clock position in left anterior oblique view.
Following ablation, a unique phenomenon was demonstrated with evidence of infra-AP potential block and AP-ventricular dissociation during atrial pacing without pre-excitation (Figure 3B). During Three-months follow-up the patient was free of any arrhythmias.
Accessory pathways with decremental properties connect the right atrium or the AV node with the right ventricle or the right bundle branch. Although the historical report by Mahaim refers to a nodo-ventricular accessory pathway, the term Mahaim has been adapted to describe other decremental AP’s with different anatomical features, including atrio-fascicular and atrio-ventricular pathways.1Further classification of decremental AV pathways into “long” pathways that insert into or near the right bundle branch and “short” pathways that insert into the base of the right ventricle was later described by Haı̈ssaguerre in 1995.3 The latter pathways are characterized by atrial and ventricular insertion immediately contiguous to the tricuspid annulus as seen in the present case in which the distal insertion is adjacent to the Tricuspid annulus.3
Thus far, Mahaim potential, described as an AV nodal His like structure, has been reported in ”long” atrio-fascicular and atrio-ventricular pathways 1,2, where it also facilitates successful ablation. Typically, catheter ablation of these pathways is accomplished by identifying the proximal and distal insertions and the recording of a Mahaim potential at the tricuspid annulus or on the right ventricular free wall.
In the present case, we have found that Mahaim-like potential can be also identified in short decremental accessory pathway in which the earliest ventricular activation is adjacent to the Tricuspid annulus rather than the right bundle branch or the right ventricular free wall. The location of the Mahaim-like potential was correlated with the earliest site of ventricular activation during maximal pre-excitation and assisted in localization of the AP and ultimately with successful ablation. Post ablation, a unique phenomenon of an infra-AP potential block was documented with evidence of AP-ventricular dissociation, which proves our concept that the Mahiam-like potential was indeed associated with the localization and ventricular insertion of the accessory pathway.