Discussion
It was in 1941 that Mahaim and Winston described the histology of anomalous connections that arise from the AV node and insert into the right ventricle (RV) [5]. This was the first description of NV or so-called Mahaim’s APs. Mahaim’s bundle with decremental conduction properties that connected the atrium to the right bundle branch were subsequently mapped mainly at the lateral wall of the tricuspid annulus, thus the term atriofascicular bundle appeared[6-7]. Decrementally conducting connections can exist between the right atrium or the AV node and the RV in or close to the right bundle branch [6-8]. There are five types of Mahaim’s bundle as presently defined, including decremental antegradely conducting atrio-ventricular fiber, atrio-fascicular fiber, NV fiber, NF fiber and fasciculo-ventricular fiber[9,10]. Since the last one does not cause reentry, it rarely needs ablation [10].
The distal part of Mahaim’s bundle usually connects to the RV in the majority cases [11]. Thus QRS morphologies related to Maihaim’s bundle are usually left bundle branch block pattern. But there are some exceptions. Nodoventricular fiber is a rare type of AP that connects the AV node and the right ventricular myocardium. Since atrium is out of the reentrant circuit of NVRT, NVRT can show VA dissociation which will not appear in other AP-mediated reentrant tachycardias except for nodo-fascicular reentrant tachycardia (NFRT). A tachycardia with VA dissociation could be misdiagnosed as a ventricular tachycardia (VT), especially when it shows a broad QRS pattern. Methods to differentiate between VT and SVT include comparing H-V intervals during tachycardia and sinus rhythm and recording conduction sequence of H potential during tachycardia. For orthodromic SVT, H-V interval during tachycardia should equal to that during sinus rhythm. Conduction sequence of H potential during orthodromic SVT should be from the proximal to the distal part. Otherwise, the tachycardia should be diagnosed as VT.
By excluding VT, a tachycardia with V-A dissociation should mainly focus on differentiation among AVNRT, NVRT and NFRT. Since all these tachycardias depend on the existence of dual AV nodal pathways, an A-H interval “jump-up” phenomenon is not enough for definite diagnosis. Further more, once the patient had an NV/NF fiber, he/she actually had 3 pathways that concern the AV node. Permutation of 3 pathways obtains 2 possible patterns of tachycardia, one is AVNRT complicated with a bystander NV/NF fiber as Patient 1, the other is NV/NF-RT as Patient 2 & 3. So, one should not rush to diagnosis of AVNRT with VA dissociation after VT is excluded. Further differential maneuvers are necessary. His bundle refractory period ventricular stimuli resetting the tachycardia points to the existence of a retrogradely conducting AP. For concealed NV fibers, His bundle refractory period ventricular stimuli reset the H-H interval earlier than the V-V interval because the conduction sequence of an orthodromic SVT is from His bundle to the ventricle. But para-Hisian pacing at different current intensity during sinus rhythm which is used to differentiate between ordinary septal AP and retrograde V-A conduction via AV node could be less helpful for differentiation between an NV/NF fiber and retrograde V-A conduction via AV node, because the retrograde VA conduction in NV/NF fiber could not bypass AV node and form a much shortened VA interval as ordinary AP.
As we know, CL of orthodromic AVRT can prolong if accompanied with ipsilateral bundle branch block, because it takes more time to go along the contralateral bundle branch and then go through the interventricular septum than just go along the ipsilateral bundle branch[12]. In Patient 3, we noticed that CLs were equal during normal QRS pattern and CLBBB pattern, while it became longer during CRBBB pattern. This suggested that the NV fiber located on the right side. We wonder that might all NV fibers locate on the right side because the AV node is mainly a right heart structure.
Since all the above phenomena were shared by concealed NV fiber and concealed NF fiber, we need further differentiations between them. It has been reported that the presence of QRS fusion (defined as any QRS complex morphology other than that of a fully paced morphology) during entrainment of SVT by ventricular pacing proves that the ventricular myocardium is participating in the circuit [13]. That means if there were QRS fusion, it should be an NV fiber, otherwise it should be an NF fiber. Our understanding of this point of view is as the following: An NFRT is a closed loop reentry within the conduction system, while an NVRT is an open loop reentry that concern the ventricular myocardium. To entrain an NFRT, RV stimulation has to enter the distal part of right bundle branch retrogradely before supraventricular wavefront activates it antegradely and then goes into ventricular myocardium to form fused QRS, because antegrade conduction within the right bundle branch would cause refractory period itself that hinders entrainment capture. So, NFRT entrainment causes a fully paced morphology at the pacing site. For NVRT, RV stimuli can drive into the tachycardia via RV myocardium at any time, forming progressive QRS fusion. The shorter the entrainment CL, the closer the morphology to the fully paced one, as was seen in Patient 2 and 3. In our opinion, His bundle refractory period ventricular stimulus is like a one-beat entrainment of SVT. Thus, if an SVT could be reset by His bundle refractory period ventricular stimuli with only one QRS morphology at a fixed position which should be the full paced one, ventricular myocardium is ruled out from the reentrant circuit, and vise versa. That’s why we deduced Patient 1 also had an NV fiber, but not an NF fiber.