A manifest nodoventricular accessory pathway with unusual electrophysiological manifestationsLonghai Tian MM，Long YANG MD, Yidong ZHAO MM，Qifang Liu MDDepartment of Cardiology，Guizhou Provincial People’s Hospital, Guiyang , ChinaCorresponding author: Qifang Liu E-mail：liuxu19782000 @163.comDisclosures:（None）Funding：This study was supported by Clinical Research Center Project of Department of science and technology of Guizhou Province.Key Words: Nodoventricular pathway, Preexcitation, Decremental conductionAbstractWe present an unusual case of a nodoventricular accessory pathway showing the degree of anterograde decremental conduction was more than the atrioventricular node decremental Conduction. In the patient，there was no rapid antegrade conduction and inducible reentrant tachycardias during electrophysiologic examine and no ablation was performed.Case reportA 24-year-old male was diagnosed to have preexcitation suggestive of a septal accessory pathway by a 12-lead ECG (Figure 1). Given his occupation, he was referred to our hospital for electrophysiologic testing and radiofrequency catheter ablation. During sinus rhythm, intracardiac electrograms revealed preexcitation with an A-H interval 80ms and H-V interval of 20ms. Ventricular overdrive pacing at a cycle length of 400ms resulted in ventriculoatrial dissociation. Atrial extra-stimuli (AES) were introduced during sinus rhythm with a cycle length of 500 ms, The coupling interval was reduced from 400 to 280 ms. This resulted in a gradual prolongation of the atrio-His interval that was associated with a prolongation of the H-V interval and a decrease in the degree of preexcitation (figure2). A decremental atrioventricular pathway was noted. Interestingly, This electrophysiologic phenomenon of decremental conduction suggests that the degree of anterograde decremental conduction of accessory pathway was more than the atrioventricular node (AVN).Atrial overdrive pacing (AOD) resulted in intermittent loss of preexcitation, PR prolongation, and Mobitz II atrioventricular block. At atrial pacing with cycle length of 300 ms, the H-V interval was suddenly increased to 102 ms and the QRS complex morphology showed a right bundle branch block pattern. This suggests anterograde conduction over accessory pathway and the QRS complex morphology presenting fully preexcitation (figure3). The ventricular insertion site of accessory pathway could have been predicted by the fully pre-excited QRS complex morphology. Spontaneous junctional beat was seen during electrophysiologic study. Consequently, the H-V interval was still 28ms and the QRS complex morphology remained minimal preexcitation，indicating the proximal insertion site of accessory pathway is AV node (figure 2).DISCUSSIONNodoventricular pathways（NVP） are rare accessory pathway that connect the posterior extensions of the AVN to the crest of the ventricular septum [1]. These pathways were anatomically described in 1941 by Mahaim and Winston. The diagnosis of manifest NVP is based on the following evidence: (1) upper insertion site is AVN, (2) lower insertion site is ventricular, (3) proving accessory pathway presence with decremental conduction manifestations and excluding fasciculoventricular pathway.In our case，the patient present preexcitation without symptoms of arrhythmias. The electrophysiologic study demonstrated evidence of anterograde decremental conduction in response to programmed atrial pacing. The QRS morphology and H-V intervals remained changed during AES, excluding a fasciculoventricular fiber. If the pathway was fasciculoventricular, preexcitation should remained fixed and H-V remained constant when conducting over either fast or slow pathways. Catheter pressure over AVN region resulted in junctional ectopy that remained preexcitation, indicating the proximal insertion site of accessory pathway is AV node. The sudden appearance of infra-His conduction block by AOD resulted in complete ventricular preexcitation，we can confirmed the distal insertion site is the septal of left ventricular based on fully pre-excited QRS morphology. All these findings indicate the presence of a manifest NVP pathway in the patient.In this case, on programmed atrial stimulation testing, there was decremental conduction over the pathway, resulting in a longer A–V and H-V time as the A1–A2 coupling interval decreased. As far as we are aware, this case is unique since the nodoventricular fiber show the degree of anterograde decremental conduction was more than the atrioventricular node. In contrast, most of the NVP bypass only a portion of the AV node with decremental conduction properties. Therefore, the preexcitation degree may increase and the H-V interval may decrease even to negative values during AES [2]. As far as our patient is concerned，The electrophysiologic features of anterograde decremental conduction may be due to the anisotropic conduction of the transitional area of the AV node and variability in the space construction of tissue[3]. In addition, the differential expression of connexin isoforms in the nodal area also may be responsible for more degree decremental conduction of NVP than AVN[4].[1] Nazer B, Walters TE, Dewland TA, et al. Variable Presentations and Ablation Sites for Manifest Nodoventricular/Nodofascicular Fibers. Circ Arrhythm Electrophysiol. 2019;12(9):e007337.[2] Ali H, Sorgente A, Lupo P, et al. Nodo- and fasciculoventricular pathways: Electrophysiological features and a proposed diagnostic algorithm for preexcitation variants. Heart Rhythm. 2015 ;12(7):1677-1682.[3] Katritsis DG, Marine JE, Latchamsetty R, et al. Coexistent Types of Atrioventricular Nodal Re-Entrant Tachycardia: Implications for the Tachycardia Circuit. Circ Arrhythm Electrophysiol. 2015;8(5):1189-1193.[4] Anderson RH, Quintana DS, Mori S, et al. Re-evaluation of the structure of the atrioventricular node and its connections with the atrium. Europace. 2020 ;22(5):821-830.Figure 1：Twelve-lead electrocardiographic recordings showing fixed and minimal preexcitation during sinus rhythm.Figure 2：Intracardiac recordings during programmed atrial stimulation demonstrating decreased preexcitation with a prolongation of the H-V interval. The last spontaneous junctional beat still showing a preexcitation morphology. Note that the QRS complex morphology change of the lead I and V1. His electrograms are recorded on the RV pole and red arrows indicate His potentials.Figure 3：Intracardiac recordings during atrial overdrive stimulation, demonstrating a longer HV interval and QRS complex morphology with fully preexcitation. Red arrows indicate His potentials.