Surface potentials from the region of the atrioventricular node and their relation to dual pathway electrophysiology

Anatomical Ablation of the Atrioventricular Node

Background

Atrioventricular (AV) conduction ablation has been achieved by targeting the area of penetration of the conduction axis as defined by recording a His bundle potential. Ablation of the His bundle may reduce the possibility of a robust junctional escape rhythm. It was hypothesised that specific AV nodal ablation is feasible and safe.

Methods

The anatomical position of the AV node in relation to the site of penetration of the conduction axis was identified as described in dissections and histological sections of human hearts. Radiofrequency (RF) ablation was accomplished based on the anatomical criteria.

Results

Specific anatomical ablation of the AV node was attempted in 72 patients. Successful AV nodal ablation was accomplished in 63 patients (87.5%), following 60 minutes (IQR 50-70 minutes) of procedure time, 3.4 minutes (IQR 2.4-5.5 minutes) of fluoroscopy time, and delivery of 4 (IQR 3-6) RF lesions. An escape rhythm was present in 45 patients (71%), and the QRS complex was similar to that before ablation in all 45 patients. Atropine was administered in six patients after the 10-min waiting period and did not result in restoration of conduction. In nine patients, AV conduction could not be interrupted, and AV block was achieved with ablation of the His after delivery of 12 (IQR 8-15) RF lesions. No cases of sudden death were encountered, and all patients had persistent AV block during a median 10.5 months (IQR 5-14 months) of follow-up.

Conclusion

Anatomical ablation of the AV node is feasible and safe, and results in an escape rhythm similar to that before ablation.

High-density mapping of Koch's triangle during sinus rhythm and typical atrioventricular nodal re-entrant tachycardia, integrated with direct recording of atrio-ventricular node structure potential

BACKGROUND

The mechanism of typical slow-fast atrioventricular nodal re-entrant tachycardia (AVNRT) and its anatomical and electrophysiological circuit inside the right atrium (RA) and Koch's Triangle (KT) are not well known.

OBJECTIVE

To identify the potentials of the compact AV node and inferior extensions and to perform accurate mapping of the RA and KT in sinus rhythm (SR) and during AVNRT, to define the tachycardia circuit.

METHODS

Consecutive patients with typical AVNRT were enrolled in 12 Italian centers and underwent mapping and ablation by means of a basket catheter with small electrode spacing for ultrahigh-density mapping and a modified signal-filtering toolset to record the potentials of the AV nodal structures.

RESULTS

Forty-five consecutive cases of successful ablation of typical slow-fast AVNRT were included. The mean SR cycle length (CL) was 784.1 ± 6 ms and the mean tachycardia CL was 361.2 ± 54 ms. The AV node potential had a significantly shorter duration and higher amplitude in sinus rhythm than during tachycardia (60 ± 40 ms vs. 160 ± 40 ms, p < .001 and 0.3 ± 0.2 mV vs. 0.09 ± 0.12 mV, p < .001, respectively). The nodal potential duration extension was 169.4 ± 31 ms, resulting in a time-window coverage of 47.6 ± 9%. The recording of AV nodal structure potentials enabled us to obtain 100% coverage of the tachycardia CL during slow-fast AVNRT.

CONCLUSION

Detailed recording of the potentials of nodal structures is possible by means of multipolar catheters for ultrahigh-density mapping, allowing 100% of the AVNRT CL to be covered. These results also have clinical implications for the ablation of right-septal and para-septal arrhythmias.

New insights into atrioventricular nodal anatomy, physiology, and immunochemistry: A comprehensive review and a proposed model of the slow-fast atrioventricular nodal reentrant tachycardia circuit in agreement with direct potential recordings in the Koch's triangle area

Atrioventricular nodal reentrant tachycardia (AVNRT) is the most frequent regular tachycardia in humans. In this review, we describe the most recent discoveries regarding the anatomical, physiological, and molecular biological features of the atrioventricular junction that could underlie the typical slow-fast AVNRT mechanisms, as these insights could lead to the proposal of a new theory concerning the circuit of this arrhythmia. Despite several models have been proposed over the years, the precise anatomical site of the reentrant circuit and the pathway involved in the slow-fast AVNRT have not been conclusively defined. One possible way to evaluate all the hypotheses regarding the nodal tachycardia circuit in humans is to map this circuit. Thus, we tried to identify the slow potential of nodal and inferior extension structures by using automated mapping of atrial activation during both sinus rhythm and typical slow-fast AVNRT. This constitutes a first step toward the definition of nodal area activation in sinus rhythm and during slow-fast AVNRT. Further studies and technical improvements in recording the potentials of the atrioventricular node structures are necessary to confirm our initial results.

Development of the Cardiac Conduction System

The cardiac conduction system initiates and propagates each heartbeat. Specialized conducting cells are a well-conserved phenomenon across vertebrate evolution, although mammalian and avian species harbor specific components unique to organisms with four-chamber hearts. Early histological studies in mammals provided evidence for a dominant pacemaker within the right atrium and clarified the existence of the specialized muscular axis responsible for atrioventricular conduction. Building on these seminal observations, contemporary genetic techniques in a multitude of model organisms has characterized the developmental ontogeny, gene regulatory networks, and functional importance of individual anatomical compartments within the cardiac conduction system. This review describes in detail the transcriptional and regulatory networks that act during cardiac conduction system development and homeostasis with a particular emphasis on networks implicated in human electrical variation by large genome-wide association studies. We conclude with a discussion of the clinical implications of these studies and describe some future directions.

Superior-Type Fast-Slow Atrioventricular Nodal Reentrant Tachycardia Phenotype Mimicking the Slow-Fast Type

BACKGROUND

Superior-type fast-slow (sup-F/S-) atrioventricular nodal reentrant tachycardia (AVNRT) is a rare AVNRT variant using a superior slow pathway (SP) as the retrograde limb. Its intracardiac appearance, characterized by a short atrio-His (AH) interval and the earliest site of atrial activation in the His-bundle, is an initial indicator for making a diagnosis.

METHODS

Among 22 consecutive patients with sup-F/S-AVNRT, 3 (age, 68-81 years) patients had an apparent slow-fast (S/F-) AVNRT characterized by a long AH interval and the earliest site of atrial activation in or superior to the His-bundle region (tachy-long-AH).

RESULTS

The diagnosis of sup-F/S-AVNRT was based on the standard criteria in 2 patients and on the occurrence of Wenckebach-type atrioventricular block during tachycardia, which was attributable to a block at the lower common pathway (LCP) below the circuit of the AVNRT, detected owing to the lower common pathway potentials, in one patient. As with the typical S/F-AVNRT, tachy-long-AH was induced after a jump in the AH interval. In contrast to typical S/F-AVNRT, fluctuation in the ventriculoatrial interval was observed during the tachy-long-AH. Ventricular overdrive pacing was unable to entrain or terminate the tachy-long-AH. Moreover, the tachy-long-AH reciprocally transited to/from sup-F/S-AVNRT spontaneously or was triggered by ventricular contractions while the atrial cycle length and earliest site of atrial activation remained unchanged. Both tachycardias were cured by ablation at a single site in the right-side para-Hisian region of 2 patients and the noncoronary aortic cusp of one patient. Collectively, the essential circuit of both tachycardias was identical, and the tachy-long-AH was diagnosed as another phenotype of sup-F/S-AVNRT accompanied by sustained antegrade conduction via another bystander slow pathway breaking through the His-bundle owing to the repetitive antegrade block at the lower common pathway, thus representing a long AH interval during the ongoing sup-F/S-AVNRT.

CONCLUSIONS

An unknown sup-F/S-AVNRT phenotype exists that apparently mimics the typical S/F-AVNRT and is also an unknown subtype of apparent S/F-AVNRT.

Ventricular-triggered atrial pacing: A new maneuver for slow-fast atrioventricular nodal reentrant tachycardia

BACKGROUND

Atrioventricular (AV) node duality is suggested by several electrophysiological findings, none of which are strong predictors of AV nodal reentrant tachycardia (AVNRT).

OBJECTIVE

The purpose of this study was to propose a novel maneuver to study conduction over the AV node and attempt to induce slow-fast AVNRT.

METHODS

Ventricular-triggered atrial pacing (VTAP) with decremental VA delay was carried out in 36 consecutive patients with slow-fast AVNRT and in 21 controls after conventional electrophysiology study. Maneuvers were repeated after ablation in patients with AVNRT.

RESULTS

VTAP resulted in a hysteretic conduction curve in 21 of 36 patients (58%) in the AVNRT group but only 4 of 21 patients (19%) in the control group (sensitivity 58; specificity 81%). This finding demonstrates sustained conduction in a slow conducting pathway and concealed retrograde conduction over a fast pathway. VTAP resulted in AVNRT induction in 15 of 25 inducible patients at baseline (60%), 4 of which were not inducible with incremental pacing or programmed atrial stimulation. VTAP resulting in a suspended p wave had 51% (39%-64%) sensitivity and 100% (89%-100%) specificity for predicting noninducibility in a given autonomic state.

CONCLUSION

VTAP is useful in patients with suspected slow-fast AVNRT. A hysteretic conduction curve demonstrates sustained conduction over a slow pathway and concealed retrograde conduction through the fast pathway, a finding in favor of slow-fast AVNRT. VTAP may facilitate AVNRT induction and proves to be an interesting complement to classic maneuvers. A suspended p-wave response specifically predicts noninducibility of slow-fast AVNRT in a given autonomic state, providing an interesting surrogate to noninducibility as a procedural end point.

Should fast pathway ablation be reconsidered in typical atrioventricular nodal re-entrant tachycardia?

INTRODUCTION

Atrioventricular nodal re-entry tachycardia (AVNRT) is the most common, regular narrow-complex tachycardia. The established treatment is catheter ablation of the AV nodal slow pathway (SP). However, in a select group of patients with long PR intervals in sinus rhythm, SP ablation can lead to AV block due to the absence of robust anterograde conduction through the fast pathway (FP). This report aims to demonstrate that AV nodal FP ablation is a reasonable approach in patients with AVNRT and poor or absent anterograde FP conduction.

METHODS AND RESULTS

Standard electrophysiology study techniques were used in the electrophysiology laboratory. Catheter ablations were performed using radiofrequency energy. Mapping of intracardiac activation was performed with electroanatomical mapping systems. Outcomes were assessed acutely during the procedure and during routine clinical follow-up. Six patients with first-degree AV block and recurrent AVNRT who underwent ablation of their tachycardia at our institution are presented. One patient underwent ablation of AV nodal SP resulting in high-degree AV block necessitating pacemaker implantation. The remaining five patients underwent ablation of the AV nodal FP guided by electroanatomical mapping of the earliest atrial activation in tachycardia. These five had successful treatment of the tachycardia with preservation of anterograde AV nodal conduction. Mapping and ablation approach to eliminate retrograde FP conduction are described.

CONCLUSION

In select patients with AVNRT and poor anterograde FP conduction, retrograde FP ablation is reasonable and is less likely to result in AV block and pacemaker dependency.

Acidosis slows electrical conduction through the atrio-ventricular node

Acidosis affects the mechanical and electrical activity of mammalian hearts but comparatively little is known about its effects on the function of the atrio-ventricular node (AVN). In this study, the electrical activity of the epicardial surface of the left ventricle of isolated Langendorff-perfused rabbit hearts was examined using optical methods. Perfusion with hypercapnic Tyrode's solution (20% CO₂, pH 6.7) increased the time of earliest activation (T_act) from 100.5 ± 7.9 to 166.1 ± 7.2 ms (n = 8) at a pacing cycle length (PCL) of 300 ms (37°C). T_act increased at shorter PCL, and the hypercapnic solution prolonged T_act further: at 150 ms PCL, T_act was prolonged from 131.0 ± 5.2 to 174.9 ± 16.3 ms. 2:1 AVN block was common at shorter cycle lengths. Atrial and ventricular conduction times were not significantly affected by the hypercapnic solution suggesting that the increased delay originated in the AVN. Isolated right atrial preparations were superfused with Tyrode's solutions at pH 7.4 (control), 6.8 and 6.3. Low pH prolonged the atrial-Hisian (AH) interval, the AVN effective and functional refractory periods and Wenckebach cycle length significantly. Complete AVN block occurred in 6 out of 9 preparations. Optical imaging of conduction at the AV junction revealed increased conduction delay in the region of the AVN, with less marked effects in atrial and ventricular tissue. Thus acidosis can dramatically prolong the AVN delay, and in combination with short cycle lengths, this can cause partial or complete AVN block and is therefore implicated in the development of brady-arrhythmias in conditions of local or systemic acidosis.

Functional mathematical model of dual pathway AV nodal conduction

Dual atrioventricular (AV) nodal pathway physiology is described as two different wave fronts that propagate from the atria to the His bundle: one with a longer effective refractory period [fast pathway (FP)] and a second with a shorter effective refractory period [slow pathway (SP)]. By using His electrogram alternance, we have developed a mathematical model of AV conduction that incorporates dual AV nodal pathway physiology. Experiments were performed on five rabbit atrial-AV nodal preparations to develop and test the presented model. His electrogram alternances from the inferior margin of the His bundle were used to identify fast and slow wave front propagations. The ability to predict AV conduction time and the interaction between FP and SP wave fronts have been analyzed during regular and irregular atrial rhythms (e.g., atrial fibrillation). In addition, the role of dual AV nodal pathway wave fronts in the generation of Wenckebach periodicities has been illustrated. Finally, AV node ablative modifications have been evaluated. The model accurately reproduced interactions between FP and SP during regular and irregular atrial pacing protocols. In all experiments, specificity and sensitivity higher than 85% were obtained in the prediction of the pathway responsible for conduction. It has been shown that, during atrial fibrillation, the SP ablation significantly increased the mean HH interval (204 ± 39 vs. 274 ± 50 ms, P < 0.05), whereas FP ablation did not produce significant slowing of ventricular rate. The presented mathematical model can help in understanding some of the intriguing AV node mechanisms and should be considered as a step forward in the studies of AV nodal conduction.

Functional characteristics and inducibility of atrioventricular nodal re-entry in rabbits of different ages

AIMS

Many issues regarding atrioventricular nodal re-entry (AVNR) remain unexplored; however, no stable animal model for the study of AVNR has yet been developed. Clinically, AVNR is found more commonly in adults than children. We attempt to characterize AV nodal functional properties and inducibility of AVNRT using rabbits of three different age groups since we hypothesize that the inducibility of AVNR may increase as the subject ages.

METHODS AND RESULTS

We evaluated the inducibility of AVNR and the functional characteristics of the AV node in 6-month-old (Group 1), 2-month-old (Group 2), and at 1-week-old (Group 3) rabbits using a well-established rabbit heart model. We found that the inducibility of AVNR was higher in the 2-month-old group, compared with the 1-week-old group (30%). There was no functional difference between the two older groups (6-month-old and 2-month-old groups), however the youngest group (Group 3) showed better AV nodal functional properties. Regarding the correlation between inducibility and nodal functional properties, subgroups with AVNR demonstrated a longer AH maximum (AHmax) compared with the non-re-entry group, although there is no difference in age-adjusted AHmax. Regarding the correlation between inducibility and nodal functional properties, for the 1-week-old rabbits, subgroups with AVNR demonstrated a shorter AH minimum compared with the non-re-entry group (39.0 vs. 61.0, P=0.017).

CONCLUSION

Our results demonstrate that both 2-month-old and 6-month-old rabbits represent stable models for AVNR. Longer AH conduction time may lead to greater re-entry inducibility.

One-dimensional mathematical model of the atrioventricular node including atrio-nodal, nodal, and nodal-his cells

Mathematical models are a repository of knowledge as well as research and teaching tools. Although action potential models have been developed for most regions of the heart, there is no model for the atrioventricular node (AVN). We have developed action potential models for single atrio-nodal, nodal, and nodal-His cells. The models have the same action potential shapes and refractoriness as observed in experiments. Using these models, together with models for the sinoatrial node (SAN) and atrial muscle, we have developed a one-dimensional (1D) multicellular model including the SAN and AVN. The multicellular model has slow and fast pathways into the AVN and using it we have analyzed the rich behavior of the AVN. Under normal conditions, action potentials were initiated in the SAN center and then propagated through the atrium and AVN. The relationship between the AVN conduction time and the timing of a premature stimulus (conduction curve) is consistent with experimental data. After premature stimulation, atrioventricular nodal reentry could occur. After slow pathway ablation or block of the L-type Ca(2+) current, atrioventricular nodal reentry was abolished. During atrial fibrillation, the AVN limited the number of action potentials transmitted to the ventricle. In the absence of SAN pacemaking, the inferior nodal extension acted as the pacemaker. In conclusion, we have developed what we believe is the first detailed mathematical model of the AVN and it shows the typical physiological and pathophysiological characteristics of the tissue. The model can be used as a tool to analyze the complex structure and behavior of the AVN.

Noncontact three-dimensional mapping guides catheter ablation of difficult atrioventricular nodal reentrant tachycardia

BACKGROUND

Atrioventricular nodal reentrant tachycardia (AVNRT) is the most common supraventricular tachycardia in adulthood. Although selective ablation of the slow AV nodal pathway can cure AVNRT, accidental AV block may occur. The details on the electrophysiologic characteristics, quantitative data on the voltage inside Koch's triangle, and the use of three-dimensional noncontact mapping to facilitate the catheter ablation of AVNRT associated with a high-risk for AV block or other arrhythmias have been limited.

METHODS AND RESULTS

Nine patients (M/F=5/4, 34+/-23 years, range 17-76) with clinically documented AVNRT were included. All patients had undergone previous sessions for slow AV nodal pathway ablation but they had failed, because of repetitive episodes of complete AV block during the RF energy applications. Further, one patient had a complex anatomy and 4 patients were associated with other tachycardias, respectively. The electrophysiologic studies revealed that 4 patients had the slow-fast, 4 the slow-intermediate and one the fast-intermediate form of AVNRT. Noncontact mapping demonstrated two types of antegrade AV nodal conduction, markedly differing sites of the earliest atrial activation during retrograde VA conduction, and a lower range of voltage within Koch's triangle. The lowest border of the retrograde conduction region was defined on the map, and the application of the RF energy was delivered below that border to prevent the occurrence of AV block. The distance between the successful ablation lesions and the lowest border of the retrograde conduction region was significantly shorter in the patients with the slow-intermediate form of AVNRT than in those with the slow-fast form (5.5+/-3.4 vs. 15+/-7.6 mm; p<0.05). After the ablation procedure, either rapid pacing or extrastimulation could not induce any tachycardia, and there was no recurrence during the follow-up (10.3+/-5.4, 2 to 22 months).

CONCLUSIONS

Noncontact mapping could effectively demonstrate the antegrade and retrograde atrionodal conduction patterns, electrophysiologic characteristics of Koch's triangle, and guide the successful catheter ablation in difficult AVNRT cases.

Connexin-mediated cardiac impulse propagation: connexin 30.2 slows atrioventricular conduction in mouse heart

In mouse heart, four connexins (Cxs), Cx30.2, Cx40, Cx43, and Cx45, form gap junction (GJ) channels for electric and metabolic cell-to-cell signaling. Extent and pattern of Cx isoform expression together with cytoarchitecture and excitability of cells determine the velocity of excitation spread in different regions of the heart. In the SA node, cell-cell coupling is mediated by Cx30.2 and Cx45, which form low-conductance (approximately 9 and 32 pS, respectively) GJ channels. In contrast, the working cardiomyocytes of atria and ventricles express mainly Cx40 and Cx43, which form GJ channels of high conductance (approximately 180 and 115 pS, respectively) that facilitate the fast conduction necessary for efficient mechanical contraction. In the AV node, cell-cell coupling is mediated by abundantly expressed Cx30.2 and Cx45 and Cx40, which is expressed to a lesser extent. Cx30.2 and Cx45 may determine higher intercellular resistance and slower conduction in the SA- and AV-nodal regions than in the ventricular conduction system or the atrial and ventricular working myocardium. Cx30.2 and its putative human ortholog, Cx31.9, under physiologic conditions form unapposed hemichannels in nonjunctional plasma membrane; these hemichannels have a conductance of approximately 20 pS and are permeable to cationic dyes up to approximately 400 Da in molecular mass. Genetic ablation of Cxs confirmed that Cx40 and Cx43 are important in determining the high conduction velocities in atria and ventricles, whereas the deletion of the Cx30.2 complementary DNA led to accelerated conduction in the AV node and reduced the Wenckebach period. We suggest that these effects are caused by (1) a dominant-negative effect of Cx30.2 on junctional conductance via formation of low-conductance homotypic and heterotypic GJ channels, and (2) open Cx30.2 hemichannels in non-junctional membranes, which shorten the space constant and depolarize the excitable membrane.

The elusive extracellular AV nodal potential: studies from the canine heart, ex vivo

Various forms of extracellular recordings from the AV node (AVN) have been reported. However, lack of consistent validation have precluded the use of such recordings in experimental and clinical studies. In 14 Langendorff perfused dog hearts, the triangle of Koch (TOK) was exposed and an octapolar electrode catheter (2 mm rings, 2 mm spacing) was inserted under the endocardium so that the bipolar pairs recorded electrograms from the apex to the base of the TOK. All recording were filtered between 0.05 and 250 Hz, except for a His bundle (Hb) recording (30-250 Hz) made from another bipolar electrode catheter placed in the aortic root. Transmembrane action potentials (AP) were recorded close to the sites of extracellular electrograms. Pin electrodes at the periphery of the bath were arranged to register two ECG leads from the volume conductor. During recovery of electrical activity 11 of 14 preparations developed a junctional rhythm that initially manifested only an AV nodal extracellular and corresponding intracellular AV nodal potentials followed gradually by conduction to the Hb and ventricles but no retrograde atrial activation; 3 preparations initially produced Hb rhythms based on extracellular and transmembrane AP recordings from the AVN and Hb. The amplitude and duration of the AVN extracellular potentials (average: 97 +/- 26 microV and 92 +/- 25 msec, respectively) during AVN rhythms, significantly differed from those during atrial pacing (262 +/- 185 microV and 78 +/- 26 msec, p < 0.05). Histologic sections of the sites underlying the electrodes recording AVN potentials showed AVN tissue throughout. We conclude that extracellular AV nodal potentials are independent waveforms with specific qualitative and quantitative characteristics that distinguish them from adjacent atrial, transitional, Hb or ventricular potentials.