Demonstration of the Anatomical Tachycardia Circuit in Sinoatrial Node Reentrant Tachycardia: Analysis Using the Entrainment Method

Three-dimensional functional anatomy of the human sinoatrial node for epicardial and endocardial mapping and ablation

The sinoatrial node (SAN) is the primary pacemaker of the human heart. It is a single, elongated, 3-dimensional (3D) intramural fibrotic structure located at the junction of the superior vena cava intercaval region bordering the crista terminalis (CT). SAN activation originates in the intranodal pacemakers and is conducted to the atria through 1 or more discrete sinoatrial conduction pathways. The complexity of the 3D SAN pacemaker structure and intramural conduction are underappreciated during clinical multielectrode mapping and ablation procedures of SAN and atrial arrhythmias. In fact, defining and targeting SAN is extremely challenging because, even during sinus rhythm, surface-only multielectrode mapping may not define the leading pacemaker sites in intramural SAN but instead misinterpret them as epicardial or endocardial exit sites through sinoatrial conduction pathways. These SAN exit sites may be distributed up to 50 mm along the CT beyond the ∼20-mm-long anatomic SAN structure. Moreover, because SAN reentrant tachycardia beats may exit through the same sinoatrial conduction pathway as during sinus rhythm, many SAN arrhythmias are underdiagnosed. Misinterpretation of arrhythmia sources and/or mechanisms (eg, enhanced automaticity, intranodal vs CT reentry) limits diagnosis and success of catheter ablation treatments for poorly understood SAN arrhythmias. The aim of this review is to provide a state-of-the-art overview of the 3D structure and function of the human SAN complex, mechanisms of SAN arrhythmias and available approaches for electrophysiological mapping, 3D structural imaging, pharmacologic interventions, and ablation to improve diagnosis and mechanistic treatment of SAN and atrial arrhythmias.

Cryoablation for atrial tachycardia with cycle length variability originating from the vicinity of the sinus node

This case is about atrial tachycardia with cycle length variability originating from the vicinity of the sinus node and diagnostic pacing maneuvers to assess the tachycardia circuit were not achieved. Activation mapping revealed that the origin of atrial tachycardia was 15 mm away from the sinus node and the phrenic nerve was captured by pacing at the posterior portion of atrial tachycardia. A multipolar catheter was placed in the right brachiocephalic vein to capture the right phrenic nerve by pacing. The absence of phrenic nerve palsy was confirmed by palpation of constant diaphragmatic movement. The cryoablation could be safely and efficiently performed without ablation-induced injury of sinus node and phrenic nerve palsy by confirming constant diaphragmatic movement. The efficacy of cryoablation in the vicinity of the conduction system and phrenic nerve will be increasingly confirmed in the future.

Learning Objective

Cryoablation for atrial tachycardia might be more safe and effective in terms of ablation-induced injury of conduction system and phrenic nerve palsy compared with conventional radiofrequency ablation when diagnostic pacing maneuvers are not able to estimate the circuit due to variability of tachycardia cycle length.

Structural and Functional Properties of Subsidiary Atrial Pacemakers in a Goat Model of Sinus Node Disease

Background

The sinoatrial/sinus node (SAN) is the primary pacemaker of the heart. In humans, SAN is surrounded by the paranodal area (PNA). Although the PNA function remains debated, it is thought to act as a subsidiary atrial pacemaker (SAP) tissue and become the dominant pacemaker in the setting of sinus node disease (SND). Large animal models of SND allow characterization of SAP, which might be a target for novel treatment strategies for SAN diseases.

Methods

A goat model of SND was developed (n = 10) by epicardially ablating the SAN and validated by mapping of emergent SAP locations through an ablation catheter and surface electrocardiogram (ECG). Structural characterization of the goat SAN and SAP was assessed by histology and immunofluorescence techniques.

Results

When the SAN was ablated, SAPs featured a shortened atrioventricular conduction, consistent with the location in proximity of atrioventricular junction. SAP recovery time showed significant prolongation compared to the SAN recovery time, followed by a decrease over a follow-up of 4 weeks. Like the SAN tissue, the SAP expressed the main isoform of pacemaker hyperpolarization-activated cyclic nucleotide-gated channel 4 (HCN4) and Na⁺/Ca²⁺ exchanger 1 (NCX1) and no high conductance connexin 43 (Cx43). Structural characterization of the right atrium (RA) revealed that the SAN was located at the earliest activation [i.e., at the junction of the superior vena cava (SVC) with the RA] and was surrounded by the paranodal-like tissue, extending down to the inferior vena cava (IVC). Emerged SAPs were localized close to the IVC and within the thick band of the atrial muscle known as the crista terminalis (CT).

Conclusions

SAN ablation resulted in the generation of chronic SAP activity in 60% of treated animals. SAP displayed development over time and was located within the previously discovered PNA in humans, suggesting its role as dominant pacemaker in SND. Therefore, SAP in goat constitutes a promising stable target for electrophysiological modification to construct a fully functioning pacemaker.

Comparison of the catheter ablation outcome in patients between targeting the entrance and exit of the reentry circuit in verapamil-sensitive atrial tachycardia originating from the atrioventricular-node vicinity

Verapamil-sensitive atrial tachycardia originating from the atrioventricular node vicinity (AVN-AT) can be eliminated with radiofrequency energy (RF) deliveries targeting either the entrance or exit of its reentry circuit. However, the outcome of these different approaches has not been clarified well. Thus, we compared the catheter ablation outcome targeting the entrance of reentry circuit, identified by the entrainment method (Ent-Group; 21 patients) with that targeting the earliest atrial activation site (EAAS) during AT (Exit-Group; 16 patients). There was no significant difference in the tachycardia cycle length (441.4 ± 87.4 vs. 392.8 ± 64.8 ms, p = 0.0704) or distance from the His bundle (HB) site to the EAAS (6.5 ± 2.0 vs. 7.6 ± 1.8 mm, p = 0.0822) between the Ent- and Exit-Groups. However, distance from the successful ablation site to the HB site in the Ent-Group was significantly longer than that in the Exit-Group (13.4 ± 3.1 vs. 7.6 ± 1.8 mm, p < 0.0001), resulting in more frequent transient atrioventricular block episodes in the Exit-Group than Ent-Group (31.3 vs. 0%, p < 0.01). Initial ATs (AT1s) were terminated in all patients in both Groups. However, ATs accompanied by shifting in the EAAS (AT2) were induced more frequently in the Exit-Group than Ent-Group (50.0 vs. 14.3%, p < 0.02) after eliminating AT1. RF deliveries to the EAAS eliminated all AT2s. The number of RF deliveries was greater in the Exit-Group than Ent-Group (6.9 ± 3.3 vs. 3.9 ± 1.6, p < 0.001). In conclusion, RF ablation targeting the entrance sites can avoid AVN injury and is superior in reducing the number of RF deliveries and occurrence of different ATs than targeting the exit sites in the AVN-AT.

Ablation Success in Various Arrhythmias: When It Is Appropriate to Recommend Ablation?

Cardiac arrhythmia is an abnormal electrical activity of the heart. It can be divided into rhythms with increased electrical activity (tachyarrhythmia) and those with reduced electrical activity (bradyarrhythmia). Ablation therapy has a role in tachyarrhythmia, but this role varies from being limited in inappropriate sinus tachycardia to being a class 1 indication in typical atrial flutter. A balanced approach in recommendation for ablation therapy for the management of tachyarrhythmias involves knowledge of the interplay between the risk, benefit, success rate, and alternatives with advances in mapping and ablation therapy.

Demonstration of the Anatomical Tachycardia Circuit in Sinoatrial Node Reentrant Tachycardia: Analysis Using the Entrainment Method

Background

The anatomical tachycardia circuit of sinoatrial node reentrant tachycardia (SANRT) has not been well clarified. This study aimed to elucidate the tachycardia circuit of SANRT.

Methods and Results

Exit and entrance of the intranodal sinoatrial node conduction (I‐SANC) of the reentry circuit were identified in 15 SANRT patients. After identifying the earliest atrial activation site (EAAS) during the tachycardia (EAAS‐SANRT), rapid atrial pacing was delivered from multiple atrial sites to identify the entrainment pacing site where manifest entrainment and orthodromic capture of the EAAS‐SANRT were demonstrated. Radiofrequency energy was then delivered starting at a site 2 cm proximal to the EAAS‐SANRT in the direction of the entrainment pacing site and gradually advanced toward the EAAS‐SANRT until tachycardia termination to localize the I‐SANC entrance. The EAAS‐SANRT was orthodromically captured by pacing delivered from the distal coronary sinus (n=7), high posteroseptal right atrium (n=2), low posteroseptal right atrium (n=2), low anterolateral right atrium (n=2), or coronary sinus ostium (n=2). Radiofrequency energy delivery to the entrance of the I‐SANC, 10.4±2.8 mm away from the EAAS‐SANRT, terminated tachycardia immediately after onset of energy delivery (3.4±2.3 seconds). The successful ablation site was located further from the EAAS during sinus rhythm (EAAS‐sinus) than the EAAS‐SANRT (12.8±4.5 versus 7.2±3.1 mm; P<0.0001).

Conclusions

The reentry circuit of SANRT was composed of the entrance and exit of the I‐SANC being located at distinctly different anatomical sites. SANRT was eliminated by radiofrequency energy delivered to the I‐SANC entrance, which was further from the EAAS‐sinus than I‐SANC exit.