Introduction
Congenitally corrected Transposition of the great Arteries (cc-TGA) is an uncommon congenital heart defect marked by double discordance of atrioventricular (AV) and ventriculoarterial connections. This dual discordance naturally corrects the cardiopulmonary circulation. However, the embryological right ventricle (with its right bundle branch) becomes connected to the aorta, while the left ventricle (with its left bundle branch) is attached to the pulmonary artery. Reentrant supraventricular tachycardias (SVTs) occur in patients with cc-TGA but their frequency and mechanisms are not well known. The description of SVTs in these patients has been largely limited to case reports and small case series.
In this editorial, we will review the mechanisms underlying these arrhythmias and explain how the abnormal embryological development can become the substrate for reentrant SVTs despite otherwise normal systemic and pulmonary circulations.
The most common form of cc-TGA is situs solitus with L-looped ventricles {S,L,L} with atria located in their usual respective positions. The right atrium, which receives venous blood, is connected to the morphological left ventricle located on the right side of the heart, which in turn pumps blood into the pulmonary artery. Conversely, the left atrium, which receives arterial blood from the pulmonary veins, is connected to the left-sided morphologically right ventricle, which pumps blood into the aorta, supplying the systemic circulation. A less common form of cc-TGA with sinus inversus and D-looped ventricles {I,D,D} is also possible that is characterized by mirror-imaged atria.
Due to the fact that the tricuspid and mitral valves follow their respective ventricles, there is often a misalignment between the atrial and ventricular septa, disrupting the normal embryological development of the conduction system (see Figure 1). As a result, an accessory superior (anterior) AV node is formed in most patients that is in continuity with the penetrating AV bundle. The usual AV node located inferiorly may fail to fully develop and may or may not connect to the His bundle. The specialized conduction system is also inverted, with the bundle branches tracking their respective ventricles [1].
Schematic Representation of the Conduction System in Congenitally Corrected Transposition of the Great Arteries (cc-TGA).
Abbreviations: MA, mitral annulus; TA, tricuspid annulus; RBB, right bundle branch; LAF, left anterior fascicle; LPF, left posterior fascicle; AP, accessory AV pathways; RAA, right atrial appendage; LAA, left atrial appendage; PA, pulmonary artery; Ao, aorta.
In addition to the abnormal development of the specialized conduction system, there is frequently an Ebstenoid malformation of the tricuspid valve (left AV valve) that can be associated with left-sided accessory AV pathways [2]. This anomaly predisposes patients with cc-TGA to AV reentrant tachycardias using the accessory pathway as one limb of the reentrant circuit and the AV node-His axis as the other.
Limited information is available regarding the incidence of reentrant SVTs in patients with cc-TGA. In an Asian study, 11% of patients with cc-TGA developed SVTs, with the majority of cases secondary to AV reentrant tachycardia using an accessory A-V pathway and less commonly due to AV nodal reentrant tachycardia involving one or two AV nodes [3].
Atrioventricular Nodal Reentrant Tachycardia in CC-TGA
In patients with cc-TGA, reentry utilizing inputs to the AV nodal tissue may involve one or two AV nodes.
Anatomical and Histological Observations
The presence of two separate AV nodes was originally reported by Monckeberg in 1913 in a patient with AV discordance and double outlet right ventricle [4]. This finding, subsequently called “twin AV nodes”, has also been described in cc-TGA [5, 6]. In a dissection of eleven hearts with cc-TGA reported by Anderson and colleagues, the anterior AV node was present in all patients, located on the superior margin of the mitral annulus with a long AV bundle passing lateral to the pulmonary outflow tract, and reaching the anterior aspect of the interventricular ventricular septum, eventually connecting to its respective bundle branches [5]. A hypoplastic inferior AV node was found in ten of eleven examined hearts that failed to connect with the specialized intraventricular conduction system. Only one specimen, with better septal alignment, demonstrated intact connection from both AV nodes to the ventricular septal tissue. A good septal alignment is usually associated with preserved inferior (posterior) AV node connection to the penetrating AV bundle [7].
Electrophysiologic Observations
Gilette et al reported in 1979 the feasibility of His-bundle recording in these patients [8]. Thereafter, it was reported that AV nodal tachycardia may involve one AV node for antegrade conduction and another AV node for retrograde conduction [9]. The presence of two distinct non-preexcited QRS morphologies is an indication of two AV nodes in these patients. During an electrophysiology study, the presence of two separate AV nodes can be confirmed by two separate His-bundle signals associated with distinct ventricular conduction patterns, decremental properties of the atrioventricular conduction with stable HV intervals, change of QRS morphology when one AV node is refractory and complete transient AV block following adenosine administration.
In 1998, Tada reported the first known case of AV nodal reentrant tachycardia in cc-TGA that was successfully treated by ablation of the input to the slow AV nodal pathway [10]. Interestingly, ablation near the inferior AV node was not successful and required ablation near the superior AV node. Several other reports have shown that elimination of AV nodal reentrant tachycardia can be challenging in these patients. Patients may have one or two anterogradely conducting AV nodes. One AV node may have poor antegrade conduction and therefore should be targeted first for ablation. During the procedure, recording of one or two His bundle electrograms facilitate location of the AV nodes [11–14].
Atrioventricular Reentrant Tachycardia in cc-TGA
As mentioned before, in about half of patients with cc-TGA the left-sided tricuspid valve has features consistent with Ebstein’s anomaly, which is known to be associated with accessory pathways [6, 15]. Anderson et al. found that 9 out of 20 examined hearts with ventricular inversion, including 8 with cc-TGA, had Ebstein’s anomaly affecting the left-sided tricuspid valve [15]. In addition, they noticed that the AV sulcus circumference is not increased as it occurs with right-sided Ebstein’s anomaly with the anterior leaflet of the valve often clefted. Bharati and Lev et al. reported similar histological and anatomical findings in patients with cc-TGA [16].
Successful ablation of accessory pathways is feasible in patients with cc-TGA [17–19]. Left infero-paraseptal (postero-septal), left inferior and left infero-lateral (posterolateral) are the most frequent locations [19]. Patients with a left-sided accessory A-V pathway requiring surgery for the tricuspid valve, should undergo ablation before surgical repair [20].
Conclusions
Reentrant SVTs in cc-TGA patients often involve complex reentrant circuits, including twin AV nodes and multiple accessory pathways, frequently associated with Ebstein’s anomaly of the tricuspid valve. Successful management requires a thorough understanding of the unique anatomy and physiology of the conduction system in cc-TGA. Curative interventions like ablation require clear understanding of the anatomical challenges that present in these patients and the potential for multiple and complex reentrant circuits.