In atrial isomerism, so-called heterotaxy syndrome, the atrial appendages are mirror images of each other, either morphologically right or left atrial appendages. A terminal crest, which works as an anatomical barrier for the reentrant circuits, is reported to exist bilaterally in 87% of the patients with right atrial isomerism but is absent bilaterally in 88% of those with left atrial isomerism [6]. The presence of bilateral terminal crest is the reason of a high incidence of intra-atrial reentrant tachycardia (IART) in right atrial isomerism [7]. Atrial fibrillation (AF) and non-reentrant supraventricular tachycardia (SVT) were frequently observed rather than macro-reentrant tachycardia in left atrial isomerism [8]. The lack of anatomical barriers and the high prevalence of sinus node dysfunction with aging may account for the higher incidence of AF and non-reentrant SVT in left atrial isomerism [8, 9].

Various conduction abnormalities are often observed in hearts with atrial isomerism. In the pathological findings of 35 hearts (10 with right atrial isomerism and 25 with left atrial isomerism), bilateral sinus nodes were present in 90% of the hearts with right atrial isomerism, whereas solitary and hypoplastic sinus nodes were present in 44% and not found in 56% of the hearts with left atrial isomerism [10]. Twin AV node physiology was present in all of hearts with right atrial isomerism but in only 32% of those with left atrial isomerism [10]. Conduction bundles under the AV node were also reported to be unique. Some had a common conduction bundle between two separate AV nodes (twin AV nodes), and some had separate bundles in the twin AV nodes, with or without a connecting sling [10, 11]. In the cases with twin AV nodes, SVT involving two AV nodes (twin AV node reciprocating tachycardia) is sometimes observed. Wu et al. [12] reported that twin AV nodes were more frequent in right atrial isomerism with balanced ventricles instead of a dominant ventricle and would increase the risk of SVT. SVT in such patients could be managed by ablation or medication and was not associated with mortality. They also mentioned that SVT episodes were uncommon after the age of 15 years, maybe because the reentry between twin AV nodes is less sustained in adult patients with larger hearts and slower AV conduction owing to the maturation of the AV nodal conduction. Further, another unique form of SVT, adenosine-sensitive focal junctional tachycardia, was reported [13]. There are several controversial issues in these unique SVTs, such as their incidence, prognosis, and the participation or not of connecting slings.

AV connection is related to the development of the AV conduction system. The conduction system in AV discordance (AVD) is thought to be related to septal alignment. In congenitally corrected transposition of the great arteries (ccTGA), which is a representative anomaly of AVD, malalignment of the interatrial and interventricular septa usually results in a gap that prevents a normally located AV node (posterior AV node) from communicating with the ventricular conduction bundles, forming an anterior AV node [14]. The anterior AV node is situated in the right atrium at the lateral junction of the pulmonary and mitral valves, and the bundle courses anterior to the pulmonary artery to reach the anterior part of the interventricular septum. In cases of AVD with the presence of a small or atretic pulmonary trunk, the septa were well aligned and the posterior positioned AV node, the so-called posterior AV node, connected to an AV bundle [15]. The superiority of a posterior or anterior AV node is determined by whether the septa are aligned or maligned, and some have both AV nodes (twin AV nodes) with a connecting sling [15]. AV block spontaneously develops with aging because of the abnormal AV conduction system. However, in a study of a small number of cases (n = 8), AV block was reported to rarely occur in ccTGA with situs inversus because of the predominant posterior AV node, which might be related to good septal alignment [16]. Further, twin AV node reciprocating tachycardia and adenosine-sensitive focal junctional tachycardia could occur in this setting.

Representative diseases of the subpulmonary right ventricle (RV) are repaired tetralogy of Fallot (TOF) and double-outlet right ventricle (DORV). Atrial and ventricular arrhythmias and exertional dyspnea from progressive RV dilatation due to chronic pulmonary regurgitation or severe residual RV outflow tract obstruction occur in 10–15% of patients within 20 years after the initial repair [17]. The prevalence of these arrhythmias increases with age; in particular, atrial fibrillation and ventricular arrhythmias markedly increase after 45 years of age [18]. The arrhythmogenicities of this disease are reported in many papers.

Regarding ventricular arrhythmias, histological findings in repaired TOF with ventricular tachycardia (VT) showed degeneration, adiposis, fibrosis, and scattered myocyte islets in the area of the RVOT [19, 20]. Late gadolinium enhancement (LGE), cardiovascular magnetic resonance (CMR), and a fragmented QRS complex are useful markers to observe myocardial fibrosis in the RV of repaired TOF and are related to the arrhythmic events [21, 22]. Further, common anatomical isthmuses for VT are present in repaired TOF and DORV, and a VT isthmus ablation was reported to be possibly curative only in patients with a preserved ventricular function and isthmus-dependent reentry [23].

To the best of our knowledge, there have been no histological evaluations of atrial tachyarrhythmia, but the right atrial area index measured by CMR was a strong predictor of new-onset atrial tachyarrhythmia [24]. The arrhythmogenic substrate of atrial tachyarrhythmia is mostly related to the right atrial side, especially the right atrial free wall [25, 26].

The representative diseases of systemic RV are transposition of the great arteries after an atrial switch operation (AtSO-TGA) and ccTGA. Both diseases carry a risk of atrial and ventricular tachyarrhythmia, which may be related to ventricular dysfunction and fibrosis deteriorating with age [27–30]. However, the arrhythmias that occur within these unique hemodynamics were mostly evaluated in patients with AtSO-TGA.

Regarding ventricular arrhythmias in AtSO-TGA, to the best of our knowledge, there have been no histological findings or evaluations of common anatomical isthmuses. The risk factors of sustained VT and/or sudden death have been reported to be age, systemic ventricular functions, and duration of the QRS complex [29, 31]. The RV ejection fraction was negatively correlated with age and LGE CMR, and QRS duration was negatively correlated with RV ejection fraction and positively correlated with LGE CMR [29, 32]. Moreover, LGE CMR was associated with clinical outcome, mainly new-onset atrial and ventricular tachyarrhythmia [33]. Given these results, the etiology of ventricular arrhythmias in AtSO-TGA was suspected to be related to RV fibrosis. Another possible etiology is that the rapid ventricular response in SVT induces ventricular arrhythmias.

Regarding atrial arrhythmias in AtSO-TGA, the common anatomical isthmuses of IART and origin of focal atrial tachycardia have been well described [34]. The recent technique of catheter ablation promised a good success rate [35]. However, SVT often becomes life-threatening, and its mechanism is still controversial [36]. There are several hypotheses: atrial tachycardia rates tend to be slower than with atrial flutter, leading to 1:1 conduction, which in turn may result in hemodynamic instability; SVT itself may induce VT or ventricular fibrillation with a rapid ventricular response [37, 38, 34]. Further, the incidence of sinus node dysfunction increases with age as well as atrial tachyarrhythmia [39].

With the modification of the Fontan procedure and progress in surgical techniques, the mortality rate and incidence of arrhythmias have decreased in patients with Fontan circulation [40]. In the original Fontan procedure, the pulmonary artery is directly connected to the atrium, the so-called atriopulmonary connection (APC). In this type of procedure, the atria are exposed to both pressure and volume overload. In a total cavopulmonary connection (TCPC), both the supra vena cava and the infra vena cava connect to the pulmonary artery. Several types of TCPC exist: intra-atrial rerouting or intra-atrial lateral tunnel, intra-atrial graft, and extracardiac rerouting (ECR). In the modern era, the ECR type of TCPC is performed most frequently because ECR has several benefits to avoid atrial suture lines and preserve low atrial pressure compared with other types of TCPC [41].

In patients with Fontan circulation, the incidence of atrial tachyarrhythmia and sinus node dysfunction increases with age [42–44]. The most frequent atrial tachyarrhythmia is IART followed by AF, focal atrial tachycardia, and reentrant SVT [43, 44]. In a report of long-term outcomes of 1052 patients at the Mayo Clinic, the proportion of IART, AF, focal atrial tachycardia, and reentrant SVT among 412 patients who were diagnosed new arrhythmias after the Fontan operation were 74%, 39%, 26%, and 9%, respectively [44]. There is no doubt that the APC-type Fontan procedure has the highest risk of atrial tachyarrhythmia among the different types of Fontan procedure. In the report of catheter ablation for IART in APC Fontan, both age and time since Fontan surgery were correlated to the extent of low-voltage areas [45]. The incidence of atrial arrhythmias in TCPC is lower than that in APC; however, it still occurs over time. Whether ECR is of greater benefit in avoiding atrial tachyarrhythmia and sinus node dysfunction than intra-atrial lateral tunnel is a controversial issue [41, 43, 46, 47]. We also need to know that the atrial tachyarrhythmia deteriorates the hemodynamics of the Fontan circulation, and the presence of atrial tachyarrhythmia was associated with higher morbidity and mortality [48, 49].

Ventricular tachyarrhythmia has also been reported as a late comorbidity of the Fontan procedure, but the incidence is lower than that of atrial tachyarrhythmia (3.5–10%) [42, 44]. The ventricular myocardial fibrosis measured by LGE CMR was reported to be related to the incidence of ventricular tachyarrhythmia [50].

Atrial and ventricular arrhythmias also increase in patients with unrepaired cyanotic congenital heart disease, although there are no reports that discuss this in detail thus far. However, this population accounted for approximately 10% of sudden cardiac deaths in a cohort of patients with ACHD [4, 51]. And we found that unrepaired cyanotic congenital heart disease accounted for 13 (43%) of the 30 ACHD patients given oral amiodarone and eight (15%) of the 54 ACHD patients with AF (unpublished data). Arrhythmia events are supposed to be related to the myocardial damage due to the long-term hypoxemia and the atrial and ventricular overload due to the high pulmonary blood flow.