Discussion

Catheter ablation for atrioventricular nodal reentrant tachycardia associated with complex CHD after total cavopulmonary connection Fontan surgery is challenging, because of the lack of anatomical indication of the slow pathway as well as the difficulty in catheter access. Moreover, persistent tachycardia after Fontan surgery can easily lead to low cardiac output and elevated central venous pressure associated with increased ventricular filling pressure. Therefore, the risk of haemodynamic collapse is very high in cases requiring slow pathway mapping during tachycardia. This is the first report demonstrating the usefulness of verapamil for stabilisation of haemodynamics during tachycardia. The optimal dosage of verapamil for this purpose is unknown. In our case, although a transient ventriculoatrial block was observed, 1 mg (0.06 mg/kg) of verapamil was effective. Haemodynamics stability during tachycardia induced within minutes after ventriculoatrial block recovery was maintained without blood pressure decrease due to the prolonged tachycardia cycle length. Verapamil prolonged the tachycardia cycle length by extending the atrioventricular interval without affecting the ventriculoatrial interval during tachycardia (Figures 2(b), 2(c)). It remains unclear whether this is simply a result of conduction delay of the fast pathway or indicates a transition of the reentrant circuit from fast-slow to slow-slow atrioventricular nodal reentrant tachycardia. Half-life of verapamil is less than 15 minutes; hence, the risk of verapamil use during electrophysiological study is not high under conditions where a ventricular pacing backup is ensured. Therefore, verapamil would be useful for activation mapping during supraventricular tachycardias associated with the atrioventricular node in patients after Fontan surgery. More fortunately, verapamil also contributed to 1:1 ventriculoatrial conduction during tachycardia, although it is not clear whether this was associated with a slowing of the tachycardia rate or a direct effect on the conductivity of the upper common pathway. When the upper common pathway complicates the diagnosis of tachycardia, administering verapamil may have diagnostic value and could be worth attempting. Landiolol, an ultra-short-acting beta-1 blocker, was another option to avoid haemodynamic collapse. Although it is generally easy to adjust to optimal dosage, we did not select it in this case due to the possibility of adverse effects such as an asthma attack and an increase in pulmonary vascular resistance because of his history of asthma and relatively high central venous pressure after Fontan surgery.

Catheter ablation for atrioventricular nodal reentrant tachycardia in a normal structural heart is usually performed within the triangle of Koch during sinus rhythm. On the other hand, electrophysical features of atrioventricular nodal reentrant tachycardia and anatomical location of the slow pathway associated with complex CHDs are still unknown.^{Reference Papagiannis, Daniel and Ulrich1} Especially, the cardiac conduction system in criss-cross hearts with complex anatomical atrioventricular connections, as in the present case, has not been reported. Therefore, atrial activation mapping during tachycardia is usually required. In our case, the coronary sinus was relatively enlarged because the persistent left superior caval vein perfused the right atrium with venous blood from the left upper body via the coronary sinus until bilateral bidirectional Glenn surgery at 6 months of age. The relationship between persistent left superior caval vein and supraventricular tachycardia has been reported.^{Reference Uhm, Choi and Baek2,Reference Hwang, Park and Kim3} Uhm et al. reported that a slow pathway at the coronary sinus ostium was significantly more frequent in patients with persistent left superior caval vein rather than in those without persistent left superior caval vein.^{Reference Uhm, Choi and Baek2} Although the location of the slow pathway in patients with complex CHD is unknown, we should start mapping from the coronary sinus ostium position, especially in cases with persistent left superior caval vein.