entry point of the isthmus These areas of slow conduction usually develop around areas of scar either surgical or due to progressive atrial fibrosis (Fig 73.16) After all forms of Fontan, the commonest position for such an isthmus is between the bottom end of a right atriotomy scar and the inferior vena cava (“pericaval origin”).116 This is different to other postoperative congenital heart groups and the structurally normal heart with atrial flutter, where the isthmus commonly runs across the anatomic cavotricuspid junction It has been proposed that an additional surgical line should be made at the time of the lateral tunnel surgery, to prevent such an isthmus developing Unfortunately, given the time lag in the development of IART after the Fontan procedure, it will be decades before we know if this has been successful.117 As mentioned before, some modifiable surgical techniques may help to prevent IART Certainly, the move away from atriopulmonary connection has been beneficial, as has reduced age at the time of the Fontan operation FIG 73.16 An anteroposterior projection of a three-dimensional electroanatomic map in a patient with intraatrial reentrant tachycardia (IART) post Fontan The white arrows show the IART circuit, with the critical zone located in a gap in a scar (gray areas) on the lateral wall The intracardiac signals taken at this zone (blue arrows) show long, lowvoltage, fractionated signals A single radiofrequency lesion in that area interrupted the tachycardia Other congenital arrhythmia substrates such as accessory pathway–mediated tachycardias and atrioventricular nodal reentrant tachycardias account for up to 30% of the tachycardias in Fontan patients treated in a tertiary electrophysiology laboratory.115 These tachycardias are more responsive to medical and ablative therapy than IART In the acute setting, medical management can be difficult, and the patient may have decompensated cardiac failure, as both a cause and effect of the IART Direct current cardioversion can fail in a quarter of patients, with increased success rate if type I or III antiarrhythmic medications are started prior.118 Medication for rate control can be difficult to manage because of the commonly associated sinus node dysfunction, and although amiodarone can be effective, side effects can be harmful when this medication is used long term Thus, in the adult patient with an atriopulmonary connection, medical management is frequently unsuccessful Interventional strategies involve a choice of (or combination of) a catheter ablation strategy versus a surgical takedown to a lateral tunnel or extracardiac conduit with concomitant surgical ablation techniques, usually a maze procedure Catheter ablation for IART in the atriopulmonary Fontan can be successful in the short term, but there is a high recurrence rate.119 This is not surprising given the fact that the underlying substrate—the atrial dilation and wall thickening with large areas of scarred and electrically inhomogeneous tissue—is not altered The grossly dilated atrium is also a nidus for thrombus formation and is hemodynamically inefficient The early Fontan conversion experience was one of considerable mortality outside of several high-volume centers The results of this surgery are improving, and this improvement relates at least in part to a better appreciation of the indications for operation.107,120 Many centers have published favorable results, with an early mortality rate of approximately 5%, improved NYHA functional class, and reduction in arrhythmia incidence over 10 years (see later, “Surgical Management of Fontan Failure”).121–124 Atrial Fibrillation Atrial fibrillation commonly occurs earlier in the Fontan population than in other patients with postoperative congenital heart disease and is generally poorly tolerated Onset often occurs in the third decade, usually as an intermittent arrhythmia that commonly coexists or alternates with other atrial tachycardias Progression to sustained atrial fibrillation is common within 5 years of the first episode The inclusion of left atrial (Cox) maze with right atrial maze at the time of Fontan conversion may prove effective in reducing the recurrence rate of this arrhythmia, especially in older patients and those who already have atrial fibrillation However, it is not known what proportion of atrial fibrillation has a left atrial/pulmonary vein origin in the Fontan circulation, even though this is the commonest mechanism in the structurally normal heart There is anecdotal evidence, and it makes intuitive sense, that some atrial fibrillation in these patients has a right atrial origin.125 Role of Catheter Ablation Although a surgical approach may be most appropriate for those with atrial tachycardia with an atriopulmonary Fontan, catheter ablation has a role in other cases An ablation can be a useful palliation where conversion is contraindicated, atrial dilation is not excessive, or the patient has declined surgery Focal atrial tachycardias can be relatively straightforward to ablate, along with congenital arrhythmia such as accessory pathways, atrioventricular node reentrant tachycardia, and rare cases with twin atrioventricular nodes.115 With the extracardiac conduit or lateral tunnel Fontan, the critical isthmus is usually on the cardiac side of the baffle, so that access for ablation catheters is difficult However, there has been increasing confidence in the use of transbaffle puncture technique in these cases because there is commonly a safe puncture point at the lower end of the baffle at the junction with the inferior vena cava/atrial border.126