FIG 73.14 Chest radiography of the patient in Fig 73.13 after placement of an epicardial pacemaker system with one atrial and five unipolar ventricular leads, one of which is disconnected Two sets of unipolar leads were placed to resynchronize the functional single ventricle because of ventricular dysfunction The atrial lead is an endocardial lead placed epicardially Implantable Cardioverter-Defibrillators Sudden death after the Fontan procedure is not that uncommon,12,103 usually occurring in the context of end stage of the Fontan circulation failure It may be related to events such as pulmonary and cerebral embolism or poorly controlled atrial tachycardias Implantable cardioverter-defibrillators are a class IB indication for secondary prevention following resuscitated cardiac arrest due to sustained VT or ventricular fibrillation.111 However, implantation carries a significant risk in those with Fontan failure given that it entails thoracic surgery If pacing is not required, a subcutaneous implantable cardioverter-defibrillator may be an option in some patients.112 Careful consideration should be made regarding defibrillation threshold testing at the time of implantation because this process can be lethal in those with severe ventricular failure Atrial Tachyarrhythmias Intraatrial Reentrant Tachycardias The association of atrial tachycardias with poor outcome is at least partly correlated to the underlying substrate of the arrhythmia, rather than the arrhythmia per se In the atriopulmonary Fontan, there is an electromechanical correlation between the occurrence of arrhythmia and the degree of atrial dilation and thickening.113 Risk factors also include previous pulmonary artery banding,98 isomerism, and a systemic right ventricle The observation that atrial dilatation was associated with IART led in part to the adoption of the lateral tunnel procedure The prevalence of IART has proven to be lower with this type of Fontan connection.98 The external cardiac conduit approach reduces intracardiac surgery and avoids progressive dilation of the atrial wall, but it is not evident that this operative strategy is associated with a reduced prevalence of IART compared with the lateral tunnel Invasive electrophysiologic studies demonstrate that the mechanism of IART commonly involves surgical scars created during suturing of the lateral tunnel.114–116 These studies reveal large areas of low-voltage diseased atrial myocardium, with fractionated signals demonstrating delayed and nonhomogeneous electrical conduction This substrate is ideal for the development of intraatrial reentry (Fig 73.15).113,114 FIG 73.15 (A) ECG demonstrating intraatrial reentrant tachycardia in a 53-year-old female with tricuspid atresia who underwent a modified Fontan with right atrium to right ventricle valved conduit The red arrows mark P waves (B) Intracardiac electrograms of the same patient demonstrating a second inducible intraatrial reentrant tachycardia following successful ablation of the tachycardia seen in Fig 73.11A There was an area of slow conduction in the superior right atrium with low amplitude, fractionated signals best depicted in the T2 position of a multielectrode catheter (red arrows) Successful ablation was performed at this site Note also the diffuse low voltage/absent signals at other electrodes on this scarred atrium Atrial reentry circuit depends on areas of slow conduction in diseased atrial myocardium with electrically silent tissue on each side This results in a slow conducting bridge, or isthmus An electrical signal enters the isthmus, and by the time the electrical signal is released from this isthmus, the healthy myocardium is able to conduct again; the signal propagates around the atrium and back to the