Introduction
Compared with patients with structurally normal hearts, there is an even greater need to suppress pathologic tachycardias in patients with congenital heart disease. The decrease in diastolic filling time and potential loss of atrioventricular (AV) synchrony are more poorly tolerated in these patients, proportionate to the degree of underlying systolic and diastolic dysfunction. Moreover, the cardiac side effects of anti-arrhythmic drugs may be amplified in these patients, especially in the presence of sinus node,AV junctional, or ventricular dysfunction.Although common forms of supraventricular tachycardia, such as AV nodal re-entry and AV reciprocating tachycardia certainly may occur in these patients, atrial or ventricular muscle-based tachycardias represent the greatest challenges in performance of successful catheter ablation. Therefore, the following comments will be limited to catheter ablation of primary atrial tachycardias that occur following atrial surgery, AV valve surgery, and ventricular surgery with secondary poor diastolic function; and primary ventricular tachycardias following right ventricular (RV) outflow tract surgery.
Primary Atrial Tachycardias
Macrore-entry atrial tachycardias and focal atrial tachycardias represent the two forms of atrial tachycardia in this category. Macrore-entry tachycardias are variously called intra-atrial re-entry tachycardia, scar-related and incisional atrial tachycardia, and atypical atrial flutter. These are synonymous so long as principles of re-entry can be demonstrated, and a finite region of slow conduction that participates in the tachycardia circuit can be demonstrated. Illustration of the entire circuit is frequently not possible in these patients. The precise mechanism of focal atrial tachycardia is usually not demonstrated, but because these tachycardias tend to also be paroxysmal, they most likely represent 'microre-entryÔÇÖ or perhaps triggered automaticity.They tend to localize along a suture line.
The outcome from catheter ablation is optimized when the operator has a full understanding of the congenital and post-surgical anatomy.This requires access to prior surgical reports; review of prior imaging studies, including echocardiograms, magnetic resonance imagery (MRI), and angiography (MRA); discussion with a congenital heart surgeon if possible; and even review of preserved heart specimens. The procedure should be performed with biplane fluoroscopy, especially in the presence of complex structural defects, such as post-Mustar, post-Senning, or post-Fontan operation.
The conduct of the electrophysiologic study in these patients varies according to vascular access and the anatomy. Some of these patients lack femoral venous access to their heart due to illiofemoral venous thrombosis caused by prolonged hospital stays in childhood. Transhepatic access can be of value in these cases, and in cases of heterotaxy, in which the inferior vena cava (IVC) is congenitally interrupted.1 It has been demonstrated that ablation success is improved by the utilization of contemporary three-dimensional (3D) electroanatomic mapping systems.2 These systems require a stable electrical reference, which, in the normal heart, is usually achieved from a coronary sinus catheter. In the absence of coronary sinus access following congenital heart surgery, an esophageal electrode catheter or a transvenous active fixation catheter (Medtronic Inc.) may be used.
The electrophysiologic principles that are used for identifying target areas for ablation in patients not having congenital heart disease are usually applicable in these patients as well. For macrore-entry atrial tachycardias, regions of slow conduction may usually be identified by application of principles of concealed entrainment with a short post-pacing interval and the presence of complex atrial diastolic electrograms.There are caveats. Atrial muscle that is heavily scarred may have very low voltage bipolar electrograms.While these may be critical to the tachycardia circuit, their presence may be underappreciated when using electroanatomic mapping systems, especially non-contact systems, which are dependant upon mathematical reconstruction from intracavitary unipolar electrograms. Also, very dilated and slowly conducted atria, especially after the Fontan operation, may present to the operator a confusing activation sequence. Knowledge of potential areas of slow conduction between sites of conduction block (such as valve annuli, venous ostia, and artificial conduits) are of paramount importance when selecting regions for ablation.
Following childhood repair of sinus venosus or ostium secundum atrial septal defect, the incidence of symptomatic atrial tachyarrhythmias is about 4%.3 This incidence increases with age at repair. The majority of atrial tachyarrhythmias are atrial fibrillation, but atrial flutter may also occur.When post-surgical atrial flutter does occur in this patient group, it seems to be related to slow conduction within either the low lateral right atrium (between the atriotomy scar and the orifice of the IVC) or the subtricuspid isthmus. Catheter ablation within these zones is highly effective for long-term relief of tachycardia in all reported series. Success in well over 90% of such cases is expected.4
Mustard and Senning operations were developed in the early 1960s to provide venous redirection to the opposite AV valves in patients with d-transposition of the great arteries. These operations were often destructive to the sinus node or its blood supply, and always required complex suture lines. This operation has been replaced with the arterial switch operation beginning in the mid 1980s. Nevertheless, it is estimated that there are still several thousand adults in the US and Canada who have had one of these operations. Within 10 years of having undergone a Mustard or Senning operation, it is estimated that at least 10% of patients will have had one or more episodes of macrore-entry atrial tachycardia. In the electrophysiology laboratory, the majority of these circuits utilize the subtricuspid isthmus and can be ablated there. Unfortunately, this region is bisected by a portion of the surgically placed atrial baffle. Access to the pulmonary venous side requires either a retrograde approach (arterial catheter across the aortic valve and then retroflexed across the tricuspid valve) or needle/sheath perforation of the baffle, analogous to a transeptal approach. Sometimes, ablation lesions must be extended in the pulmonary venous atrium as far posteriorly as the posterior atrial septal remnant. Demonstration of conduction block is seldom achieved in these patients, but, rather, interruption of intraatrial re-entry tachycardia during lesion generation, followed by inability to reinduce tachycardia are usually the end-points for success. Other reported zones of slow conduction that can be expected to benefit from catheter ablation include areas in the systemic venous atrium below the superior vena cava and the region between the medial mitral valve annulus and the medial subtricuspid isthmus on the systemic venous side. Among 70 patients in 10 published reports, the acute success rate for this procedure is 84% with recurrence in 17% of these patients at 12-28 months follow-up.5
The Fontan operation was devised for patients having a functionally single ventricle. This operation has gone through several iterations. The style of operation in which there is a direct atriopulmonary connection is no longer performed, but a large number of adults have survived with this type of anatomy. One of the unforeseen problems with this type of operation is progressive dilatation and hypertrophy of the anatomic right atrium. The combination of gratuitous right atrial hypertension with secondary dilatation, surgical scar, natural conduction obstacles, and secondary interstitial fibrosis conspire to predispose these patients to macrore-entry atrial tachycardia, often multiple, at a high incidence.
At 10 years follow-up, the incidence ranges from about 15->50% following the aforementioned style of Fontan operation. More recent Fontan-type operations, including 'lateral tunnelÔÇÖ variants and extracardiac conduits appear to have a much lower incidence, but follow-up is still incomplete. Concerning radiofrequency catheter ablation, recent reports tout acute success rates of up to 90%, but the recurrence rate by three years remains close to 50%.2,6 In TriedmanÔÇÖs report in 2002, predictors of an unfavorable 'clinical arrhythmia scoreÔÇÖ following catheter ablation in this patient group included increased number of macrore-entry circuits, failure of complete procedural success, and (at borderline significance) failure to use an electro-anatomic mapping system.2 It is believed that this disappointing clinical experience is related to the increased atrial wall thickness and difficulty in achieving reliable transmural lesions. Irrigated-tipped or large-tipped catheters and high-energy generators are now being used in an effort to overcome these obstacles. Even more daunting are pathologic circuits that are not accessible to the systemic venous return, due to their location in the pulmonary venous atrium. Transbaffle perforation may be attempted, and a recent experience demonstrated potential efficacy of a transthoracic approach.7 This is a relatively high-risk procedure, however, and requires immediate surgical back-up.
Ventricular Tachycardia
Sudden death following congenital heart surgery most commonly results from a primary ventricular tachyarrhythmia. Sample incidences of this dread outcome include 5.4/1,000pt-yr for aortic stenosis, 4.9/1,000pt-yr for Mustard/Senning operation, and 1.5/1,000 pt-yr for tetralogy of Fallot.8 The efficacy of implantable cardioverter-defibrillators (ICDs) as a means of secondary prevention has been demonstrated in this patient group, and current investigations of risk factors are on-going so that primary prevention may be provided. That said, a small number of patients who have undergone RV outflow tract surgery for tetralogy of Fallot, double outlet RV, d-transposition/VSD, and truncus arteriosus may also have monomorphic macrore-entry ventricular tachycardia. This may even occur in the absence of other risk factors for sudden death. By applying the principles described above to identify zones of slow conduction, there are several reports of successful ablation with an enduring outcome.The anatomic 'hot spotsÔÇÖ for successful ablation appear to be the isthmus of tissue between the RV outflow tract patch/conduit and the tricuspid valve annulus, ventricular septal defect patch and the tricuspid valve annulus, or the ventricular septal defect patch and the RV outflow tract patch/conduit. Care must be taken to avoid the perimembranous region in the vicinity of the specialized AV conduction system. Application of this technology to this patient group should be limited to those patients whose ventricular tachycardia was clinically well-tolerated. Furthermore, unless an ICD is also implanted, there should be adequate documentation of successful ablation from follow-up electrophysiological testing, probably up to six to nine months following the original procedure.