Palliated and repaired congenital heart disease is associated with a significant risk of developing arrhythmias. These patients are at risk for both atrial and ventricular arrhythmias and while the treatment options available are the same as for those with morphologically normal hearts there are unique differences that present challenges. The full breadth of congenital heart disease anatomy is beyond the scope of this article however there are arrhythmia associations between various congenital anomalies that can be discussed and general principles of an approach to catheter ablation in congenital heart disease is outlined. Figure 1 gives an outline of important considerations for each patient.
Atrial tachycardia in patients with congenital heart disease is most often found in the morphological right atrium. These arrhythmias are usually reentrant but can rarely be triggered or micro-reentrant with a focal origin.35 Reentrant tachycardia is termed intra-atrial reentrant tachycardia or incisional atrial reentrant tachycardia (IART) to distinguish it from typical right atrial flutter in which there is a similar mechanism but more predictable origin. Typical atrial flutter involves a reentrant circuit with counterclockwise conduction around the tricuspid valve and is treated by creating a line a block in a critical portion of the circuit at the cavo-tricuspid isthmus (CTI). IART may also involve the CTI but often involves other areas of slow conduction to create a reentrant circuit.36 Congenital heart surgery usually involves an atriotomy and may include septal patches or intracardiac baffles with long suture lines that can create other areas of delayed conduction and arrhythmia substrates. Moreover, congenital heart patients often have abnormal hemodynamics or varying degrees of atrial-ventricular valve leak creating a higher pressure atrium that is often dilated, thick and with varying degrees of fibrosis all contributing to arrhythmia mechanism.
IART is usually slower than typical atrial flutter with cycle lengths of 270-450msec and often presents as a 2:1 tachycardia with what appear to be discrete p waves rather than the typical saw-tooth pattern of atrial flutter.35,36 Clinicians should hold a high index of suspension for these arrhythmias when treating patients with CHD. Often a mild increase in baseline heart rate and mild symptoms are the only clinical indicator of the arrhythmia. ECG will often appear to be of an ectopic atrial rhythm or sinus tachycardia when the true underlying rhythm is 2:1 conducted IART with the second p wave buried in the QRS complex or T wave. Figure 2 is an ECG of a patient in IART with 2:1 conduction and ventricular rate of 111.
ECG of a 28y/o with Noonan’s syndrome ASD and pulmonary valve stenosis post repair in 2:1 IART. Note the discrete p waves and relatively slow tachycardia p-p interval of 270 msec.
Ablation of IART requires an in depth knowledge of the patients anatomy including surgical scars and suture lines and thorough review of the patient’s clinical data and operative reports is essential. Special attention should be paid to the location of the conduction system that can be located in atypical locations. Care should be taken in each ablation case to find and mark the location of the AV node and His signals.
While the arrhythmia circuit in these patients can be quite complex, 67% of IART circuits involve the CTI in patients with straight forward and repaired CHD such as Tetralogy of Fallot, ASDs and VSDs.36,37 In these cases ablation requires similar techniques as typical atrial flutter. Other more complex congenital heart lesions such as single ventricle patients with Fontan palliation were found to have circuits throughout the atrium with a large proportion (51%) on the lateral right atrial wall and (25%) anterior right atrial wall.36,37 Patients with Mustard and Senning baffles for transposition of the great arteries were found to have a large proportion (57%) of circuits involving the CTI however the presence of the baffle from Inferior vena cava to the mitral valve creates further complexity often requiring ablation on both sides of the baffle.36,38 In addition to finding and marking the AV node special care should be made on the right lateral atrial wall to find and mark locations of phrenic nerve stimulation.
Three-dimensional (3D) mapping systems have become an important resource for ablations in congenital heart disease.37 These systems include both contact and noncontact mapping and allow for better collection, assimilation, and visualization of arrhythmia data. 3D mapping not only facilitates success but simplifies ablation procedures. The newest iterations of contact mapping collect geometry, timing and voltage data simultaneously. Noncontact mapping can facilitate further simplification of the procedure in select patients however the severely dilated atrium of many congenital heart patients is a limiting factor as illustrated in Figure 3. Future improvements of noncontact mapping may eliminate this problem with directable sensing electrodes that incorporate contact mapping technology. It is particularly important to make sure the geometry created in the 3D mapping system represents the chamber of interest. Often an angiogram is useful to make sure that all areas of these complex and dilated chambers are included in the geometry. In the most difficult cases a segmented CT or MRI scan can by fused or merged with the geometry collected by the 3-D mapping system.
3D mapping of a severely dilated right atrium in a 28 year old male with Ebstein Anomaly who is post tricuspid valve replacement and right atrial maze. The first panel shows a noncontact mapping image displaying the areas out of the 4cm accuracy range in red. The second panel shows a contact mapping geometry. In this case contact mapping revealed that the low voltage area and scar likely created by a right atrial maze procedure was a target for ablation that was out of range for noncontact mapping. Ablation in this area with a large (8mm) tip RF catheter resulted in arrhythmia termination, non-induciblilty and bidirectional conduction block.
While 3D mapping systems have improved the ability to assimilate information and identify corridors of arrhythmia conduction, traditional techniques of entrainment pacing aid in identifying critical areas of arrhythmia circuits. Potential areas for ablation can be evaluated and marked with the mapping system and include locations of low amplitude fractionated electrograms, local timing preceding a discrete p-wave by 50-80 msecs, or areas that demonstrate entrainment (post pacing intervals – tachycardia cycle length <= 30 msec, perfect concealed entrainment, or electrogram to p time = stimulus to p time).35,39 Without these techniques success can be limited in all but the simplest cases.