Mapping of Atrial Arrhythmias in Complex Congenital Heart Disease

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Citation
European Cardiovascular Disease 2007 - Issue 2;2007:3(1):104-106
DOI
http://dx.doi.org/10.15420/ecr.2007.0.2.104

Advances in the treatment of complex congenital heart disease over the past 20 years have led to a dramatic improvement in long-term survival.1 However, the incidence of cardiac arrhythmias as a sequel of palliative heart surgery has also increased significantly in adolescent and adult survivors of congenital heart disease surgery.1,2 Cardiac arrhythmias are the leading cause of emergency admissions in adult congenital heart disease patients with or without surgical intervention.3
The incidence of late arrhythmias has been associated with long-term abnormal pressure–volume load, hypoxia, surgically created scars or suture lines.4–6 Residual haemodynamic problems are frequently encountered in these patients and may be aggravated by the arrhythmia, resulting in heart failure, acute collapse or cardiac arrest. The risk of sudden death is reported to be as high as 6–10% in long-term survivors after surgery for congenital heart disease.1
As patients with congenital heart disease form a heterogeneous population, an optimal treatment strategy for the individual patient has to be defined. This may include medical therapy, catheter ablation or implantation of a pacemaker or automatic internal defibrillator. Medical therapy is often insufficient to control the arrhythmia and exposes patients to the risk of serious side effects, especially with long-term treatment.7 Catheter ablation offers an alternative treatment strategy in many of these patients. New mapping and ablation techniques have improved the understanding of complex arrhythmia mechanisms.
There are some crucial steps to take when approaching an ablation therapy in patients with congenital heart disease. These include adequate preparation of the procedure, reliable identification of the tachycardia circuit and the appropriate ablation site and creation of an effective tissue-altering lesion. This article will focus on the identification of the intra-atrial tachycardia circuit by looking at the most recent developments in mapping, and will also present the two main patient populations that are encountered in the electrophysiology (EP) lab: patients after the Fontan procedure and patients with D-transposition of the great arteries after an atrial switch procedure (Mustard or Senning procedure).

The Use of Mapping Systems

In contrast to a ‘healthy’ atrium with defined anatomical structures, scarring, pressure overload or hypoxia-related changes can lead to the development of complex intra-atrial re-entrant tachycardias (IARTs) in patients after congenital heart disease surgery. Often, conventional mapping fails to depict these re-entrant circuits.8
In most cases IART is dependent on areas of surgical scarring, which function as lines of conduction slowing or block. Therefore, it is important to identify areas of absent or low voltage (<0.03mV), which are catalogued as incisional scars or patches (e.g. coloured grey on CARTO® maps). Other important insights into tachycardia mechanisms are given by ‘double potentials’ – two discrete electrocardiograms separated by at least 20msec. For example, a line of double potentials situated on the free wall of the right atrium (RA) may correlate with an atriotomy scar (a line of conduction block with asynchronous activation on either side). ‘Fractionated’ potentials show continuous low amplitude (<0.1mV) with more than two separate positive or negative deflections. These may also correspond to areas of slow or blocked conduction, or may indicate the site of origin of a focal atrial tachyardia.9
Currently, two systems allow the creation of 3D colour-coded activation maps: the CARTO system (Biosense Webster) and the NavX® System (St Jude Medical). Both are able to create a virtual 3D map in which local activation time on the map catheter is displayed in relation to a reference catheter. In addition to the colour-coded activation map, a voltage map can be displayed to show areas of low voltage or scarring. Examples of colour-coded activation maps during tachycardia are shown in Figures 1 and 2. Both systems have the ability to integrate 3D data sets from a computed tomography (CT) or magnetic resonance (MR) scan. A ‘merge’ of a CT or MR tomography (MRT) heart scan of the individual patient with the acquired electro-anatomical map is possible; this allows a more precise incorporation of the underlying anatomy and facilitates mapping and ablation.

Remote Magnetic Navigation

Remote magnetic navigation offers a new approach for mapping and ablation of different arrhythmia substrates. Remote catheter movement over a computer-controlled catheter advancer system (Cardiodrive®, Stereotaxis Inc.) is achieved within a uniform magnetic field (0.08 Tesla), which is created by two magnets positioned on either side of the fluoroscopy table. A small permanent magnet at the tip of the mapping and ablation catheter allows easy manoeuvring along all relevant structures within the RA or even the left atrium (LA), including movement into and inside the coronary sinus (CS).10
The exact methodology of the system has been described before.11 By providing exact and reliable catheter steering, remote magnetic navigation allows mapping of different supraventricular tachycardia (SVT) substrates in patients with small or wide cardiac chambers or complex anatomy (see Figure 3). Reduction of fluoroscopy time seems possible. Catheter ablation can be performed in selected subgroups, but the lack of irrigated tip catheters still limits its use in the majority of cases.12

Patients After the Fontan Operation

The Fontan procedure has been performed by connecting the RA directly to the pulmonary arteries to create a definite palliation in patients with single ventricle anatomy. With time, the RA volume increases and atrial re-entrant tachycardias are common.2
After a Fontan procedure, the re-entrant circuit usually lies in the RA and involves tissue anterior to the crista terminalis, which is often displaced posteriorly. The site of ablation will often be found to be the isthmus between the inferior vena cava (IVC) and the tricuspid valve, as in typical atrial flutter. It will also commonly be a region near scars on the lateral or anterior right atrial wall.1,13–15 Especially in those patients who lack a right-sided valve annulus and who have undergone a Fontan operation, successful ablation sites are more likely to be scattered around the lateral wall14 and anterior aspect of the RA.

In patients with an intra-cardiac total cavo–pulmonary connection (TCPC), antegrade access to the residual RA can be accomplished through a pre-existing (surgically created) opening or by puncture.16 Identification of a re-entrant tachycardia on the right atrial wall with only small, focal areas of entrainment has been reported.17 In our experience, focal tachycardia (including a micro re-entrant mechanism) is not rare in patients after the Fontan procedure, accounting for up to 30% of cases.18 Foci are found preferentially adjacent to a scar area or the atrio-pulmonary conduit.

Patients After the Mustard or Senning Operation

Before the introduction of the arterial switch operation, an atrial switch using the Mustard or Senning operation was performed in patients with transposition of the great arteries (TGA). Extensive surgery and the prolonged time of hypoxia after birth may facilitate the development of atrial tachycardia.
One of the main findings in patients following the Mustard procedure is that electro-anatomical mapping has to be performed in both the systemic venous (SVA) and the pulmonary venous (PVA) atrium during IART to outline the re-entrant circuit and to perform successful ablation.19,20
To map the SVA, the catheter is advanced via the right femoral vein in order to map the PVA via the right femoral artery across the aortic valve and across the tricuspid valve (see Figure 2), or via trans-septal puncture. The His region is usually found in the SVA and the ostium of the coronary sinus in the PVA (depending on the baffle position).
In the vast majority of patients, IART is a single-loop re-entrant circuit, commonly with the tricuspid annulus (TA) as a central barrier. The IART circuits are constrained inferiorly by the IVC and septally by the atrial baffle. Patch, incisional scars or atriotomy can serve as additional boundaries. Peri-tricuspid re-entry tachycardias can rotate clockwise or counterclockwise around the TA, and the site of ablation in these patients is most often the TA–IVC isthmus (see Figure 2).

The intra-atrial baffle is usually positioned in such a way that the septal TA–IVC isthmus is located in the PVA. However, the suture line at the mid-isthmus is placed in such a way that the mid-isthmus is confined to either the PVA or to both the SVA and the PVA. Therefore, it may be necessary to ablate not only in the PVA, but in both atria (see Figure 2). It is important to remember that the created isthmus line must not be too septal to avoid damage to the AV node.

Summary

In patients with complex congenital heart disease, extensive cardiac surgery as well as residual defects may lead to abnormal anatomy, scarring and dilatation of one or both atria. Subsequently, a variety of intra-atrial flutter circuits may occur. Conventional ablation techniques in these patient groups show low success and high relapse rates. The introduction of 3D mapping systems has allowed the depiction of the anatomical–electrophysiological correlation, leading to a better understanding of the tachycardia mechanisms.
The integration of data sets from 3D imaging techniques (CT, MRT) may provide further insight into the mechanism of these tachycardias and thus may facilitate ablation. Remote magnetic navigation allows the limitations of catheter range in large or unusually formed atria to be overcome.
A more precise and reproducible movement and further reduction of fluoroscopy time seems to be within reach. The introduction of irrigated tip catheters in combination with remote navigation may offer the chance of further improving precise mapping and successful ablation of atrial tachycardias in this patient population.

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