American College of Cardiology

Radiofrequency Catheter Ablation of Complex Atrial Arrythmias in Patients with Congenital Heart Disease
Ed.note: In this issue of ‘Current Opinion’, Dr. Andrew Blaufox provides an update on innovative 3-dimensional mapping systems that are being used to guide radiofrequency ablation in patients with congenital heart disease.

Andrew D. Blaufox, M.D.

Children's Heart Program of South Carolina
Charleston, South Carolina

There has been a tremendous increase in the number of patients who are reaching adulthood after having undergone palliative surgery for congenital heart disease, thus there has been a growing need to focus attention on the late term morbidity suffered by these patients. Among the more prevalent, disruptive, and potentially lethal problems experienced in this population is intra-atrial reentrant tachycardia (IART). The cumulative occurrence rate for this arrhythmia approaches 50% at 12 years following palliation procedures, such as the Fontan or Mustard procedures. Although the incidence is not as high after surgery for other defects, such as Atrial Septal Defects or Tetralogy of Fallot, it is still significant. Because tachycardia cycle lengths are typically longer in IART than those seen with typical adult atrial flutter and because patients with IART often have fairly well preserved AV conduction, there is a greater chance that these patients will have 1:1 AV conduction with rapid ventricular conduction. This may lead to more severe symptoms or sudden cardiac death. Thus, there is a strong impetus to treat IART aggressively.

Pharmacologic therapy has proven to be ineffective for long term control with recurrence rates greater than 75% at 5 years. The results being reported for Fontan revision (total cavopulmonary artery conversion combined with right-sided maze or Maze-Cox III surgery) have been very promising with recurrence rates as low as 12.5%. However, these results are only applicable to Fontan patients, especially those with significant hemodynamic issues requiring intervention with a substantial morbid risk. On the other hand, radiofrequency catheter ablation of atrial arrhythmias can be safely undertaken in a wider set of patients with a much lower morbidity. Although cumulative recurrence rates are approximately 40%, patients who have recurred have improvement in their symptoms and quality of life. As experience and knowledge are growing rapidly, current practices are likely to yield greater success. Advances in mapping utilizing three dimensional techniques and advances in lesion formation utilizing catheter tip cooling promise even greater improvement in ablation results.

These atrial arrhythmias consist of electrical circuits around barriers to electrical conduction, such as scars or incision lines, or between areas of electrical isolation, such as the orifices of the caval veins or the atrioventricular valve annuli. Before the advent of advanced mapping techniques, identification of these structures relied heavily on descriptions from surgical notes and other conventional mapping techniques. Various specific diagnostic catheters have been used. Typically, one or two catheters with 20 electrodes were laid out in the chamber of interest so that atrial activation patterns could be carefully analyzed. Areas of slow conduction were sought. The ability to produce concealed entrainment and post-pacing intervals within 20 msec of the tachycardia cycle length indicated that these areas of slow conduction were critical to the tachycardia circuit and should be targeted for ablation. Ablation results utilizing this type of mapping varied from center to center with reported acute success rates of 70-100% while recurrence rates were approximately 50% at 2 years follow-up.

Although there was an appreciation for the complexity of these atrial circuits, knowledge of the underlying electrophysiological and anatomical substrate of the critical protected zones of the reentrant circuit was limited with conventional mapping techniques. The use of innovative mapping systems, such as CARTO (Biosense-Webster, Johnson-Johnson) and ENSITE3000 (Endocardial Solutions) have greatly facilitated better understanding of the circuits and have been associated with a greater long-term outcome after radiofrequency ablation procedures. Each system operates from a different technology and has benefits and disadvantages in comparison to the other.

CARTO operates via an electromagnetic field to provide spatial orientation data for anatomic reconstruction and catheter positioning without the need for fluoroscopy. Electrograms are acquired point by point with a single roving catheter and are retained with easy ability for review. Activation timing is calculated in comparison to a reference catheter. Activation is color coded for display and interpolated between collected points. In addition, voltage potentials are recorded with each point, stored, and can be displayed in a similar fashion as activation timing.

This system has several advantages:

1. activation timing that is easily be seen and updated with numerous points and can readily be correlated with anatomy,
2. catheter position can be easily and accurately marked during mapping and ablation so that repositioning can occur more accurately,
3. scars, prosthetic material, and anatomic obstacles can easily be identified and marked as barriers to electrical conduction,
4. voltage maps can provide valuable information regarding potentially critical parts of the circuit,
5. the completeness of bidirectional conduction block following radiofrequency applications can be readily assessed, and
6. the electroanatomic data can be obtained in relatively small or narrow chambers, such as systemic or pulmonary venous atria in Mustard or Senning patients.

The disadvantages of the system are in part due to limitations of the technology as well as the problems with the substrate.

1. In order to obtain more detail about activation, more points need to be collected.
2. As data between points is interpolated, the potential for error is dependent upon the number of points taken, thus prompting the user to spend more time gathering as many as hundreds of data points.
3. Because many of these patients, particularly Fontan patients, have multiple circuits, data collection for one circuit may be interrupted and confused by initiation of another circuit.
4. Time constraints make complete data collection difficult for nonsustained or hemodynamically compromising tachycardias.
   Nonetheless, CARTO has proven to be a very valuable tool.

Figures 1 and 2 are CARTO propagation maps of an IART circuit and results from its ablation in a 5 year old patient s/p Tetralogy of Fallot repair who had previously failed Amiodarone as well as Sotalol. Figure 1 demonstrates a counterclockwise IART around an anterior atriotomy scar through a channel between the scar and the IVC. An ablation line was placed between the scar and the IVC as well as between the scar and an ASD patch. Figure 2 shows conduction block during medial pacing after completion of the RF line.

ENSITE relies upon an array with approximately 3,000 electrodes that is opened around a fluid-filled balloon enabling it to capture electrograms that are virtually projected, via Laplace’s law, onto an anatomy which was constructed using a single roving catheter. This system has some very distinct advantages:

1. activation is recorded, displayed, and correlated with anatomy on a single beat so that extensive amounts of time for data acquisition are not required allowing one to map multiple circuits, nonsustained tachycardias, and hemodynamically compromising tachycardias,
2. electrogram amplitude is readily seen so that inferences about tissue health can easily be made,
3. catheter position can be easily marked during mapping and ablation, and 4) the completeness of bidirectional conduction block following radiofrequency applications can be readily assessed.

Although this system has proven to be accurate, several disadvantages exist:

1. interference from farfield signals may influence electrogram interpretation,
2. the array is thrombogenic and requires vigilant attention to aggressive anticoagulation,
3. scars, incision lines, and other barriers to conduction are somewhat more difficult to identify,
4. the balloon size limits the use of this system in smaller chambers seen in smaller patients or certain anatomies, such as in the Mustard or Senning patients, and
5. the anatomic and spatial accuracy is limited in very large chambers such as those seen in some older APC-Fontan patients.
   Nevertheless, ENSITE has helped tremendously in dealing with these arrhythmias.

Figures 3 – 7 demonstrate the use of ENSITE to delineate the IART circuits and results from their ablation in a 15-year-old boy with Tricuspid Atresia s/p APC Fontan. Figure 3 shows tachycardia #1. This tachycardia travels in a counterclockwise direction: anterior and inferior then superior and posterior through a gap in the mid cristae terminalis. Figure 4 shows tachycardia #2. This tachycardia travels in a counterclockwise direction: starting superior and posterior then heading anterior by passing below the cristae terminalis through a channel between the cristae and the IVC. A line of radiofrequency applications was made along the posterior aspect of the cristae terminalis through its gap and down to the IVC. Figure 5 shows unidirectional conduction block with posterior pacing following ablation. Figure 6 shows that a undirectional gap in the ablation line exists in the mid cristae region when anterior pacing is performed. After this is realized, three more lesions are placed along the anterior aspect of the cristae in order to close this gap. Figure 7 demonstrates that the gap is closed with these final lesions.

In addition to these important advances in mapping, the advent of catheter tip cooling promises to allow physicians to create more complete and transmural lines of conduction block. Cooling at the catheter tip actively with irrigation or passively with larger tip surface areas allows for lower impedance and greater energy transfer and heating into the tissue. Initial reports indicate that acute success is augmented by catheter tip cooling in adults with atrial flutter as well as patients with congenital heart disease and IART.

While management strategies for patients with IART should be individualized according to patient need, the recent advances in radiofrequency catheter ablation make it a very attractive option for many of these patients.