Atrioventricular nodal re-entrant tachycardia (ANRT) is a common arrhythmia in children and young adults.1 Since the early 1990s, radiofrequency catheter ablation of the slow atrioventricular nodal pathway has been the treatment of choice for definitive therapy of this arrhythmia.2 More recently, cryoablation catheter technology has become available and is an alternative treatment.3-9 Should cryoablation replace radiofrequency catheter ablation as the first-line ablative therapy for ANRT in the pediatric population?
Benefits and Risks of Radiofrequency Ablation
The benefits and risks of radiofrequency catheter ablation for ANRT have been well established, as the procedure has been conducted for nearly 20 years.The benefit of radiofrequency ablation is that it provides a potential long-term cure, with initial success rate for ANRT in a pediatric population of 99% and a recurrence rate of approximately 5%.10-12 The primary risk of the procedure is inadvertent atrioventricular block, which has been reported in 2.1%.11 Atrioventricular block causes significant morbidity, especially to a young child who is otherwise healthy. With atrioventricular block, the child would be required to undergo the placement of a permanent pacemaker and then would need future pacemaker revisions and generator changes. The child would likely be made pacemaker-dependent for the remainder of his/her life.
Theoretic Benefits and Risks of Cryoablation
Cryoablation has several potential advantages over radiofrequency catheter ablation. First, cryoablation has been shown to produce smaller lesions in the heart compared with lesions created by radiofrequency ablation.13 The advantage of smaller lesions is that a potential cure can be provided with less damage to the normal myocardium, an important advantage in very small hearts that will continue to grow and develop. Second, cryoablation is theoretically safer with respect to the risk of atrioventricular block. With cryoablation, an ice ball develops at the tip of the ablation catheter, and the catheter adheres securely to the cardiac tissue. This eliminates the possibility of inadvertent atrioventricular block due to catheter dislodgement. With cryoablation, one is also able to create a reversible lesion by freezing to -30┬║C prior to making a permanent lesion by freezing further to -70°C.14 With the reversible 'test' lesion, one can assess both the effect of the therapy and the risk of damage to the normal conduction system prior to placing a permanent lesion. Third, cryoablation is theoretically safer with respect to the risk of damage to adjacent structures, such as coronary arteries.
Case reports have provided examples of radiofrequency ablation resulting in temporary or permanent damage to the coronary arteries.15,16 Cryoablation is not expected to produce these type of complications.14,17,18 Finally, cryoablation has been reported as being painless.19 Painless lesions have an advantage for the patient, with less discomfort and anesthetic agents, and perhaps a faster recovery. All these characteristics are potentially even more important in the pediatric population as compared with the adult population.
Clinical Outcomes of Cryoablation
Initial outcomes for cryoablation of ANRT in a pediatric population are promising (see Table 1).3-8 The report by Kirsh et al.5 is an international registry of the first cryoablation procedures. Others have reported initial single-center experiences with cryoablation. Initial success rates for cryoablation of ANRT to date have been 85-96%.3-8 These rates are only marginally lower than the success rates for radiofrequency ablation.11 Data from our institutional experience have shown similar findings, with an initial success rate for cryoablation for ANRT of 95% compared with an initial success of radiofrequency ablation 100%, a finding which did not meet statistical significance.3
The initial studies of cryoablation in pediatrics have shown improved safety profile for cryoablation. There have been no reports of permanent atrioventricular block in any of the studies.3-8,20-23 Transient atrioventricular block has been documented, but all episodes of atrioventricular block recovered following catheter rewarming.3-8,24
The primary risk of cryoablation appears to be in the higher recurrence rates following an initially successful procedure. In reports to date, the overall arrhythmia recurrence rate is about 10% (0-23%).3-8
Risk to Benefit Ratio for Cryoablation Versus Radiofrequency Ablation
In comparing cryoablation with radiofrequency ablation for the treatment of ANRT in children, the initial procedural success rates of the two techniques are comparable. The tradeoffs appear to be an improved safety profile for cryoablation with respect to inadvertent atrioventricular block versus a higher arrhythmia recurrence rate. The primary rationale for using cryoablation for ANRT is the safety concern. However, since the risk of inadvertent atrioventricular block is relatively low with radiofrequency catheter ablation, the safety advantage is difficult to demonstrate, even in a randomized clinical trial.
There have been two randomized controlled trials of cryoablation versus radiofrequency ablation for ANRT in the adult population, with a total of 263 patients.25,26 These studies showed no atrioventricular block in either the radiofrequency ablation or the cryoablation groups. In the study by Kimman et al.,25 the recurrence rates for both modalities were similar, while in the study by Zrenner et al.26 the recurrence rate for cryoablation was higher than that for radiofrequency ablation.
The principal argument against using cryoablation as the first-line therapy is the risk of arrhythmia recurrence. One should be reminded, however, that the studies currently published on cryoablation of ANRT in pediatrics represent outcomes from initial experiences with a new technology. As operators use the technology and progress on the learning curve, it is anticipated that outcomes will improve. Also, in centers that are routinely utilizing cryoablation for ANRT, changes in the technical aspects of the procedure are currently being made. For example, most of the published reports reflect the use of the 4mm-tip cryoablation catheter while many electrophysiologists are now moving to the 6mm-tip cryoablation catheter. The larger tip catheter is thought to create larger, and potentially more permanent, ablation lesions. There have also been proposed changes in approaches to ablation, such as the placement of multiple cryoablation applications (versus a single application) or longer duration of ablation lesions.3,27 A few reports have evaluated the electrophysiologic changes of the atrioventricular node during or following all cryoablation lesions in order to better delineate cryoablation end-points in this arrhythmia substrate.28,29
The goal of most laboratories performing cryoablation for ANRT in a pediatric population is to decrease recurrence rates while maintaining the current safety profile. Arguably, an arrhythmia recurrence after a successful procedure causes less morbidity and is less detrimental to the patient than a complication of inadvertent atrioventricular block requiring a pacemaker. Yet, the recurrence rates of cryoablation have been higher than those of radiofrequency ablation procedures. Further study is warranted in order to identify specific patient, procedural, or technologic factors associated with recurrence that can then be modified for the benefit of our patients.