Special Considerations for Ablation in Pediatric Patients

Arrhythmia Mechanisms

The arrhythmias seen in pediatric patients are typically the more treatable varieties. Ventricular tachycardia is relatively rare in children, accounting for less than 5% of all tachycardias and only 20% of wide-complex tachycardias. In addition, when ventricular tachycardia does occur in a child without structural heart disease, it is more likely than in an adult to have one of the ablatable mechanisms. Supraventricular tachycardia (SVT), which accounts for the vast majority of arrhythmias in children, is most likely to result from a concealed or manifest accessory AV pathway ( Fig. 40.1 ), again portending well for possible catheter ablation therapy. In the absence of a history of surgery for structural CHD, APs probably underlie about 75% of all SVTs in children and account for about 95% of SVTs in neonates, compared with 30% to 40% of SVTs in adults. Atrioventricular nodal reentrant tachycardia (AVNRT) and primary atrial tachycardias (both reentrant and automatic) each appear to account for about half of the remaining SVTs (see Fig. 40.1 ). Atrial fibrillation is seen uncommonly in pediatric patients in the absence of structural or CHD (so-called “lone” AF) and only rarely is problematic enough to justify catheter ablation with pulmonary vein (PV) isolation. However, there are circumstances under which either ablation of an isolated ectopic PV focus or full wide area PV isolation are reasonable approaches. Further, an increasing number of older patients with CHD are developing atrial fibrillation as a consequence of aging and left ventricular dysfunction, leading to a need for more invasive approaches to management.

The Decision to Ablate: Safety Versus Efficacy

One overriding theme in the management of arrhythmias in children compared with adults is an emphasis on safety over efficacy. Although there are few cases at any age in which safety is not an important concern, the relatively benign course of many arrhythmic conditions in childhood, the potential disruption that therapies such as permanent pacing cause for a child, and the fact that parents are usually the decision-making surrogate for the child often lead to a different decision tree for children than for adults. Further, for some situations and technologies (e.g., the potential for coronary damage with RF energy application at the AV groove), the size of the patient and of the heart may be important.

Ablation in patients with Wolff-Parkinson-White (WPW) syndrome is an excellent example of how decision making may be highly age dependent. In this chapter, the term WPW will be used to describe the condition of preexcitation on the surface electrocardiogram (ECG), with or without coexisting tachycardia. Because of a variety of concerns, even the most symptomatic infant with WPW and paroxysmal SVT is only rarely a candidate for ablation. Myocardial injury and potentially severe coronary injury are more likely with RF ablation in this age group. Although the introduction of cryoablation has changed this situation somewhat, there remain a number of reasons to be cautious in managing infants with ablation procedures. Perhaps most important is that about 40% of APs in infants spontaneously stop functioning during the first year of life, and an additional third of patients are unlikely to have symptoms between infancy and early childhood. In children larger than 15 kg who have symptomatic arrhythmias, the balance between risks and benefits clearly shifts toward ablation therapy, but usually only if the ablation can be performed safely. In contrast to the situation in infants, even asymptomatic WPW patients between the ages of 10 and 18 years may be managed more aggressively than adults. Unlike asymptomatic adults older than age 28 years, who are unlikely to ever have symptoms, the older child with a high-risk pathway is exactly the type of patient who may present with sudden arrhythmic death as their initial symptom, leading to the recommendation that such patients should undergo risk stratification, with those who are high risk being offered catheter ablation as a therapeutic option. Further, the guidelines for sports participation in patients with WPW recommend considering risk stratification before approval for this age group. Other age-dependent differences in management decisions are addressed in the discussions of individual arrhythmias.

Use of Cryoablation in Children

Catheter-based cryotherapy was approved for use in the United States in 2003 for ablation of a variety of cardiac arrhythmias. Since then there have been a variety of reports in children, primarily focused on its use in AVNRT and other septal substrates. Cryoablation has several potential advantages over RF ablation, including (1) reversible cryomapping before the production of a permanent lesion; (2) adherence of the catheter tip to the endocardium upon freezing; (3) a well-defined edge of the cryolesion; (4) minimal effects on adjacent coronary arteries; and (5) a lower incidence of thrombus. The first four of these issues are particularly relevant to small children because of the close proximity of a variety of critical cardiac structures to the ablation target and the reported potential for RF lesion growth in immature myocardium. In fact, the most common major complication during RF ablation in pediatric patients is AV block, and there appears to be a higher potential for coronary artery injury in this patient group, even during slow pathway modification (see full discussion later under AVNRT).

Typical cryotherapy systems allow for both ice mapping at a tip temperature of –30 to –40°C where the catheter adheres and nearby tissue loses electrical activity but few cells are killed, and ablation at a tip temperature of less than –65°C where a lesion will be formed. Once cells freeze, they expand and burst. After 4 minutes at the ablation temperature, a typical lesion size is 3 to 6 mm in diameter, smaller than those seen for RF. One of the contrasting features of cryoablation compared with RF is that there is a much larger zone of reversibility as the lesion expands because tissue cooling above the freezing point will lead to loss of electrical activation well before the loss of viability. This feature has dramatically enhanced the safety profile in clinical trials to date. In fact, despite frequent use of the technology for septal tachycardia substrates, there are no reports of AV block with cryoablation, even in children as small as 20 kg, in the presence of a His potential ( Fig. 40.2 ).

The primary disadvantage of cryoablation is that inherent in its high level of safety is a smaller lesion size than for RF ablation. For ablation of septal tachycardia substrates (AV node modification, anterior and posterior septal pathways), cryoablation success rates have been statistically similar to those for RF techniques. However, most operators are less aggressive with RF in septal areas and have had to be highly aggressive with cryotherapy to achieve success. Further, with few exceptions, even aggressive application of cryoenergy has not yielded similar success rates to RF for ablation of nonseptal accessory pathways. Although the data is limited in very small children and infants, anecdotal evidence suggest that cryoablation may be effective for all accessory pathway locations in these unique patient groups. Given the aforementioned considerations, as discussed later for individual arrhythmia substrates, the use of cryoablation is most important for septal substrates, small children, and patients with abnormal anatomy where the precise location of the AV conduction system is not known.

Atrioventricular Nodal Reentry Tachycardia in the Child

The issue, addressed earlier, of rebalancing safety and efficacy in the decision-making process for a child is particularly important in the management of AVNRT. A number of factors that are distinctly different in children compared with adults must be considered. These factors are related to particular risk in children that may affect the decision to ablate, the diagnosis of dual AV nodal physiology and AVNRT, and the technical aspects of the ablation procedure.

Decision to Ablate and Safety Issues

Once a decision has been made to perform slow pathway modification in a child, identification of appropriate locations for modification is not particularly different from that in adults. Further, the ablation technique and end points for either RF ablation or cryoablation do not vary significantly in children and adults, with the exception that a smaller catheter should generally be used in smaller children (<20 kg or so) to minimize the lesion size. With the use of such techniques, it appears that more than 95% of AVNRT can be eliminated in adults or children. However, two safety issues and their implications should make a significant difference between adults and children in the decision to use RF ablation: the risk of AV block and the risk of coronary damage.

Atrioventricular Block

The possibility of ablation-induced complete heart block deserves special attention in the pediatric patient for two reasons. First, the risk of heart block is theoretically higher in smaller patients, simply because of a number of geometric differences. Because the size of an average RF lesion does not depend on patient size, the typical lesion is relatively large compared with the size of the heart in a smaller patient. The smaller the patient, the closer the AV node is to the area of the slow pathway and to the posterior and anterior septum, common locations for APs. Further, the AV node itself is smaller in small hearts and therefore can be included in a typically sized lesion more easily than in an adult heart. Despite these potential problems, the reported incidence of ablation-induced complete heart block does not appear to be significantly higher in infants and children than in adults. However, a second consideration is the technical details of pacing if heart block occurs. Superior vena cava thrombosis complicating transvenous pacing can occur with both single and dual chamber systems, leading most clinicians to place epicardial pacing systems in children who weigh less than 10 to 20 kg, and only single-chamber transvenous systems in patients weighing under 20 to 25 kg. For this reason, pacing may require chest surgery for epicardial lead placement and may leave the patient physiologically compromised by asynchronous pacing. In addition, the pacing system in a child may need to be maintained for as many as 70 to 80 years, and its presence will almost certainly restrict the child from many competitive sports, thereby setting lifestyle limits that are of less concern for most adult patients. These issues should affect both the decision to ablate and the energy choice for the procedure itself when ablating near the normal AV conduction system, if either the AV node or an AP is targeted. In particular, there are still no reported cases in the literature of permanent AV block with the use of cryoablation; however, a theoretical risk still exists.

Coronary Artery Damage—General Considerations for Atrioventricular Nodal Reentrant Tachycardia and Accessory Pathway Ablation

Acute and late coronary artery injury has been reported after RF ablation for AP-mediated tachycardias in children, adults, and animals. These cases have involved both left- and right-sided coronary arteries, as well as a posterior descending branch of the right coronary artery during ablation of a right posteroseptal AP.

In 2004 we reported a case of coronary damage during slow pathway modification in a 30-month-old, 15.5 kg child with recurrent AVNRT resistant to drug therapy. Approximately 100 seconds after a fourth application of RF energy, ST elevation was noted ( Fig. 40.3 ); it lasted about 15 minutes and was not accompanied by hemodynamic compromise or echocardiographic abnormalities. Selective coronary angiography revealed a dominant right coronary artery giving off a posterior left ventricular branch artery that had an 80% stenosis ( Fig. 40.4 ). The vessel course was within 2 to 3 mm of where the catheter tip had been placed during the last RF application ablation. Acute management was conservative, and after 2 days, repeat angiography revealed some improvement, with an approximately 50% stenosis. Repeat selective right coronary angiography 2 months later revealed complete resolution of narrowing (see Fig. 40.4 ), and repeat electrophysiology studies with and without isoproterenol failed to induce any AVNRT. ST segments remained normal with the isoproterenol infusion. Follow-up off medical therapy has been unremarkable.

Although this case is anecdotal, it reveals several important issues regarding the potential for coronary artery injury during RF ablation in children. First, coronary artery injury may occur with slow pathway ablation for AVNRT. Second, acute coronary artery injury has the potential to be missed and is probably an underreported phenomenon. Third, infants and young children may be at particular risk. The inflammatory component of tissue injury caused by RF energy has been shown to invade layers of the right coronary artery, leading to acute narrowing, when RF energy is applied to the atrial side of the lateral tricuspid annulus in pigs. Further maturation of this injury can result in significant late coronary stenosis. Therefore with RF energy application, coronary stenosis may occur acutely or be delayed. Our patient’s injury was almost missed because ST segment changes did not occur until 100 seconds after the last RF application and resolved spontaneously within minutes, despite a significant persistent stenosis of the involved artery. Other instances of coronary artery injury after RF ablation were also nearly missed because of this delay. In large retrospective and prospective studies in which there were no coordinated attempts to investigate coronary injury after RF ablation, the reported incidences of injury were 0.03% in children and 0.06% to 0.1% in adults. However, in a study in which coronary angiography was performed before and after RF ablation for AP-mediated tachycardias, Solomon et al. reported a 1.3% incidence of coronary artery injury in 70 patients. In fact, we have had an unreported case of 95% occlusion of the posterior descending artery in a 41-kg, 10-year-old patient undergoing ablation of a left posteroseptal AP in the proximal coronary sinus. The stenosis was completely asymptomatic without ECG changes and was discovered only because we performed routine coronary angiography pre- and post-RF ablation for posteroseptal AP locations. Therefore unless evidence for coronary artery injury is actively sought, it will go undiagnosed and underreported.

Smaller children may be at particular risk for coronary injury, because the distance between the ablation catheter and the coronary arteries is significantly less than in adults. Although coronary blood flow probably helps reduce this risk, the flow in small coronaries in any patient may be inadequate to prevent damage. Understanding these factors is critical to preventing injury.

Ablation Energy Cryo Versus Radiofrequency

Although RF energy is the time-honored ablation technology for slow pathway modification to treat AVNRT, over the last 5 years, since the publication of the first edition of this book, the use of cryothermal energy has gained gradual acceptance, particularly in the pediatric population for AVNRT. This growth in acceptance has come from a combination of the safety features of cryoablation discussed in the introductory aforementioned sections, and increasing evidence of equivalent acute success and recurrence rates compared with RF. Earlier reports did find lower acute success rates and higher recurrence rates for AVNRT therapy in both children and adults. However, as experience has accumulated and technology has progressed both acute and long-term results have improved to the point that multiple authors have reported acute success rates above 95% and recurrence rates below 5% with cryoablation, equivalent to those with RF energy. This improvement in outcomes has been caused in part by additional experience with cryo techniques, which appear to have a longer learning curve than that for RF techniques, but also to further refinements in the use of cryothermal energy with the use of additional freeze-thaw-freeze cycles and larger catheter tips. Further, as noted earlier, there are no reports of permanent AV block with cryoablation, despite its most common use being for ablation of septal arrhythmia substrates. The data discussed previously have led to multiple debates at scholarly meetings and conferences on the use of RF versus cryo energy for slow pathway modification in children. Consistent with a formal survey in the literature from 2011, informal surveys at these meetings suggest that despite the higher safety of cryo, because of longer procedure times with cryo and an excellent safety experience with RF, about half of the operators queried continue to use RF as first choice in all larger patients and reserve cryo for smaller patients or specific concerns with RF safety during individual procedures.

Optimal End Point for Slow Pathway Modification in Children

The optimal end point for slow pathway modification ablation in children is not particularly different from that in adults when RF energy is used—a maximum of single AV nodal echo beats. However, when the same end point was used in a large multicenter prospective trial comparing RF to cryoablation in adults, the recurrence rate for cryoablation was more than double that seen in the RF group (9.4% vs. 4.4%, P = .029). Despite some cryoablation studies showing similar recurrence rates with and without complete slow pathway ablation and elimination of dual AV node physiology as a procedural end point, many clinicians do use complete slow pathway ablation as an end point to reduce recurrences. Eliminating sustained SP conduction when it is present before ablation also appears to reduce recurrences. In summary, following RF or cryoablation, the presence of single AVN echo beats in the absence of sustained slow pathway conduction is an acceptable endpoint. My interpretation of available literature along with my personal experience suggest that eliminating all slow pathway conduction as an endpoint, which is safely achievable with cryoablation, helps prevent recurrences.

It is not uncommon that during EP studies performed under general anesthesia in pediatric patients with clinical ECG documented SVT, or palpitations consistent with SVT but without documentation, it is not possible to induce SVT during the procedure. Further, although AVNRT is strongly suspected, it may be difficult to conclusively demonstrate dual AV nodal physiology or AV nodal echo beats. Thus, when indicators of SP conduction are subtle or absent at the start of the case, determining when to terminate the procedure becomes much more challenging. As an example, a single AV nodal echo beat at the end of a case is a much less tolerable end point when this was the only positive finding at the start of the procedure. Multiple reports have demonstrated that anatomic slow pathway modification or complete elimination of all slow pathway conduction are end points that predict clinical success in eliminating recurrences of either documented SVT or sustained palpitations. In such cases, voltage mapping of the AV node region to identify the slow pathway may be useful.

Atrioventricular Nodal Reentrant Tachycardia in Children—Summary and Recommendations

Based on many of the aforementioned discussion points, as of the 2016 Consensus Statement, the indications for ablation to treat AVNRT in larger patients, defined as greater than 15 kg, are very similar to those in adults. In particular, for patients greater than 15 kg, the guidelines provide a class I recommendation: “ablation is recommended for documented SVT, recurrent or persistent, when the family wishes to avoid chronic antiarrhythmic medications” with a Level of Evidence C. However, in smaller patients less than 15 kg, the recommendation is class III if the SVT can be controlled with medication, and only becomes class I after failure of multiple Vaughan-Williams classes of antiarrhythmic medications.

When AVNRT is not inducible at the ablation study, the recommendation is class IIa for slow pathway modification, if there has been clinically documented SVT and there is evidence of dual AV nodal physiology during the study. However, it is class IIb if there are similar findings but without a history of clinically documented SVT. In both of these cases, it is recommended that cryotherapy be considered as first choice for the ablation. These recommendations are based on the data presented earlier that if an operator wishes to absolutely minimize the chance of coronary injury and AV block in children undergoing slow pathway modification for AVNRT, cryoablation is the preferred ablation methodology. Although RF energy is a reasonable choice for patients over 15 kg because of its high safety experience in many centers, cryoablation is strongly recommended for patients under 15 kg. In addition, if RF energy is to be used for children who weigh less than 15 kg ( Table 40.2 ), selective coronary angiography of the artery supplying the posterior septum should be considered before ablation. If a small coronary artery is within 2 to 3 mm of the expected ablation location, RF energy should not be delivered, and if any RF energy is delivered, acute follow-up angiography should be performed after ablation.

TABLE 40.2

Precautions for Ablation of Atrioventricular Nodal Reentry Tachycardia in Children

Cryoablation is the safest energy modality. For children weighing <15 kg: Before radiofrequency (RF) ablation, coronary angiography of the artery supplying the posterior septum should be considered. If a small coronary artery is <2–3 mm from an RF ablation site, delivery of RF energy should be used with a high level of caution. After RF delivery, postablation coronary angiography should be considered.

Preexcitation Syndromes in Infants

There are three special considerations in infants that make management of symptoms secondary to an AP different from those in an older child or adult. First, the risk of a sustained reentrant primary atrial tachycardia, such as atrial fibrillation, is very close to zero in the small, structurally normal heart, making the risk of sudden death in infants with WPW very low. Second, as noted earlier, AP function may spontaneously disappear by 1 year of age. Finally, the known risks of any catheterization, combined with the specific risks of catheter ablation in this age group, suggest that pharmacologic control should be aggressively pursued before ablation is attempted. This last issue deserves further discussion.

In humans, myocardial cell division probably occurs through approximately 6 months of age. Although this finding could potentially protect the myocardium from long-term complications secondary to early injury, ventriculotomy scars produced in newborn puppies, RF ablation lesions in immature lambs, and cryoablation lesions in infant swine appear to increase in size during subsequent development. In addition, in contrast to mature ablation scars from adult animals, late lesions from the neonatal lambs were often histologically invasive and poorly demarcated from the surrounding tissue. The potential clinical importance of these results is underscored by a reported sudden death 2 weeks after an AP ablation in a 5-week-old, 3.2 kg infant. An echocardiogram from the infant at the time of a brief resuscitation, as well as autopsy findings, revealed relatively large lesions extending into the left ventricle from the intended mitral groove ablation site ( Fig. 40.5 ). Another heightened risk in infants is coronary artery damage because of the potentially close proximity of the coronaries to the ablation catheter and the reduced capacity for protective cooling during RF application in any small coronary artery. Although most reports of coronary damage in the literature have been limited to the posterior septum or a nondominant right coronary artery, complete occlusion of the left circumflex artery was reported in a 5-week-old, 5.0 kg infant undergoing RF ablation of a left lateral AP.

Despite these disturbing cases, nonpharmacologic therapy will be necessary in a small subset of infants with AP-mediated tachycardia. Until accurate methods are available to assess lesion size in real time, alternative methodologies should be used in all infants. As discussed previously, data on the effects of cryotherapy suggest that this form of energy is at least less harmful to coronary arteries, even when they are in very close contact, because of the differing effects of cold and heat on connective tissue and the vascular inflammatory response. Coronary artery flow also protects the vessel through local warming during cryotherapy, similar to the way in which blood flow protects through local cooling during RF energy application.

Cryotherapy may offer other advantages for small children. Cryolesions are smaller than RF lesions, contributing to the ability to safely create cryolesions even adjacent to the His bundle. In addition, cryoablation allows for reversible loss of tissue function. In the largest cryoablation clinical study in small children, defined as children less than 15 kg or younger than 5 years of age, cryoablation was found to have a success rate of 74% and recurrence rate of 20%, whereas RF, which was used after failed cryoablation, had a success rate of 81% and a recurrence rate of 30%. Regardless of technique, these outcomes are not as good as in older children; however, importantly no complications were found in the cryoablation alone group, which was statistically lower than the 12.5% major complication rate for the RF after failed cryoablation group.

If technical or other considerations require the use of RF energy, considerable caution should be used. RF lesion size is related to catheter tip size, RF power, tip temperature, and lesion duration. Therefore the following technical modifications should be adopted ( Table 40.3 ): (1) deliver energy in as atrial a location as possible; (2) use a 5 French (5 F) catheter tip; (3) use low temperature mapping (50–55°C) to identify the correct location before higher-temperature RF application; (4) use a lower temperature set point of 60°C for the ablation lesion; and (5) use lesions of shorter duration (7–10 times the time to effect, with a maximum of 30–40 seconds). The future development of real-time ultrasonographic or other modality monitoring of lesion size may also help reduce the procedural risks.

The 2016 Consensus Statement provided multiple recommendations related to ablation in infants and small children. Most importantly, based on a variety of data, the document recommends an approximate cutoff of 15 kg to differentiate indications for ablation, recognizing that an exact size or age should not be specified for individual patients. Further, it specifies that catheter ablation in the very smallest children (3–7 kg and <6 months of age) should be reserved for life-threatening arrhythmias, or refractory ones after multiple failed attempts at medical management, which may include various combination therapies. Some of the general recommendations are that ablation should be performed by an experienced pediatric electrophysiologist who undertakes the various strategies to reduce risk described earlier. Further, the use of alternate sources of energy like cryoablation before RF should be strongly considered.