Key Points
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Cryoablation has advantages because of the unique properties of lesion formation in small, immature hearts.
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Radiofrequency ablation may have a small chance of a complication that may be reduced by cryoablation including atrioventricular (AV) block and coronary artery injury.
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Cryoablation has similar efficacy in ablation of pediatric AV node reentry tachycardia.
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Cryoablation is not as effective for ablation of accessory pathways in pediatrics, except in accessory pathways in proximity to the AV node.
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Cryoablation may be useful in other pediatric arrhythmias such as permanent junctional reciprocating tachycardia and junctional ectopic tachycardia.
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Epicardial application of linear cryoablation lesions may reduce atrial arrhythmias in patients with complex structural heart disease.
Cryoablation has been used in pediatrics since the initial inception of ablations. In fact, some of the initial surgical ablations in children were done with cryoablation. The ability to perform transvenous radiofrequency ablations revolutionized the treatment of not only adults but children and adolescents with supraventricular tachycardia in the 1990s. Now that the technology has evolved to allow transvenous ablations using cryoenergy, the number of applications in pediatric ablations has greatly increased. The first study of transcutaneous transcatheter cryoablation as a treatment for pediatric supraventricular arrhythmia was conducted in 2004, and the use of cryoablation in pediatrics has greatly expanded since that time.
Perhaps one of the most compelling uses of cryoablation is in pediatric ablations. In some pediatric catheterization laboratories, cryoablation is considered to be an essential element to safely and effectively perform ablations. Although ablations in pediatrics can be performed with a high success rate and a low complication rate (4.3% for electrophysiology studies and 2.9% for the ablation procedure) compared with adults, adverse events can happen during the procedure, the most common being hematoma formation at the catheter entry site, atrioventricular (AV) block, and damage to cardiac valves. There are multiple advantages to using cryoablation in children. With the smaller size of the heart in addition to the relative thinness of the walls of both the atria and ventricles in the pediatric population, the potential for inadvertent damage of tissue both within and outside the heart may be decreased with cryoablation compared with radiofrequency ablation.
Atrioventricular Block
The major advantage of using cryoablation in pediatrics is the decreased incidence of complete AV block. Because most patients with supraventricular tachycardia have no other structural or functional limitations, the potential need for a pacemaker and subsequent follow-up, as well as lead and generator replacement for multiple decades, makes the additional safety of cryoablation desirable. To date, there has not been a reported case of permanent AV block using cryoablation, if energy application is terminated immediately after an effect on the AV node is seen. A prolonged PR interval and bundle branch block after cryoablation have been reported in pediatric patients.
Although adult studies show that the risk rate for complete AV block with radiofrequency ablation is between 0.1% and 1%, the risk is around 1% to 2% in children. The greatest risk for AV block is ablation of an accessory pathway in the parahisian region. The risk rate for AV block with radiofrequency ablation in adults in this region is around 2%. In the pediatric registry, the risk rate is around 3% in the midseptal region and is around 9% if the pathway is truly parahisian. AV block has also been reported in other locations in the pediatric population, with an incidence rate of around 1% in the right posteroseptal region and 3% in the right anterior/anteroseptal region. AV block has also reported in right posterior, right lateral, left lateral, and left paraseptal regions. In pediatric radiofrequency ablations, AV block has been seen from 5 seconds to 2 months (mean, 4.1 days; median, 15 seconds) after the onset of the energy application and may be permanent or transient, lasting 1 hour to 1 month.
Another potential advantage is in ablation of congenital heart disease where the conduction system may be displaced into an abnormal position such as in patients with an AV septal defect or in patients who have right atrial isomerism and have two distinct AV nodes (twin AV nodes or “Mönckeberg sling”). Another potential advantage is in patients with underlying deficiencies in the function of their AV node, such as patients with corrected transposition of the great arteries. The reversible nature of cryoablation may prevent inadvertent AV block in these unusual circumstances.
Coronary Artery Damage and Thrombus Formation
Because of the documented safety of cryoablation adjacent to the coronary arteries and its limited endothelial disruption, ablation in areas around the coronary arteries is likely to be safer than with radiofrequency ablation. There are multiple case reports of coronary artery injury in children during radiofrequency ablation. Some of these coronary artery injuries from radiofrequency ablation are related to thrombus formation. Other incidences may be caused by spasm or damage to the artery itself. Many cases of coronary artery injury in children may not be detected because general anesthesia is frequently used during the procedure (no acute chest pain). Frequently, there are also T-wave changes from T-wave memory in patients with Wolff–Parkinson–White after their ablation that may make determination of ischemia difficult. In a systematic study of coronary artery effects during ablation procedures, 2 of 117 children who underwent radiofrequency ablation for an accessory pathway (all posteroseptal) had an acute reduction in diameter of coronary artery adjacent to the ablation site on angiography. ST-segment changes were seen on the electrocardiogram with normalization in all of the patients within 1 week. The two patients who were noted to have coronary artery narrowing were clinically asymptomatic with a normal echocardiogram, once again emphasizing the difficulty in detecting coronary artery injury in children in the periablation period. In this same study, there was no coronary artery damage after ablation for AV nodal reentrant tachycardia or after ablation with cryoenergy. Cryoenergy can likely be used in very small patients with minimal risk to the coronary artery circulation. In a study of 5-week-old piglets receiving cryoablation with 6-mm tip catheters, there was no evidence of stenosis by coronary angiography or ultrasound after the ablations.
Although the most common area of injury to the coronary arteries occurs in the right posteroseptal region where the right coronary runs very close to the tricuspid valve annulus, damage to coronary arteries has been seen in multiple locations, including the left circumflex coronary artery.
An additional area of interest for using cryoablation is in the coronary sinus. Because of the low blood flow in the coronary sinus, thin wall, and proximity to the right coronary artery, cryoablation may be particularly useful in this region in adults. This is further amplified in children because of the minimal distance between the right coronary artery and the coronary sinus, and the relatively small size and low flow in smaller sized patients.
Because of the documented safety of cryoablation, ablation even within the branches of the coronary sinus is likely to be safer than with radiofrequency energy. There is as high as a 25% incidence rate of mural thrombus formation with radiofrequency ablation lesions placed in the coronary sinus. Cryoablation lesions can be produced in the absence of significant thrombosis within the coronary sinus or the adjacent artery. In addition, there appears to be no long-term development of stenosis. In patients who underwent cryoablation distally in the coronary sinus, angiography of the left coronary circumflex artery and coronary venous system was performed at 12-month follow-up using cardiac multislice computed tomography, and no coronary stenosis or anatomic anomaly was visualized. A study of cryoablation of accessory pathways in the coronary sinus in children showed no complications from these ablations. There was, however, a relatively low acute success rate with cryoablation (71%) and a high recurrence rate (40%).
Another potential advantage of cryoablation is to minimize thromboembolic risk. Approximately 60% of accessory pathways are on the mitral valve annulus. Ablation on this annulus disrupts the endothelium and has the potential for forming a thrombus that may then embolize to cause a cerebral infarction. The reported risk for a cerebrovascular embolus after a radiofrequency ablation on the left side in pediatrics is around 7 in 10,000 (0.07%). The stroke may not manifest until 8 to 10 hours after completion of ablation lesions. Cryoablation has the advantage of a decreased potential for acute thrombus formation and late thrombus formation that likely occurs from disruption of the endothelium up to several hours after completion of a radiofrequency lesion. The decreased potential for thrombus formation may also be important in patients with structural congenital heart disease and an intracardiac shunt, patients with a congenital or acquired hypercoaguable state, or patients with decreased blood flow that may be at risk for coagulation, such as patients with cardiomyopathy or ablations within a Fontan circuit.
Unique Lesion Properties in Pediatrics
There are some unique concerns about radiofrequency ablation pertaining to the pediatric population. The first is lesion expansion of radiofrequency lesions, which is likely more pronounced in the immature heart. A study on infant lambs undergoing radiofrequency energy application showed that although acute lesion formation in immature sheep myocardium is similar to that seen in the adult myocardium, there is late lesion enlargement and fibrous tissue invasion of the normal myocardium. Although not seen with lesions placed on the AV groove, atrial lesions expanded in width (average, 164%) and ventricular lesions expanded in both width (171%) and depth. Although there are no similar studies with cryoablation, the amount of fibrosis and local tissue damage is less with cryoablation and likely does not expand to the same degree as radiofrequency lesions. Because both the atrial and ventricular myocardium are thinner in younger patients, the potential for collateral damage to surrounding tissues may also occur with radiofrequency ablation. In an animal study in piglets, there was clear damage to lung tissue overlying an ablated intracardiac target by the radiofrequency energy delivered. Although there has not been any defined long-term consequence of this collateral injury, there have been reports of both pleural and pericardial effusions after radiofrequency ablation. Although damage to collateral structures including the phrenic nerve has been reported with cryoablation, the incidence is likely decreased and there may be a greater incidence of long-term recovery because of the lack of extensive cellular destruction with cryoablation. In adult studies, inadvertent damage to esophageal tissue is minimized using cryoablation when compared with radiofrequency ablation. This is likely to be magnified in the pediatric population where the amount of tissue between the ablation catheter and surrounding collateral structures is markedly decreased. In multiple patients at Texas Children’s Hospital who have undergone a cardiac surgical procedure within 1 month after cryoablation, there has been no evidence of cryoablation lesions placed either on the intracardiac surface or in the surrounding tissues including the lung and esophagus.
Cryoablation of Accessory Pathways
One of the main targets of ablation in the pediatric age group is ablation of accessory pathways. Accessory pathways may be manifest (such as Wolff–Parkinson–White or Mahaim fibers) or concealed (unidirectional retrograde accessory pathway).
Cryoablation has been used successfully for ablation of accessory pathways in the pediatric population. The acute success for cryoablation of accessory pathways in pediatrics combining all of these studies is 237 of 317 (75%). The initial success rate with radiofrequency ablation of accessory pathways in pediatrics was 83%, but subsequently the larger cohort studied in the Pediatric Radiofrequency Ablation Registry showed success in 1696 of 1801 (94%) ablations. Although the overall success rate of ablation of accessory pathways is lower with cryoablation, the success rates for pathways located in the anteroseptal, midseptal, or parahisian region in the above studies was 105 in 126 (83%). In comparison, the success with radiofrequency ablation in these locations is lower (74%) in the initial multicenter report of pediatric radiofrequency ablation. When taking the number of overall cases, the relative success of cryoablation may be even greater because radiofrequency ablations may not even be attempted in pathways with close proximity to the AV node. With cryoablation, it is possible to attempt ablations even in positions where there is a large His signal seen on the ablation catheter as the chance of permanent AV block with immediate termination of the lesion after noting an effect on the AV node is very low ( Figure 18–1 ). Often, the site of successful ablation of the accessory pathway and the site of effect on the AV node are only millimeters apart. The acute success of ablation for both cryoablation and radiofrequency ablation is dependent on the location of the accessory pathway ( Table 18–1 ). One of the potential reasons for the decreased success of cryoablation compared with radiofrequency ablation is the nature of some accessory pathways. It is not uncommon to have an orthogonal orientation in some accessory pathways. In many cases, it is possible to eliminate antegrade conduction while leaving retrograde conduction intact or vice versa. Radiofrequency ablation catheters move as the heart beats, and respirations occur during the ablation lesions, making a broader lesion. Because the cryocatheter does not move during ablation, it may be necessary to place multiple lesions around the initial lesion to ensure that all inputs are permanently eliminated. However, the adherence of the cryoablation catheter to the tissue during the lesion may also have advantages in pediatrics. As the AV node frequently functions very well in pediatrics, mapping of an accessory pathway can be challenging because of fusion between the AV node signals and the accessory pathway signals. When mapping in supraventricular tachycardia (vs. delta-wave mapping or mapping with ventricular pacing), the retrograde atrial activation sequence obtained is purely from the accessory pathway and may therefore be mapped more precisely. When ablating in supraventricular tachycardia, the ablation catheters, with curves designed for adult hearts, frequently will move significantly with the abrupt termination of tachycardia at a successful ablation site, not allowing placement of a full lesion at the site an effect is seen. It may be challenging to attempt to replace the ablation catheter in the exact position of the initial success. With cryoablation, mapping and ablation can be performed while the patient is in supraventricular tachycardia without concern about catheter dislodgement.
| CRYOABLATION | RADIOFREQUENCY | ||
|---|---|---|---|
| INITIAL (1994) | LATE (2004) | ||
| Right antero/midseptal | 78% | 74% | Right septal 89% |
| Right posterior/posteroseptal | 64% | 85% * | |
| Right free wall | 60% | 69% | 90% |
| Left free wall/posterior | 90% | 89% | 98% |
| Left posteroseptal | 55% | 85% * | 88% |
The recurrence rate after cryoablation of an accessory pathway in children is around 25% (range, 3% to 45%) and varies widely from center to center. The reported incidence rate of recurrence with radiofrequency ablation is 11% at 1 year (range is 5% to 25% depending on pathway location). Table 18–2 compares recurrence with cryoablation and radiofrequency ablation.
| RECURRENCE | CRYO (1 yr FOLLOW-UP) | RFA (1 yr FOLLOW-UP) |
|---|---|---|
| Right anter/midseptal | 18% | 25% |
| Right posterior/posteroseptal | 17% | 25% |
| Right free wall | 12% | 16% |
| Left-sided | 40% | 9% |
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