AVRT is the most common type of SVT in neonates and infants, while focal AT and permanent junctional reciprocating tachycardia are less common chronic forms of SVT. Postoperative atrial flutter within the first year after neonatal CHD surgery, as seen in case 6, is extremely rare. Generally, these conditions have a favourable prognosis and often resolve spontaneously over time, though RFCA may be necessary in selected cases [2, 3, 5, 13, 14]. At our institution, we perform approximately 80 to 100 pediatric ablation procedures annually; however, RFCA has only been conducted in thirteen infants under one year of age in the past 20 years, including this recent consecutive series of six patients. The youngest was a premature newborn, weighing 1.9 kg, with an incessant AVRT (not included in this series) [15].

The number of RFCA in neonates and infants is relatively high at our institution, as we receive referrals from other centers in the Netherlands for the treatment of fetuses and neonates with drug-resistant SVT. These referrals are made for direct fetal therapy by intra-umbilical anti-arrhythmic drug treatment or postnatally for ablation, if necessary. However, based on the population-based study of Turner et al. [1] (incidence of infant SVT 22 cases per 100,000 live births), approximately 800 cases of neonatal SVT can be expected over 20 years in the Netherlands. The 13 infants who underwent ablation in our centre make up around 1 to 2% of the total infant SVT cases in the Netherlands.

The success of RFCA in infants is well-established in experienced medical centres and can be lifesaving [8, 16]. However, safety concerns remain, particularly for smaller infants and neonates, due to insufficient data and conflicting results. Initial findings from the Pediatric RFCA Registry suggested that the risk of severe complications—including complete AV block, perforation and/or pericardial effusion, valvular or coronary damage—is higher in children weighing less than 15 kg compared to older children [17]. This was confirmed in subsequent research [9]. Contributing factors include the smaller heart size, proximity of critical structures to the arrhythmia substrate, limited vascular access, thin-walled vessels and cardiac chambers, and catheter designs that are not ideal for this population. Severe complication rates as high as 4–8% have been reported in the youngest patients [13, 18,19,20], although these were not significantly different compared to older children, possibly due to the relatively small sample size in this age group. Nonetheless, there is general agreement that the risk of severe complications is usually higher in the youngest patients [8, 21].

While AV valve damage caused by RF energy or coronary lesions is a major but infrequent complication in infants [12] mild and transient AV valve regurgitation—particularly from the mitral valve—is relatively common following paediatric ablation procedures, as also evidenced in one of our cases, which completely resolved during follow-up [22,23,24].

Animal studies highlight the concerns regarding RF ablation in small hearts. Early research involving RF use in lambs showed significant expansion of lesions in the atria and ventricular free wall myocardium as the animals grew, while demonstrating only a limited increase in RF-equivalent lesions in the AV annuli [17, 25]. Similar findings were observed when cryo-energy and RF energy were applied to piglets, as AV groove lesions did not significantly increase with either energy modality. This offers some reassurance regarding the safety of AVRT ablation in infants [11]. However, the close proximity of the right and left circumflex coronary arteries to the endocardial AV annuli remains a concern [12, 22, 26]. Studies in piglets using RF ablation at the tricuspid and mitral annulus have shown a high incidence of both acute and late coronary lesions [27]. Notably, these lesions appeared to be independent of the number of RF applications and the maximum temperature achieved (60 °C) [28]. In contrast, cryoablation studies in animals with immature myocardium reported a significantly lower risk of coronary lesions [29,30,31].

Given these data, RF energy should be used with caution in infants. For example, using 5 Fr RF catheters and ablation cycles of 15–30 s with a low power limit of 20–30 W and temperature-controlled mode of 55 °C may reduce risks [15, 21, 25, 32, 33]. However, due to the limited steerability of the flexible 5 Fr design, we often had to switch from 5 Fr ablation catheters to 7 Fr catheters. Based on our experience, we now prefer to start with a 7 Fr small curve RF catheter. Prioritizing safety by using smaller RF catheters in combination with lower power settings (less than 50 W), along with shorter RF application durations, may reduce the risk of complications, but this approach can compromise efficacy, as some studies suggest a higher recurrence rate [34]. In one case, we also initially achieved successful ablation of a left posterior AP using a 5 Fr catheter with low RF energy (55 °C, max 20 Watts), but it recurred within a week. For the redo procedure, we used a 7 Fr RF catheter for better stability, with low temperature and power settings (30 Watts) to permanently interrupt the pathway. Cryoablation is potentially safer than RFCA and our first-line approach for ablating AV nodal re-entrant tachycardia and APs near the His region in children. However, while cryoablation appears particularly promising for infants, the cryo-catheter’s long radial curvature and limited flexibility make it challenging and precarious to navigate within small cardiac chambers. As a result, RFCA remains the preferred option in most cases [23, 35,36,37].

Another adjustment to ablation procedures in infants to avoid vascular damage and cardiac perforation is to reduce the number of catheters [7, 15, 19]. In young infants, an ablation catheter and an additional diagnostic catheter are typically sufficient, utilizing both the left and right femoral vein. A second diagnostic catheter can be positioned in the esophagus to replace the RA catheter or, as we did in most cases, a single 5 Fr six- or ten-pole catheter can be placed in the RV apex for pacing and recording RA, His bundle, and RV signals. In infants with Wolff-Parkinson-White syndrome or focal atrial tachycardia, a single RF catheter may suffice [15]. If the tachycardia mechanism is not easily identifiable, multiple catheters may be necessary, and additional vascular access, such as the jugular vein, should be considered. For left-sided substrates, an antegrade approach through a PFO or via transseptal puncture is recommended; we prefer using a J-shaped transseptal guidewire (SafeSept, Pressure Products, Inc., USA) or a RF wire to minimize the risk of complications during the puncture. In infants, the retrograde approach should be avoided due to the increased risk of arterial and aortic valve complications.

EAM systems significantly reduce radiation exposure by providing real-time visualization and guidance for catheters, as well as enabling anatomical reconstruction. The EnSite Precision system (Abbott, St. Paul, MN, USA), in particular, supports mapping and ablation catheters smaller than 7 Fr, making it potentially more suitable for newborns and infants. However, challenges remain in this age group, such as the large size of adhesive patches. It is important to carefully plan the placement of ECG electrodes, impedance field patches, and RF grounding pads to prevent suboptimal data acquisition due to patch overlap. Another concern is the heightened risk of signal and EAM distortion in very small patients, which may arise from limitations in field scaling or changes in impedance fields during procedures [38]. Most paediatric ablation programs, including ours, routinely utilize EAM systems and aim for (near) zero fluoroscopy. However, we believe that fluoroscopy remains necessary in these very small hearts to mitigate the risk of complications such as atrial or ventricular perforation, in addition to monitoring of endocardial signals.

Given the specific requirements for the use of catheters, energy settings, and EAM mapping system in neonates and infants, we think these procedures should only be performed by operators who have experience in interventional catheterisation in this young age group and electrophysiological procedures in older children. These procedures should therefore be embedded in a larger program with a dedicated team for interventional and electrophysiological procedures in children.