Methods

We performed an institutional review board–approved retrospective review of the pediatric electrophysiology database at the Medical University of South Carolina (Charleston, South Carolina). Data were collected on all patients <21 years old who had been diagnosed with AVNRT and treated with cryoablation. The patients had to have had ≥1 year of follow-up after ablation for inclusion in the present study (procedure dates from May 2003 to October 2008). Patients who did not attend follow-up or receiving follow-up out of state were excluded.

All electrophysiologic (EP) studies were performed with the patient under general anesthesia. EP catheters were routinely placed in the coronary sinus, in the right ventricular apex, and at the His bundle. Atrial extrastimuli pacing protocols were performed to identify dual atrioventricular (AV) nodal physiology (defined by an increase in the AH interval of >50 seconds after a 10-ms decrement in atrial extrastimuli pacing or PR > RR during incremental atrial pacing ) and to induce tachycardia. When necessary, isoproterenol infusion was used to aid in the induction of AVNRT. The slow pathway was targeted anatomically for each case. The immediate effect of cryotherapy was evaluated by either ablating during tachycardia or performing atrial extrastimuli protocols during the application. After successful ablation, additional EP testing was performed for a minimum of 30 minutes to ensure success. Acceptable end points for the ablation procedure were an inability to induce AVNRT and either no evidence of slow pathway conduction or some residual slow pathway conduction with or without single AVNRT echo beats. After 5 hours of postprocedure observation, a 12-lead electrocardiogram and echocardiogram were performed, and the patients were discharged.

The cryocatheter used for each procedure was determined by patient size and electrophysiologist preference (7Fr Freezor, 7Fr Freezor Xtra, or 9Fr Freezor Max, Cryocath Technologies, Quebec, Canada). For the purposes of the present study, a cryomap lesion was defined as a cryo-application reaching a temperature of 0 to −30°C. A cryoablation lesion was defined as a cryo-application reaching a temperature of about −70°C. Cryoablation applications of 2-, 4-, and 6-minute durations were separately tallied for the study data.

The primary outcome was freedom from recurrence of supraventricular tachycardia (SVT) after ablation. Recurrence was defined as documented SVT on the electrocardiogram, Holter, or event monitor at any point after procedure completion. The secondary outcome measures included immediate and late-occurring complications.

Routine follow-up with a pediatric cardiologist was scheduled for all patients at 2 and 6 months and 1 and 2 years after ablation. Electrocardiograms were routinely performed at these visits. Patients with symptoms of palpitations were evaluated with a Holter monitor and/or cardiac event monitor. Patients free of SVT recurrence at the 2-year follow-up visit after ablation underwent electrocardiogram, echocardiogram, exercise testing, and Holter monitoring and were typically discharged from pediatric cardiology care if no specific reasons were found for continued follow-up.

Results

During the 5 years of cryoablation use at the Medical University of South Carolina, 73 patients had AVNRT treated with cryoablation. All patients had acute procedure success (100%); however, 3 patients required both cryoablation and radiofrequency (96% cryoablation success rate) and were excluded from the present review. Another 9 patients were excluded, because they were lost to follow-up. The remaining 61 patients were included in the present study ( Table 1 ).

The mean interval since ablation was 3 ± 1 years (range 1 to 5). During this period, 4 patients had a documented recurrence of AVNRT ( Table 2 ). One patient developed recurrence 2 months after ablation, 2 patients at 1 year after ablation, and 1 patient at 2 years after ablation. Of the 4 patients with recurrence, 3 underwent successful repeat ablation. The patients with recurrence were statistically similar to those not experiencing recurrence in age, weight, presence of congenital heart disease, use of 3-dimensional mapping, number of cryomapping lesions used, number of cryoablation lesions used, and cryocatheter tip size. Before the initial ablation, 2 of the 3 patients with later recurrence had experienced SVT multiple times per month, and the third had experienced SVT every couple of months. The 4 patients with recurrences had had no evidence of slow pathway conduction during postablation testing.

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