Zook N, DeBruler K, Ceresnak S, Motonaga K, Goodyer W, Trela A, Dubin A, Chubb H.Heart Rhythm. 2022 Feb;19(2):262-269. doi: 10.1016/j.hrthm.2021.09.031. Epub 2021 Sep 30.PMID: 34601128
Take Home Points
- Cryoablation for acute slow pathway modification or elimination can be accomplished safely and with very high acute success
- Recurrence of SVT following cryoablation for AVNRT remains a measurable mid-term outcome despite procedural refinements and high initial success rates
- In this study, the degree of pre-ablation SVT inducibility and post-ablation slow pathway activity were not predictive of post-ablation SVT recurrence
Commentary by Dr. Philip Chang (Gainesville, FL, USA) Congenital and Pediatric Cardiac EP section editor:
Catheter ablation is considered a safe and effective treatment for SVT in otherwise healthy individuals with low risk of complications as well as reasonably low incidence of recurrence. In the case of AVNRT, cryoablation has widely been adopted given its favorable safety profile in pediatric patients, however SVT recurrence has historically been higher than what has been seen following radiofrequency ablation for slow pathway modification or elimination. Efforts to further refine the procedure remain an active area of interest.
Zook et al examined their experience with cryoablation for slow pathway modification in pediatric AVNRT. This study is retrospective in design with review of procedural experiences and data from a single pediatric EP center. Patients <21yo were included if they underwent first-time cryoablation for the treatment of typical AVNRT with acute success between 2011-2019. Data and findings from ablation procedures were reviewed to identify factors that were associated with SVT recurrence following initial ablation success. The authors noted and included the incorporation of low voltage bridge mapping and ablation beginning in 2013. Subjects were further classified and studied based on pre-ablation SVT inducibility characteristics and degree of residual slow pathway activity, both being numerically designated (see Tables 1 and 2). A scale was developed to quantify the degree of change in AVNRT inducibility following ablation by subtracting the pre-ablation inducibility value from the post-ablation residual slow pathway value.
A total of 256 patients were included. There were no significant differences in general demographic characteristics between those with and without AVNRT recurrence. AVNRT recurrence occurred in 5% of patients (14 patients) with 64% of the recurrences occurring within the first year after ablation. A total of 44 cases were performed prior to the incorporation of low voltage bridge mapping and 5 of the 14 total recurrences were noted in this subgroup of patients. Among all included patients, a majority received isoproterenol before ablation (57%). There was no correlation between degree of inducibility pre-ablation and post-ablation recurrence. Most patients fell into either of the 2 extremes for post-ablation residual slow pathway activity – 152 patients (59%) without any identifiable slow pathway activity and 76 patients (30%) with some slow pathway activity defined as an echo beat without isoproterenol. Those without post-ablation slow pathway activity received significantly less cryoablation time/treatment compared to those who had some post-ablation slow pathway activity. Recurrence of SVT occurred in all 4 post-ablation slow pathway activity categories with the highest absolute number of recurrences among those with the lowest degree of residual slow pathway conduction after ablation. However, there was no statistically significant difference in recurrence rates between each of the 4 post-ablation endpoint groups. (see Figure 3) Furthermore, on multi-variate analysis looking at procedure duration, total cryolesion time, use of low voltage bridging, cryocatheter tip size, pre-ablation inducibility, and post-ablation slow pathway activity, there were no factors that were clearly predictive of future recurrence. Using the scale derived from pre-ablation inducibility and post-ablation slow pathway activity (ie. degree of change in AVNRT inducibility post-ablation), no association was seen between level of inducibility and actual recurrence.
This single center retrospective experience with exclusive cryoablation treatment for AVNRT demonstrates the midterm outcomes for this approach to ablation therapy in the hands of experienced operators at a well-established and moderate-to-large volume center. As with previously published data, acute success rates of ablation for AVNRT are very high. However, recurrences remain an observed outcome and have remained a perceived limitation of cryoablation vs. radiofrequency ablation for AVNRT. Zook et al have shown that cryoablation can achieve excellent outcomes without post-ablation heart block in pediatric patients, with a recurrence rate that falls toward the lower range of recurrence rates that have been reported in the literature.
There are multiple limitations to this study. First, the endpoint criteria evaluated are rather narrow, focusing primarily on pre-ablation inducibility vs. post-ablation slow pathway activity. Further definition of slow pathway activity before and after ablation, such as the presence or absence of sustained slow pathway conduction, was not incorporated as an endpoint. There was inconsistency in the performance of EP testing and ablation delivery as evidenced by the variable use of isoproterenol before and after ablation and the varying durations of cryolesions ranging between 4-8 minutes, which are not trivial differences. Follow-up duration was carried out to an upper limit of 3.7 years, though late recurrences beyond this timeframe are known to occur (which the authors do acknowledge as well) but were not included in this study.
The authors also noted that achieving an ablation goal of complete slow pathway elimination can lead to longer ablation times and longer procedure times. Yet, from their data, the shortest ablation times were in patients who achieved acute slow pathway elimination. The assumption is that further ablation was not delivered once slow pathway elimination was observed. However, the authors reported the highest absolute number of recurrences among patients that initially had elimination of slow pathway conduction. This brings into question the durability of lesions in these cases and, while lesions may have been placed in the critical sites, tissue damage and injury may have been insufficient to result in long-term elimination. As such, the issue may be less about acute post-ablation slow pathway activity and more an issue of long-term lesion durability.
The authors mentioned the vastly different cryoablation times between published pediatric vs. adult studies on AVNRT ablation but provided no suggestions from their findings as to how to understand this difference or potentially reduce cryoablation times in pediatric cases. Finally, a comparison to radiofrequency ablation using similar endpoint measures was not performed, which could potentially have been useful to determine if lesion durability may be a more dominant contributor to recurrence risk versus functional slow pathway behavior following ablation.
Endpoints following ablation for slow pathway modification or elimination for AVNRT can be viewed as either well established and predictive or debatable depending on practice patterns and experience and results from formal investigation. Both the location of ablation therapy and durability of lesions likely contribute to early and late findings following slow pathway ablation and likely impact the incidence of recurrence.