Should all-comers with atrioventricular nodal re-entrant tachycardia or selected patients only be treated with cryoablation?

This editorial refers to ‘Cryoablation of atrioventricular nodal re-entrant tachycardia: 7-year follow-up in 515 patients—confirmed safety but very late recurrences occur’ by P. Insulander et al., on pages 1038–1042

Atrioventricular nodal re-entrant tachycardia (AVNRT) is one of the most encountered arrhythmias in otherwise healthy people. Among supraventricular tachycardias, AVNRT accounts for >50% of the cases, and it is therefore one of the most frequently targeted arrhythmias in the electrophysiology (EP) lab. However, AVNRT is not completely innocent and may lead to syncope and predisposes to atrial fibrillation in the elderly.

There was a reason why more serious arrhythmias were initially tackled after the introduction of radiofrequency (RF) ablation than AVNRT: early investigators reported an incidence of up to 20% complete heart block when targeting the fast pathway. The observation that a selective ablation of the slow pathway was a possibility opened the road to safe interventional treatment of AVNRT with RF ablation.¹ This reduced the incidence of atrioventricular (AV) nodal block from >5% to <2%.² With growing experience, the incidence of this side effect became even lower than 0.5% in the observational series.

However, the risk did not become zero, due to various reasons. Furthermore, it is clear that procedures are sometimes not performed, as a risk of AV block is judged to be present, at the initial contact, or even during the procedure. Apart from experience, other factors may play a role, such as an abnormal anatomical position of the fast pathway. It remains difficult with RF technology to identify the place of the slow pathway with certainty. The never-resolved discussion about the character of the slow pathway potentials is an illustration of this fact; junctional tachycardia during RF delivery is explained as itching of the fast pathway and careful observation of normal conduction during this event is mandatory to avoid AV block.

In contrast, with modest freezing, one can identify the site of the slow pathway in a very reliable way, and concomitant preservation of normal conduction can be demonstrated with the so-called cryomapping approach.³ Therefore, focal cryoablation came as a real promise for safer ablation. In our hands, it was always as effective as RF ablation, and the uncommon late recurrences were often associated with the use of larger tips (which make local recording less accurate).^3,4 In >173 consecutive patients, not a single heart block out of the EP lab was observed, even when temporary block was observed while freezing. A much more impressive series without lasting heart block was published by Insulander et al. in 2014.⁵ The fact that no late AV block occurred over 24 months while this occurred in a similar RF series at a rate of 0.4% seems not enough to convince the RF adepts to change their minds.⁶

The Swedish study published in this issue of EP-Europace reports (after an observational, uncontrolled study design) very late recurrences after cryoablation.⁷ I have recently observed the same after RF, and this seems to me the natural history of this disease. The existence of multiple slow pathways is one explanation; the paroxysmal (or occasional) character of the arrhythmia is another one.

Let us analyse the randomized trials, and the more recent meta-analyses, directly comparing RF and cryoablation. The only potential disadvantage of cryotherapy is a slightly higher recurrence rate.⁸ This is balanced by some advantages. The actual rate of procedure-related heart block is 0.75–0.87% in RF vs. 0% in cryoablation.^9–12 As these data come from reliable, experienced centres and controlled trials or studies, the first number might be rounded off to 1% in the real world, a rate high enough to discuss with every patient before the procedure. Late arrhythmia, including the small amount of late heart block, is not uncommon. It is striking that on top of excluding high-risk patients (the young, women, etc.) from an RF procedure, cryoablation is used after failing RF by very experienced investigators.¹³

Recommendations to keep cryoablation for the young and those at high risk for heart block seem to be valid, as no one wants to pace 20-year-olds for the next 80 years. However, learning curves exist for all procedures. In order to perform an effective cryoablation, one should be experienced, able to interpret signals, and understand cryobiology, including cryomapping during programmed electrical stimulation and tachycardia. This aspect was clearly part of the approach of the Swedish group.⁷

I do wonder how we will get there if cryoablation is kept only for the exceptional patient and the young. If we really want to make progress with ablation of this very prevalent arrhythmia and if we want to treat all symptomatic patients with this disease (predisposing at higher age to atrial fibrillation), we should become very liberal with performing cryoablation—the safer approach.

This will lead to 4% redo ablation—a technique with less radiation than RF—(and not to multiple pacemaker re-interventions in 1%), but who cares?

Funding

Conflictof interest: none declared.

References

Author notes