An accessory for pathway ablation

Abbreviations

RF

Radiofrequency

A student of electrophysiology feels unbridled joy when after detailed mapping and performing appropriate diagnostic maneuvers, a single ablation lesion regardless of energy source successfully ablates an accessory pathway with easily recognized loss of antegrade pathway conduction. Indeed, a generation of electrophysiologists can trace when they fell in love with our field to such a procedure early in their training or career.

While still an expectation, such results are not universal, and electrophysiologists need to have a well-organized approach for redo pathway ablation to be successful at a second or third procedure.

In this issue of Indian Pacing and Electrophysiology journal, Leung et al. [1], report on the use of two cryo catheters concurrently to treat the resistant accessory pathway. They report on a series of seven patients where, in general, results were significantly improved compared to a first procedure.

Where in our armamentarium of tools and techniques for the invasive management of recalcitrant arrhythmias do we place the approach they describe? To answer this specifically for accessory pathway ablation, we need to understand the modes for failure at first attempt.

1. Why do we fail with pathway ablation?

Early in the history of catheter ablation, several reports taught us that success is an expectation largely borne out with individual operator experience [2]. However, difficulties remain.

1.1. Inexact mapping

Much like hunting wild game, pulling the trigger with an effective shotgun is an easy part in the exercise. Careful and detailed mapping to know exactly where the culprit or target lies and placing the catheter at exactly that site is critical. Despite advanced mapping techniques, many contemporary electrophysiologists fail to take the time and effort to identify the pathway potential at the annular location prior to delivering energy. Indeed, in this report, there is mention of prior ablation targeting the earliest atrial electrogram during orthodromic tachycardia. Further, even if the pathway potential is identified, clarifying whether the signal of interest is recorded from the distal or ring electrode can make the difference between success and failure since energy delivery is only from the distal electrode [3].

1.2. Contact

Stability of the catheter with adequate tissue contact is critical for annular ablation. The annulus is at a slightly different plane from the larger ventricle and the thinner atrial tissue. In addition, valve movement, especially in patients with prolapse or dysjunction, can make catheter stability extremely challenging. Further, stability even if meticulously obtained while mapping with energy delivery such as during tachycardia may change to suboptimal or no contact when the tachycardia terminates and the geometry of the heart changes. Several maneuvers have been described including ventricular pacing to entrain orthodromic reciprocating tachycardia or analogous maneuvers in antidromic reciprocating tachycardia, but challenges still remain.

1.3. Experience

With the emergence of pediatric electrophysiology and many well-trained, outstanding colleagues now practicing, adult electrophysiologists see less of accessory pathways, particularly in the United States and indeed may be much more comfortable with pulmonary vein isolation or epicardial substrate ablation than they are with the nuances of pathway ablation.

1.4. Neighboring sensitive structures

Despite perfect mapping and excellent contact, even the most experienced proceduralist may choose to not ablate if pathway ablation is near a sensitive structure such as the compact atrioventricular node or a coronary artery [4]. At times, nuanced mapping such as defining the pathway slant or isolating atrial or coronary sinus tissue without actually ablating the pathway may be effective but represent highly challenging procedures [5].

2. McIntosh apples vs Mandarin oranges

For medical practitioners to understand the value of a specific approach or technology, a valid comparison, preferably where a single variable changes, is ideal for us to know when to use the new approach best.

As the authors acknowledge in a limited, nonrandomized study, several variables change from the index procedure and the redo procedure. It is difficult to compare or control for site of energy delivery, adequacy of contact, completeness of mapping, etc. However, a fundamental issue in this report is that single catheter radiofrequency (RF) ablation is compared to two catheter cryotherapy. In other words, we don't know if the second site of energy delivery is what made the difference or whether cryotherapy was just the right energy source for pathways in these specific locations. If studied further, even in a nonrandomized study, complete understanding of prior mapping and perhaps ablating with cryoenergy using a single catheter first when RF was used previously and then the second catheter after showing that indeed the critical accessory to success was the dual catheter approach may be more revealing.

3. Two electrodes, one target

Whether we use RF or cryoenergy, ablation is affected with thermal energy (heat or cold) at the site of tissue electrode contact. Thus, is there a difference with concurrent two electrode, two catheter cryo (or RF) energy delivery vs. sequential lesions at the two sites of interest?

While impossible to know the answer to this question from this study, we may hypothesize that one catheter maintains contact at the annulus and valve while the second is incrementally more stable at the site of pathway location.

An alternative hypothesis akin to bipolar RF ablation [6] is whether by using two catheters a penumbra of sub tissue necrosis level cooling has additive effects from the penumbra of the second catheter allowing tissue death and simply a fundamentally larger lesion. Indeed, if two electrodes are better than one, what about three or four to simply create large lesions? It is perhaps telling that the authors chose to title their paper as “Multi-catheter cryo therapy” rather than dual-catheter therapy, which they demonstrate in the paper.

Although a nonthermal approach, bipolar electroporation where two disparate electrodes (on one or two catheters) can potentially modulate the electrical field and thus energy density for electroporation and indeed several trials and clinical use registries are presently examining this question [7,8].

4. Learning from nonrandomized small studies

Given the multiplicity of variables, lack of control, and thus the impossibility to conclude exactly what the benefit of the additional catheter was in these patients, it is likely that authors of similar studies may find it difficult to publish their work in most leading electrophysiology journals. However, in practice, single reports or uncontrolled studies teach us as the authors clearly state in their limitations that a new approach is safe and can be effective when we deal with the difficult redo accessory pathway procedure.

Leung et al. [1], approach on such a proof-of-concept finding that may well be a go-to technique for ablationists around the world when confronted with recurrent pathway conduction after first procedure. The editors of this journal should also be commended for publishing a small series which leaves us with many questions but clear, recognized, potential, practical value.

Funding

None.

Disclosures

Speaking and research: Johnson&Johnson, Biotronik, Boston Scientific, Medtronic Mayo Clinic has pursued protection of intellectual property in the form of patents and patent applications naming Dr. Asirvatham as an inventor related to devices for cardiac ablation, rhythm management, autonomic modulation, including the use of deep learning in cardiology.