Frozen too: Adopt the new or let it go?

Pulmonary vein isolation (PVI) has been established as the mainstay of interventional management of atrial fibrillation, as has the utility of cryoablation as one of two standard thermal techniques for achieving PVI.1 The efficacy of cryoballoon ablation is known to be comparable to radiofrequency ablation, however novel technologies within the realm of contemporary cryoablation have yet to be explored.2, 3 Ellenbogen et al. report the results of an international multi-center, open-label, prospective, single-arm study on the safety and performance of PolarX™ (BosSci), a new cryoballoon system for treatment of paroxysmal atrial fibrillation.3 Compared to the current standard of care Medtronic cryoablation system, the Boston Scientific system differs in its design. The FlexCath™ (Medtronic) sheath has a 135° angle of deflection and a cryoballoon which does not afford constant pressure through the inflation and ablation process. In contrast, The POLARsheath™ (BosSci) does not have a step-up between the dilator and sheath, and has a 155° angle of deflection with the goal of affording more maneuverability particularly for occlusion of the often challenging right inferior pulmonary vein, or other difficult anatomies.4 The PolarX™ thermoplastic balloon itself is designed to maintain a constant pressure, stiffness and shape throughout inflation and ablation. As well, the balloon is available in a 28 and 31 mm size compared to the 28 mm size of the Arctic Front™ (Medtronic) although the present study primarily examined the 28 mm sized balloon with a 50 patient extension arm examining the interchangeable balloon size (28 and 31 mm). The mapping catheter in theory can be withdrawn without compromising balloon stability.4 Thus, the overarching question for the present analysis was whether the differences in design of the PolarX™ balloon and ablation system afford procedural ease or better efficacy compared to previously available technology. In the present study, 325 patients underwent cryoablation with a 28 mm PolarX™ balloon, and 50 patients were treated with a variable 28/31 mm sized balloon, analyzed separately. Investigators reported a 79.9% freedom from documented atrial arrhythmia and 10.1% usage of class I/III antiarrhythmic drugs at 12 months. The overall complication rate was low at 4.0%. These results are comparable to those of STOP-AF PAS, a study that bears most resemblance to the FROZEN AF trial3, 5 (Table 1). While the overall efficacy reported is high, it is not significantly higher than that noted with prevalent technologies, and was noted in absence of continuous rhythm monitoring with 24 h holter monitoring provided only at 12 months with twice a month trans-telephonic monitoring, thus introducing the possibility of underdiagnosing subclinical recurrence. Subjects in the STOP AF PAS received rhythm monitoring, with a 24 h holter monitor at 6 months, 48 h holter monitor annually, and a 30 day event monitor for symptomatic episodes.5 The current manuscript is a well written narrative of the safety and efficacy of the PolarX™ system, however clinical implications are limited by the absence of a comparator group with Arctic Front™ to prove that the theoretical improvements to procedural efficiency or clinical efficacy based on design improvements, truly translate to real world practice. By extension, the possibility of harm from heightened maneuverability for example in context of phrenic nerve injury also warrants larger studies with long term follow up. Furthermore, high density 3D anatomical mapping of the left atrium, which could portend better procedural outcomes was not performed in all patients in this study and could impact outcomes. While there was a recommended dosing strategy, it was not followed in the majority of patients such that conclusions about efficacy outcomes are somewhat limited, with a confounding variable of operator discretion. With respect to variant pulmonary vein anatomy, prior data has shown no difference in clinical efficacy with the Arctic Front™ system.7 While there is enhanced maneuverability in the PolarX™ sheath, no objective parameters were tracked such that the impact of this feature is unknown. Most available studies comparing the two technologies are retrospective, nonrandomized and often focused on acute procedural success alone. A meta-analysis of eight studies from 11 European centers comparing PolarX™ system and Arctic front™ Advanced Pro revealed no differences in acute PVI, procedure time, fluoroscopy time, ablation time, minimal esophageal temperature, and risk of phrenic nerve palsy or thromboembolic events.8 However, this study did not examine long term clinical outcomes of arrhythmia recurrence. Results of current randomized studies comparing the two technologies with blinded end point adjudication and rigorous rhythm monitoring including during the blanking period, in addition to data on arrhythmia burden are likely to provide more definitive guidance for clinical practice.9 Nevertheless, it does appear based on the current study that safety and efficacy outcomes are similar. The advent of newer technologies such as pulsed field ablation which may afford significant safety and efficiency thus prompting a therapeutic paradigm shift in AF ablation therapy further underscores the importance of a head-to-head comparison of available cryoballoon technologies to determine the true value of new entrants into the thermal ablation space.