For many highly symptomatic patients with atrial fibrillation catheter ablation offers an efficacious means to maintaining sinus rhythm [Citation1,Citation2]. Over the past 15 years large-scale observational studies and randomized controlled trials have demonstrated that the success rate of catheter ablation in maintaining sinus rhythm is universally superior to that of drug therapy, thus moving it from an ‘experimental therapy’ to the standard of care for the maintenance of sinus rhythm [Citation3–Citation11]. In addition, catheter ablation has been shown to be superior to antiarrhythmic drugs (AADs) for the improvement of symptoms, exercise capacity, and quality of life [Citation5,Citation12–Citation14].
The early percutaneous catheter ablation procedures were designed to compartmentalize the left atrium (LA) into smaller regions incapable of sustaining the critical number of circulating wavelets, functionally replicating the lines of a surgical Cox maze procedure. While AF maintenance by randomly propagating wavelets may occur in some cases, the identification of sites of AF initiation and/or perpetuation within the pulmonary veins (PV) led to the development of percutaneous procedures designed to electrically isolate the PV (PVI) from the vulnerable LA substrate. The contemporary AF ablation procedure, as recommended by major heart rhythm societies, involves the creation of wide circumferential ablative lesions within the LA myocardium outside of the tubular veins with a goal of electrical PVI, effectively targeting not only the initiating triggers of AF (the PVs) but also the mass of electrically active LA tissue capable of sustaining the fibrillatory wavelets responsible for perpetuating AF.
However, despite considerable success PVI has limitations. Recurrence due to a failure to effectuate a lasting transmural lesion is not infrequent [Citation3–Citation11]. As such, considerable effort has been directed toward developing technologies to achieve safer and more durable PV isolation. The two biggest advances in the last few years have centered on: (1) the development of dedicated ‘single-shot’ PVI tools, the most mature of which is the Arctic Front cryoballoon (Medtronic, Minneapolis, MN), and (2) the integration of an accurate real-time quantitative assessment of catheter contact force into focal point-by-point radiofrequency (RF) ablation catheters.
Cryoballoon ablation
The cryoballoon system consists of a steerable 10.5-Fr catheter with distally mounted polyurethane and polyester balloons specifically designed to facilitate PVI. Cryorefrigerant is delivered to the distal aspect of the inner balloon from an external CryoConsole® via an ultrafine injection tube where it is pressurized through a restriction tube before undergoing a liquid-to-gas phase change as it enters the distal aspect of the inner balloon. The cryorefrigerant then absorbs heat from the tissue before returning to the console through a central lumen maintained under vacuum.
In a recent observational meta-analysis, we reported that cryoballoon ablation resulted in a high procedural success rate (> 98% of patients achieving complete PVI) and a 1-year freedom from recurrent AF (one-year single procedure success of 60% off AAD; 73% if a 3-month blanking period was included) [Citation15]. A subsequent comparative meta-analysis reported similar freedom from recurrent AF at a mean follow-up of 16.5 months vs. standard (‘first-generation’) RF ablation (66.9% vs. 65.1%; RR 1.01; 95% CI: 0.94 to 1.07, P = 0.87).
Since the publication of these analyses the cryoballoon has undergone a significant iterative evolution. In the first-generation catheter, 6.2 L/min of vapor was sprayed to the distal face of the balloon through four jets positioned 90° from one another, slightly distal to the cryoballoon equator. The second-generation (Arctic Front Advance) catheter refined the design by increasing and repositioning the jets more distally resulting in an increased uniformity of cooling around the entirety of the distal surface of the cryoballoon.
Recent studies have examined the short- and long-term success with the second-generation cryoballoon. Studies of planned remapping procedures have demonstrated that the durability of PVI at three months post index ablation procedure has improved from 67% of PVs (23% of patients with all 4 PVs isolated) with standard RF, to 88% of PVs (67% of patients with all 4 PVs isolated) with the first-generation cryoballoon, to 91% of PVs (79% of patients with all 4 PVs isolated) [Citation16–Citation21]. Clinically this has translated into a one-year freedom from recurrent AF of 82% with the second-generation cryoballoon (11 studies; 1725 patients), which was significantly improved compared to the first-generation cryoballoon in a separate comparative meta-analysis (OR of arrhythmia recurrence 0.34 [0.26–0.45] when compared to first-generation cryoballoon; 10 studies, 2310 patients) [Citation22]. While there was no significant difference in access site complication or pericardial effusion, significantly more phrenic nerve palsies (transient and persistent) were observed with the first-generation cryoballoon.
Contact force
The most commonly used point-by-point RF contact force sensing catheter systems are TactiCath (St. Jude Medical, St Paul, USA) and SmartTouch (Biosense Webster, Diamond Bar, CA). TactiCath estimates axial and lateral contact force by examining the wavelength of the light emitted from fiber-optic tubes embedded within an open irrigated-tip ablation catheter. Forces applied at the tip of the catheter result in micro-deformation of optical fibers, changing in the wavelength of the light proportional to the force applied (with a sensitivity of 1 g). SmartTouch uses electromagnetic location technology to detect movement between a precision spring (mounted within the tip of a 3.5 mm externally irrigated RF ablation catheter) and three location sensor coils are also mounted within the shaft of the catheter. These movements are sampled every 50 msec and calibrated to produce a contact force reading (in grams) that is averaged over 1 s.
Recent data suggests that incorporating real-time contact force assessment into the ablation procedure results in a reduction in procedure time, ablation time and total energy delivery, with a comparable safety profile to that observed with standard irrigated RF [Citation18,Citation23]. However, the two largest multicenter trials evaluating this technology demonstrated a one-year success of 68% (TactiCath, TOCCASTAR [Citation24]) and 74% (SmartTouch, SMART-AF [Citation25]). In the case of the former the success was no different from that observed with standard noncontact force RF ablation. Interestingly, post-hoc analyses of these studies have suggested that the outcomes were improved when the procedure was performed with adequate contact force parameters (84% one-year freedom from AF in the 47% of patients in whom ablation was in the target range ≥80% of the time in SMART-AF, and 76% one-year freedom from AF in the 57% of patients in whom ≥90% of the lesions were > 10 g in TOCCASTAR).
No differences in the incidence of complications have been reported between patients undergoing ablation with the contact force vs. noncontact force sensing RF ablation catheters in randomized (TOCCASTAR) or non-randomized studies [Citation24,Citation26,Citation27].
Conclusion
Second-generation AF ablation technologies have resulted in improved clinical outcomes, while achieving a reduction in procedure time.
Financial and competing interests disclosure
J. Andrade is an advisor to and has received grants from Medtronic. M. Dubuc has received grants from Medtronic. The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.
Additional information
Notes on contributors
Jason Andrade
Marc Dubuc
Laurent Macle
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