This editorial refers to ‘Comparison between radiofrequency with contact force-sensing and second-generation cryoballoon for paroxysmal atrial fibrillation catheter ablation: a multicentre European evaluation’ by F. Squaraet al., on page 718–724.
Since the landmark publication by Haissaguerre et al.1 on the importance of the pulmonary veins (PVs) as triggers for the induction and maintenance of paroxysmal atrial fibrillation (AF), significant efforts have been made to further improve available catheter technologies and ablative techniques. Recently, the second-generation cryoballoon (CB2) was introduced, featuring improved cooling characteristics and better long-term clinical success rates than the first-generation device.2–4 Catheter ablation using radiofrequency (RF) energy is dependent on contact force (CF) as a key parameter for transmural lesion formation.5 Currently, two CF catheters, the TactiCath Quartz (St Jude Medical, St Paul, MN, USA) and the Thermocool Smarttouch CF (Biosense Webster Inc., Diamond Bar, CA, USA), allow direct measurement of applied CF during ablation. The EFFICAS I study demonstrated a correlation between low CF and gap formation along circumferential lesion sets following PV isolation.6 Two recent prospective non-randomized trials showed better long-term clinical outcome after CF-guided PV isolation compared with conventional RF ablation, with 88 and 89.5% of patients remaining in sinus rhythm after 12 months and off antiarrhythmic drugs.7,8
In this issue of Europace, Squara et al.9 report the results from a multicentre cohort of 376 patients with paroxysmal AF treated with either the CB2 (178 patients) or the TactiCath or Thermocool CF catheter (198 patients). The mixed design of the study included 276 patients enrolled prospectively and 100 patients included retrospectively. Procedure duration was shorter in the CB2 group with similar overall complication rates (7.1% for CB2 and 7.3% for CF). However, severe non-fatal complications such as embolic events, cardiac tamponade, or oesophageal injury at a rate of 2.5% occurred only in the CF group, while transient right phrenic nerve palsy was noted only in the CB2 group (5.6 vs. 0%). The investigators found similar single-procedure long-term success rates for both technologies, with 82.2 and 83.9% of patients free from recurrent AF or atrial tachycardia during 12-month follow-up, respectively. Extending follow-up to 18 months, 73.3% of patients in the CB2 group and 76.0% in the CF group remained in sinus rhythm.
This study adds useful information to the currently available literature. The incidence of cardiac tamponade reported in the CF group is similar to earlier studies using CF catheters.10,11 The incidence of phrenic nerve palsy as a specific complication during CB-based procedures is also in line with previous studies2,12,13 but may have been further reduced by continuous monitoring of the right diaphragmatic compound motor action potential.14 In addition, foregoing an additional freeze application after successful PV isolation has shown comparable outcome to a conventional bonus freeze protocol15 while possibly lowering the rate of untoward events.
Squara et al.9 confirm earlier studies that have demonstrated excellent single-procedure outcome following CB2 and CF-based PV isolation in patients with paroxysmal AF. Furthermore, no significant difference in the long-term clinical success rate was found between groups. Our group and others have reported 1-year single-procedure success rates off antiarrhythmic drugs of 80, 83, and 83.6%, respectively, using the CB2 in patients with paroxysmal AF.2,12,16 The recently published SMART-AF trial has shown a 1-year single-procedure success rate of 72.5% using the Smarttouch Thermocool CF catheter.10 Importantly, the success rate increased to 81% if the measured CF was >80% of the time within the predefined range compared with only a 66% success rate in the remaining patients. Recently, a prospective, single-centre study enrolling 150 patients with paroxysmal AF has shown a 1-year single-procedure success rate of 86.3% for CB2 and 88% for the Smarttouch CF catheter.17
But do these results provide enough evidence to accept both ablation technologies as equal? In order to answer this question, a prospective randomized multicentre trial would need to enrol a high number of patients. The present study utilized a non-randomized, mixed prospective, and retrospective design. Furthermore, only a small proportion of patients completed 18-month follow-up. A potential bias may have been introduced because not every ablation technology was utilized at every participating centre. The use of two different sizes, the 23 and 28 mm CB2, and choice of two different CF catheters may have also influenced outcome. Finally, the investigators state that CF ablation was guided by using the force-time integral (FTI). However, FTI as a surrogate marker for predicting effective lesion formation has only been evaluated for the TactiCath catheter,6 while clinical data using the Thermocool Smarttouch catheter are lacking.
In conclusion, the ablative armamentarium available to the electrophysiologist has widened with the development of CF catheters and the improvements made to the CB2. Confirmation of their appropriate use may come from the multicentre FIRE AND ICE trial, a prospective randomized study that completed enrolment of a total of 768 patients with paroxysmal AF earlier this year comparing conventional and CF-based catheter ablation with the CB2.18 The initial results are expected for 2016.
Conflict of interest: K.H.K. and E.W. are serving as consultant/advisory board member to Medtronic and Biosense Webster.