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Novel ablation tools with innovative catheter designs have been introduced to overcome potential limitations of currently available radiofrequency-based ablation catheters
Thomas Fink MD
Karl-Heinz Kuck MD
Andreas Metzner MD
Department of Cardiology,
Asklepios Klinik St Georg, Hamburg, Germany
Catheter ablation has become a standard treatment option for patients suffering from cardiac arrhythmias. Within the last two decades many methodological efforts have been made to facilitate ablation procedures and to make them safer and more time efficient. This article summarises recent developments in catheter ablation technologies focusing especially on the treatment of atrial fibrillation (AF).
Catheter ablation of atrial fibrillation
AF is the most common cardiac arrhythmia with an incidence of 1.5–2% in the general population.1 Recent guidelines recommend catheter ablation for patients suffering from symptomatic drug-refractory AF and suggest catheter ablation as a first-line therapy in patients with paroxysmal AF (PAF) when performed in experienced centres.1 All electrophysiological interventions target the elimination of focal triggers or arrhythmogenic substrates being essential for the initiation and/or the maintenance of AF, respectively. Most ablation strategies for AF include a wide area circumferential ablation (WACA) of the pulmonary veins (PV) since the PVs have been identified as the most common site of triggers inducing and maintaining AF.2
PV isolation (PVI) is performed by creating transmural myocardial lesions inside the left atrium (LA) around each or both ipsilateral ostia of the PVs.3 The procedure is carried out by transvenous access of the diagnostic and ablation catheters to the atria guided by fluoroscopy and 3D-mapping systems. Additional ablation strategies to PVI were developed to increase the acute and long-term success especially for patients suffering from persistent (more than one week duration) and long-standing persistent (more than one year duration) AF. These strategies include the creation of additional linear lesions4,5 and/or ablation of complex fractionated atrial electrograms (CFAE) inside the LA and/or the right atrium.6
Data from our EP laboratory show five-year clinical success rates between 46.6% (after a single procedure) and up to 79.9% (after multiple procedures) in patients suffering from PAF.7 Further data from our centre show significantly lower five-year success rates for the ablation of long-standing persistent AF with 20.3% of patients maintaining sinus rhythm after a single procedure and 45.0% after multiple procedures, respectively, when performing the Hamburg sequential ablation approach.8 Interestingly, in both cohorts the majority of patients with recurrent AF showed a recovered LA-PV conduction.
Consequently, novel ablation technologies focus first on reducing the significant rate of electrical reconnection in previously isolated PVs and second to facilitate the ablation procedures. The formation of contiguous and transmural lesions is a prerequisite for permanent electric isolation of the targeted areas. However, the manual radio frequency current (RF)-based ablation approach for AF in a point-by-point fashion is technically challenging, time-consuming and it is difficult to create durable transmural lesions. To solve these difficulties, new catheter-based ablation systems incorporating alternative energy sources were introduced and evaluated during recent years, including new RF ablation catheters, circular ablation catheters and balloon-based ablation devices.
Contact force catheters
As described above, the creation of durable transmural lesions is of crucial importance for the success of ablation procedures. However, it depends on several factors such as wall contact, catheter stability, delivered temperature, power settings and impedance. Contact force (CF) between the ablation electrode and the myocardium has been found to be a new key factor to effective lesion formation. Low CF results in inadequate lesion formation, while high CF may increase the risk for complications such as perforation of the myocardium, steam pop or thrombus formation. Although CF has been recognised as an important parameter during ablation, measurements were only possible applying indirect surrogate parameters such as impedance. Recently, different catheters were introduced which allow for direct measurement of the CF.
The TactiCath Quartz (Endosense/ St. Jude Medical, St. Paul, Minnesota, USA) is an irrigated ablation catheter with a fibre optic sensor to measure CF. This catheter can determine CF changes with a sensitivity of 1g and can be incorporated into the 3D-mapping system EnSite NavX (St. Jude Medical, St. Paul, Minnesota, USA). Two recent multicentre clinical trials evaluated the TactiCath system for AF ablation. The TOCCATA study and the EFFICAS I study could prove direct correlation between low mean CF and high rates of electrical reconduction in previously isolated PVs and recurrence of AF after PVI using the TactiCath system.9 The EFFICAS I study demonstrated direct correlation of areas of low CF and gap formation in ablation lesions after PVI.10 The second CF catheter is the Thermocool Smarttouch catheter (Biosense Webster Inc., Diamond Bar, California, USA). It uses a spring connection between the catheter tip and the shaft to measure CF via microdeflection recordings. In the recently published prospective multicentre SMART-AF trial, Natale et al. could demonstrate a promising 12-month single-procedure success rate of AF-freedom of 72.5% in a cohort of patients suffering from PAF.11 Furthermore, they could show a significantly higher success rate in cases where on average more than 80% of CF within the predefined range was applied. The 12-month success rate raised to 81% after one procedure compared to only 66% if procedures were performed with CF values out of the operators selected working ranges.11 Therefore, the present trial shows a direct correlation between procedural long-term success and correctly applied CF during ablation.
Circular and balloon ablation catheters
During recent years, several new circular and balloon-based ablation devices have been introduced. In general, these types of catheters aim to facilitate PVI and to shorten procedure times. Two circular multi-electrode RF catheters are currently in clinical use. The pulmonary vein ablation catheter (PVAC, Medtronic Ablation Frontiers, Carlsbad, California, USA) is a non-irrigated circular catheter using duty-cycled RF energy. The system allows energy delivery via all or selected electrodes. Although being able to reduce both procedure and fluoroscopy times, the system failed in proving superiority to conventional RF ablation in smaller randomised, prospective trials, as demonstrated in a recently published meta-analysis.12
Of note, recent studies raised safety issues due to a higher rate of observed silent cerebral embolic lesions after PVAC ablation as compared to other catheter designs.13 The clinical significance of these finding remains unclear, nonetheless this issue needs to be solved before routine clinical use of this device can be recommended. Another circular multi-electrode catheter, the nMARQ (Biosense Webster, Diamond Bar, California, USA), allows for either uni- or bipolar RF ablation via 10 selected electrodes with simultaneous recordings of ostial PV electrograms to guide RF application (Figure 1). The use of irrigated RF energy offers possible reduction of silent cerebral microembolisation and improved lesion formation. Two initial studies were able to demonstrate a high acute procedure success and a beneficial safety profile using the nMARQ catheter.14,15
Reports on the incidence of cerebral embolisms describe 33% and 0%, respectively. These inconsistent observations may be attributable to different energy settings and possible electrode overlapping in the ablation attempts of the former group of patients. In another study by Rillig et al. the incidence of oesophageal thermal injury after nMARQ-based PVI was described with 50% after use of the initially recommended energy settings.16 After modification of the ablation protocol, oesophageal lesions were observed in 6.7%, which is comparable to other ablation systems.16 Data on mid- and long-term efficacy is sparse. In conclusion, further studies focusing on safety issues and long-term clinical results are necessary before a final evaluation can be made.
In addition to these technical developments using RF energy, three different balloon-based ablation systems incorporating various energy sources were introduced. The Endoscopic Ablation System (EAS) (CardioFocus, Marlborough, MA, USA) is equipped with a laser energy source and allows visually guided ablation via endoscopic imaging (Figure 2). The system consists of a deflectable sheath of 12F diameter and a balloon ablation catheter comprising an endoscope with a diameter of 2F and a 980nm laser generator. The endoscope enables the operator to apply laser applications and ablation lines under visual control. Different to other balloon-based systems, the compliant balloon allows for adaptation according to the individual PV diameter. The EAS offers point-by-point ablation under optical control without the challenge of achieving a stable catheter position at each ablation spot.
Additionally, the EAS allows for energy titration between 5.5W and 12W. Accordingly the usage of lower ablation energy along the posterior atrial wall contributes to avoid damage to adjacent extracardiac anatomical structures such as the phrenic nerve or the oesophagus, while along the thicker anterior wall higher energy settings can be applied.17 One-year clinical follow-up data from our institution and an international multicentre study could assess a success rate of 60% after a single procedure of antiarrhythmic medication in patients with PAF.18,19 This success rate is comparable to reported results after conventional RF ablation. Concerning the safety profile, a higher degree of ulcered oesophageal lesions in patients treated with the EAS as compared to patients treated with RF energy20 and a rate of 2.5% of patients suffering from phrenic nerve palsy was observed.18 However, no case of significant PV stenosis was described.
The most established balloon-based ablation system is the Arctic Front cryoballoon (Medtronic CryoCath LP, Quebec, Canada). The Cryoballoon (CB) was introduced about 10 years ago. Since then, multiple studies assessed high acute success rates and promising long-term outcomes after CB-based PVI in PAF. One-year clinical long-term success after PVI using the first generation CB was 72.83%, as shown in a recent meta-analysis.21 In addition Neumann et al. reported a five-year clinical success rate of 53% after a single procedure, which is comparable to a five-year result of 46.6% obtained after RF ablation at our centre.7
In a prospective observational study by Vogt et al. rates of freedom from atrial tachyarrhythmias after first generation CB-based PVI were 61.6% after a single procedure and 76.2% after multiple procedures as seen in a 30-month clinical follow-up.22 Several smaller, non-randomised studies compared the first generation CB to conventional and phased RF ablation strategies and proved non-inferiority of the balloon device in PAF.21 Currently, there is a lack on data of CB-based ablation of persistent AF.
Beneath this favourable efficacy, the safety profile of the first generation CB seemed to be comparable to RF ablation with low rates of oesophageal lesions and PV stenosis. However, higher incidences of phrenic nerve palsy were documented.21 Meanwhile, the second generation CB was introduced and features an improved refrigerant injection system with additional distal injection jets providing a more homogenous cooling of the complete northern balloon hemisphere (Figure 3).
Recently published results of 12-month clinical follow-up after ablation of patients suffering from PAF seem to be promising. Data from our centre demonstrated freedom from AF after a single procedure without antiarrythmic drugs in 80% (39 out of a total of 49) patients.23 Other groups assessed even better results with 83% of patients holding sinus rhythm after one year.24,25 The incidence of phrenic nerve paralysis is reported as 3.5–5.4%25,26 and thus comparable to the incidence after first generation CB ablation.20 The incidence of oesophageal thermal lesions increased to 12–19%.27,28 However, safety cut-offs having a high sensitivity and specificity were developed to prevent these oesophageal lesions28,29 and could successfully reduce the incidence of thermal lesions to 3.2%.29 The currently ongoing “Fire and Ice” study is the first comparing long-term efficacy after second generation CB ablation to conventional RF ablation in a prospective, randomised, multicentre fashion. Recruitment is not expected to be finished until the end of 2015. This study will help to clarify the future role of the second generation CB in catheter ablation of PAF.
Another balloon device incorporating high-intensity focused ultrasound (ProRhythm, Ronkonkoma, NY, USA), is not in clinical use anymore after the occurrence of severe clinical complications, including fatal outcomes after atrio-oesophageal fistula have occurred.30
Catheter ablation of AF is still challenging. Several technological efforts were made and supported by multiple study results to overcome the limitations and to improve procedure feasibility, safety and success.