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Decebal Gabriel Latcu1, Bogdan Enache1, Ahmed Moustafa Wedn1, Sok-Sithikun Bun1, Nadir Saoudi1

1 Centre Hospitalier Princesse Grace, Avenue Pasteur, 98000 Monaco

Robotics has been used for radiofrequency ablation of human arrhythmias for more than 15 years; among 2 widely distributed systems1,2, only Stereotaxis (Saint-Louis, Missouri, USA) is still commercially available. Our experience with the Stereotaxis system goes back more than a decade3,4 and its advantages still make it the system of choice, in our center, for a number of arrhythmias.

THE REMOTE MAGNETIC NAVIGATION (RMN) SYSTEM
RMN uses a steerable magnetic fi eld which allows the remote manipulation inside the heart chambers of a very soft magnetic catheter embedded with an ablation electrode. The RMN system is composed by two giant magnets (Niobe ES, Stereotaxis) positionned each side of the fl uoroscopy table (Axiom Artis, Siemens, Germany), which create a magnetic fi eld of a 0.1 T maximal intensity (Figure 1). The orientation of the magnetic field is remotely controlled by the operator (Figure 2) via a dedicated software (Navigant, Stereotaxis). Additional dedicated systems (V-Drive / V-drive Duo) and disposables (Quick-Cas / V-Cas / VCas Defl ect5, Stereotaxis) connected to the ablation catheter allow the advancement and the retraction of the catheter, of the sheath, as well as defl ection / undeflection / rotation of a remotely controlled fixed curve or steerable sheath. These may be completed by a remotely controlled system for a rigid circular catheter (V-Loop, Stereotaxis). In the following paragraphs some evidence-based data for specific arrhythmias ablation with RMN will be presented.

RMN FOR ABLATION OF ATRIAL FIBRILLATION (AF)
AF ablation with RMN has been performed since 2008 when the fi rst irrigated magnetic catheter became available. Retrospective comparison6 with manual ablation of AF did not show any difference in the ablation result. The longer procedure time for RMN (223 vs 166 min) is compensated by a shorter fluoroscopy time (13 vs 34 min) and possibly inferior complication rate (without any cardiac perforation in the RMN group vs 2.4% in the manual group) but this difference did not reach signifi cance since the study was underpowered. These results were confirmed by another comparative series7; even more, a dedicated prospective study on RMN8 showed comparable results to historical manual ablation data and lack of serious adverse events. A large international multicenter survey9 among RMN users does not report any atrio-esophageal fi stula when using the system, whilst this complication, even rare, is still present while using manual catheters. Persistent AF could represent an elective indication for RMN, as longer procedure times are warranted; also, common left atrial dilation facilitates magnetic navigation. In our initial experience10 on 28 patients having persistent AF ablation with RMN (mean duration of actual AF episode of 10±16 months), with a follow-up 11±6 months after 1.25 procedures/patient, 68% of the cases didn’t have any arrhythmia recurrence. No major complication occurred. The advantage for the operator to perform these lengthy procedures (235±68 min) in a seated position without the lead coat is undeniable. It is worth noting that RMN also renders possible AF ablation by aortic retrograde approach11, which may be useful in case of impossible transeptal approach in congenital abnormalities with inferior vena cava agenesis / interruption. AF ablation with RMN may be further optimized by the use of a remotely controlled steerable sheath (VCas Defl ect); this improves long-term results, allows faster right pulmonary vein isolation and diminishes radiofrequency delivery time and procedure time5. We recently investigated whether lesion creation with magnetic catheters is comparable with the contemporary gold standard manual catheters with contact force assessment. We showed that during radiofrequency delivery, the electrical modifications suggesting transmurality is faster achieved with remote magnetic catheters than with optimal use of contact force catheters12. This may be in relation with a more stable tissue contact while using magnetic technology13.

Figure 1. Electrophysiology lab with the RMN: the magnets (Niobe ES), the fluoroscopy tube, the remote catheter control system (Vdrive) and the Odyssey screen.

Figure 2. The control room with the Odyssey screen, the Cardiodrive and the Vdrive controller.

RMN FOR ABLATION OF OTHER SUPRAVENTRICULAR ARRHYTHMIAS.
Post AF ablation atrial tachycardia (AT) was until recently another elective indication for RMN technology. Indeed, without having to continuously hold the catheter, the operator could concentrate on annotation and activation mapping, since it’s accuracy determines the procedural success. We compared14 our initial experience of 25 AT patients (RMN) with a control group of manual ablations (32 AT patients). There was no difference in what concerns acute or long-term success (80% vs 78%, p=ns) between the 2 techniques. Nevertheless, if no serious adverse event occurred in the RMN group, in the control group and transitory ischemic attack and a cardiac perforation requiring drainage were reported. The difference in procedure duration between the 2 techniques did not reach signifi cance (RMN 236±67 min, control group 201±72 min). Atrio-ventricular nodal reentrant tachycardia ablation has been feasible with RMN technology from the beginning, since no irrigation is necessary. Finely tuned mapping of the atrio-ventricular node extensions, especially of the rightward inferior extension (commonly the slow pathway), with 1 mm step advancement/ retraction movements of the ablation catheter with direction changes in 1 degree steps, may be fully exploited in this indication. We reported15 a 100% success rate for these procedures, without serious adverse events and with a number of junctional beats inferior to manual technique, favoring a better tissue contact with the magnetic catheters. Typical flutter is a challenge for RMN, possibly because of insuffi cient catheter pressure on the cavo-tricuspid isthmus. Magnetic non-irrigated catheters were proven inferior to manual technique16 but irrigation improved results and seems mandatory in case of anatomical complexity17. For cost-effectiveness reasons RMN might be an alternative to manual catheters for cavo-tricuspid isthmus ablation only in case of concomitant AF ablation or in case of superior approach18. Procedural success of CTI ablation may be warranted with the RMN technology if concomitant use of a steerable sheath. AT in case of congenital heart disease is diffi cult for complex anatomies with limited catheter access. Direct robotic manipulation of the distal tip of a soft catheter, specific for RMN, is a great advantage in comparison to rigid, manually driven catheters, in case of twisting path from the puncture site to the ablation target19. Fluoroscopy exposure is thus signifi cantly reduced20,21. Retrograde transaortic approach for AT ablation in Mustard-Senning or cavo-pulmonary derivation patients seems particularly successful with RMN22,23,24. Accessory pathways, incisional flutters and sinus node reentry have all been reported with the successful use of the RMN.

RMN FOR VENTRICULAR TACHYCARDIA (VT) ABLATION
Feasibility and safety of catheter ablation with the RMN have been reported for right ventricular outfl ow tract VT25, fascicular VT26, ischemic scar-related VT27,28, including epicardial VT29, as well as in other heart disease- related VT30. An increasing amount of data31,32 seem to suggest even superior results for VT ablation with RMN in comparison to manual technique. A randomized study is currently including VT patients and will assess whether substrate-based ablation of VT with RMN has clinical advantages over manual catheter manipulation33.

CONCLUSION: ACHIEVEMENTS AND CHALLENGES
Ablation using RMN has similar effi cacy compared to the manual technique in a wide range of arrhythmias. RMN has the advantages of improved safety and an undeniable increased comfort for the operator. Complex procedures became feasible with RMN for a stand-alone operator, manipulating both the ablation catheter and the mapping system. Congenital heart disease arrhythmias are an elective indication for RMN; RMN might be superior to manual technique also for VT ablation. In our center, AVNRT and AF ablation are other procedure types for which RMN is systematically considered. Nevertheless, RMN is facing today several challenges. First, the irrigated magnetic-tip catheter, available for almost one decade, has not been upgraded. Several technological improvements have been embedded into manual catheters (contact force measurement, more effi cient cooling with less irrigation fl ow) but are still lacking for magnetic catheters. Shortening the rigid part of the distal tip of the magnetic catheters and approaching the three magnets towards the distal end might improve navigation, catheter stability and electrode- tissue contact. A contact assessment module (“eContactTM”) will be shortly available from Stereotaxis; added to the current catheters it may overcome some of these limitations. Second, for several years, electrophysiology entered the era of multielectrode mapping (MEM) with automatic annotation. More recently, ultra-high-density mapping became the gold-standard for mapping of complex arrhythmias34. Except for the use of the V-loop disposable allowing the use of the circular catheter (LassoTM) for MEM, but with the magnets in the stowed position and less reliably than multielectrode catheters like the PentaRayTM or OrionTM, RMN allows only “point-by-point” mapping. Moreover, also RMN has been used in junction with other mapping systems like Rhythmia35 and Navex36, integration is currently available only with CartoTM (Biosense-Webster, Inc.), which might also be considered a limitation.

Acknowledgements: Dr. Laţcu and Dr. Bun received in the past year consulting fees and speaking honoraria from Boston Scientifi c and Pfizer. Dr. Laţcu is also a consultant for Stereotaxis.

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