Remote Magnetic Navigation and Catheter Ablation of Ventricular Tachycardia: From Concept to Reality

DOI: 10.19102/icrm.2011.020503

ADAM E. BERMAN, MD

Section of Cardiac Electrophysiology, Division of Cardiology, Georgia Health Sciences University, Augusta, GA

Download PDF

Follow Us >> Tweet

ABSTRACT. Radiofrequency catheter ablation (RFCA) of ventricular tachycardia (VT) remains a challenging electrophysiologic procedure, particularly in the setting of underlying structural heart disease. Remote magnetic navigation (RMN) is an emerging technology uniquely suited for catheter ablation of VT. This article describes key differences between manual and RMN catheter mapping and ablation of VT, including RMN catheter techniques facilitating ablation within the LV. Published data on success rates of RMN guided VT ablation in the structurally normal and abnormal heart are reviewed, highlighting an increase in RMN success rates with the introduction of an RMN open irrigation ablation catheter.

KEYWORDS. Radiofrequency ablation, catheter, ventricular tachycardia, remote magnetic navigation, mapping.

The authors report no conflicts of interest for the published content.
Manuscript received March 4, 2011, final version accepted March 21, 2011.

Address correspondence to: Adam E. Berman, MD, 1120 15th Street, BBR 6513A Augusta, GA 30809. E-mail: aberman@georgiahealth.edu

Introduction

The incidence of ventricular tachycardia (VT) radiofrequency catheter ablation (RFCA) is growing worldwide. Key factors driving this increase in this procedural volume include

• expanded indications for the use of implantable cardio-defibrillators (ICDs) in the setting of underlying structural heart disease;

• a growing number of cardiac electrophysiologists (EPs) and non-EP physicians implanting ICD devices;

• improvements in catheter and mapping technology, facilitating shorter ablation procedures with higher success and lower complication rates.

The recently revised European Heart Rhythm Association/Heart Rhythm Society (EHRA/HRS) expert consensus statement for the ablation of VT reflects an increasing awareness of the potential benefits of RFCA of VT in the structurally normal and the abnormal heart.¹ Consequently, as these complex procedural volumes grow, newer technologies facilitating successful RFCA of these often challenging arrhythmias appear both attractive and necessary.

Magnetic catheter navigation and VT ablation

In the United States, one remote magnetic navigation (RMN) system has received FDA approval for clinical use in humans, the Stereotaxis RMN system (Stereotaxis, St. Louis, MO). The Stereotaxis RMN system is uniquely suited for RFCA in all four cardiac chambers and the epicardial surface owing to its very flexible catheter design and catheter stability.^2,3 Whereas much attention has been paid to the ablation of supraventricular tachycardia (SVT) using RMN, recently published studies have highlighted the utility of this system in the ablation of VT.^4–6

RMN and RFCA

Remote magnetic versus manual catheter manipulation

The Stereotaxis system operates by remotely navigating a magnetically tipped catheter within the heart via two large permanently affixed external magnets on either side of the patient table.³ Intracardiac catheter manipulation is accomplished by directionality of the catheter tip and advancement/withdrawal of the catheter using an external automated catheter advancement system. This is typically coupled with a three-dimensional (3D) electroanatomic mapping system such as Carto RMT (Biosense Webster, Diamond Bar, CA,) or the Ensite system (St. Jude Medical, Minnetonka, MN). Figure 1 demonstrates a typical left ventricular (LV) endocardial map created the Carto RMT mapping system during an RMN ablation procedure.

When transitioning from manual to magnetic catheter ablation, the operator must learn to adjust his or her technique of catheter manipulation within the heart. With manual catheter positioning, proper catheter placement couples the manipulation of the catheter shaft to place the catheter tip in the desired location. With magnetic navigation, catheter shaft manipulation is far less critical than obtaining the appropriate magnetic vector for the catheter tip to be stably held in the proper location. Once the catheter tip arrives at the desired location, it may typically be maintained in this position indefinitely, with exceptional stability during both ventricular systole and diastole. This locational fidelity distinctly contrasts to the challenge of maintaining accurate endocardial contact within a contracting and torsing cardiac chamber throughout the cardiac cycle. This stability, however, comes at the expense of RMN catheter contact force, which rarely exceeds 15 g.^3,6 Consequently, higher powers are typically required during RF application, particularly with the RMN open irrigation catheter (OIC) catheter to achieve lesion efficacy. Whereas traditional manual catheters are characterized by predefined unidirectional or bidirectional curvatures, the floppiness of RMN ablation catheters allows nearly unlimited flexibility within the cardiac chambers, potentially obviating the need for intraprocedural catheter swap-outs.

RMN catheters available for clinical use in the United States include non-irrigated 4-mm and 8-mm tips, as well as a 3.5-mm OIC tip available from a single manufacturer (Biosense Webster). These catheters contain three magnets located at their distal tips permitting RMN.^3,7 Features of the RMN system critical to catheter ablation of VT are outlined in Table 1.

Ventricular access and catheter considerations specific to RMN

For endocardial LV VT RFCA procedures, the preferred route of LV access is antegrade through the mitral annulus (MA) via a long sheath traversing the interatrial septum. The trans-septal sheath is typically positioned within the left atrium (LA) at the level of the MA to provide sufficient catheter support as it courses into the LV. In patients with depressed LV systolic function, catheter stability within the LV is excellent. Additionally, the presence of LV dilatation also promotes magnetic catheter stability. Intraventricular RMN mapping and ablation may be accomplished with the “end-on” direct positioning technique, as well as the creation of catheter loops within the LV with the floppy catheter's shaft. Several examples of loop techniques we employ in our laboratory are illustrated in Figure 2. Loops within the LV promote shaft stability of the soft catheter. Once a stable loop is created, density mapping and RF delivery within a circumscribed territory may be accomplished by changes in vector orientation of the catheter tip.

For right ventricular (RV) procedures, we typically use a long curved sheath with the sheath tip positioned beyond the tricuspid annulus. Within the RV outflow tract (RVOT), the magnetic ablation catheter may be used in isolation or in conjunction with a basket catheter, or with the non-contact Ensite array mapping catheter (St. Jude Medical).^8,9 While ablation in the RVOT has been reported with a 4-mm-tip RMN catheter, we routinely utilize the RMN OIC with careful attention to power settings and frequent catheter tip repositioning during energy delivery. RMN within the RVOT minimizes barotrauma-mediated ventricular ectopy owing to the catheter's floppy nature while simultaneously promoting precise density mapping either during VT or during pace-mapping.^3,9

Access to the aortic cusps may be accomplished easily via a retrograde route using a long sheath with the tip positioned in the proximal descending aorta.¹⁰ Alternatively, a series of complex curves may be placed on the catheter if the transmitral route is selected to reach the cusps via the LV outflow tract across the aortic valve. Epicardial access is achieved in the conventional fashion via a percutaneous subxiphoid route, and epicardial motion of the RMN catheter is typically free and unrestricted.^3,4,6

Overview of RMN and VT RFCA studies

Basu Ray et al.¹¹ compared manual and RMN catheter mapping of LV scar in a porcine model of healed myocardial infarction. In this study, conventional manual mapping was not significantly different from RMN for the calculated scar area or the degree of error in reaching prespecified endocardial targets. Notably, fluoroscopy times and mapping-induced premature ventricular contractions (PVCs) were significantly lower in the RMN group.

Aryana et al.⁴ reported a total of 27 cases of ablation of VT using the Stereotaxis RMN system. This series featured RV, LV and epicardial mapping and ablation utilizing a 4-mm tip non-irrigated RMN catheter. Eighty-one per cent of targeted VTs were able to be mapped and ablated successfully; when RMN ablation failed to successfully ablate VT, a manual OIC was used for further ablation. The complication rate related to RMN mapping and ablation in this series was essentially zero, further underscoring the potential safety advantage of RMN in these procedures.

Di Biase et al.⁵ reported a case series of mixed population of patients undergoing VT ablation with RMN using either the 4-mm or 8-mm-tip non-irrigated catheters.⁵ Comparing these catheters, no significant difference was found in efficacy when ablating in the structurally normal heart, primarily targeting RVOT VT. However, in patients with structurally heart disease (e.g. coronary heart disease, idiopathic dilated cardiomyopathy), the 8-mm-tip catheter was found to be significantly more effective than the 4-mm catheter (59% vs 22% respectively, p<0.05). Nevertheless, the crossover rate from RMN to manual ablation with an OIC was 48% in this series.

Given the limitations of non-irrigated RMN catheters, recent work has focused on the use of the newly approved open irrigation RMN ablation catheter in the setting of VT ablation (Thermocool RMT, Biosense Webster). Early work by Haghjoo et al.¹² reported the use of the RMN OIC with good success and a low complication rate in a small population of four patients with electrical storm and ICD therapies despite medical therapy. Using the RMN OIC, all clinical VTs were successfully ablated, with one residual non-clinical VT remaining inducible in one patient. One patient required repeat ablation within 1 week of the initial procedure. At 4 months' follow-up, all patients remained free of VT and ICD therapies.

More recently, Di Biase et al.⁶ reported a much larger case control series of 110 patients undergoing RMN ablation of VT. These RMN patients were compared with a similar cohort of 92 patients having undergone manual ablation of VT by the same operator. In both groups, a majority of patients did not have structural heart disease. Patients with an LV or aortic cusp origin of VT were included in the analysis while RV VT was excluded. Most patients exhibited inducible VT, and in some cases template PVCs were targeted for ablation. In the RMN group, 100% of patients were reported to be free of inducible or sustained VT at the conclusion of the procedure. Crossover from RMN to manual occurred in 14% of cases, with 11% requiring manual OIC ablation to achieve procedural success. Overall complication rate was low, despite the use of higher powers (typically 40 W) during RF application. At 1 year of follow up, 85% of patients in the RMN group remained free of clinical VT (82% of these being off of antiarrhythmic drugs), comparable to the manual ablation control group (p = NS).

Limitations of RMN

One feature of existing RMN systems is its physical size. There are mandatory structural and magnetic shielding requirements necessary for the installation of the Stereotaxis Niobe system. This may limit its location within a hospital facility. Another drawback of the current technology is the relative lack of options in catheter selection for use with the RMN system. Once installed, the learning curve for the physician operator transitioning from manual to RMN catheter manipulation may be steep, prompting some operators to abandon RMN in favor of traditional ablation methods.

Conclusions

A growing body of clinical data supports the routine use of RMN in the ablation of VT in structurally normal and abnormal hearts. The advent of the open irrigation RMN catheter has led to increased procedural success rates and reduced intraoperative conversion to manual catheter manipulation. The safety profile of RMN in the ventricles, the outflow tracts, the epicardial space, and the aortic cusps appears excellent. Coupled with reduced fluoroscopy times and diminished operator fatigue, RMN appears uniquely situated as a valuable asset to centers performing VT ablation.

References

1. Aliot EM, Stevenson WG, Almendral-Garrote JM, et al. expert consensus on catheter ablation of ventricular arrhythmias. Europace 2009; 11:771–817. [CrossRef] [PubMed]
2. Ernst S, Ouyang F, Linder C, et al. Initial experience with remote catheter ablation using a novel magnetic navigation system. magnetic remote catheter ablation. Circulation 2004; 109:1472–1475. [CrossRef] [PubMed]
3. Faddis MN, Blume W, Finnery J, et al. Novel, magnetically guided catheter for endocardial mapping and radiofrequency catheter ablation. Circulation 2002; 106:2980–2985. [CrossRef] [PubMed]
4. Aryana A, d'Avila A, Heist EK, et al. Remote magnetic navigation to guide endocardial and epicardial catheter mapping of scar-related ventricular tachycardia. Circulation 2007; 115:1191–1200. [CrossRef] [PubMed]
5. Di Biase L, Burkhardt JD, Lakkireddy D, et al. Mapping and ablation of ventricular arrhythmias with magnetic navigation: comparison between 4- and 8-mm catheter tips. J Interv Card Electrophysiol 2009; 26:133–137. [CrossRef] [PubMed]
6. Di Biase L, Santangeli P, Astudillo V, et al. Endo-epicardial ablation of ventricular arrhythmias in the left ventricle with the remote magnetic navigation system and the 3.5-mm open irrigated magnetic catheter: results from a large single-center case–control series. Heart Rhythm 2010; 7:1029–1035. [CrossRef] [PubMed]
7. Faddis MN, Chen J, Osborn J, et al. Magnetic guidance system for cardiac electrophysiology. A prospective trial of safety and efficacy in humans. J Am Coll Cardiol 2003; 42:1952–1958. [CrossRef] [PubMed]
8. Friedman PA, Asirvatham SJ, Grice S, et al. Noncontact mapping to guide ablation of right ventricular outflow tract tachycardia. J Am Coll Cardiol 2002; 39:1808–1812. [CrossRef] [PubMed]
9. Thornton AS, Jordaens LJ. Remote magnetic navigation for mapping and ablating right ventricular outflow tract tachycardia. Heart Rhythm 2006; 3:691–696. [CrossRef] [PubMed]
10. Burkhardt JD, Saliba WI, Schweikert RA, et al. Remote magnetic navigation to map and ablate left coronary cusp ventricular tachycardia. J Cardiovasc Electrophysiol 2006; 17:1142–1144. [CrossRef] [PubMed]
11. Basu Ray I, Dukkipati S, Houghtaling C, et al. Initial experience with a novel remote-guided magnetic catheter navigation system for left ventricular scar mapping and ablation in a porcine model of healed myocardial infarction. J Cardiovasc Electrophysiol 2007; 18:520–525. [CrossRef] [PubMed]
12. Haghjoo M, Hindricks G, Bode K, et al. Initial clinical experience with the new irrigated tip magnetic catheter for ablation of scar-related sustained ventricular tachycardia: a small case series. J Cardiovasc Electrophysiol 2009; 20:935–939. [CrossRef] [PubMed]