Catheter Ablation for Atrial Fibrillation: An Attempt at Abating the Epidemic
The evolution of catheter ablation for atrial fibrillation. Over the past decade, atrial fibrillation ablation (AF) has evolved from an investigational therapy to one that many electrophysiologists have in their arsenal. Catheter ablation for AF originally attempted to recreate the Cox-Maze procedure by performing linear lesions in the atrium using radiofrequency or cryothermy. While this technique altered the atrial substrate, there was limited knowledge concerning the importance of the pulmonary veins in AF.
The discovery that ectopic firing from the pulmonary veins (PVs) was responsible for initiating AF was a watershed moment in the history of interventional electrophysiology.1 By 1999, initial efforts at targeting the pulmonary veins focused on ablating select triggers. Early endeavors of catheter ablation for AF were frequently complicated by pulmonary vein stenosis; therefore, today, ablation is performed proximal to the pulmonary veins in the antrum.2 By 2002, the majority of all ablation strategies included pulmonary vein ablation. A number of centers reported success rates ranging from 31–88% in symptomatic patients with mostly paroxysmal AF (PAF).3 Differences in outcomes may have been biased by patient populations; ablation techniques, different endpoints and the number of re-do ablations. In 2007, The Venice Chart International Consensus Group and the Consensus of Catheter and Surgical Ablation of AF organized the relevant literature and practices.4,5
Today’s Prevailing Strategies for AF Catheter Ablation
Presently, catheter ablation is considered a reasonable alternative to pharmacologic therapy to prevent recurrent AF in patients with symptomatic AF with little or no atrial dilation (Class 2A recommendation, level of evidence C); however, the efficacy of catheter ablation has been demonstrated in patients with heart failure and reduced ejection fraction.
The two dominant strategies for catheter ablation of AF are electrical pulmonary vein antrum isolation (PVAI) and anatomical circumferential pulmonary vein ablation. The foundation of most ablation strategies is to target the pulmonary veins (PVs). In 60–85% of PAF patients, this strategy will suffice, but at least 10–15% of all PAF patients will have non-PV triggers that initiate AF. It is unclear what form of ablation endpoint should be used in PAF patients. Noninducibility of AF can be used as an endpoint for PAF; however, it is not always a good predictor of success and may require significant ablation. An alternate approach is to challenge with high doses of isoproterenol and adenosine to elicit triggered firing sites outside the PV antrum6–8 (Figures 1 and 2).
Longstanding persistent atrial fibrillation (LSPAF) is challenging to treat, thus, there are continued efforts made to identify supplemental sites for ablation. Presently, many centers target non-PV sites that include the posterior wall of the left atrium, superior vena cava, crista terminalis, fossa ovalis, coronary sinus ligament of Marshall and others. These sites are selected based on the presence of fractionated electrograms, stimulation of the vagal ganglia or analysis of the local electrograms with specific software.
The goal of PVAI is to electrically isolate the funnel-shaped portion of the PV and the posterior wall. Confirming isolation of the PVs and the posterior wall with a circular catheter is imperative. In LSPAF cases, adjunctive lesions are often necessary. Frequently, ablation is performed on the left side of the septum, with the addition of defragmentation in the right, left atrium and coronary sinus. The patients are then cardioverted and a high dose (20–30 µg/min) of isoproterenol challenge is performed to assess for spontaneous firing. Any potential triggers are ablated. The prevalence of non-PV triggers range between 30–55% in LSPAF patients.
The other predominant strategy for AF is circumferential PV ablation (CPVA) using electroanatomical mapping. CPVA encircles the PVs 1–2 cm away from the ostia, except for near the left atrial appendage. Often, linear lesions are made along the mitral isthmus and between the right-sided and left-sided encircling lesions. The endpoints of the lesions are reduction of the electrogram amplitude measured by the ablation catheter.9
Advances in Tools and Techniques for Atrial Fibrillation Catheter Ablation
AF catheter ablation is a challenging procedure and success is closely associated with operator skill and experience. Therefore, recent advances are geared towards improving ablation and mapping technology.
Radiofrequency ablation. Radiofrequency (RF) is the most common energy source used to create lesions in AF ablation. In the past, conventional 4 or 5 mm-tip ablation catheters were used to create lesions either by point-by-point or by dragging the catheter along the endocardial surface. Solid-tipped catheters did not reliably make transmural lesions and were associated with char and thrombosis. Many electrophysiologists now use cool-tip saline irrigation catheters. By cooling the electrode tissue interface, the presence of coagulated proteins is reduced and energy is more efficiently delivered. When a cooled catheter tip is used, more radiofrequency energy can be delivered without exceeding 100ºC at the electrode-tissue interface, thus creating deeper and wider lesions.10,11
Other technologies have been used in conjunction with RF ablation to improve the ability to create transmural lesions and avoid complications. Recently, a pressure sensor at the tip of the catheters has entered clinical investigation and it carries the potential of improving efficacy and safety of complex ablation procedures.
Alternate energy sources for AF ablation. While radiofrequency energy is the “bread and butter” of AF ablation, there are some potentially serious risks such as PV stenosis, stroke and atrio-esophageal fistula. The three main alternate energy sources include: cryoenergy, high-intensity ultrasound and laser. The aim of these alternate sources of energy is to create transmural circumferential lesions with a single application by using balloon systems. The major limitation of all balloon-based technologies is how to account for variations in the PV anatomy. Imaging prior to the procedure can provide information on anatomical anomalies. Since balloons are designed specifically to isolate the PVs, it might suffice in select PAF patients. At present, the most promising technologies are balloon catheters that use cryothermal or laser energy. The laser system has been recently redesigned and the newest version is being tested in humans (Figure 3).12–15
The latest generation of high-intensity focused ultrasound (HIFU) balloons are steerable and can isolate the majority of PVs. However, complications such as phrenic nerve paralysis and atrio-esophageal fistula have been seen. This has limited the enthusiasm about this system which is currently not available for clinical use.
Cryothermy can be delivered by either a conventional catheter tip or with a balloon device. Cryoablation balloons are able to isolate 97% of the PVs, however, some patients require two different balloon sizes or a touch-up with a cryo-point ablator. While there is no risk of PV stenosis with cryoablation, some patients may have phrenic nerve paralysis, particularly when small balloons were used to target the right superior PV (Figure 4). Longer follow-up periods are required to confirm the efficacy of this system.
Imaging. In the past, fluoroscopy was the main imaging technique used by electrophysiologists. While fluoroscopy still remains an excellent imaging modality, it has several drawbacks which include 2-dimensional (2-D) representation, the inability to visualize soft tissue, a long learning curve and radiation exposure to both the patient and the operator. Intracardiac echocardiography facilitates accessing the left atrium, reduces complications and ensures that lesions are made at intended target sites.
Most centers worldwide use 3-D electroanatomical mapping systems for AF ablation in addition to fluoroscopy to better understand individual atrial anatomy and accurately create and record lesions. Three-dimensional electroanatomical imaging is being integrated with other imaging modalities such as computed tomography (CT) and magnetic resonance imaging (MRI), which allow the operator to visualize the real left atrial anatomy. There are limitations with this technology from being the sole navigation guide. The actual size and geometry may be slightly different at the time of acquisition compared to that of the procedure day including differences in volume status, respiratory phases between CT/MRI, cardiac rhythm differences and registration errors (Figure 5).
Recently, an intracardiac echocardiography (ICE)-guided registration strategy for integration of electroanatomical mapping with 3-D CT/MRI images was successfully used for AF ablation.16
Remote navigation. Guiding the catheter to the intended sites and maintaining adequate contact is a critical part of a successful procedure. To limit the impact of the operator, robotic and magnetic navigation systems have been developed (Stereotaxis, St. Louis, Missouri and Hansen Medical, Mountain View, California). Early clinical experience with these devices is promising, and it may improve both the ease-of-procedure and outcomes of AF ablation.
Remote magnetic navigation (Stereotaxis) entered clinical use for electrophysiology interventional procedures approximately 5 years ago. Over 20,000 cases have been performed in the world. Changing the orientation of the two large magnets located on the side of the patient will cause the tip of the catheter to align with the composite magnetic field in the chest of the patient. Presently, the remote magnetic system is compatible with solid 4 mm and 8 mm catheters. Recently, a 3.5 mm irrigated catheter was released. The remote magnetic system is compatible with the LocaLisa system (Medtronic, Inc., Minneapolis, Minnesota) and the NAVx mapping system (St. Jude Medical, St. Paul, Minnesota). However, a fully integrated 3-D electroanatomical CARTO RMT system (Biosense Webster, Brussels, Belgium) is available. Location information is shared between the systems, allowing for navigation from the electroanatomical mapping.
The CoHesion 3-D visualization electromechanical robotic system (Hansen Medical) operates by guiding standard catheters through an articulated robotic sheath. The guiding sheaths work on a mechanical pull-wire mechanism that moves the catheter. The catheters are manipulated by a “joystick” that the operator uses with the remote console. The IntelliSense technology mathematically estimates the pressure the catheter exerts on the heart to reduce perforation; this is limited in non-straight orientations. As of December 30, 2008, over 1,500 procedures have been performed worldwide. The electromechanical robotic system is compatible with all mapping systems and all standard catheters. In addition, integrated software between the NAVx mapping system and the CoHesion 3-D visualization module robotic system is available (Table 1).
The Future of Catheter Ablation for Atrial Fibrillation
The number of patients undergoing catheter ablation for AF is expanding. There is significant benefit in achieving sinus rhythm without the adverse effects of pharmacological therapy. Moreover, it has been shown that while catheter ablation for AF was initially more expensive than medical therapy, it is considered equivalent in cost in about 4 years after ablation suggesting that it is a “fiscally sensible alternative.”17 Currently, there are still some drawbacks to catheter ablation which include complications, procedure times, operator-dependent success rates and the possibility of needing a repeat ablation. Nevertheless, as we gain a deeper understanding of the AF mechanism and experience with catheter ablation increases, it is possible that it may be considered a firstline treatment for AF.
Catheter Ablation for Ventricular Tachycardia: An Attempt at Abating the Ravages of the Coronary Artery Disease Epidemic
The evolution of catheter ablation for ventricular tachycardia and fibrillation. Treatment of ventricular tachycardia (VT) has evolved from aneurysm resection and subendocardial resection to catheter ablation. RF ablation can be used in patients with left ventricular (LV) dysfunction due to prior myocardial infarction (MI), cardiomyopathy and the various forms of idiopathic VT.
Different mapping and ablation tools are used based on the type of VT. Patients with no structural heart disease constitute a small percentage of VT patients. VTs in these patients usually originate from the right ventricular outflow tract (RVOT), left ventricular outflow tract (LVOT) or aortic cusps. Ablation may be curative in these patients and is generally associated with a good prognosis.
Sudden cardiac death from life-threatening ventricular arrhythmias is the most common cause of death in developed countries. The mechanism for VT in patients with cardiac structural damage is predominantly re-entry. Today, the majority of VT/VF catheter ablations are performed in patients with recurrent VT refractory to medications and who have received multiple implantable cardioverter defibrillator (ICD) shocks; however, the indications are expanding.18,19
VT ablation is challenging and dependent on operator skill and patient selection. Patients with structural cardiac damage may have multiple isthmuses and the substrate that maintains re-entry may involve large areas. Mapping is often time-consuming and difficult. Many patients with coronary artery disease are hemodynamically unstable during VT, which limits the time necessary to sequentially map the ventricular chamber in tachycardia. In addition, at times it is difficult to induce the clinically relevant VT during the ablation procedure. Many different approaches have been used to map and ablate hemodynamically unstable VTs. Recently, we have become aware that it is possible to eliminate VF storms by targeting the initiating pre-ventricular contractions (PVCs) which usually are related to the His-Purkinje System.20,21
Mapping ventricular tachycardias. Non-contact mapping, pace-mapping, substrate mapping, identification of channels within scar tissue and VT mapping with hemodynamic support have been used to identify ablation targets.
Substrate ablation based on transcatheter endocardial electroanatomical mapping is a strategy for patients with hemodynamically unstable VT, which obviates the need to map during VT in patients with cardiac structural damage. Localized RF lesions can be applied to the tissue bordering the scar or areas of delayed activation within a scar.22 Today, most centers use mapping systems that recreate the geometry of the ventricle while providing continuous display of the catheter location. Activation time and electrogram amplitude are provided with a color scale. Papillary muscles and other fine anatomical details are not depicted, which makes electroanatomical integration with ICE promising.23,24 More centers have become familiar with percutaneous epicardial instrumentation, which is a valuable approach to improve success in patients with VT.21
Advances in tools and techniques for ventricular tachycardia ablation.The most common energy used for VT ablation is radiofrequency. Most idiopathic VTs can be resolved with solid 4 or 5 mm tip electrodes; however, scar-related VTs may require deeper, more extensive lesion sets necessitating irrigated catheters. With an open irrigated catheter post-MI VT can be suppressed 60–65% of the time.25 Experience with cryoablation in VT is limited. Alcohol ablation with injection in the coronary tree could be considered for selected cases refractory to conventional ablation therapy. In the future, delivering intramyocardial RF using a needle electrode or the use of alternative energy such as high focused ultrasound might even overcome some of the limitations of conventional endocardial delivery.26,27
Conclusion
AF and coronary artery disease are considered to be cardiovascular epidemics. Ablation appears to be a promising treatment strategy for AF and VT/VF patients. As we gain better insight into the nature of these arrhythmias, develop better catheter and imaging technologies and equalize operator dependence with robotic technology, ablation procedures will be increasingly performed and considered routine interventions.
1. Haïssaguerre M, Jaïs P, Shah DC, et al. Spontaneous initiation of atrial fibrillation by ectopic beats originating in the pulmonary veins. N Engl J Med 1998;339:659–666.
2. Di Biase L, Fahmy TS, Wazni OM, et al. Pulmonary vein total occlusion following catheter ablation for atrial fibrillation: Clinical implications after long-term follow-up. J Am Coll Cardiol 2006;48:2493–2499.
3. Cappato R, Calkins H, Chen SA, et al. Worldwide survey on the methods, efficacy, and safety of catheter ablation for human atrial fibrillation. Circulation 2005;111:1100–1105.
4. HRS/EHRA/ECAS Expert Consensus Statement on catheter and surgical ablation of atrial fibrillation: Recommendations for personnel, policy, procedures and follow-up. A report of the Heart Rhythm Society (HRS) Task Force on catheter and surgical ablation of atrial fibrillation. European Heart Rhythm Association (EHRA); European Cardiac Arrhythmia Society (ECAS); American College of Cardiology (ACC); American Heart Association (AHA); Society of Thoracic Surgeons (STS), Calkins H, Brugada J, Packer DL, Cappato R, Chen SA, Crijns HJ, Damiano RJ Jr, Davies DW, Haines DE, Haissaguerre M, Iesaka Y, Jackman W, Jais P, Kottkamp H, Kuck KH, Lindsay BD, Marchlinski FE, McCarthy PM, Mont JL, Morady F, Nademanee K, Natale A, Pappone C, Prystowsky E, Raviele A, Ruskin JN, Shemin RJ. Heart Rhythm 2007;4:816–861.
5. Natale A, Raviele A, Arentz T, et al. Venice chart international consensus document on atrial fibrillation ablation. J Cardiovasc Electrophysiol 2007;18:560–580.
6. Wazni OM, Marrouche NF, Martin DO, et al. Radiofrequency ablation vs. antiarrhythmic drugs as first-line treatment of symptomatic atrial fibrillation: A randomized trial. JAMA 2005;293:2634–2407.
7. Haïssaguerre M, Sanders P, Hocini M, et al. Catheter ablation of long-lasting persistent atrial fibrillation: Critical structures for termination. J Cardiovasc Electrophysiol 2005;16:1125–1137.
8. Nademanee K, McKenzie J, Kosar E, et al. A new approach for catheter ablation of atrial fibrillation: Mapping of the electrophysiologic substrate. J Am Coll Cardiol 2004;43:2044–2053.
9. Pappone C, Rosanio S, Oreto G, et al. Circumferential radiofrequency ablation of pulmonary vein ostia: A new anatomic approach for curing atrial fibrillation. Circulation 2000;102:2619–2628.
10. Bai R, Fahmy TS, Patel D, et al. Radiofrequency ablation of atypical atrial flutter after cardiac surgery or atrial fibrillation ablation: A randomized comparison of open-irrigation-tip and 8-mm-tip catheters. Heart Rhythm 2007;4:1489–1496.
11. Wazni OM, Rossillo A, Marrouche NF, et al. Embolic events and char formation during pulmonary vein isolation in patients with atrial fibrillation: Impact of different anticoagulation regimens and importance of intracardiac echo imaging. J Cardiovasc Electrophysiol 2005;16:576–581.
12. Cummings JE, Pacifico A, Drago JL, et al. Alternative energy sources for the ablation of arrhythmias. Pacing Clin Electrophysiol 2005;28:434–443.
13. Reddy VY, Houghtaling C, Fallon J, et al. Use of a diode laser balloon ablation catheter to generate circumferential pulmonary venous lesions in an open-thoracotomy caprine model. Pacing Clin Electrophysiol 2004;27:52–57.
14. Phillips KP, Schweikert RA, Saliba WI, et al. Anatomic location of pulmonary vein electrical disconnection with balloon-based catheter ablation. J Cardiovasc Electrophysiol 2008;19:1–18.
15. Van Belle Y, Janse P, Theuns D, et al. One year follow-up after cryoballoon isolation of the pulmonary veins in patients with paroxysmal atrial fibrillation. Europace 2008;10:1271–1276.
16. Fahmy TS, Mlcochova H, Wazni OM, et al. Intracardiac echo-guided image integration: Optimizing strategies for registration. J Cardiovasc Electrophysiol 2007;18:276–282.
17. Khaykin Y, Wang X, Natale A, et al. Cost comparison of ablation versus antiarrhythmic drugs as first-line therapy for atrial fibrillation: An economic evaluation of the RAAFT pilot study. J Cardiovasc Electrophysiol 2009;20:7–12.
18. Reddy VY, Reynolds MR, Neuzil P, et al. Prophylactic catheter ablation for the prevention of defibrillator therapy. N Engl J Med 2007;357:2657–2665.
19. Estes NA 3rd. Ablation after ICD implantation — Bridging the gap between promise and practice. N Engl J Med 2007;357:2717–2719.
20. Marrouche NF, Verma A, Wazni O, et al. Mode of initiation and ablation of ventricular fibrillation storms in patients with ischemic cardiomyopathy. J Am Coll Cardiol 2004;43:1715–1720.
21. Tedrow U, Stevenson WG. Strategies for epicardial mapping and ablation of ventricular tachycardia. J Cardiovasc Electrophysiol 2009 Feb 2 (ahead of print).
22. Verma A, Marrouche NF, Schweikert RA, et al. Relationship between successful ablation sites and the scar border zone defined by substrate mapping for ventricular tachycardia post-myocardial infarction. J Cardiovasc Electrophysiol 2005;16:465–471.
23. Khaykin Y, Skanes A, Whaley B, et al. Real-time integration of 2D intracardiac echocardiography and 3D electroanatomical mapping to guide ventricular tachycardia ablation. Heart Rhythm 2008;5:1396–1402.
24. Fahmy TS, Wazni OM, Jaber WA, et al. Integration of positron emission tomography/computed tomography with electroanatomical mapping: a novel approach for ablation of scar-related ventricular tachycardia. Heart Rhythm 2008;5:1538–1545.
25. Stevenson WG, Wilber DJ, Natale A, et al; Multicenter Thermocool VT Ablation Trial Investigators. Irrigated radiofrequency catheter ablation guided by electroanatomic mapping for recurrent ventricular tachycardia after myocardial infarction: the multicenter thermocool ventricular tachycardia ablation trial. Circulation 2008;118:2773–2782.
26. Kim SS, Knight BP. Alcohol ablation for ventricular tachycardia: A distillation of modern cases. Heart Rhythm 2008;5:69–70.
27. Delacrétaz E, Seiler J, Tanner H, Hess OM. Ablation of ventricular tachycardia: Neither inside nor out, thus back to alcohol. Heart Rhythm 2006;3:1230–1231.
