Abstract
Atrial fibrillation, the most common cardiac arrhythmia requiring treatment, has significant morbidity and mortality consequences. Pharmacologic therapy consisting of anticoagulants, AV nodal blocking agents and antiarrhythmics, remain the primary treatment. However, several nonpharmacologic therapies for the treatment of atrial fibrillation have been developed. In this review, we provide a detailed discussion of the most promising nonpharmacologic approach to the treatment of atrial fibrillation—catheter-based ablation.
Introduction
Atrial fibrillation (AF), the most common arrhythmia requiring treatment, has significant morbidity and mortality consequences. AF may lead to reduced quality of life, stroke, heart failure and premature death. In excess of two million Americans are affected. More than 5% of individuals over 65 years of age will develop AF.1
AF can be classified into three categories: paroxysmal, persistent, and permanent. Paroxysmal AF is recurrent AF that spontaneously terminates within seven days. Persistent AF is AF lasting more than seven days or lasting less than seven days but requiring pharmacologic or electrical cardioversion. Permanent AF is continuous AF lasting more than one year.2
Pharmacologic therapy, the mainstay of AF treatment, consists of anticoagulation (for stroke prevention), AV nodal blocking agents (for improved ventricular rate control) and antiarrhythmics (for maintenance of sinus rhythm). While antiarrhythmic agents have been shown to improve maintenance of sinus rhythm, they have variable success rates, significant long-term cost, and are associated with side-effects including the potential development of life-threatening ventricular tachydysrhythmias. Recently, the AFFIRM trial suggested that a pharmacologic rhythm control strategy had no benefit in terms of mortality than a rate control and anticoagulation strategy.3
Since the initial development and subsequent evolution of AF ablation, our understanding of the mechanisms underlying AF has advanced. Atrial fibrillation is most commonly initiated by electrical discharges originating from the pulmonary veins (PVs). Additionally, AF may be sustained by rotors (high-frequency micro-reentrant circuits) primarily located at the junction between the left atrium (LA) and the PVs. Lastly, autonomic inputs from LA autonomic ganglia, also clustered at the LA-PV junction, may initiate and sustain AF (See Figure 1. )4
Figure 1.
Acronym description in Figure 1.
- SVC = superior vena cava
- IVC = inferior vena cava
- LSPV = left superior pulmonary vein
- LIPV = left inferior pulmonary vein
- RSPV = right superior pulmonary vein
- RIPV = right inferior pulmonary vein
(Adapted from Heart Rhythm6)
Atrial fibrillation ablation attempts to address these, and other potential mechanisms by (1) isolating the PVs from the LA, (2) eliminating rotors, (3) eliminating LA autonomic connections, and (4) eliminating other potential initiating AF sites (superior vena cava, crista terminalis, fossa ovalis, ligament of Marshall, and adjacent AV valve annuli).5,6
The majority of patients with paroxysmal AF can be successfully treated by simply isolating the PVs from the LA. In contrast, patients with persistent or permanent AF require, in addition to PV isolation, further substrate modification.7 Since the initial focal PV ablation of AF performed in 1998, the technique of catheter-based ablation of AF has evolved. A general approach to the performance of catheter-based ablation of AF follows.
Method
Antiarrhythmic medications are discontinued five or more half-life’s prior to the AF ablation procedure (with the possible exception of Amiodarone due to its half-life of approximately two months). Studies frequently performed prior to AF ablation include transesophageal echocardiography (to exclude the presence of LA thrombus) and CT or MR imaging of the LA and PV anatomy (to improve procedure success and avoid procedure-related complications).
Vascular access may be gained percutaneously using femoral, left subclavian and right internal jugular veins. Puncture of the intra-atrial septum, using fluoroscopy and/or intra-cardiac echocardiography (ICE), is performed to access the LA. Diagnostic catheters may be positioned within the right atrium, right ventricle and coronary sinus. Using transseptal sheaths, mapping and ablation catheters are advanced to the LA. Following transseptal access, intravenous unfractionated heparin is administered.
Most centers employ a 3D mapping system to allow for nonfluoroscopic catheter manipulation, electrical activation and voltage mapping of LA structures, and the recording of ablation lesion location. Many centers perform PV venography to better define PV ostia size and location, thereby improving ablation catheter manipulation. ICE, after initially facilitating transseptal puncture, may allow for improved catheter manipulation, identify thrombus formation and avoid tissue overheating.6
Radiofrequency (RF) energy is the most commonly used energy source for catheter ablation of AF. RF energy delivered at the electrode-tissue interface produces heat that conducts to myocardial tissue resulting in coagulation necrosis and subsequent myocardial scar formation.8
Initially, PV isolation is performed by delivering RF energy at discrete points about individual PVs or ipsilateral PV pairs (depending upon venous anatomy and operator preferences). Subsequent ablation may include ablation of (1) the LA roof by delivery of RF energy between the superior PVs, (2) the mitral isthmus by delivery of RF energy from the mitral annulus to the left inferior PV ostium, and (3) discrete LA regions demonstrating complex electrical activity (See Figure 2).7
Figure 2.
Acronym description in Figure 2.
- LSPV = left superior pulmonary vein
- LIPV = left inferior pulmonary vein
- RSPV = right superior pulmonary vein
- RIPV = right inferior pulmonary vein
(From Curr Opin Cardiol 4)
Post Procedure Management
The HRS/EHRA/ELAS Consensus Statement recommends warfarin use for at least two months in all patients following an AF ablation procedure. After two months, a decision to discontinue warfarin should be based on patient risk factors for stroke. The consensus task force recommends long-term warfarin in patients following AF ablation with a CHADS2 score ≥ 2.6
To minimize the early recurrence of AF and/or new LA tachycardias frequently seen during the first three months following AF ablation, many centers temporarily reinitiate suppressive antiarrhythmic drug therapy.
Results
Success rates for atrial fibrillation ablation vary greatly and are dependent upon center experience and patient selection. Success rates will be higher for patients with paroxysmal AF (rather than persistent or permanent AF) and minimal structural heart disease (absence of large LA, systolic LV dysfunction and/or significant valvular disease). Single procedure success rates from experienced high-volume ablation centers for paroxysmal AF range from 70 to 75% and for persistent/permanent AF from 30 to 50%.4,6,10
Early recurrence of AF or a LA tachycardia is common during the first three months following an AF ablation. Since both may resolve spontaneously, repeat ablation should be deferred for three months. There will be a 5 to 10% incidence of very late AF recurrence (greater than 12 months) following AF ablation and a 20 to 40% need for a second ablation procedure. Most commonly, the underlying mechanism for both early and late recurrence of AF following catheter ablation is ectopic foci from reconnected PVs.6,9
In addition to maintaining sinus rhythm, AF ablation may have beneficial effects on the LA and left ventricle (LV). Studies have shown a decrease in LA size and an improvement in LA transport following AF ablation. Studies involving systolic heart failure patients have shown a decrease in LV size and an improvement in LV ejection fraction (from 5 to 20%) following AF ablation.6
Complications
Due to its complexity, the risk associated with AF ablation is higher than for ablation of other cardiac arrhythmias. As with success rates, complications and their frequency following AF ablation are dependent upon center experience and patient selection. Complications from AF ablation include (1) vascular injury, (2) cardiac perforation/tamponade, (3) TIA/stroke, (4) systemic embolism, (5) PV stenosis, (6) LA tachycardia, (7) phrenic nerve injury, (8) atrio-esophageal fistula, (9) acute pyloric spasm/gastric hypomotility, and (10) death. The incidence of complications from AF ablation varies widely but experienced high-volume centers have reported rates of serious complications of less than 5%. 4,6
Indications
In general, catheter ablation of AF should not be considered first-line therapy for the treatment of AF. The 2006 ACC/AHA/ESC Atrial Fibrillation Guidelines treatment algorithim for maintenance of sinus rhythm lists catheter ablation as second-line therapy for all categories of patients. Catheter ablation has a IIa recommendation as a “reasonable alternative to pharmacologic therapy to prevent recurrent atrial fibrillation in symptomatic patients with little or no left atrial enlargement”.2
The 2007 HRS/EHRA/ECAS Expert Consensus Statement on ablation of AF supports these recommendations. They state that “the primary indication for catheter AF ablation is the presence of symptomatic AF refractory or intolerant to at least one class 1 or 3 antiarrhythmic medication.” However, they recognize that catheter ablation of AF may be beneficial as first-line therapy in rare clinical situations.6
The presence of LA thrombus is the primary contraindication to catheter ablation of AF.
Future
The approach to catheter-based ablation of AF continues to evolve. Several new technologies are under development to improve procedural success and reduce complications. These new technologies can be separated into three categories: remote navigational technology, 3D imaging technology, and balloon ablation catheter technology.12
Remote navigation technology may allow for improved ablation catheter control, thereby improving ablation lesion contiguity. Currently, the two remote navigation systems available for clinical use are a magnetic system (Niobe II, Stereotaxis, St. Louis, MO) and a robotic system (Sensei, Hansen Medical, Mt. View, CA). While both navigation systems are becoming more widely utilized, long-term efficacy and safety trial data for use during catheter-based AF ablation have yet to be completed.12
Many centers currently integrate pre-procedurally acquired patient-specific 3D anatomic models of the LA and PVs obtained by CT or MRI with electroanatomic mapping systems. However, chamber geometry and size may vary intra-procedurally due to changes in heart rate, rhythm and volume status. Efforts are underway to achieve real-time or near real-time 3D imaging of LA anatomy during the ablation procedure using rotational angiography, intracardiac ultrasound and MRI.12
There is significant ongoing technological development of alternative energy sources and balloon-based catheter systems to achieve PV isolation. The four major balloon-based catheter ablation modalities undergoing evaluation are cryoballoon, endoscopic laser, ultrasound and balloon-based RF ablation. All have unique characteristics that may prove advantageous in the future. However, long-term efficacy in achieving PV isolation has yet to be established.12
At present, AF ablation is considered second-line therapy for patients who have failed antiarrhythmic drug therapy. However, some have proposed using AF ablation as first-line therapy in selected patients. Potential justification for using AF ablation as first-line therapy include the (1) ineffectiveness of antiarrhythmic drugs, (2) side effects of antiarrhythmic drugs, (3) superior outcomes, with potential for cure, of AF ablation, (4) low serious complication rate of AF ablation at experienced centers, and (5) possibility for discontinuation of Coumadin in some patients. However, others have argued against the use of AF ablation as first-line therapy due to the (1) lack of long-term safety and efficacy data and (2) potential for significant procedure-related complications.12
Summary
Atrial fibrillation, the most common cardiac arrhythmia requiring treatment, has significant morbidity and mortality consequences. Pharmacologic therapy, using anticoagulants, AV nodal blocking agents, and antiarrhythmics, remains the mainstay of treatment for AF. However, AF ablation, when performed by experienced centers on selected patients, has a significant potential for cure with a low rate of serious complications.
Presently, catheter ablation of AF is not considered first-line therapy for the treatment of AF. Instead, it should be reserved for patients with recurrent symptomatic AF refractory to at least one class 1 or 3 antiarrhythmic agent. While currently a second-line therapy, continued technological advances to achieve greater success and lower complication rates, along with pending studies prospectively comparing AF ablation with antiarrhythmic drugs, may lead to greater initial use of AF ablation in the future.
Biography
Richard Weachter, MD, MSMA member since 2004, and Sara Baig, MD, are at the University of Missouri Hospital and Clinics, Division of Cardiovascular Medicine.
Contact: weachterr@health.missouri.edu
Footnotes
Disclosure
None reported.
References
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