Atrial fibrillation (AFib) is an enormous clinical problem: it is the most common sustained cardiac arrhythmia, affecting approximately 2% of the adult population, and the most common arrhythmia necessitating hospital admission in the United States. The risk of stroke associated with AFib (estimated at 5% per year in nonrheumatic AFib) also makes the disease and its sequelae potentially devastating. While anticoagulation has been shown to decrease this risk, not all patients are candidates for warfarin therapy and anticoagulation has a risk of serious bleeding. 

As a result, aggressive therapeutic measures to restore and maintain sinus rhythm remain a frequent objective of clinical care. However, the use of antiarrhythmic medications has been called into question because of low efficacy (around 50% at one year for most agents), potentially serious cardiac and noncardiac side effects and even possible increases in mortality. This leaves medical therapy as a limited and suboptimal treatment for many patients. As such, nonpharmacological treatment alternatives, especially those incorporating the recent advances made in catheter ablation technology using radiofrequency energy,have gained wider acceptance. Those treatment options are the focus of this issue of the Arrhythmia News. 

AV Junctional Ablation 

In patients refractory to medical control of ventricular heart rate response in AFib or in whom symptoms such as palpitations or dyspnea are severely limiting, ablation of AV conduction in association with implantation of a permanent pacemaker may be an excellent treatment option. Pacemakers can be dual chamber with mode-switching capability in patients with paroxysmal AFib or single-chamber in patients with chronic AFib. To optimize heart rate response to exercise, the rate-responsive function of these pacemakers should be optimized using exercise testing. Several studies have documented a reduction in symptoms and an improved quality of life in the majority of patients. In patients with congestive heart failure, increases in the duration of exercise on standard stress testing as well as improvements in NYHA classification and left ventricular ejection fraction have also been demonstrated. While sudden cardiac death has been reported in the weeks to months following these procedures, the advent of radiofrequency ablation technology has made this risk small (<2%). While the mechanism of sudden death remains unclear, torsade de pointes associated with the newly-created, relative bradycardia has been described. Programming the lower rate of pacing to at least 70 beats/minute can avoid this potential complication. 

AV Nodal Modification 

Because AV junctional ablation creates a life-long dependency on permanent pacing, attempts have been made to modify AV nodal conduction have been developed. As a result of the experience gained from the use of catheter ablation for supraventricular tachycardias, conduction properties of specific AV nodal tissues have been determined. The posterior or "slow" pathway of the AV node is capable of producing and sustaining more rapid ventricular rates in AFib. Ablation of the slow pathway has been shown to produce significant decreases in the mean ventricular rates associated with AFib. Early published data supported the clinical efficacy of this technique; adequate rate control was said to be achieved in 70-80% of patients undergoing the procedure. However, data on long-term follow-up is more sketchy and less convincing. In addition, the incidence of complete heart block associated with the procedures is relatively high (approaching 20%). The Arrhythmia Service suggests that all patients undergoing AV nodal modification be prepared for the possibility of permanent pacing. However, in some patients, this treatment alternative can preserve native AV conduction, reduce the ventricular rate in AFib, and decrease symptoms. 

Curative Catheter Ablation of Atrial Fibrillation 

 Recently, significant interest has been generated by reports of curative radiofrequency ablation procedures for AFib. To clarify discussion regarding these procedures, published reports can be divided into those targeted at a single focus and those designed to reproduce the surgical maze procedure. 

Single Focus Ablation. As early as 1994, reports of single focus ablation of AFib have come from Haissaguerre and coworkers in France. In subsequent reports, these authors have continued to describe successful focal ablations in a well-defined subset of patients with AFib. These patients are young with structurally normal hearts; their AFib is paroxysmal; and most important diagnostically, on 24-hour Holter monitoring they have brief bursts of atrial tachycardia and frequent (usually monomorphic) APCs. During catheter mapping, the authors demonstrated that single foci were responsible for the APCs, the atrial tachycardias and the AFib. Lesions delivered to these foci produced immediate and long-term success. The distribution of these foci seems to be quite similar to that previously described for atrial tachycardia with a predominance in the region of the upper pulmonary veins. Using the careful diagnostic guidelines described above, curative ablation can be realistically offered to appropriate patient candidates. 

Transcatheter Maze Procedure. Unfortunately, much of the AFib encountered clinically is due to multiple reentrant wavelets within the atria; as such, mapping for a single focus for ablation is not possible. The maze procedure, which electrically segments the atria eliminating reentrant activity, requires extensive, open-heart surgery. Recent reports form several centers have suggested that catheter-delivered radiofrequency lesions may be able to produce an effective electrical maze.

Significant published experience for the catheter maze also comes from Haissaguerre. His technique, which initially focuses on the right atrium, also includes sequential lesions delivered linearly between the pulmonary vein ostia (see illustration on page 1). The Haissaguerre technique utilizes a specially designed catheter with multiple electrodes capable of delivering radiofrequency energy along the length of the catheter, thus eliminating the need for frequent catheter repositioning. As a result, procedural times and fluoroscopic exposure are in acceptable ranges. Extensive testing documenting lines of conduction block within the atria is not performed. Success rates for these procedures, defined as the ability of medication to control previously refractory AFib, are promising, on the order of 80%. Of note, however, is that follow-up periods are short (months) and drug-free success rates are less than remarkable. Other investigators, most notably Swartz, have utilized techniques which painstakingly create and documented multiple lines of block reproducing the surgical maze in both atria. While success rates have been promising, the procedures are time-consuming and seems to be associated with an intolerably high rate of stroke complicating the procedure.

Summary 

 Catheter-ablation techniques applied to the treatment of AFib offer promising alternatives in difficult to treat patient. In a small, highly selected subset of patients with AFib, a curative procedure can be offered.

Candidates for Single Focus Ablation of Atrial Fibrillation

• Young Patients
• No Structural Heart Disease
• Paroxysmal Atrial Fibrillation

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