Guest Commentary
  by Vinod H. Thourani, MD and John D. Puskas, MD
Emory University School of Medicine, Atlanta, GA, United States
 
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Atrial Fibrillation: A Brief Review of Classification Systems

Introduction

Atrial fibrillation represents the most common clinically significant cardiac arrhythmia; it is estimated that over two million Americans are affected.  Atrial fibrillation is characterized by uncoordinated supraventricular (atrial) activation associated with an irregular rapid ventricular response.  Consequently, this irregularly, irregular rhythm leads to deterioration of normal atrial mechanical function.  Although the prevalence of atrial fibrillation remains under one percent in the general population for those less than 60 years old, it is estimated to affect six to ten percent of those patients over the age of 80.  It has been clearly shown that the presence of atrial fibrillation may convey significant morbidity and mortality.

Outside of a few renowned surgical centers, atrial fibrillation has generally been treated medically; by drug therapy designed to achieve rate control, rhythm control, and anticoagulation or by percutaneous ablative intervention.  Sub-optimal results of long-term medical management combined with the increase in elderly patients undergoing cardiac surgery with concomitant atrial fibrillation has led to a renewed interest in the elimination of atrial fibrillation utilizing surgical modalities.

The lack of a universally accepted classification scheme for atrial fibrillation has led to confusion in the medical and surgical literature and has undermined the validity of direct comparisons of treatment outcomes.  The purpose of this paper is to succinctly review the definitions and classification schemes of atrial fibrillation. 

The treatment of atrial fibrillation should be based on an understanding of its pathophysiology.  As noted by Dr. James L. Cox, atrial fibrillation may result when a premature electrical signal originating from within the pulmonary veins discharges into the left atrium, inducing multiple macroreentrant circuits.  This atrial fibrillation may may be self-limited or may continue until these circuits are thwarted by pharmacological therapy, electrical cardioversion, or interventional ablative therapies.  Subsequent aberrant premature atrial beats are required to re-initiate atrial fibrillation.  While these triggering impulses have been shown to originate from the pulmonary veins in 90% of cases, approximately 10% originate elsewhere in the atria. The entity of continuous atrial fibrillation requires the ability of the macroreentrant circuits in the atrial myocardium to sustain themselves without the requirement for additional aberrant impulse(s) from the pulmonary veins.  It has been hypothesized that the ability of macroreentrant circuits to be self-sustaining may be due to electrical remodeling within the atria.

Nomentclature

A variety of nomenclature terms have previously been used to describe atrial fibrillation, including “lone atrial fibrillation”, “idiopathic atrial fibrillation”, and “nonvalvular atrial fibrillation”.  The most recent executive summary endorsed by the American College of Cardiology, the American Heart Association, the European Society of Cardiology, and the North American Society of Pacing and Electrophysiology has classified atrial fibrillation paroxysmal, persistent, and permanent.  These terms are defined below:

1) Paroxysmal: recurrent, intermittent atrial fibrillation that previously terminated without specific therapy.  Paroxysmal atrial fibrillation is self-limited.

2) Persistent: recurrent, sustained atrial fibrillation that was previously terminated by therapeutic intervention.  Persisent atrial fibrillation may be the first presentation,  a culmination of recurrent episodes of paroxysmal atrial fibrillation or long-standing atrial fibrillation (greater than one year). Persistent atrial fibrillation is not self-limited, but may be converted to sinus rhythm by medical or electrical intervention.

3) Permanent: Continuous atrial fibrillation which cannot be converted to normal sinus rhythm by pharmacologic or electrical techniques. 
 
This ACC/AHA classification scheme differs from the simpler classification system proposed by Dr. Cox, and largely accepted by the surgical community.  In the Cox classification system, if atrial fibrillation is present all of the time, it is defined as continuous atrial fibrillation; if the atrial fibrillation is not present all of the time, it is defined as intermittent atrial fibrillation.  (This important distinction is directly linked to therapeutic decision-making.  Simple pulmonary vein encirclement may provide an adequate cure for those patients with intermittent atrial fibrillation since in this subset of patients, pulmonary veins usually provide the necessary aberrant electrical signal.  However, for patients with continuous atrial fibrillation, simple pulmonary vein isolation procedure is not usually adequate therapy.)

Atrial fibrillation may be viewed as a disease continuum, in which patients often progress from intermittent self-limiting paroxysms to more persistent episodes requiring medical intervention to refractory atrial fibrillation unresponsive to medical intervention.  Thus, the categorization of patients with late-stage “persistent” atrial fibrillation may be contentious.  Nonetheless, in general terms the ACC/AHA and Cox surgical classification systems may be roughly accommodated by recognizing that “paroxysmal” and “persistent” atrial fibrillation are by definition “intermittent”.  “Permanent” atrial fibrillation is by definition “continuous”. 

Although the Cox-Maze procedures have been used with excellent success by surgeons as the classic surgical ablation procedure, the complexity of these procedures has precluded their wide spread adoption.  As a compromise, the mini-maze procedure has been described as a simple, yet effective treatment of both continuous or intermittent atrial fibrillation.  Lesions in the mini-maze procedure include a pulmonary vein encircling incision, left atrial isthmus lesion with its attendant coronary sinus lesion, and a right atrial isthmus lesion.  A variety of alternative energy sources and devices have been recently utilized to create these lesions effectively both endocardially and epicardially.

References
  1. Cox JL. Atrial fibrillation I: A new classification system.  J Thorac Cardiovasc Surg. 2003;126:1686-92.
  2. Cox JL. Atrial fibrillation II: Rationale for surgical treatment.  J Thorac Cardiovasc Surg. 2003;126:1693-9.
  3. Fuster V, Ryden LE, Asinger RW, Cannom DS, Crijns HJ, Frye RL, et al. ACC/AHA/ESC guidelines for the management of patients with atrial fibrillation: executive summary. A report of the American College of Cardiology/American Heart Association Task Force on Practice Guidelines and the European Society of Cardiology Committee for Practice Guidelines and Policy Conferences (Committee to Develop Guidelines for the Management of Patients with Atrial Fibrillation) developed in collaboration with the North American Society of Pacing and Electrophysiology.  J Am Coll Cardiol. 2001;38:1231-65.
  4. Song HK and Puskas JD. Recent advances in surgery for atrial fibrillation.  Posted at http://www.ctsnet.org/doc/8509 in December, 2003.