What is Atrial Fibrillation?
Atrial fibrillation (atrial fibrillation or afib) is an abnormal heart rhythm (cardiac arrhythmia) which involves the two small, upper heart chambers (the atria). Heart beats in a normal heart begin atrial fibrillationter electricity generated in the atria by the sinoatrial node spread through the heart and cause contraction of the heart muscle and pumping of blood. In atrial fibrillation, the regular electrical impulses of the sinoatrial node are replaced by disorganized, rapid electrical impulses which result in irregular heart beats.
Atrial fibrillation is the most common cardiac arrhythmia. The risk of developing atrial fibrillation increases with age. Atrial fibrillation effects four percent of individuals in their 80s. An individual may spontaneously alternate between atrial fibrillation and a normal rhythm (paroxysmal atrial fibrillation) or may continue with atrial fibrillation as the dominant cardiac rhythm without reversion to the normal rhythm (chronic atrial fibrillation). Atrial fibrillation is often asymptomatic, but may result in symptoms of palpitations, fainting, chest pain, or even heart failure. These symptoms are especially common when atrial fibrillation results in a heart rate which is either too fast or too slow. In addition, the erratic motion of the atria leads to blood stasis which predisposes to blood clots which may travel from the heart to the brain and other areas. Thus, atrial fibrillation is an important risk factor for stroke, the most feared complication of atrial fibrillation.
The symptoms of atrial fibrillation may be treated with medications which slow the heart rate. Several medications as well as electrical cardioversion may be used to convert atrial fibrillation to a normal heart rhythm. Surgical and catheter-based therapies may also be used to prevent atrial fibrillation in certain individuals. People with atrial fibrillation are often given blood thinners such as warfarin to protect them from strokes.
Atrial Fibrillation: Prognosis
Atrial fibrillation can usually be controlled with treatment. The natural tendency of atrial fibrillation, however, is to become a chronic condition. Chronic atrial fibrillation leads to an increased risk of death. Patients with atrial fibrillation are at significantly increased chance of stroke (about 2 to 7 times the regular population).
Pathophysiology
The normal electrical conduction system of the heart allows the impulse that is generated by the sinoatrial node (SA node) of the heart to be propagated to and stimulate the myocardium (muscle of the heart). When the myocardium is stimulated, it contracts. It is the ordered stimulation of the myocardium that allows efficient contraction of the heart, thereby allowing blood to be pumped to the body.
In atrial fibrillation, the regular impulses produced by the sinus node to provide rhythmic contraction of the heart are overwhelmed by the rapid randomly generated discharges produced by larger areas of atrial tissue. It can be distinguished from atrial flutter, which is a more organized electrical circuit usually in the right atrium that produces characteristic saw toothed waves on the electrocardiogram.
Often, the rhythm produced is more rapid than normal, but the difficulty is in obtaining control of the heart rate both at rest and with exercise. Good rate control will usually require two drugs, and can only be checked by observing heart rate response to exercise.
An organized electrical impulse in the atrium produces atrial contraction; the lack of such an impulse, as in atrial fibrillation, produces stagnant blood flow, especially in the atrial appendage and predisposes to clotting. The dislodgement of a clot from the atrium results in an embolus, and the damage produced is related to where the circulation takes it. An embolus to the brain produces the most feared complication of atrial fibrillation, stroke, while an embolus may also lodge in the mesenteric circulation (the circulation supplying the abdominal organs) or digit, producing organ-specific damage.
Atrial Fibrillation: Treatment
The main goals of treatment of atrial fibrillation are to prevent temporary circulatory instability and to prevent stroke. Rate and rhythm control are principally used to achieve the former, while anticoagulation may be required to decrease the risk of the latter.
Atrial fibrillation can cause disabling and annoying symptoms. Palpitations, angina, lassitude (weariness), and decreased exercise tolerance are related to rapid heart rate and inefficient cardiac output caused by atrial fibrillation. There are two ways to approach these symptoms: rate control and rhythm control. Rate control treatments seek to reduce the heart rate to normal, usually 60 to 100 beats per minute. Rhythm control seeks to restore the normal heart rhythm, called normal sinus rhythm. Studies suggest that rhythm control is mainly a concern in newly diagnosed atrial fibrillation, while rate control is more important in the chronic phase. Rate control with anticoagulation is as effective a treatment as rhythm control in long term mortality studies, the atrial fibrillation FIRM Trial (Wyse et al., 2002).
Atrial fibrillation with a persistent rapid rate can cause a form of heart failure called chronotropic cardiomyopathy. This can significantly increase mortality and morbidity. The early treatment of atrial fibrillation through either rate-control or rhythm-control can prevent this condition and thereby improve mortality and morbidity.
Atrial Fibrillation and the Cox Maze Procedure
James Cox, MD, and associates developed the Cox maze procedure, an open-heart surgical procedure intended to eliminate atrial fibrillation, and performed the first one in 1987. "Maze" refers to the series of incisions made in the atria (upper chambers of the heart), which are arranged in a maze-like pattern. The intention was to eliminate atrial fibrillation by using incisional scars to block abnormal electrical circuits (atrial macroreentry) that atrial fibrillation requires. This procedure required an extensive series of endocardial (from the inside of the heart) incisions through both atria, a median sternotomy (vertical incision through the breastbone) and cardiopulmonary bypass (heart-lung machine). A series of improvements were made, culminating in 1992 in the Cox maze III procedure, which is now considered to be the "gold standard" for effective surgical cure of atrial fibrillation. The Cox maze III is sometimes referred to as the "traditional maze", the "cut and sew maze", or simply the "maze".
Atrial Fibrillation and the Minimaze Procedure
Minimaze surgery is minimally invasive cardiac surgery intended to cure atrial fibrillation. Minimaze refers to "mini" versions of the original maze procedure. These procedures are less invasive than the Cox maze procedure and do not require a median sternotomy (vertical incision in the breastbone) or cardiopulmonary bypass (heart-lung machine). These procedures use laser, microwave, radiofrequency, or acoustic energy to ablate atrial tissue near the pulmonary veins.
AV Nodal Ablation
In patients with atrial fibrillation where rate control drugs are ineffective and it is not possible to restore sinus rhythm using cardioversion, non-pharmacological alternatives are available. For example, to control rate it is possible to destroy the bundle of cells connecting the upper and lower chambers of the heart - the atrioventricular node - which regulates heart rate, and to implant a pacemaker instead. A more complex technique involves ablating groups of cells near the pulmonary arteries where atrial fibrillation is thought to originate, or creating more extensive lesions in an attempt to prevent atrial fibrillation from establishing itself.
Radiofrequency Ablation
Radiofrequency ablation (RFA) uses radiofrequency energy to destroy abnormal electrical pathways in heart tissue. It is used in recurrent atrial fibrillation. The energy emitting probe (electrode) is placed into the heart through a catheter. The practitioner first "maps" an area of the heart to locate the abnormal electrical activity before the responsible tissue is eliminated. Ablation is a newer technique and has shown some promise for cases unresponsive to conventional treatments. New techniques include the use of cryoablation (tissue freezing using a coolant which flows through the catheter), and microwave ablation, where tissue is ablated by the microwave energy "cooking" the adjacent tissue. The abnormal electrophysiology can also be modified in a similar way surgically, and this procedure referred to as the Cox maze procedure, is commonly performed concomitantly with cardiac surgery. More recently, minimally invasive surgical variations on the Cox Maze procedure (" minimaze " procedures) have also been developed.
This is an area of active research, especially with respect to the RF ablation technique and emphasis on isolating the pulmonary veins that enter into the left atrium.