Atrial fibrillation

Atrial fibrillation (AF) is an electrical rhythm disturbance of the heart affecting the atria. Abnormal electrical impulses in the atria cause the muscle to contract erratically and pump blood inefficiently. The atrial chambers are thus not able to completely empty blood into the ventricles.

Pooling of blood in the atria can cause red blood cells to stick together and form a clot. Most clots within the heart form in a portion of the left atrium known as the left atrial appendage. The most worrysome complication of atrial fibrillation is dislodgement of a clot and embolism of the clot material to one of the major organs of the body (e.g., the brain). A clot that embolizes to the brain can interrupt blood flow to a portion of the brain, resulting in a cerebrovascular accident, more commonly known as a stroke. Most individuals with atrial fibrillation are advised by their doctors to take one of a number of medications to prevent clot formation within the heart (and therefore reduce the risk of stroke secondary to clot embolism from the heart). The most commonly used medications are warfarin and aspirin.

A type of AF, termed paroxysmal, is when the arrhythmia occurs on an unexpected, intermittent basis. These patients tend to have less heart damage than those with chronic (sustained) AF.

Causes

AF is an intermediate-phase disease, meaning that the condition usually develops in response to damage to the heart, or by changes in the cardiovascular system. Causes include:

• Untreated hypertension
• Abnormal mitral valve function
• Heart surgery
• Coronary heart disease
• Excessive alcohol consumption ("binge drinking")
• Hyperthyroidism

In turn, untreated AF can precipitate further damage to the heart by weakening the muscles of the ventricles. This weakened condition of the ventricles is termed cardiomyopathy, which in turn leads to an end-stage condition called heart failure.

Treatment

There are three distinct clinical issues that may require treatment in AF. Firstly, AF dramatically increases the risk of stroke. This is due to embolization of thrombus material that forms in the left atrium during atrial fibrillation. Treatment of AF patients over age 60 with warfarin (also known as Coumadin) results in a significant reduction in the subsequent risk of stroke. Patients under age 60 who have any structural heart disease (ie: valvular heart disease, ejection fraction <= 35%, history of heart attack) also benefit from warfarin. Patients under age 60 who do not have structural heart disease do not require warfarin, and can be treated with aspirin.¹

Secondly, AF can cause disabling and annoying symptoms. Palpitations, angina, lassitude, and exercise intolerance are related to rapid heart rate and inefficient cardiac output caused by AF. 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.

Thirdly, AF can induce a reduction in left ventricular ejection fraction, called cardiomyopathy. This can significantly increase mortality and morbidity. Controlling the rate and possibly the rhythm can improve mortality and morbidity.

Rate control methods include:

• beta blockers
• calcium channel blockers
• digoxin

These medications work by slowing the generation of impulses from the atrium and the conduction of those impulse from the atrium to the ventricle.

Rhythm control methods include:

• electrical cardioversion ? An external defibrillator, applied to the chest while the patient is sedated, works in principal by a sudden electrical shock to the heart, which can induce a normal rhythm. This procedure is moderately successful in the short term, but most patients will relapse within one year. The key risk factor for relapse is duration of AF
• radio frequency ablation ? this procedure commonly uses radiofrequency energy to destroy abnormal electrical pathways in heart tissue. 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.
• Amiodarone
• Propafenone
• Sotolol
• Flecainide

These anti-arrhythmic medications alter the flux of ions in heart tissue, making them less excitable, setting the stage for spontaneous and durable cardioversion. These medications are often used in concert with electrical cardioversion. However, there is no difference in risk of stroke in patients who have converted to a normal rhythm, compared to those who have not, whether or not they are treated with anti-arrhythmic agents. Therefore, the only rationale for rhythm control methods is to control symptoms. If the patient is asymptomatic, there is no benefit for rhythm control. This is an area of active research, especially with respect to the RF ablation technique

References

1. . J Am Coll Cardiol 2001;38:1266i-1xx