Rapid, uncoordinated contractions of the atria, the upper chambers of the heart
Atrial fibrillation is the most common type of rapid, irregular heart rate. It most often occurs in people over 60, and up to 1 in 20 very elderly people in the UK may be affected. In atrial fibrillation, the atria contract weakly and in an uncoordinated way at 300–500 beats per minute. Some of the electrical impulses that cause this rapid beating are conducted through the heart to the ventricles (the lower chambers), which also beat more rapidly than normal, at up to 160 beats per minute. Since the atria and ventricles are not beating in rhythm, the strength and timing of the heartbeat become irregular and less blood is pumped.
The most dangerous complication of atrial fibrillation is stroke, the risk of which increases with age. Since the atria do not empty properly during contractions, blood stagnates in them and may form a clot. If a part of the clot breaks off and enters the bloodstream, it may block an artery anywhere in the body (see Thrombosis and embolism). A stroke occurs when part of a clot blocks an artery supplying the brain.
Atrial fibrillation may occur for no apparent reason, especially in the elderly, but it is usually due to an underlying disorder that causes the atria to enlarge. Such disorders include heart valve disorders, coronary artery disease, and high blood pressure (see Hypertension). Smoking, lack of exercise, a high-fat diet, and being overweight are risk factors for many of these disorders. Atrial fibrillation is also common in people with an overactive thyroid gland (see Hyperthyroidism) or low potassium levels in the blood. It may occur in people who drink excessive amounts of alcohol.
Symptoms do not always develop, but, if they do, their onset is usually sudden. The symptoms may be intermittent or persistent and typically include:
Palpitations (awareness of an irregular or abnormally rapid heartbeat).
Shortness of breath.
Stroke and acute or chronic heart failure may be complications.
Your doctor may suspect atrial fibrillation if you have a fast, irregular pulse. To confirm the diagnosis, you will have electrocardiography (see ECG). You may have blood tests to look for an underlying cause such as hyperthyroidism. If a cause is found, treating it often cures the arrhythmia. If atrial fibrillation is diagnosed early, it may be treated successfully using cardioversion, in which a brief electric shock is applied to the heart.
Atrial fibrillation is usually treated with antiarrhythmic drugs such as beta-blockers, diltiazem (see Calcium channel blocker drugs), or digitalis drugs. These slow the conduction of electrical impulses from the atria to the ventricles, giving the ventricles time to fill with blood between heartbeats. Other antiarrhythmics may then be used to treat the irregular rhythm. You may also be prescribed warfarin, which reduces the risk of clots (see Drugs that prevent blood clotting) and thus lowers the risk of a stroke. If drug treatment is ineffective, your doctor may suggest a cardiac electrophysiological study with radiofrequency ablation, which can permanently cure certain cases of atrial fibrillation.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.