Atrial Fibrillation

We keep learning more ways in which women's risk of heart disease is different from men's. A recent report indicating that atrial fibrillation, a condition in which the upper chambers of the heart quiver rather than contract, has provided a new lesson. According to data from the Framingham Heart Study, an ongoing project involving more than 5,000 people, atrial fibrillation is associated with a higher risk of death for women than for men. Atrial fibrillation not only increases the risk of dying when it occurs with other cardiovascular conditions, such as heart attack, congestive heart failure, or stroke, it also doubles the risk of death in otherwise healthy women.

Atrial fibrillation is the result of a disturbance in the electrical conductivity of the heart. The myocardium, or heart muscle, is threaded with nerve fibers that transmit electrical impulses throughout it. These impulses are generated in the right atrium by a bundle of nerves called the sinus node. They flow along a prescribed pathway, stimulating thousands of individual muscle fibers to contract in a synchronized fashion, thus producing rhythmic contractions of the myocardium.

The heart's contractions determine the direction of blood flow. The resting heart accepts oxygenated blood from the lungs into the left atrium. The left atrium contracts, opening the mitral valve and squeezing blood into the left ventricle (arrow). The nerve impulses flow from the atria, triggering contractions of muscle fibers in the left ventricle, thereby stimulating that chamber to contract. The contraction of the left ventricle is what we know as a heart beat.

Atrial fibrillation is one of several types of heart-rhythm disturbances, or arrhythmias. It occurs when the muscle fibers in the upper chambers of the heart contract in an unsynchronized fashion. As a result, the atria merely quiver rather than contract. Not only are the atria unable to squeeze out all the blood they contain, but nerve impulses also reach the ventricles in an uncontrolled fashion. The left ventricle contracts rapidly and erratically, and heart rate accelerates.

A number of factors -- including the excessive production of thyroid hormone (hyperthyroidism), a previous heart attack, high blood pressure, congestive heart failure, valve disease, and congenital disorders like Wolff-Parkinson-White syndrome that affect the heart's conductivity --can overstimulate the sinus node. Alcohol and caffeine consumption and high fevers can also produce bouts of atrial fibrillation. In many cases, particularly in younger people, there is no apparent cause.

Some people with atrial fibrillation have no symptoms whatsoever. Those who have sporadic episodes of atrial fibrillation commonly report feeling palpitations, or merely having an acute awareness of their heart beats. Some people experience the symptoms of inadequate blood flow and low oxygen levels -- dizziness, chest pain, and fainting. Chronic atrial fibrillation may cause congestive heart failure, which is often experienced as shortness of breath during exercise and fluid accumulation in the feet and legs.

The first priority in treating atrial fibrillation is to reduce heart rate, and treatment depends on the severity of symptoms. Most people can be treated with oral medications such as digoxin (Lanoxin), verapamil (Calan, Isoptin, Verelan), and beta-blockers like atenolol (Tenormin) and propranolol (Inderal). If hyperthyroidism is diagnosed and treated, heart rate is likely to stabilize as well. Patients with symptoms of heart failure usually require hospitalization to bring heart rates under control. Cardioversion -- treatment with an electric shock or high doses of drugs that are designed to restore normal rhythms -- is sometimes necessary.

In general, people with chronic atrial fibrillation are at an increased risk of stroke. The recent Framingham report indicated that although atrial fibrillation is associated with a risk of stroke of only 1.5% annually for women ages 50-59, the annual risk increases to 23.5% for those 80-89. The cause has been traced to the increased likelihood of clot formation as blood pools in the left atrium. Such clots may break off and travel to and obstruct arteries that feed the brain.

There is increasing evidence that regular treatment with low doses of the anticoagulant warfarin (Coumadin) reduces stroke risk in people with atrial fibrillation. However, Coumadin isn't for everyone; in some people, it may increase the risk of strokes caused by bleeding. Thus, patients most likely to benefit from Coumadin treatment are those with the greatest risk of clotting. An echocardiogram -- an ultrasound image of the heart -- may reveal an enlarged atrium, which fosters clot development.

A single episode of atrial fibrillation, while usually not life-threatening, can serve as a wake-up call. It's a good idea to see your clinician to determine whether an overactive thyroid or other condition was responsible. It's also a reminder to take care of yourself: Get lots of sleep, avoid stress, watch your alcohol and caffeine consumption, and explore cessation programs if you're a smoker.

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