Atrial fibrillation, the abnormal heart rhythm Democratic presidential candidate Bill Bradley suffers from, presents risks that range from the trivial to the very serious, depending on numerous other variables in a person's health.

The condition is the most common arrhythmia, or heart rhythm disorder, in industrialized society. Its prevalence rises sharply with age, from about 0.5 percent in people Bradley's age, to more than 10 percent in men over age 75. It has many causes, and in a large percentage of cases, no cause is found.

The arrhythmia rarely causes death. However, it often produces a fast, irregular heartbeat that is unpleasant and can leave a person breathless or weak. The main worry is that in some--but not all--patients, atrial fibrillation markedly increases the risk of stroke.

Bradley--who apparently has "lone atrial fibrillation," unaccompanied by other heart problems--is in a group in which fewer than 1 percent of people have a stroke in any given year, according to several studies.

Stroke-preventing treatment with warfarin, an oral blood-thinner, is not advised for patients like him because it can cause as many problems as it prevents. Many take aspirin, which also has anticoagulant effects but is safer. Bradley, however, does not.

At highest risk for stroke are people with atrial fibrillation who are over age 75 and have one or more specific "risk factors": high blood pressure, a heart misshapen or damaged by disease, congestive heart failure or previous strokes. Each year, as many as 12 percent of such patients have a stroke. Their risk can be reduced by about 60 percent if they take warfarin and by about 20 percent if they take aspirin.

Bradley's physician, Robert H. Heissenbuttel, said yesterday that four recent episodes of atrial fibrillation "have not, in any way, interfered with [Bradley's] busy schedule." He said the normal course of Bradley's type of atrial fibrillation "includes sporadic increases in frequency of episodes that do not indicate any change in heart status."

Heissenbuttel, who practices at Columbia Presbyterian Medical Center in New York, said he has advised Bradley "to continue all normal activity without limitations."

Atrial fibrillation is a disease of the highly specialized electrical "wiring" of the heart.

Normally, a heartbeat arises as a spontaneous electrical signal in a part of the heart known as the sinoatrial node, which is in the right atrium, one of the thin-walled upper chambers of the heart. The signal causes the right atrium to contract, along with its twin, the left atrium. Blood then flows through valves connecting the atria to the ventricles, the main pumping chambers of the heart, which lie below them.

The movement of blood from atria to ventricles, however, takes time. To prevent the ventricles from getting the electrical signal before the blood arrives, the electrical signal that stimulates the muscle tissue to contract is slowed by an area of specially "insulated" tissue, called the atrioventricular node, at the border between the upper and lower chambers.

In atrial fibrillation, the pace-setting electrical signals no longer arise from the sinoatrial node. Instead, they come from dozens of spontaneously firing regions of tissue in one (or both) atria.

Often, there are hundreds of signals per minute. That rate is far too fast to stimulate contraction of the atria, and they stop working. They become passive holding chambers for blood, which drains into the ventricles by gravity and suction.

During an episode of atrial fibrillation, hundreds of electrical signals race down the heart's conduction system. Many are blocked and extinguished by the atrioventricular node. In most cases, however, an abnormal number get through, pushing the heart rate from about 80 beats per minute to up to 200 per minute.

The discomfort of atrial fibrillation comes from this rapid, irregular heartbeat. The risk of stroke derives not from the fact that the heartbeat is unusually fast, but from the fact that when the atria stop contracting, blood clots can form in them. Those clots--often quite large--can dislodge and travel to the brain.

When atrial fibrillation first occurs, physicians usually try to convert the rhythm to normal by giving the heart either a lower-power electrical shock, or by prescribing a course of "anti-arrhythmic" medicine. In the last several years, Bradley has had at least two electrical "cardioversions." He also now takes procainamide, a drug intended to lower the chance that he will go into atrial fibrillation again.

Procainamide is not the first, or even the second, drug recommended for that purpose. In a survey of cardiologists and electrophysiologists (cardiologists with special training in arrhythmias), it was ranked by each as the sixth drug they would choose to prevent "paroxysms" of atrial fibrillation, of the sort Bradley has.

Patients who are permanently in atrial fibrillation, or who go in and out of it frequently, often take medicines whose main purpose is to slow the rate and ease sensation of a pounding or palpitating heart. The recommended drugs are those in the beta blocker or calcium channel blocker class.

Staff writer Mike Allen in Iowa contributed to this report.

Bradley's Heart Condition

Atrial fibrillation makes it difficult for a heart to pump blood and can cause blood clots.

Normal heart: The sinoatrial node sends electrical signals into the heart's conduction system, stimulating regular muscle contractions.

Atrial fibrillation: Fast, irregular electrical impulses arise from many sites in the atria, delivering rapid and irregular signals to the conduction system.

SOURCES: Staff reports, Advocate Health Care