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Sunday, January 20, 2013

How does atrial fibrillation increase stroke risk? - Ask Doctor K, Harvard Medical School


I have atrial fibrillation. It doesn’t bother me, but I still have to take medications because the condition apparently increases my risk of stroke. How does it do that? 


Atrial fibrillation is a heart rhythm disorder that causes a rapid and irregular heartbeat.

The atria are the upper two chambers of the heart; they receive blood from the rest of the body. The atria pump blood into the lower two chambers of the heart (the ventricles). Then the ventricles pump blood to the rest of the body. During atrial fibrillation, the atria do not beat normally. Instead, they quiver or “fibrillate.”

Normally, electrical impulses from a small part of the atrium cause the atria to beat. The electrical signal then travels down to the ventricles and causes them to beat. The atria pump, filling the ventricles with blood, and then the ventricles pump. It is coordinated so that the heart pumps blood efficiently.

In contrast, during atrial fibrillation, the electrical impulses come from all over the atria. Instead of pumping efficiently, the atria just quiver. There is an irregular and rapid heartbeat, and no coordination between the atria and the ventricles. That means that the ventricles may pump when they don’t have a lot of blood in them.

Most important, in terms of your question, the atria do not pump all of their blood into the ventricles. They’re very inefficient. As a result, blood pools inside the left atrium. When blood isn’t moving, it tends to form clots. Those clots can break free and travel out of the heart.

That’s when the trouble begins. As a clot travels in the blood, it moves through progressively narrower blood vessels. Finally, the clot reaches a blood vessel that’s so narrow it gets stuck. When it’s stuck, none of the blood behind it can get past the obstruction. As a result, some tissue will be deprived of its blood supply.

Some organs are big enough that it doesn’t matter much if the blood supply to a small part of the organ is stopped. But when the clot gets stuck in an artery to the brain, it can obstruct blood flow and cause a stroke. (See illustration.)

Atrial fibrillation and stroke

During atrial fibrillation, the upper chambers of the heart (atria) quiver rapidly rather than contract forcefully. Blood pools along the walls of the left atrium, eventually forming clots that may break free to travel through the left ventricle to the aorta. If the clot lodges in an artery to the brain, it may obstruct blood flow downstream and cause an ischemic stroke.

That’s why you are most likely taking blood-thinning medications. Most people with atrial fibrillation should be. These drugs prevent blood clots, thereby reducing the risk of a stroke and other serious complications.

Atrial fibrillation is linked to stroke also through atherosclerosis, the buildup of plaque in blood vessel walls. Atherosclerosis of the coronary arteries is one cause of atrial fibrillation, and atherosclerosis of the arteries of the brain can cause strokes.

The saddest thing for me to witness as a doctor is a patient who has chosen not to take the medicine he or she has been prescribed and pays a price. Unfortunately, I’ve had several patients who had strokes — unnecessarily. It breaks my heart.

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