Excerpted and adapted from "When the Beat is Off," Stroke Connection January/February 2004 (Science update May 2008)

An estimated 15 percent of strokes are a result of untreated atrial fibrillation (AF or A-fib), but most people don’t know what AF is – much less know if they have it. AF occurs in an estimated 2 million Americans and is the most common type of irregular heartbeat (arrhythmia). AF dramatically increases the risk of stroke – approximately five times over the general population.

To the beat of a different drum

The heart has four chambers. The bottom two, called the ventricles, are thick, powerful pumps that force 

• The upper two, the atria, serve as collecting chambers for the returning blood.
• The muscles of the atria are not thick because the atria only transport the blood to the ventricles next door.
• To contract and pump, the atria and ventricles must receive electrical stimulation.
• In a normal heart, a steady electrical impulse travels from a node in the atrium to the ventricle, signaling their successive contractions.
• Atrial fibrillation is a condition in which the atria quiver (or fibrillate) instead of fully contracting.
• Instead of a steady electrical impulse, a virtual blitzkrieg of electrical energy storms the atria.
• As a result of this quivering, only a portion of the atria’s blood is released into the ventricles.

Think of an atrium as a wet kitchen sponge. A few short squeezes would get only a little water out – even if you squeeze the sponge multiple times. But one hard squeeze (contraction) would empty the sponge more efficiently.

When blood has the opportunity to pool, it also has the opportunity to clot. If a blood clot forms in the atria, it can be pumped out of the heart, to the brain, blocking off the blood supply to an artery in the brain, causing a stroke.  This type of stroke is called an embolic stroke or some doctors call it a cardioembolic stroke. 

Healthy atria contract 60-80 times per minute, but fibrillating atria quiver at 300-400 times per minute.

Essentially, the heart of an AF patient works as if it’s enduring a marathon, even if the patient is relaxing in a chair. Because of the overactive heart, symptoms of AF most commonly include fatigue, lightheadedness, shortness of breath and even fainting. Some atrial fibrillation patients feel a “flopping” or pounding sensation in their chest. Some patients don’t feel a thing.

• The AV node serves as a “gatekeeper” for all of the electrical pulses going though the atrium.
• The electrical pulses are delayed at the AV node before they are allowed to move into the ventricles and signal them to contract.
• In AF, the AV node does its best to protect the ventricle from extra electrical impulses, but it can’t stop all of them.
• As a consequence, the ventricle beats more often than it should – giving rise to the noticeable symptoms of breathlessness and fatigue.

Risk Factors for Atrial Fibrillation