Atrial fibrillation occurs when stray electrical pulses set off muscle contractions that disrupt the coordinated top-to-bottom pumping action of the atria: two small blood-receiving chambers atop the heart.

That disruption can cause the atria to beat more than 300 times a minute and hamper the ability of the atria to carry out their function of filling the heart’s main pumps, the ventricles, with blood at the start of every heartbeat.

Fibrillation is more chaotic than another common rhythm irregularity known as atrial flutter. And it is harder to treat because it is far less likely than the flutter condition to be confined to the right atrium. The fibrillation can transform the normally coordinated pumping into a disorganized clash of localized mini-contractions that doctors say makes the muscle fibers of the atria look like a bucket of writhing worms.

Until the 1990s, atrial fibrillation was noted mostly for the number of false alarms it generated among patients who showed up at emergency rooms fearing they were having heart attacks.

Doctors viewed it as relatively benign because the most common symptoms — palpitations, dizziness and shortness of breath — were tolerable and often short-lived. No matter how bad patients may have felt, enough blood still flowed into the ventricles to sustain adequate circulation, as long as the ventricles remained healthy.

But doctors now recognize that atrial fibrillation allows blood to pool in the atria and form clots, which in turn may explain why such patients are prone to strokes and heart attacks.

About a third of strokes in patients 80 years or older are attributable to atrial fibrillation, and such strokes are more likely to be deadly than other types, according to studies summarized by Dr. Andrew E. Epstein, a researcher at the University of Alabama in Birmingham, at a meeting of heart rhythm specialists last month in Denver.

And research suggests the added burden that inefficient atrial pumping puts on the ventricles may contribute over time to heart failure.