The study presented Monday found that women who have atrial fibrillation were more than four times more likely to suffer a stroke than were men with the disorder.
The findings by scientists at Bispebjerg University Hospital in Copenhagen, Denmark, were presented Monday at the annual meeting of the European Society of Cardiology in Vienna.
Atrial fibrillation is the most common type of heart rhythm abnormality. It is estimated to affect up to 2 percent of all people, and up to 14 percent of the elderly.
Symptoms include palpitations, breathlessness and tiredness; the major hazard is an increased risk of stroke. It is about three times more common among men than women.
Nearly 5.7 million Europeans are estimated to have atrial fibrillation, a condition in which the two small upper chambers of the heart quiver instead of beating properly. In the United States, about 2 million people have it.
"This (study) raises awareness of the impact of cardiovascular disease among women," said Dr. Sidney Smith, a professor of cardiology at the University of North Carolina at Chapel Hill and former president of the American Heart Association. "It's one more example that women are not protected from cardiovascular disease. We should do everything we can to dispel that myth that if you're a woman you don't need to worry."
Smith, who was not connected with the research, noted that doctors had long believed that women were less vulnerable to heart disease than men, but that evidence is accumulating that they may not be.
For instance, scientists suspected that the reason heart disease deaths are lower among women was because the female hormone estrogen offered protection. However, recent studies have shown that hormone replacement therapy does not reduce women's risk of heart attacks or strokes.
The study involved 29,310 Danes over age 40 who had never had a stroke. They were followed for about five years.
Among them, 166 men and 110 women had atrial fibrillation.
During the study, there were 635 strokes, with 35 of them occurring in people with the irregular heartbeat.
Women with the disorder were nearly eight times more likely to have a stroke than those with a normal heart beat and four times more likely than men with the disorder. Men with the problem were twice as likely as other men to have a stroke.
Researchers also found women with the disorder were more than twice as likely to die from heart or blood vessel problems than men with an irregular heart beat. The disorder was far more likely to lead to stroke in women than other factors, such as diabetes or high blood pressure, which ranked higher among men.
The study's leader, Dr. Jens Friberg, said it is unclear why women with irregular heartbeats fare worse than their male counterparts.
However, he said that since atrial fibrillation is more common in men, it might be that women who develop the problem have sicker hearts.
"We should be especially worried when atrial fibrillation occurs in women," Friberg said.
Dr. Robert Hatala, a professor of the Slovak Cardiovascular Institute in Bratislava, said Friberg's study means that doctors probably need to treat women much more vigorously than they have been, especially with anticoagulants, or blood thinners, which dissolve clots.
Atrial fibrillation results when some of the blood in the upper chambers of the heart stays still, leading to a high risk of blood clot formation.