Add another cardiovascular problem to the list of life-threatening conditions associated with obesity: atrial fibrillation.

The risk of this irregular heartbeat, which can lead to stroke and heart failure, is directly related to an increase in body mass index, the standard measure of obesity, said a report from the long-running Framingham Heart study appearing in the Nov. 24 Journal of the American Medical Association.

"If you are obese, with a body mass index of 30 or higher, you have a 50 percent higher risk of developing atrial fibrillation compared to those with a body mass index of 21 to 25," said Dr. Thomas J. Wang, a research associate with the study who is a cardiologist at Massachusetts General Hospital in Boston.

Body mass index (BMI) is a person's weight in kilograms divided by height in meters. Someone with a BMI of 25 or higher is regarded as overweight; someone with a BMI 30 or over is obese. A 5-foot, 4-inch woman with a BMI of 30 weighs 174 pounds, while a 6-foot man with a BMI of 30 weighs 221 pounds.

The study was not designed to determine why obesity leads to atrial fibrillation, in which the upper chambers of the heart lose the ability to pump blood effectively, Wang said, but other studies offer clues to that relationship.

"The thought is that this is mediated by differences in the size of the upper chambers of the heart," he said. "It is known that obese persons tend to have larger heart chambers, and separate studies have shown that enlargement of the heart is associated with atrial fibrillation."

There are other relationships between obesity and the condition, said Dr. Michael Argenziano, assistant professor of surgery at Columbia University Medical Center in New York and an expert in the surgical treatment of atrial fibrillation.

Obesity can cause high blood pressure, which puts a strain on the heart, Argenziano said. It also can lead to sleep apnea, a condition in which breathing is interrupted frequently at night and which also strains the heart.

Treatment of atrial fibrillation starts with an effort to eliminate conditions that promote it, such as thyroid disease and high blood pressure, he said. "Until now, no one has included obesity on that list," he said. "Now we can say it might be worth trying weight loss."

Drug treatment of atrial fibrillation often is unsuccessful because adverse side effects can outweigh their benefits, Argenziano said. Surgery has become a commonly used treatment in recent years, because of new techniques that make it less stressful for the patient.

The aim of surgery is to isolate areas of the heart that are known to initiate atrial fibrillation and to create pathways through which normal beat-controlling electrical signals can pass, Argenziano said. When the operation was first done about 15 years ago, it required open-heart surgery.

Now, the procedure can be done introducing catheters, thin tubes that deliver microwave impulses, "through nothing but puncture wounds," Argenziano said. Some incisions are no larger than a third of an inch, he said.