WEDNESDAY, Jan. 11 (HealthDay News) -- Many people may not feel their heart race when they are having an irregular heartbeat known as atrial fibrillation, but these silent symptoms double their risk of stroke, a new study finds.

Of 2,580 study participants with pacemakers who did not have a history of atrial fibrillation, more than one-third experienced pacemaker-documented episodes that lasted for more than six minutes, researchers said. However, 85 percent of these people did not realize it because there were no obvious symptoms. Pacemakers are placed in the chest to control abnormal heart rhythms. The results appear in the Jan. 12 issue of the New England Journal of Medicine.

The findings don't mean that everyone should be tested or treated for silent atrial fibrillation (AF), but they do argue for awareness and tighter control of known stroke risk factors such as high blood pressure.

"In patients with pacemakers, we do see a very high prevalence of silent AF that is not recognized by the patient," said study author Dr. Jeff Healey, an associate professor of medicine of the Michael G. DeGroote School of Medicine at McMaster University in Ontario, Canada. "Even though they are silent, these episodes are clearly associated with risk of stroke."

Individuals who had one silent atrial fibrillation episode within the first three months of the study were twice as likely to have a stroke, when compared to their counterparts who did not experience any bouts, the study showed. An episode of atrial fibrillation was defined as an irregular heartbeat that lasted at least six minutes.

This risk increased with each additional risk for stroke such as high blood pressure and diabetes, Healey said. All participants were 65 or older and had a history of high blood pressure. Researchers followed the patients for about 2.5 years.

"We know that high blood pressure is a very important risk factor for stroke, and this study reinforces the importance of good primary care to pick up these risk factors," he said. "Further research will tell us if it makes sense to screen for silent AF in certain high-risk populations."

Another expert said common sense should prevail.

"We should not check everyone for silent AF," said Dr. Marc Gillinov, a heart surgeon at the Cleveland Clinic. "If you feel palpitations or your heart racing, let your doctor know, but otherwise I would not rush to the doctor. The cause of the stroke is unknown in about 25 percent of people, and a lot of us think maybe subclinical AF plays a role. This study helps answer that piece of the puzzle."

Many questions remain, he said. "The big question is how much AF is too much because the drugs used to treat AF are very powerful and have their own issues with safety," he says. "Does one six-minute episode buy you a lifetime of anticoagulants, or do you need 10 times that amount?"

Dr. Neil Sanghvi, an electrophysiologist at Lenox Hill Hospital in New York City, said the new study was "thought-provoking." He said people should discuss their personal risk factors for stroke with their doctor to come up with a plan. "The average patient who has no symptoms should just have a conversation with their physician," he added.

More information

Learn how atrial fibrillation is treated at the U.S. National Heart, Lung, and Blood Institute.