Patients who have a heart rhythm disorder that can come with a high risk of stroke often don't receive blood-thinning medications that can make this complication less likely, a U.S. study suggests.

Researchers studied almost 430,000 people with a condition known as atrial fibrillation, an irregular rapid heartbeat that can lead to stoke, heart failure and chronic fatigue.

More than half of the atrial fibrillation patients at the highest risk of stroke didn't get prescriptions for blood thinners - also called anticoagulants - the study found. Most strokes occur when a clot blocks a blood vessel supplying the brain; blood thinners can prevent clots from forming or stop existing clots from getting bigger.

"We have identified a continued treatment gap in patients with atrial fibrillation at risk for stroke," said lead study author Dr. Jonathan Hsu, a heart specialist at the University of California, San Diego.

"Our study should act as a wake-up call - we as practitioners and patients need to be aware of the stroke risk that exists with atrial fibrillation and find ways to do better in treating at-risk patients with potentially life-saving oral anticoagulants," Hsu added by email.

In atrial fibrillation, electrical impulses in the upper chambers of the heart are chaotic, and the atrial walls quiver rather than contract normally in moving blood to the lower chambers. This can lead to the formation of clots, and atrial fibrillation patients have up to seven times the stroke risk of people without the disorder.

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To see how many atrial fibrillation patients receive needed pills to avert clots, Hsu and colleagues reviewed medical and prescription data collected from 2008 to 2012 in a national registry of people with cardiovascular disease.

The patients were 71 years old on average and about 56 percent were men.

Overall, 45 percent of them received blood thinners, researchers report in JAMA Cardiology.

Another 31 percent of the patients took aspirin, which can reduce the risk of strokes by preventing platelets in the blood from attaching to debris on blood vessel walls and can also lead to clots. Some of the people on aspirin paired it with anti-platelet medications known as thienopyrdine.

Roughly one in four patients took no medication at all to avoid clots or strokes.

Side effects may partly explain the limited use of blood thinners.

Long-term use of the blood thinner warfarin (Coumadin, Jantoven) can increase the risk of severe bleeding, and it's inconvenient because it requires frequent lab tests and check-ups to ensure that patients have the right dose.

One limitation of the study is that researchers lacked data on whether patients had medical reasons that would prevent their use of blood thinners, the authors note. People with conditions that can increase the risk of bleeding problems - like high blood pressure, cancer, kidney problems or alcoholism - might not be able to take warfarin.

Another shortcoming is that they assessed blood thinner prescriptions at initial doctor visits, and didn't see whether people stopped or started medication based on follow-up exams, the researchers also point out.

"It takes time and effort to have the appropriate discussion with patients and their families and to prescribe and monitor these therapies," said Dr. Gregg Fonarow, co-director of the University of California, Los Angeles Preventive Cardiology Program and researcher at the David Geffen School of Medicine at UCLA.

Anticoagulants can reduce the risk of stroke for atrial fibrillation patients by 80 percent, Fonarow, author of an accompanying editorial, said by email.

"Patients with any history of atrial fibrillation who are not receiving an oral anticoagulant should discuss with their physicians the specific benefits and potential risks of treatment," Fonarow added.