Reuters Health - More than a third of patients with a heart rhythm problem that can cause a stroke are incorrectly given aspirin instead of the blood-thinning medications they need to make this complication less likely, a U.S. study suggests.

Most strokes occur when an artery that carries blood to the brain gets blocked by a clot. While aspirin can prevent clots, it doesn't work well as blood thinners to prevent stroke in these patients, guidelines say.

Researchers analyzed data on more than 500,000 people with atrial fibrillation, an irregular rapid heartbeat that can lead to stroke, heart failure and chronic fatigue.

Roughly 40 percent of these patients got aspirin instead of prescriptions for blood thinners - also called anticoagulants.

"By prescribing aspirin, we may be fooling ourselves that the patient may be protected from stroke when this is not the case - blood thinners have been shown to be clearly more effective than aspirin in preventing stroke for atrial fibrillation patients," lead study author Dr. Jonathan Hsu, a heart specialist at the University of California, San Diego, said by email.

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In atrial fibrillation, electrical impulses in the upper chambers of the heart are chaotic, and the atrial walls quiver rather than contracting normally. As a result, blood doesn't move as well to the heart's lower chambers. This can lead to the formation of clots that can travel through the arteries. Atrial fibrillation patients have up to seven times the stroke risk of people without the disorder.

To see how many atrial fibrillation patients receive blood thinners 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.

One group of about 210,000 people in the study were at least 75 years old and had other risk factors for stroke such as congestive heart failure, diabetes, or high blood pressure. Roughly 38 percent of these patients were treated with aspirin and nearly 62 percent were prescribed anticoagulants.

A second group of roughly 295,000 people were between 65 and 74 years old. Among this subset of atrial fibrillation patients, 40 percent were treated with aspirin and 60 percent got anticoagulants.

For both groups, patients prescribed aspirin were younger, slightly less overweight, and more likely to be female or have other medical problems such as diabetes, hypertension, high cholesterol, coronary artery disease, a prior heart attack or peripheral artery disease, researchers report in the Journal of the American College of Cardiology.

The possibility of side effects may partly explain the limited use of blood thinners.

Long-term use of the older blood thinner warfarin (Coumadin), for example, 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.

Newer anticoaulants are associated with fewer serious bleeding complications than warfarin, Dr. Samuel Wann, a cardiologist at Ascension Health in Milwaukee, Wisconsin, notes in an accompanying editorial.

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.

Wann told Reuters Health there are a wide variety of reasons why doctors might prescribe aspirin when blood thinners are the recommended treatment.

"Lack of knowledge, patient compliance and cost may all explain failure to prescribe effective treatment," Wann said by email.

In some cases, Wann added, doctors may be "taking the easier way out because the adverse consequences of not prescribing anticoagulation are not immediately apparent."