May 8, 2012— -- Elderly women diagnosed with atrial fibrillation, a common type of irregular heart beat, are at higher risk of stroke than elderly men with the same heart condition regardless of the use of a common blood thinner to prevent strokes, according to a new study published in the Journal of the American Medical Association.
People with AF have a much higher risk of stroke compared to the general population, but it's still unclear why older women with the condition are more likely to suffer a stroke, the authors wrote. One factor, they suggested, could be that women with atrial fibrillation may need to take more warfarin.
Canadian researchers compared patterns of warfarin use and later stroke incidence between more than 80,000 men and women 65 and older who were admitted to a hospital with atrial fibrillation.
Both men and women stuck to their prescribed warfarin regimen well, but stroke rates were significantly higher in women even though they were more likely to fill prescriptions for warfarin. Women who had stroke were also more likely to be 75 or older.
"Thus, women older than 75 years represent the most important target population in patients with AF, and the effectiveness of novel anticoagulants in this population in real-world practice will need to be closely monitored," wrote the authors, led by Meytal Avgil Tsadok of McGill University Health Center in Montreal, Quebec, Canada.
Preventing stroke using blood thinners can be effective, but doctors say it does carry serious risks.
"About 65 to 70 percent of strokes can be prevented using warfarin, but any anticoagulant presents a bleeding risk," said Dr. James O'Keefe, a cardiologist and clinical researcher at Mid America Heart Institute in Kansas City, Mo. O'Keefe was not involved in the study.
"The decision to give patients with a-fib blood thinners is a balancing act," said Dr. Lawrence Phillips, an assistant professor of medicine at NYU Langone Medical Center. "We have to carefully weigh the risk of getting a stroke against the risk of bleeding." Phillips was also not involved with the study.
While stressing the relationship between warfarin dosage and increased stroke hasn't been firmly established, the authors believe the study suggests older women's stroke risk may need to be addressed differently.
"These results suggest that current anticoagulant therapy to prevent stroke might not be sufficient for older women, and new strategies are needed to further reduce stroke risk in women with AF," they wrote.
"I think that the more research that comes out that talks aboutt higher stroke risk in women, the more we're going to think about being aggressive in treatint them with blood thinners," said Phillips.
There are newer blood thinners available now, and O'Keefe said these could someday be used in place of warfarin to prevent strokes.
"This is a very dynamic area. We don't really understand why older women have a higher risk of stroke than older men. We also don't know if these newer agents -- Pradaxa and Xarelto -- might be more effective."
ABC News' Dr. Murtaza Akhter contributed to this report.