Blood-Thinning Therapy Cuts Stroke Risk

More warfarin is better in atrial fibrillation, study finds

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HealthDay Reporter

WEDNESDAY, Sept. 10, 2003 (HealthDayNews) -- Intensive drug therapy to prevent blood clots in people with the heart condition called atrial fibrillation can reduce deaths and damage caused by strokes without increasing the risk of dangerous bleeding, a study finds.

It's a finding of life-and-death interest to the growing numbers of Americans with the condition, whose incidence increases with age. Atrial fibrillation reduces the heart's ability to pump blood, and the resulting sluggish flow promotes formation of clots that can block brain arteries, causing strokes.

Blood-thinning therapy with aspirin or warfarin, a much more potent anticoagulant, can prevent those clots. But doctors fear that too-intensive treatment can cause bleeding in the brain that does the same damage as a clot-caused stroke. Atrial fibrillation is the leading cause of stroke among the oldest Americans.

Clotting is measured by what is called the international normalized ratio (INR); a higher INR means freer-flowing blood with a lower risk of clots but a higher risk of dangerous bleeding. Physicians have known that an INR of 2.0 or higher can effectively reduce the risk of stroke, but there have been questions about the level at which a higher INR becomes dangerous.

The new study of more than 13,500 patients with atrial fibrillation shows that "the risk of hemorrhage does not increase until you reach an INR of about 4," says Dr. Elaine M. Hylek, an assistant professor of medicine at Harvard Medical School and lead author of a paper on the study. It appears in the Sept. 11 issue of the New England Journal of Medicine.

Previous studies have shown that an INR of 2 or higher can reduce the risk of strokes in patients with atrial fibrillation, Hylek says. "This is the first time it has been shown that the severity and mortality of strokes are affected by the intensity of anticoagulation medication," she says.

Not all atrial fibrillation patients get anticoagulant therapy. Those regarded as having a very low risk of stroke get no medication at all. Others, at some risk, are told to take aspirin. Warfarin is reserved for the highest-risk patients, because it requires careful monitoring to achieve the desired INR.

A third of the 592 strokes reported in the study occurred in patients taking warfarin. Among those patients, an INR under 2 nearly doubled the risk of having a severe stroke and more than tripled the risk of dying because of the stroke. Increased risk of bleeding in the brain was found only in INR readings of 3.9 or higher, Hylek says.

"The message is that patients with atrial fibrillation should maintain an INR at a minimum of 2.0 or higher," she says. "An INR of about 2.5 should not influence physicians toward a lower level."

The aging of the American population means that more and more people will become aware of the INR and what it means, Hylek says. An estimated 2.3 million Americans now have atrial fibrillation; one of every 20 Americans 70 or older has the condition.

The study "reinforces that we do know the optimal range" of INR readings, says Dr. Robert G. Hart, professor of medicine at the University of Texas Health Sciences Center at San Antonio, who wrote an accompanying editorial.

A reading between 2 and 3 is best, not only for people with atrial fibrillation but also for those who take warfarin because they have mechanical heart valves, he says. "For the very elderly with a high risk of bleeding, I would aim at the lower part of that range when possible," Hart says.

More information

A primer on atrial fibrillation can be found at the American Heart Association, while you can learn about warfarin (sold as Coumadin) from MedlinePlus.

SOURCES: Elaine M. Hylek, M.D., M.P.H., assistant professor, medicine, Harvard Medical School, Boston; Robert G. Hart, M.D., professor, medicine, University of Texas Southwestern Medical Center, San Antonio; Sept. 11, 2003, New England Journal of Medicine

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