A widely used blood thinner can dramatically reduce the risk of a form of stroke triggered by an irregular heartbeat, says a study released today.

The research in the New England Journal of Medicine involved the drug warfarin and strokes triggered by atrial fibrillation, abnormally rapid beating of the heart's upper chambers. Such strokes afflict 75,000 Americans annually.In March, another major study found that aspirin, also a blood-thinning drug, also significantly reduces the likelihood of these strokes.

In atrial fibrillation, the heart's rapidly beating chambers can produce blood clots that can travel to the brain and clog up blood vessels, causing strokes. Both aspirin and warfarin interfere with the formation of clots.

The study, directed by Dr. J. Philip Kistler at Massachusetts General Hospital, was based on 420 patients who were randomly assigned to take warfarin or serve in an untreated comparison group.

It found that warfarin reduced the risk of stroke by 86 percent. After two years, there were two strokes in the warfarin users and 13 in the comparison group.

Warfarin is routinely used to prevent strokes in people with severe atrial fibrillation caused by damaged heart valves. However, doctors have been reluctant to use this medicine in people with milder cases because of the chance of serious bleeding.

The new study found that the bleeding risk is slight when the drug's use is monitored by doctors.

More research is needed to determine which patients should receive warfarin and which should get aspirin, the doctors said.