NEW YORK (Reuters) -- Researchers say they have found the safest, yet still effective, dose levels of blood-thinning drug therapy for people with the condition called atrial fibrillation whose rapid and irregular heart rhythms might lead to a stroke.
Their study, published in this week's issue of The New England Journal of Medicine, found that people with atrial fibrillation taking anticoagulant therapy to prevent stroke should be closely monitored so as to keep their INR (international normalized ratio -- a measure of how fast blood clots) between 2.0 and 3.0.
"Tight control of anticoagulant therapy... is a better strategy than targeting lower, less effective levels of anticoagulation," write the authors. "The risk of stroke rose sharply at INRs below 2.0."
The authors note that previous studies "have indicated that the risk of hemorrhage rises rapidly at INRs greater than 4.0 to 5.0, so keeping the INR between 2.0 - 3.0 appears to offer the best balance between stroke prevention and avoiding hemorrhage.
The Boston researchers note that blood-thinning drugs (anticoagulant agents) have proven useful to prevent these strokes in people with atrial fibrillation -- strokes that can occur when minute blood clots (emboli) leave the heart when it suddenly beats rapidly, and enter the bloodstream, eventually lodging within an artery of the brain, cutting off the blood supply and causing an ischemic stroke.
In arriving at the optimum dose, their study looked at people taking warfarin for atrial fibrillation who were admitted to hospital for a stroke, and compared them with others with the same heart condition who had been treated as outpatients.
"In patients with atrial fibrillation, anticoagulation reduces the risk of stroke by 70%," writes Dr. F.R. Rosendaal of University Hospital Leiden in the Netherlands.
But anticoagulation therapy is very individual, Rosendaal notes. "Patients may require very different doses (up to 10-fold differences) to reach the same level of anticoagulation, and the required dose may also vary over time in an individual patient."
According to Rosendaal, other studies found that the risk of hemorrhage increased when the INR was above 4.5. So, based on the results of this and previously published studies, Rosendaal recommends that in patients with atrial fibrillation, "the INR should be maintained at all times between 2.0 and 4.5."
"The next step will be to define individualized levels of anticoagulation for patients with different risk profiles, a step that may eventually lead to individually customized anticoagulant treatment," concludes Rosendaal.
Source: The New England Journal of Medicine (1996;335:540-546)
