Warfarin given in a dosage that achieves an International Normalized Ratio (INR) between 2.0 and 3.0 or 2.5 and 4.0 has been found to be more effective than aspirin in reducing the incidence of strokes in persons with atrial fibrillation.

Aronow and associates performed an observational study to determine if the greater efficacy of warfarin holds true regardless of several comorbid factors occurring in older patients with chronic atrial fibrillation. Following 350 older adults with chronic atrial fibrillation in a long-term care facility, 209 of whom received long-term oral aspirin therapy in a dosage of 325 mg per day and 141 of whom received oral warfarin in an adjusted dose to maintain an INR between 2.0 and 3.0, thromboembolic strokes were recorded. The physicians were more likely to prescribe warfarin if they believed the subjects were at high risk for developing stroke and had no contraindications to warfarin, including a history of falls. Six of the patients taking warfarin and six of the patients in the aspirin group discontinued the medication because of adverse side effects.

Abnormal left ventricular function was demonstrated to be an independent risk factor for thromboembolic stroke in patients with chronic atrial fibrillation. In patients with chronic atrial fibrillation, warfarin in a dosage that maintained an INR between 2.0 and 3.0 compared with aspirin caused a 40 percent significant reduction in thromboembolic stroke in persons with a previous stroke, a 31 percent significant decrease in thromboembolic stroke in persons with no previous stroke, a 45 percent significant reduction in thromboembolic stroke in persons with an abnormal left ventricular ejection fraction and a 36 percent significant decrease in thromboembolic stroke in persons with normal left ventricular function.

The authors conclude that the use of warfarin was a significant independent risk factor for reducing new thromboembolic stroke by 67 percent.