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Original Article
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Volume 335:540-546 August 22, 1996 Number 8
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An Analysis of the Lowest Effective Intensity of Prophylactic Anticoagulation for Patients with Nonrheumatic Atrial Fibrillation
Elaine M. Hylek, M.D., M.P.H., Steven J. Skates, Ph.D., Mary A. Sheehan, R.N., and Daniel E. Singer, M.D.

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ABSTRACT

Background To avert major hemorrhage, physicians need to know the lowest intensity of anticoagulation that is effective in preventing stroke in patients with atrial fibrillation. Since the low rate of stroke has made it difficult to perform prospective studies to resolve this issue, we conducted a case–control study.

Methods We studied 74 consecutive patients with atrial fibrillation who were admitted to our hospital from 1989 through 1994 after having an ischemic stroke while taking warfarin. For each patient with stroke, three controls with nonrheumatic atrial fibrillation who were treated as outpatients were randomly selected from the 1994 registry of the anticoagulant-therapy unit (222 controls). We used the international normalized ratio (INR) to measure the intensity of anticoagulation. For the patients with stroke, we used the INR at admission; for the controls, we selected the INR that was measured closest to the month and day of the matched case patient's hospital admission.

Results The risk of stroke rose steeply at INRs below 2.0. At an INR of 1.7, the adjusted odds ratio for stroke, as compared with the risk at an INR of 2.0, was 2.0 (95 percent confidence interval, 1.6 to 2.4); at an INR of 1.5, it was 3.3 (95 percent confidence interval, 2.4 to 4.6); and at an INR of 1.3, it was 6.0 (95 percent confidence interval, 3.6 to 9.8). Other independent risk factors were previous stroke (odds ratio, 10.4; 95 percent confidence interval, 4.4 to 24.5), diabetes mellitus (odds ratio, 2.9; 95 percent confidence interval, 1.3 to 6.5), hypertension (odds ratio, 2.5; 95 percent confidence interval, 1.1 to 5.7), and current smoking (odds ratio, 5.7; 95 percent confidence interval, 1.4 to 24.0).

Conclusions Among patients with atrial fibrillation, anticoagulant prophylaxis is effective at INRs of 2.0 or greater. Since previous studies have indicated that the risk of hemorrhage rises rapidly at INRs greater than 4.0 to 5.0, tight control of anticoagulant therapy to maintain the INR between 2.0 and 3.0 is a better strategy than targeting lower, less effective levels of anticoagulation.


Source Information

From the General Internal Medicine Division, Clinical Epidemiology Unit, Department of Medicine, Massachusetts General Hospital and Harvard Medical School, Boston.

Address reprint requests to Dr. Hylek at the Medical Practices Evaluation Center, S 50-9, Massachusetts General Hospital, Boston, MA 02114.

Full Text of this Article


Related Letters:

Anticoagulation for Nonrheumatic Atrial Fibrillation
Hart R. G., Hylek E. M., Skates S. A., Singer D. E.
Extract | Full Text  
N Engl J Med 1997; 336:441-442, Feb 6, 1997. Correspondence

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