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LETTER

Intensity of Anticoagulation To Prevent Stroke in Patients with Atrial Fibrillation

Robert G. Hart, MD

1 March 1998 | Volume 128 Issue 5 | Page 408

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TO THE EDITOR:

In their "Update in General Internal Medicine," Sheffield and Larson [1] cite the conclusions of a case–control study [2] reporting that the lowest effective intensity of warfarin for atrial fibrillation is an international normalized ratio (INR) of 2.0. This is an oversimplification of available information.

In the case–control study by Hylek and colleagues [2], INRs of 2.0 or more seemed to provide optimal protection against stroke. However, INRs between 1.6 and 1.9 were associated with an 80% or greater reduction in risk for stroke compared with INRs of 1.0 to 1.1 (equivalent to untreated patients). This partial efficacy of INRs of 1.6 to 1.9 has been confirmed by time-dependent analysis of a recent randomized trial [3]. Two clinical trials with estimated target INRs between 1.4 and 2.8 reported the highest efficacy for stroke prevention in patients with nonvalvular atrial fibrillation (the trials were done by using prothrombin time ratios, with INRs estimated post hoc by the trial investigators on the basis of Institute for Scientific Information values).

Substantial, if incomplete, protection is afforded by INRs between 1.6 and 2.0. This may be important for selected elderly patients who have special risks for bleeding, in whom a target INR of 2.0 (range, 1.6 to 2.5) may be a sensible option. Achieved INRs between 1.6 and 2.5 seemed to provide more than 90% of the protection afforded by higher ranges (Figure 1).

View larger version (40K): [in this window] [in a new window]   Figure 1. Relative efficacy of target international normalized ratios (INRs) of 1.6 to 2.5. Percentages calculated under the presumption that maximal protection is achieved with INRs greater than 2.0 Data obtained from [2] and [3]. SPAF = Stroke Prevention in Atrial Fibrillation.

 

Author and Article Information

Top Author & Article Info References

University of Texas Health Science Center; San Antonio, TX 78284

References

Top Author & Article Info References

1. Sheffield JV, Larson EB. Update in general internal medicine. Ann Intern Med. 1997; 127:43-51.

2. Hylek EM, Skates SJ, Sheehan MA, Singer DE. An analysis of the lowest effective intensity of prophylactic anticoagulation for patients with nonrheumatic atrial fibrillation. N Engl J Med. 1996; 335:540-6.

3. Adjusted-dose warfarin versus low-intensity, fixed-dose warfarin plus aspirin for high-risk patients with atrial fibrillation: stroke prevention in atrial fibrillation III randomized clinical trial. Stroke Prevention in Atrial Fibrillation Investigators. Lancet. 1996; 348:633-8.

4. Warfarin in the prevention of stroke associated with nonrheumatic atrial fibrillation. Veterans Affairs Stroke Prevention in Nonrheumatic Atrial Fibrillation Investigators. N Engl J Med. 1992; 327:1406-12.

5. The effect of low-dose warfarin on the risk of stroke in non-rheumatic atrial fibrillation. Boston Area Anticoagulation Trial for Atrial Fibrillation Investigators. N Engl J Med. 1990; 323:1505-11.

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