Guidelines for stroke prevention in patients with atrial fibrillation

P A Howard
Drugs 1999, 58 (6): 997-1009
Atrial fibrillation (AF) is a major independent risk factor for stroke. AF is most commonly associated with nonvalvular cardiovascular disease and is especially frequent among the elderly. The annual risk for stroke in patients with AF is approximately 5% with a wide range depending on the presence of additional risk factors. For patients who cannot successfully be converted and maintained in normal sinus rhythm (NSR), antithrombotic therapy is an effective method for preventing stroke. The 2 drugs which are indicated for stroke prophylaxis in patients with AF are warfarin and aspirin. For primary prevention, warfarin reduces the risk of stroke approximately 68%. Aspirin therapy is less effective, resulting in a 20 to 30% risk reduction. Combination therapy with aspirin and low intensity warfarin adjusted to an International Normalised Ratio (INR) of 1.2 to 1.5 has not been shown to be superior to standard intensity warfarin with a target INR of 2.0 to 3.0. In patients with AF and a prior history of stroke or transient ischaemic attack (TIA), the absolute risk reduction with warfarin is even greater because of the high risk of stroke in this population. In contrast, aspirin has not been shown to significantly reduce the risk of stroke in patients with AF when used for secondary prevention. When appropriately managed, warfarin is associated with a low risk of major bleeding. In controlled trials of highly selected patients, the annual rate of intracranial haemorrhage (ICH) with warfarin was approximately 0.3%. Studies have shown that specialty anticoagulation clinics can achieve similar low rates of major bleeding. However, these results cannot be extrapolated to the general population. Factors which have been identified as predictors of bleeding include advanced age, number of medications and most importantly, the intensity of anticoagulation. INR values above 4.0 have been associated with an increased risk of major bleeding while values below 2.0 have been associated with thrombosis. Slow careful dosage titration, regular laboratory monitoring and patient education can substantially reduce the risk of complications. In patients with AF, antithrombotic therapy has been shown to be cost effective. For high risk patients, warfarin is the most cost-effective therapy, provided the risks for bleeding are minimised. In contrast, aspirin is the most cost-effective agent for low risk patients. Current practice guidelines for stroke prophylaxis recommend warfarin (target INR 2.5: range 2.0 to 3.0) for AF patients at high risk for stroke including those over 75 years of age or younger patients with additional risk factors. Aspirin should be reserved for low risk patients or those unable to take warfarin. Although these recommendations are strongly supported by the clinical trial evidence, studies show that many patients are not receiving appropriate antithrombotic therapy. In particular, warfarin is underutilised in high risk elderly patients. Additional studies are needed to identify barriers that prevent implementation of the clinical trial findings into clinical practice.

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