Placebo-controlled studies have demonstrated the efficacy of oral anticoagulant therapy for prevention of stroke in patients with atrial fibrillation. Since oral anticoagulant therapy is inconvenient and increases the risk of bleeding, three randomized placebo-controlled trials have evaluated the use of aspirin for stroke prevention in patients with atrial fibrillation. One study revealed a nonsignificant relative risk reduction of 18 percent, one found a non-significant 15 percent relative risk reduction and one found a 44 percent relative risk reduction of stroke. The Atrial Fibrillation Investigators performed a meta-analysis of pooled data from these three studies to identify any subgroups of patients who might be particularly responsive to aspirin therapy.

The three studies included the Atrial Fibrillation, Aspirin, Anticoagulation Study (AFASAK), the European Atrial Fibrillation Trial (EAFT) and the Stroke Prevention in Atrial Fibrillation 1 Study (SPAF1). A total of 1,985 patient-years were assigned to the aspirin group and 1,867 patient-years were assigned to the placebo group. The daily dose of aspirin varied, from 75 mg in AFASAK, to 300 mg in EAFT, to 325 mg in SPAF1. The primary end point in the meta-analysis was occurrence of ischemic stroke.

When data from all three studies were combined, the relative risk reduction with aspirin therapy was 21 percent. Disabling stroke was decreased by 17 percent and nondisabling stroke was reduced by 27 percent with aspirin use. Aspirin therapy did not appear to be efficacious in patients without risk factors for stroke.

Patients with a history of hypertension had a 36 percent relative reduction in the risk of stroke, although the analysis did not reveal a conventional statistical significance. When patients with a previous stroke or transient ischemic attack were evaluated, aspirin therapy was associated with a relative risk reduction of 19 percent. The data did not show convincing evidence of particular efficacy in any subgroup of patients.

The authors conclude that aspirin appeared to have a slightly protective effect against stroke in patients with atrial fibrillation, but no subgroup was identified that was particularly responsive to aspirin. Other analyses have shown that oral anticoagulant therapy offers a larger and more predictable reduction in the risk of ischemic stroke for patients with atrial fibrillation.