Risk factors for thromboembolism during aspirin therapy in patients with atrial fibrillation: The stroke prevention in atrial fibrillation study.

Although atrial fibrillation (AF) is associated with ischemic stroke, some AF patients have an inherently low rate of thromboembolism. Additional AF patients might have a low thromboembolic rate when given aspirin. We undertook exploratory analyses to identify patient features related to thromboembolism during aspirin therapy, as the basis for the design of a clinical trial. A cohort of 854 patients given aspirin 325 mg/day and followed for a mean of 2.3 years in a clinical trial was analyzed. Clinical and echocardiographic features at study entry were correlated with subsequent arterial thromboembolism by multivariate analysis. AF patients given aspirin who had high thromboembolic rates were women over age 75 years (10.4%/year) and those of either gender with systolic blood pressure > 160 T (7.6%/year), prior thromboembolism (6.4%/year), and impaired left ventricular function (5.5%/year). AF patients with one or more of these features had a thromboembolic rate of 5.9%/year (95% CI 4.5-7.8) during aspirin therapy. Patients without these features comprised 52% of the cohort and had a thromboembolic rate of 1.9% per year (95% CI, 1.2-2.9). Most strokes in all subgroups were classified as cardioembolic; the highest rate of cardioembolic stroke occurred in elderly women (6.1%/year). These exploratory analyses suggest that many AF patients may have a relatively low thromboembolic rate when given aspirin, but the findings must be independently confirmed before being applied to clinical management. Pathogenic mediators of high-risk features and aspirin responsiveness are unclear.