PLATELET-AGGREGATION IN PATIENTS WITH ATRIAL-FIBRILLATION TAKING ASPIRIN OR WARFARIN

Background and Purpose: Although warfarin and perhaps aspirin may be e ffective in preventing thromboembolism in patients with nonvalvular at rial fibrillation, some patients develop cerebral infarction despite t hese therapies. The purpose of this study was to determine inhibition of platelet aggregation in patients on aspirin and platelet reactivity in those on warfarin in the Stroke Prevention in Atrial Fibrillation study. Methods: Twenty-four patients in the Stroke Prevention in Atria l Fibrillation study at the University of Illinois at Chicago, 17 on e nteric-coated aspirin 325 mg/d and 7 on warfarin to produce an Interna tional Normalized Ratio of 2.0 to 4.5, had platelet aggregation studie s performed during a 10-month period and interpreted by an investigato r blinded to therapy. Epinephrine, adenosine diphosphate, collagen, an d arachidonic acid were used as aggregating agents. Compliance was det ermined by pill count for those patients on aspirin. Results: Seven pa tients taking aspirin had partial and 10 had complete inhibition of pl atelet aggregation. Three of seven patients on warfarin had hyperaggre gable platelets. Compliance was 80% or greater for those patients taki ng aspirin. One patient on warfarin had partial inhibition of platelet aggregation. Conclusions: Some patients in the Stroke Prevention in A trial Fibrillation trial on aspirin 325 mg/d did not achieve complete inhibition of platelet aggregation. Others had hyperaggregable platele ts. These findings suggest platelet-dependent mechanisms for aspirin a nd warfarin failure to prevent stroke in these patients.