Patients with atrial fibrillation considered high risk for stroke require anticoagulation with warfarin to maintain an INR of 2–3 to prevent stroke. An INR of 1.4–1.9 has been shown to be associated with a stroke or mortality rate similar to that for an INR of less than 1.5. The loss of atrial contraction leads to stasis of blood in the atrium and is more marked in the left atrial appendage, the most common site for clot formation. Stasis is accompanied by hypercoagulability and there is increased concentrations of fibrinogen and fibrin D-dimer and increased concentra¬tions of von Willebrand factor. These derangements all contribute to the development of a prothrombotic state and embolization. Five randomized clinical trials indicate that warfarin anticoagulation reduces the risk of stroke by 68% and lowers mortality by 33%. The risk of hemorrhagic complications, particularly cerebral hemor¬rhage, rises greatly when the INR exceeds 3.9. Patients on oral anticoagulants should have blood tests every 2–3 weeks to maintain an INR of 2–3 in order to prevent serious hemorrhagic events. In patients over 80 years of age and in those with small risk of bleeding, the INR is maintained at 1.8–2.8. Patients at high risk for bleeding are not given anticoagulants.

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