Antithrombotic therapy for atrial fibrillation: Balancing the risks

Atrial fibrillation (AF) affects about 4% of people over 60 and 9% of those over 80. The risk of ischemic stroke with AF is about six times that without AF, and increases with age and coexistent cardiovascular disease. Aside from AF associated with rheumatic heart disease or prosthetic heart valves, prior stroke or transient ischemic attack confers the greatest risk of thromboembolism. Moderate to severe left ventricular dysfunction is the only independent precordial echocardiographic predictor of stroke in patients with AF; the left atrium diameter is less useful. Among high-risk AF patients, thrombus, dense, spontaneous echo-contrast or reduced blood flow velocity in the left atrial appendage, and complex atheromatous plaque in the thoracic aorta on transesophageal echocardiography are associated with thromboembolism. Five large, randomized trials evaluated oral anticoagulation (two tested aspirin) for primary prevention of thromboembolism in non-valvular AF, and a sixth focused on secondary prevention after nondisabling stroke or transient ischemic attack. Estimating the risk of stroke is crucial in the decision to anticoagulate AF patients. If the stroke rate is low with aspirin, the benefit of warfarin may not outweigh the risk and the required monitoring. Several risk stratification schemes have been proposed for ischemic stroke in AF patients, based on analyses of clinical trial cohorts. They differ mainly in terms of patients at intermediate risk for stroke (3%-5%/year). Adjusted-dose oral anticoagulation is highly efficacious for both primary and secondary stroke prevention. However, it increases the frequency and severity of extra- and intracranial hemorrhage; age and intensity of anticoagulation are the strongest predictors of bleeding. The target intensity of anticoagulation involves balancing stroke prevention and risk of hemorrhagic complications. For primary prevention in most AF patients under age 75 and for secondary prevention, an international normalized ratio (INR) of 2.0-3.0 is generally recommended, but a target INR near 2.0 may be safer for primary prevention in patients over age 75. Low-intensity anticoagulation requires special effort to minimize time spent below the target range, during which stroke protection is sharply reduced. In clinical trials, aspirin conferred only modest and relatively inconsistent protection against stroke. In general, the greater the risk of a disabling cardioembolic stroke, the less protection provided by aspirin. Individual risk varies over time, so the need for anticoagulation must be re-evaluated at regular intervals in all patients with AE Combining low-dose oral anticoagulation (INR <1.5) with aspirin adds little protection compared to aspirin alone.