Antithrombotic Therapy for Cerebrovascular Disorders: Nonvalvular Atrial Fibrillation

The fraction of AF-associated strokes due to cardiogenic embolism vs coexistent cerebrovascular disease continues to be uncertain. Clinical estimates of the cardioembolic fraction vary widely: 19-75%. A recent angiographic study of 12 consecutive patients with AF and stroke convincingly supported a cardioembolic etiology in 75%. Detailed clinical classification in a recent stroke data bank found 78% of AF-associated strokes of presumed cardioembolic origin. Preliminary studies of ultrasonic duplex carotid imaging of AF patients with stroke have revealed a low prevalence of ipsilateral carotid stenosis. Two autopsy studies, presumably with a high proportion of large infarctions, reported 50-71% of AF-associated stroke were cardioembolic. The inability to diagnose cardioembolic mechanisms with certainty, even at autopsy, continues to cloud this issue.
Nonrheumatic AF encompasses a wide spectrum of heart disease in a population of a broad age range and with varying coexisting illnesses. Natural history data suggest that clinical subgroups of AF patients exist who have particularly high or low stroke risk. Recent studies suggest that “lone” AF (unassociated with other cardiopulmonary disease) in younger patients (mean age, 45 years) carries a very low risk of embolism. These contrast with the findings of the Framingham Heart Study, but older, hypertensive patients were included under less restrictive definitions. On balance, it appears that lone AF in young people does not carry a substantial risk of embolism. In thyrotoxic AF, embolism risk is highest in older patients with cardiac dilatation and congestive heart failure. It is thus reasonable to hypothesize that other cardiac factors or associated cerebrovascular disease contribute importantly to the risk of AF-associated stroke.

Cardiologic subgroups of AF patients at high risk of embolism are presently ill defined. AF associated with congestive heart failure appears to carry a higher risk, although uncertainty about the basis of diagnosis, the severity, and the degree of compensation of congestive heart failure limit practical application of this observation. Embolism risk may be greatest during the initial months following the onset of AF. Despite data concerning electrical cardioversion suggesting that reversion to sinus rhythm may be high risk, intermittent (paroxysmal) AF in younger patients is associated with an embolic risk of only about 2.5% yearly, and most patients with stroke have AF that is chronic and apparently sustained.