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Although atrial fibrillation has long been recognised as a risk factor for thromboembolic events, preventive treatment has, until recently, been both empirical and controversial. Only in the past few years have antithrombotic drugs been proved by prospective clinical studies to be effective against strokes in atrial fibrillation.
Atrial fibrillation and thromboembollism
The pathophysiological mechanism for thromboembolism seems to be the disturbed blood flow in the fibrillating left atrium, which predisposes to the formation of thrombi and arterial embolism, especially in the presence of underlying heart disease. About 15-20% of patients who have an acute stroke have atrial fibrillation at the time of their stroke; mortality is one and a half to three times higher in those patients than in patients with sinus rhythm at the time of stroke.
Underlying heart disease
Underlying heart disease-including valvar and hypertensive heart disease, an enlarged left atrium, and poor left ventricular function - is a contributory risk factor for stroke and thromboembolism in atrial fibrillation. The risk of thromboembolism in atrial fibrillation is 18 times greater if rheumatic heart disease is present. In the stroke prevention in atrial fibrillation study, the presence of recent (within three months) congestive heart failure and left ventricular dysfunction on echocardiography also contributed to an increased risk of thromboembolism in atrial fibrillation. This is consistent with the observation that thromboembolism is a common cause of death in patients with congestive heart failure, occurring in up to 30% of patients, which is partly preventable by anticoagulation.
Left atrial enlargement and spontaneous echo contrast
An enlarged left atrium may contribute to an increased risk of atrial thrombi and thromboembolism in patients with atrial fibrillation. An enlarged left atrium has also been associated with "spontaneous echo contrast" on transoesophageal echocardiography - a smoke-like appearance of blood in the atria, suggesting sluggish flow. It is associated with dilated left atria, intracardiac thrombi, thromboembolism, stroke, abnormal rheology, and coagulation.
Previous cerebrovascular disease or
A history of stroke, transient ischaemic attack, or other embolic events adds to the risk of stroke and mortality in atrial fibrillation. The pooled analysis by the Atrial Fibrillation Investigators showed that a previous stroke or transient ischaemic attack was an independent risk factor for further strokes. Using warfarin in such patients reduced the annual rate of stroke from 12% a year to 5%.
In a Glasgow study, among patients admitted with acute severe stroke, those with atrial fibrillation (25%) had a significantly higher hospital mortality than those in sinus rhythm (67% v 44%). …