Atrial fibrillation (AF) is the most common serious cardiac arrhythmia, with an estimated prevalence of two million in the USA.¹ AF carries significantly increased risk of morbidity and mortality, a major component of which is a fivefold increase in the frequency of stroke.² Non-rheumatic AF is probably responsible for 15–20% of cerebrovascular accidents of ischaemic origin.^3w1 The risk of embolic stroke in the general population increases with age; in people over the age of 75 years AF is one of the most important causes of embolic stroke.⁴

A majority of the cerebrovascular events in AF patients are ischaemic.⁵ A principal mechanism responsible for ischaemic stroke in AF is embolism secondary to the stasis in the left atrium, specifically in the left atrial appendage. Thrombogenesis in AF patients is correlated with low flow velocities in the appendage.^w2 The left atrial appendage is the almost exclusive location of intracardiac thrombus in patients with AF.^w3 Overall in AF, two thirds of the ischaemic cerebrovascular events and perhaps half of all vascular events are related to the atrial thrombi.⁴

The other implicated cause of stroke in AF patients is coexisting atherosclerosis of the large arteries and valvar abnormalities. Indeed, many of the patients with AF are at increased risk of significant carotid disease.^w4 Notably, the major risk factors for stroke in patients with AF are the same as the risk factors for accelerated atherosclerosis (table 1⇓). Clinically active atherosclerotic disease is very common in patients with AF. In the SPAF (stroke prevention in atrial fibrillation) trial (mean age of patients 67 years) 10% of patients had active angina, 8% had had a myocardial infarction, and 12% had significantly decreased left ventricular function.⁶ The overall mortality in placebo treated patients with AF was …