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Atrial Fibrillation and Cardioembolic Stroke

Atrial fibrillation and the need for antithrombotic therapy

Atrial fibrillation (AF) is the most common sustained arrhythmia seen in clinical practice, affecting an estimated 4.5 million people in the European Union and 2.2 million Americans.²² Atrial fibrillation is associated with a major risk of stroke, caused by a thrombus that forms within the left atrium and embolises to block a cerebral artery. The degree of stroke risk and the need for anticoagulant therapy to lower this risk varies among patients with AF.

AF — the most common arrhythmia

AF, the most common type of sustained cardiac arrhythmia, is primarily a problem of the elderly. The prevalence is less than 1% in those under 60 and almost 10% in those over 80.⁷⁹

AF is often classified based on the temporal pattern of presentation⁴:

• Recurrent AF: two or more episodes of AF
• Paroxysmal AF: episodes end spontaneously within seven days
• Persistent AF: pharmacologic or electrical cardioversion is required to terminate the arrhythmia
• Permanent AF: sustained AF despite treatment to end the arrhythmia or when cardioversion is inappropriate

AF — rhythm control vs rate control

The objectives of treating AF are to relieve symptoms (when present) and to optimise cardiac function. This can be accomplished with either a rhythm-control or a rate-control approach. Rhythm control involves efforts — electrical cardioversion or drug therapy, or both — to restore and maintain normal sinus rhythm. In addition, interventional approaches designed to ablate the source of the abnormal rhythm — known as catheter ablation procedures — have proven successful in some patients with paroxysmal AF.⁴ Rate control involves using medications to maintain a ventricular rate under 100 beats per minute without attempting to terminate the arrhythmia.⁴ Generally, studies have shown that there is no survival advantage with rhythm control as opposed to rate control.⁸⁰

AF — the role of antithrombotic therapy

Regardless of which treatment approach is pursued, antithrombotic therapy is essential. This is because cardioembolic stroke is one of the main complications of AF.⁴ Cardioembolic stroke (or thromboembolic stroke) occurs when stagnant blood in the fibrillating atrium forms a thrombus that then embolises to the cerebral circulation, blocking arterial blood flow and causing ischaemic injury. The incidence of stroke in patients with nonvalvular AF (ie, AF not caused by damage to the mitral valve) is between two- and seven-fold greater than in the general population. For patients with AF caused by valvular disease, the risk of stroke increases 17-fold.²²

Thrombus in right atrial appendage

AF — the risk of stroke

The risk of stroke is age-dependent. In the Framingham study, the annual risk was 1.5% in those 50 to 59 years old and 23.5% in those 80 to 89 years old.²² A systematic review of six cohorts of AF patients identified three other independent risk factors in addition to age: prior history of stroke or transient ischaemic attack (TIA), history of hypertension, and diabetes.⁸¹

Several scoring systems are available to help clinicians estimate the stroke risk in AF. One popular, well-validated risk assessment tool is the "CHADS₂". This system assigns single points for congestive heart failure, hypertension, age over 75, and diabetes and two points for stroke or TIA history.