THE CONTEXT
Atrial fibrillation (AF) involves abnormal automatic firing and interacting atrial re-entry circuits. Disorganised atrial activity produces an irregular ventricular response.
Initiation and maintenance
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AF is characterised by abnormal automatic firing and multiple interacting re-entry circuits within the atria.
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Episodes begin with rapid bursts of ectopic beats arising from the pulmonary veins or diseased atrial tissue.
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AF is sustained by re-entrant conduction within the atria or, sometimes, continuous ectopic firing.
Mechanical activity, pulse and ECG
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During AF, the atria beat rapidly in an uncoordinated and ineffective manner.
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The ventricles are activated irregularly at a rate determined by conduction through the atrioventricular node, producing the characteristic irregularly irregular pulse.
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The ECG shows normal but irregular QRS complexes and no P waves. Irregular fibrillation waves may be visible along the baseline.
Clinical patterns and progression
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Paroxysmal AF consists of intermittent episodes that terminate spontaneously within 7 days.
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Persistent AF consists of prolonged episodes that can be terminated by electrical or pharmacological cardioversion. Permanent AF is also a recognised category.
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The pattern may be difficult to determine at first presentation and often becomes clearer with time.
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Paroxysmal AF may become persistent as the underlying disease progresses.
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AF lasting for months leads to structural remodelling, with atrial fibrosis and dilatation that predispose to chronicity.
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Early treatment can sometimes prevent AF from becoming persistent.
Common causes and associations
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Listed cardiac causes include coronary artery disease, including acute MI; valvular heart disease, especially rheumatic mitral valve disease; hypertension; sinoatrial disease; cardiomyopathy; congenital heart disease; and pericardial disease.
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Other listed causes or associations include hyperthyroidism, alcohol, chest infection, pulmonary embolism and obesity.
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AF may also be idiopathic, termed lone atrial fibrillation.