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Atrial fibrillation is the most common type of cardiac arrhythmia. In a healthy heart,
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the sinoatrial node or SA node initiates all electrical impulses in the atria. In atrial
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fibrillation, electrical impulses are initiated randomly from many other sites called ectopic
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sites in and around the atria, commonly near the roots of pulmonary veins. These un-synchronized,
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chaotic electrical signals cause the atria to quiver or fibrillate rather than contract.
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Although the atrial rate during atrial fibrillation can be extremely high, most of the electrical
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impulses do not pass through the atrioventricular – the AV - node to the ventricles. This
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is due to refractory properties of the cells of the AV node. Those that do come through are
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irregular. Ventricular rate or heart rate is therefore irregular and can range from
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slow - less than 60 - to rapid -more than 100 - beats per minute.
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On an ECG, atrial fibrillation is characterized by absence of P-waves and irregular narrow
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QRS complexes. Reminder: P-wave represents electrical activity of the SA node that is
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now obscured by activities of multiple ectopic sites. The baseline may appear undulating
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or totally flat depending on the number of ectopic sites in the atria. In general, larger
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number of ectopic sites results in flatter baseline.
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As the atria do not function properly, the heart puts out less blood, and heart failure
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may occur. The most common complication of atrial fibrillation, however, is the formation
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of blood clots in the atria. As the atria do not empty completely into the ventricles,
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the blood may stagnate inside the atria and blood clots may form. These clots may then
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pass into the bloodstream, get stuck in small arteries and block them. When a blood clot
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blocks an artery in the brain, a stroke may result.