Irregular rhythm (arrhythmia) of contraction of the atria (upper heart chambers). The most common major arrhythmia, it may result as a consequence of increased fibrous tissue in the aging heart, of heart disease, or in association with severe infection. If it continues, it can permit formation of blood clots, which can block blood flow to essential organs. Emergency treatment consists of drugs such as beta-blockers or digitalis, which slow the heart's action, and anticoagulants. In addition, atrial fibrillation can be interrupted by administering electric shocks (defibrillation). Seealso ventricular fibrillation.

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