In matters of the heart (cardiology), fibrillation is incoordinate twitching of the heart muscle fibers. The difference between fibrillation and flutter is that fibrillation is not well organized while flutter is. For example, atrial flutter is regular, organized but over-rapid contraction of the atrium of the heart. By contrast in atrial fibrillation, the atrium quivers incordinately and ineffectually.
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1. The condition of being fibrillated. 2. The formation of fibrils. 3. Exceedingly rapid contractions or twitching of muscular fibrils, but not of the muscle as a whole. 4. Vermicular twitching, usually slow, of individual muscular fibers; commonly occurs in atria or ventricles of the heart as well as in recently denervated skeletal muscle fibers.
- atrial f., auricular f. f. in which the normal rhythmical contractions of the cardiac atria are replaced by rapid irregular twitchings of the muscular wall; the ventricles respond irregularly to the dysrhythmic bombardment from the atria. SYN: ataxia cordis.
- ventricular f. coarse or fine, rapid, fibrillary movements of the ventricular muscle that replace the normal contraction.

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fi·bril·la·tion .fib-rə-'lā-shən, .fīb- n
1) an act or process of forming fibers or fibrils
2 a) a muscular twitching involving individual muscle fibers acting without coordination
b) very rapid irregular contractions of the muscle fibers of the heart resulting in a lack of synchronism between heartbeat and pulse see ATRIAL FIBRILLATION, VENTRICULAR FIBRILLATION

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a rapid and chaotic beating of the many individual muscle fibres of the heart, which is consequently unable to maintain effective synchronous contraction. The affected part of the heart then ceases to pump blood.
Fibrillation may affect the atria or ventricles independently. Atrial fibrillation, a common type of arrhythmia, results in rapid and irregular heart and pulse rates. The main causes are atherosclerosis, chronic rheumatic heart disease, and hypertensive heart disease. It may also complicate various other conditions, including chest infections and thyroid over-activity. The heart rate is controlled by the administration of digoxin; in some cases the heart rhythm can be restored to normal by cardioversion. Anticoagulant therapy with warfarin reduces the risk of blood-clot formation, which could cause a stroke.
When ventricular fibrillation occurs the heart stops beating (see cardiac arrest). It is most commonly the result of myocardial infarction.

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fi·bril·la·tion (fib″rĭ-laґshən) 1. the quality of being fibrillar. 2. a small, local, involuntary contraction of muscle, invisible under the skin, resulting from spontaneous activation of single muscle cells or muscle fibers whose nerve supply has been damaged or cut off. 3. the initial degenerative changes in osteoarthritis, characterized by softening of the articular cartilage and development of vertical clefts between groups of cartilage cells. 4. irregular, uncoordinated contraction of cardiac myocytes, either individually or in small groups; see atrial f. and ventricular f.

Medical dictionary. 2011.

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