ATRIAL FIBRILLATION

irregular and uncoordinated rhythm of contraction of the muscles of the atria, the upper chambers of the heart. The most common of the major irregularities in heart rhythm (cardiac arrhythmics), atrial fibrillation may occur in spasms as a consequence of chest surgery, after blockage of a pulmonary blood vessel by a blood clot or some other type of embolism, or in association with serious infections or extreme fever. Otherwise, it is rarely present except in persons who have some form of physical defect, or disease, of the heart. Defects, for example, of the mitral valvewhich functions between the upper and lower chambers of the left side of the heartusually cause atrial fibrillation if they are severe enough to prevent the heart from pumping adequate quantities of blood into the circulatory system, a condition called congestive heart failure. A possible complication of continued atrial fibrillation is the formation of blood clots in the atria. The clots get into the circulation and may, by blocking of blood vessels, cause death of tissue in essential organs. A common form of treatment of atrial fibrillation when accompanied by ventricular paroxysmal tachycardia (rapid beating of the heart's lower chambers) is administration of the drug digitalis, which slows the heart's action. The fibrillation is sometimes stopped by administering electric shocks. See also ventricular fibrillation.