Heart rhythm disorder characterized by irregular heartbeat. It can occur in all kinds of heart conditions (cardiac valve defects, constriction of the coronary arteries), but also in cases of an overactive thyroid (hyperthyroidism) or excessive consumption of alcohol. Generally atrial fibrillation is without symptoms, the only manifestation being irregular heartbeat, but it can cause fatigue and listlessness. The danger is that a blood clot can form against the wall of the right atrium, causing an embolism if it works loose. Treatment is aimed at restoration of the correct ryhthm, possibly by electric shock to the heart, or digitalis may be prescribed to make the atria work more calmly, and thus more effectively. Anticoagulants may be prescribed to reduce the risk of clotting. Heart condition in which the atria contract irregularly, comparable with atrial fibrillation, but characterized by a very rapid but regular rhythm, as opposed to the irregular rhythm of fibrillation. It is usually caused by a heart disorder (constriction of the coronary arteries, inflammation of the cardiac muscles, cardiac valve defects). This disturbance of rhythm also occurs in attacks which begin for no clear reason and that disappear of their own accord in a little while. The heart beats more quickly than normal, almost always without other symptoms. Treatment is usually by electric shock to the heart, or with digitalis to slow down the rhythm. Atrial septum defect (ASD) Congenital heart defect in which there is an opening in the septum between the left and right atria. The condition has two forms: ASD I, almost always linked with a defective mitral valve, and ASD II, which occurs more frequently and is not associated with valve defects. Because the pressure in the left atrium is higher than that in the right blood flows from one to the other, causing excess blood in the vessels of the lungs, which can eventually lead to lung complaints such as shortness of breath. But in most cases ASD causes no discomfort, and is detected only by chance, from a charac- teristic associated heart murmur. Expansion of the right ventricle sometimes causes disturbances in the rhythm of the heart, especially atrial fibrillation. Whether an ASD should be closed by surgery depends on its type and size. An ASD with valve defect must be closed when the patient is young. If the ASD is large lung problems will quickly set in, and action is certainly required, whereas a small ASD hardly ever causes complications and therefore no operation is necessary.