Abnormalities of the heart rate or rhythm (arrhythmias) are caused by disturbances in the electrical system that controls the heartbeat. The heart has a natural pacemaker (called the sinoatrial node) in the right atrium.
This node sends electrical signals through the atria (upper chambers) to another node (the atrioventricular node), from where they pass to the ventricles (lower chambers). The signals regulate the heart muscle contractions and, therefore, the heart rate and rhythm.
Poor signal transmission or abnormal electrical activity can cause various arrhythmias. (Some of the main ones are shown below.) The electrical system disturbances themselves may be due to various underlying causes, including other heart disorders (a heart attack, or heart muscle or valve problems, for example), the effects of certain drugs, or overproduction of thyroid hormones.
Treatment may include electrically shocking the heart back into normal rhythm, medication, surgery, or fitting a pacemaker or similar device to maintain or restore a normal heart rhythm.

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