Abstract
Sick sinus syndrome (SSS) is the name given to several conditions in which the sinus node (also known as the sinoatrial or SA node) does not function normally. The sinus node is the normal pacemaker of the heart and is responsible for the regular, rhythmic heartbeat. It consists of a collection of specialized cells located at the top of the right collecting chamber (right atrium) (Figure). These cells generate regular electric impulses that then spread through the atria and pumping chambers (ventricles) and cause the muscular contractions responsible for the pumping function of the heart. Under normal conditions, the sinus node produces 60 to 100 impulses a minute; this is the normal resting heart rate. The sinus node can increase the heart rate during periods of stress, such as exercise or high fever. Conversely, during quiet sleep, the sinus node may slow down to below 60 impulses, or beats, per minute. Well-conditioned athletes often have a slow heart rate at rest. This is normal and does not indicate the presence of sinus node malfunction. Schematic diagram of the heart showing the sinus (SA) node. Reprinted with permission from the North American Society of Pacing and Electrophysiology (http://www.naspe.org). When the sinus node malfunctions several different abnormalities may result:
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