The heart is a hollow muscular organ – approximately the size of your clenched fist – that beats 40 million times per year (between 60 and 100 beats per minute). The hearts pumping action is controlled by its electrical system, which gives rise to the heart rhythm. Normal cardiac rhythm results from electrical impulses that begin in a special group of cells that form the sinoatrial (SA) node, also called the sinus node. Located in the right upper chamber of the heart, sinus node cells act as the heart s natural pacemaker. Impulses spread from the sinus node to the right and left atria (the upper chambers of the heart), causing them to contract at the same time. The impulses then travel to the AV (atrioventricular) node, the region that manages impulse traffic from the atria to the ventricles (the lower chambers of the heart). Here, impulses are slowed slightly to give the atria time to contract before the signal reaches the ventricles. From the AV node, the impulses travel through a system of specialized heart tissue. Located in the wall that separates the two ventricles, this conducting system splits to form the right and left bundle branches that travel to the respective ventricles. Via this conducting pathway, powerful electrical jump-start signals are delivered to the ventricular muscle of the heart. In the healthy heart, these impulses travel at the same speed so that the two ventricles contract at the same time, and oxygen-rich blood from the lungs is pumped throughout the body. Bradycardia is the term used to describe an abnormally slow heart rate of less than 60 beats per minute. This condition can be caused by certain medications, pre-existing conditions such as congenital heart disease, certain forms of acquired heart disease, or the degenerative processes of aging. In some cases, slow heart rates are considered normal, for example, in a young athlete who is in peak physical condition. In fact, most people tolerate heart rates between 50 and 60 beats per minute without difficulty. However, a slow heart rate can be considered abnormal if it causes such problems as fainting, fatigue, and light-headedness.