Each heart beat is generated by the heart's special electrical system. Specialized tissues within the heart generate electrical impulses that cause the heart muscles to contract, or squeeze tight. With each contraction (or beat), the heart sends blood through its chambers and out - either to the lungs to pick up oxygen or to the body to deliver oxygen to the cells.
The heart tissues which generate electrical activity are called the sinus node (or sinoatrial node.) The sinus node, located in heart's upper right chamber (atrium), generates an electrical impulse each fraction of a second. The impulse travels through the atrial muscles, causing them to contract. It then travels to the AV node (atrioventricular), which is located between the atria (upper chambers) and the ventricles (lower chambers). The AV node provides the only pathway for electrical impulses to travel to the ventricles.
The AV node conducts the electrical impulses more slowly than other nodes. This slower pace allows time for the ventricles to fill with blood from the atria before they contract and send it out of the heart. If the electrical impulses travel from the atria to the ventricles either early or late, the balance between blood filling the lower chambers and the timing of the heart's contraction is disturbed. The result is reduced cardiac output - or a reduced amount of blood ejected from the heart.