Normal Rhythm:Regular contraction of the heart produced by an electrical wave beginning in the heart's natural pacemaker, the sinoatrial (SA) node, which is in the right atrium. The SA node sends out regular electrical impulses that generate waves (shown in yellow in the figure) that propagate throughout the atria, causing them to contract and to pump blood into the lower chambers (the ventricles). The electrical impulses from the atria are delayed to allow time for ventricular filling and are then passed to the ventricles through a junction called the atrioventricular (AV) node. From the AV node, the electrical waves (shown in yellow in the figure) spread into the ventricles, causing the muscle to contract and to pump blood to the lungs and the body.
If this regular electrical activity is disrupted (for example, by a disturbance in the heart's rhythm known as an arrhythmia), the heart's ability to pump properly can be affected. Click on the buttons in the figure to see simulations of four different types of arrhythmias.