The heart is a muscle with a special electrical conduction system. The system is made of two nodes (special conduction cells) and a series of conduction fibers or bundles (pathways).

The normal heart begins with an electrical impulse from the SA (sinoatrial) node, located high in the right atrium. The SA node is the pacemaker of the normal heart, responsible for setting the rate and rhythm. The impulse spreads through the walls of the atria, causing them to contract. Next, the impulse moves through the AV (atrioventricular) node, a relay station, into the conduction bundles which are located in the ventricles themselves. As the impulse travels down the bundles, the ventricles contract. The cycle then repeats itself.

This regular cycle of atrial and ventricular contractions pumps blood effectively out of the heart. Problems may occur anywhere in the conduction system and interfere with effective pumping of blood. The heart may beat too fast (tachycardia), too slow (bradycardia), or irregularly. These abnormal beats are known as arrhythmias. Special studies of the heart's electrical system may be needed to accurately diagnose the type and cause of the arrhythmia. Therapy for arrhythmias is based on their type and difficulties they cause the child. Treatment may include medication, placement of a pacemaker, catheter ablation of extra pathways, or even surgery. Most arrhythmias require no treatment as they cause no ill effects to the child.