To understand the different usages of cardiac devices, it is important to look at the anatomy of the heart and the normal heart rhythm first.
The heart is a muscular pump consisting of four chambers which circulate the blood through the whole body. The two upper chambers are called atria (right and left atrium) and receive the oxygenated blood from the lungs (left atrium) or deoxygenated blood from the rest of the body (right atrium). The two lower chambers are called ventricles (right and left ventricle) and pump the blood to the lungs (right ventricle) or to the rest of the body (left ventricle).
The contractions of the heart muscle are stimulated and synchronized by electrical signals which normally follow a specific electrical circuit within the heart, the conduction system. The electrical impulse begins in a group of heart cells at the top of the right atrium called sinus node, sometimes referred to as the body’s natural pacemaker. The electrical signals from the sinus node spread through both atria, making them pump. At the bottom of the right atrium is the atrioventricular node (AV node) which acts as a junction box. From here, the electrical signals are directed across the heart valves (which are like a layer of electrical insulation) through the His Bundle and into the bundle branches and His-Purkinje system. This network of fibers act like wires to rapidly spread the electrical impulse through both ventricles.
When the electrical signals stimulate the atria, both atria contract and pump the blood into the ventricles. When the electrical signals reach the ventricles, the right and left ventricles pump the blood into the lungs and rest of the body, respectively. This sequence is completed within one heartbeat.