A permanent pacemaker or ICD has three main components:
- A pulse generator which has a sealed lithium battery and an electronic circuitry package. The pulse generator produces the electrical signals that make the heart beat. Most pulse generators also have the capability to receive and respond to signals that are sent by the heart itself.
- One or more wires (also called leads). Leads are insulated flexible wires that conduct electrical signals to the heart from the pulse generator. The leads also relay signals from the heart to the pulse generator. One end of the lead is attached to the pulse generator and the electrode end of the lead is positioned in the atrium (the upper chamber of the heart) or in the right ventricle (the lower chamber of the heart). In the case of a biventricular pacemaker, leads are placed in both ventricles.
- Electrodes, which are found on each lead.
Pacemakers can "sense" when the heart's natural rate falls below the rate that has been programmed into the pacemaker's circuitry.
Pacemaker leads may be positioned in the right atrium, right ventricle, or positioned to pace both ventricles, depending on the condition requiring the pacemaker to be inserted. An atrial arrhythmia (an arrhythmia caused by a dysfunction of the sinus node or the development of another atrial pacemaker within the heart tissue that takes over the function of the sinus node) may be treated with an atrial permanent pacemaker whose lead wire is located in the atrium.
When the ventricles are not stimulated normally by the sinus node or another natural atrial pacemaker site, a ventricular pacemaker whose lead wire is located in the ventricle is placed/used. It is possible to have both atrial and ventricular arrhythmias, and there are pacemakers which have lead wires positioned in both the atrium and the ventricle.
An ICD has a lead wire that is positioned in the ventricle, as it is used for treating fast ventricular arrhythmias. Commonly, ICDs will have an atrial lead and ventricular lead.
Pacemakers that pace either the right atrium or the right ventricle are called "single-chamber" pacemakers. Pacemakers that pace both the right atrium and right ventricle of the heart and require two pacing leads are called "dual-chamber" pacemakers. Pacemakers that pace the right atrium and right and left ventricles are called "biventricular" pacemakers.