A pacemaker is a small device that helps the heart beat at a regular rhythm and rate. The doctor places the pacemaker inside the body (implantation) during a surgical procedure. Defibrillator is a medical product that restore natural heartbeat by sending to the heart an electrical pulse or shock. These are used to avoid or correct an arrhythmia, an erratic or too slow or too fast heartbeat. If the heart suddenly stops, defibrillators can also restart the rhythm of the heart.