A defibrillator is an implanted device that is pacemaker (keeps the heart rate from getting too slow), but also is designed to treat the heart rate if it gets too fast. When it detects that the heart rate is going too fast (typically over 170 beats per minute), it will either pace the heart back to normal rhythm or deliver a shock that resets the heart to normal rhythm. The shock is a strong sensation, something one of my patients has described as "being kicked in the chest by a horse." It is given to patients who either have had a cardiac arrest, or are at high risk for having a cardiac arrest. The device is also implanted in the upper chest underneath the skin, and is about twice a big and twice as thick as a pacemaker.
Both pacemakers and defibrillators have come a long way since they were first implanted in humans (1960s for pacemakers, 1980s for defibrillators). They have become very reliable devices that have a rate of mechanical failure of less than one out of 1000. The battery does run down after 5-10 years, and has to be exchanged. The lead or leads connecting the device to the heart last several decades, but again, sometimes need to be exchanged. Most defibrillator and some pacemakers now can be followed from home using a monitor that does daily scans of the implanted device wirelessly. The patient just has to be sitting (or sleeping) within 10-15 feet of the monitor and it automatically does a "system check" and notifies the doctor if there are any signs of trouble. This is a newer feature of implantable devices that makes them even more reliable and safe to use. Even though the risk of a problem from a cardiac implanted electronic device is low, here at our program, we recommended that they are fully checked (either from home or in the office) every six months for a pacemaker, and every three months for a defibrillator.