See below. An implanted cardiac defibrillator is a pacemaker and a defibrillator. The defibrillator is needed for certain conditions that would lead to a dangerous arrythmias - heart rhythms - that could cause death. When such a rhythm is detected, within the program set by the cardiologist - the defibrillator will deliver shock(s) to the heart.
Defibrillators. The device has an antenna that monitors the electrical activity of the heart. If the recorded parameters fit those of ventricular tachycardia or ventricular fibrillation, the device will charge its capacitor and deliver programmed shocks to try and return the rhythm to normal. It is a computer and programmed to do its functions. They are very effective but not perfect.