What it does. A defibrillator provides an electrical current that is measured in joules (a measure of electrical energy). This electrical energy is used to "shock" the heart back into a normal rhythm from a terminal rhythm such as ventricular fibrillation. Thus the mechanism by which the device accomplished this is to "de" fibrillate a heart; hence defibrillator.
Shock to Heart. Untreated, ventricular fibrillation is fatal. This arrhythmia occurs when the ventricle, or lower pumping chamber of the heart, does not beat regularly but simply quivers or fibrillates. To correct this problem, a device is used to shock the heart back into a normal rhythm. This shock 'de'-fibrillates or stops the fibrillating heart, allowing the normal heart rhythm to resume.