When does someone use a defibrillator?

To stop fibrillation: "fibrillation" is when the heart muscle is not contracting efficiently, but rather twitching weakly at rapid rates. If this involves the bottom chambers of the heart ("ventricles"), it is life threatening as normal cardiac output falls drastically. The rhythm must be normalized quickly to resume normal heart output, and electrical shocks promptly defibrillate, i.e. Stop the abnormal rhythm.

For life: Threatening arrhythmia or to cardiovert rhythms like afib, svt or atrial flutter.