When will a paramedic use a defibrillator?

Cardiac Arrest/ VFib. Defibrillators are used to treat the abnormal heart rhythms ventricular fibrillation and ventricular tachycardia. The defibrillator will cause a strong electrical current to pass through the heart with the goal of acting as a "reset" button, allowing the heart's rhythm to return to a normal speed and pathway.
No pulse. When a patient isn't breathing or doesn't have a pulse. The machine will analyze the heart rhythm and the decision to shock will be made at that time during cpr.