An automated external defibrillation (AED) is a device widely available in the public which acts to defibrillate (cardiovert) a patient out of serious heart rhythm disorders, frequently ventricular tachycardia or ventricular fibrillation.
AEDs are now seen throughout many public places including airports and shopping malls. They are designed so that the normal, non-medically trained individual will be able to operate the device in the event of a cardiac arrest. Early defibrillation during cardiac arrest is crucial to reduce mortality and anoxic brain injury.
Sudden cardiac death is most commonly seen in the setting of myocardial infarction and is related to the rhythm ventricular fibrillation. Sudden cardiac death from hypertrophic obstructive cardiomyopathy (HOCM), congenital coronary anomalies or commotio cordis can occur in young athletes and thus most schools have an AED present at sporting events.
Note that implantable cardioverter defibrillators (ICDs) are implanted forms of an AED and are much smaller. These have varying indications, however in general are used in individuals at high risk for sudden cardiac death.