The shock from an automated external defibrillator addresses two kinds of abnormal heart rhythms, ventricular fibrillation and ventricular tachycardia. In ventricular fibrillation, the heart's ventricles, or lower chambers, do not beat properly. Instead, they quiver rapidly, rendering the heart unable to pump blood to the body and causing a condition called sudden cardiac arrest. In ventricular tachycardia, the ventricles beat much faster than normal, which may also lead to sudden cardiac arrest, explains the National Heart, Lung and Blood Institute.
Automated external defibrillators are able to determine if an individual is suffering from either of these two arrhythmias and therefore whether he requires a shock. Following the included instructions, the person using the AED attaches its electrodes to the patient. The AED then analyzes the victim's heart rhythm and, if it detects either ventricular fibrillation or tachycardia, uses voice commands to guide the user through administering the shock, explains the National Heart, Lung and Blood Institute.
Sudden cardiac arrest, whether caused be these two conditions or not, is fatal in 95 percent of cases. Additionally, every minute an individual suffers from sudden cardiac arrest decreases his survival chances by 10 percent. Individuals suffering from sudden cardiac arrest may collapse or pass out, breathe irregularly or stop breathing, have an irregular or nonexistent pulse or be unresponsive. If two rescuers are present, one should perform CPR while the other calls 911 and prepares the AED, states the National Heart, Lung and Blood Institute.