Why does someone having a heart attack need an AED?
When a heart attack becomes a full cardiac arrest, the heart most often goes into an uncoordinated electrical activity called fibrillation. The heart twitches ineffectively and is unable to pump blood. The AED delivers electric current to the heart muscles, momentarily stunning the heart and stopping all activity. Subsequently, the heart has an opportunity to resume beating effectively.

Will an AED always resuscitate someone in cardiac arrest?
The AED treats only a heart in ventricular fibrillation (VF). In cases of cardiac arrest without VF, the heart will not respond to electric currents but requires medical treatment and breathing support.

Will I get zapped if I shock a victim in the rain or near water?
It is remotely possible to get shocked or to shock bystanders if water is near or under the patient. Try to move the person to a dry area, and cut off wet clothing. When the SHOCK button is activated, make sure no one, including the AED operator, is touching any part of the victim.

Can an AED make mistakes?
The AED computer uses complex analysis algorithms to determine whether to shock. If the operator has attached the AED to an adult victim who is not breathing and has no pulse, the AED will make the correct "shock" decision more than 95 of 100 times and a correct "no shock indicated" decision more than 98 of 100 times. The level of accuracy is greater than the accuracy of emergency professionals.

--American Heart Association