If someone is having SCA, you may see him or her suddenly collapse and pass out. Or, you may find the person unconscious and unable to respond when you call or shake him or her.
The person may not be breathing, or he or she may have an abnormal breathing pattern. If you check, you usually can't find a pulse. The person's skin may become dark or blue from lack of oxygen. Also, the person may not move, or his or her movements may look like a seizure (spasms).
An AED can check the person's heart rhythm and determine whether an electric shock is needed to try to restore a normal rhythm