An AED is a portable device that checks the heart rhythm and can send an electric shock to the heart to try to restore a normal rhythm. AEDs are used to treat sudden cardiac arrest (SCA). SCA is a condition in which the heart suddenly and unexpectedly stops beating. Sudden cardiac arrest is not a heart attack. Heart attacks occur when there is a blockage. Sudden cardiac arrest occurs when the electrical system to the heart malfunctions and suddenly becomes very irregular. This is the largest cause of natural death in the United States.