During sudden cardiac arrest (SCA), the normally organized electrical impulses that initiate cardiac contraction (our heartbeats) discharge chaotically, and the heart muscle twitches spasmodically. Ventricular tachycardia (VT) and ventricular fibrillation (VF) are two types of these abnormal, chaotic heart rhythms that can bring on SCA. There’s rarely any warning, and a victim of SCA usually collapses and stops breathing immediately. That’s when the clock starts ticking.
Calling 911 in the event of SCA is critical. So is performing CPR on the victim. But the chances of survival from sudden cardiac arrest fall by seven to ten percent every minute, and the single most effective treatment for SCA is early defibrillation—providing a shock to the heart within the first five minutes.
Sadly, less than eight percent of people who suffer cardiac arrest outside the hospital survive. But survival rates of close to 50% have been achieved where automated external defibrillator (AED) programs have been successfully implemented. Survival rates can climb even higher when the person is treated within the first three minutes of cardiac arrest.
SCA is very different from a heart attack, or myocardial infarction, although it can cause SCA. A heart attack occurs when one or more blood vessels to the heart are blocked, causing damage to the heart muscle.
 A person experiencing a heart attack is at a much higher risk for sudden cardiac arrest. This increases his/her risk of death.
 The American Heart Association recommends that defibrillation occur within three minutes or less. Early defibrillation has been shown to significantly increase the rate of survival for SCA victims.