SCA is treatable most of the time, especially when it is due to an electrical abnormality called ventricular fibrillation (VF). Immediate treatment includes cardiopulmonary resuscitation (CPR) and defibrillation. This treatment must be provided immediately to be effective, preferably within three to five minutes. Even the fastest emergency medical services may not be able to reach a victim this quickly. That is why prompt action by bystanders is so critical and why it is so important that more laypersons learn CPR and how to use an automated external defibrillator (AED).