Sudden cardiac arrest cases are usually due to abnormal heart rhythms called arrhythmias, the vast majority of which are ventricular fibrillation. Ventricular fibrillation is a condition in which the heart's electrical impulses suddenly become chaotic, causing the heart to cease pumping blood effectively. Victims of SCA collapse and quickly lose consciousness, often without warning. Unless a normal heart rhythm is restored, death will follow within a matter of minutes.

The cause of sudden cardiac arrest is not well understood. Many victims have no history of heart disease, or if heart disease is present, it has not functionally impaired them. Unlike a heart attack, which is the death of muscle tissue from loss of blood supply, many victims of SCA have no prior symptoms. SCA can strike anyone, at any time, anywhere.

Once an AED is turned on, it provides prompts to guide the user through the process. One of the first prompts instructs the user to connect the AED to the victim via the adhesive electrodes (pads) placed on the chest.

The AED then analyzes the victim's heart rhythm through the electrodes using a built-in computer program. It then determines if a shock is "needed" or "necessary." More specifically:

1. The electrodes placed on the victim's body send the heart rhythm information (ECGs) to the AED.
2. The AED "reads" short segments of the heart's rhythm. It checks characteristics such as frequency, shape, slope, amplitude and heart rate.
3. Based on these characteristics, the AED determines whether or not a shock is needed and activates the appropriate user prompts.

If a shock is needed, the AED will prompt the user to press the button that delivers the shock. It will then re-analyze the heart rhythm to determine if more shocks are needed. If a shockable rhythm is not detected, the AED will prompt the user to check the victim for a pulse, and to perform CPR if needed.