Although the technique of mouth-to-mouth resuscitation was used in biblical times, it was forgotten until the late 1950s, according to the University of Washington. In 1960, several doctors discovered that pressing on a person's chest could help circulate the blood in the body. The two techniques -- mouth-to-mouth resuscitation and chest compression -- were then combined as CPR.

When a person goes into cardiac arrest, the heart stops beating, or stops beating effectively, halting the flow of blood, according to the AHA. CPR helps a small amount of blood to continue to flow to the heart and brain until the emergency medical help arrives. CPR doubles a person's chance of survival.

According to the AHA, you should call 911 before beginning CPR on someone who is unresponsive. But there are exceptions: if the victim is an adult who has been underwater, suffered trauma or is intoxicated, and for infants and children under age 8, a rescuer should begin CPR first, continuing for about a minute, and then call 911.

The rescuer tilts the head of the victim back to listen for breathing. CPR should not be done on a person who is breathing. If no breath is noticed, the rescuer gives two quick rescue breaths, then checks again to see if the person begins breathing, or coughs or moves. If the victim still is not breathing, CPR should begin, with about 100 compressions per minute for an adult. The cycle is 30 compressions, followed by two breaths, each breath should be given over one second and cause the chest to rise, then 30 more compressions, and two more breaths. This cycle should be repeated until emergency help arrives, or until the person begins breathing. The rescuer does not need to check for a pulse before beginning CPR.

If you know CPR and stay up to date with your training, you'll be able to help a person within that critical time frame.

The most common cause of sudden cardiac arrest is ventricular fibrillation (VF), according to the AHA. A heart in VF quivers and can't pump blood. A person in cardiac arrest needs CPR and defibrillation (shock to the heart). An AED and other forms of defibrillation stop the VF and allows normal heart rhythm to resume.

The idea behind the AED was to encourage non-medical people to assist in the rescue of someone in cardiac arrest. An AED is about the size of a laptop computer. The rescuer attaches the AED to the victim, and the device automatically analyzes the heart rhythm. If a shock is needed, the AED makes that decision and indicates that the rescuer stand away from the victim so the shock can be delivered.

If a person moves his or her arms or legs, the person most likely no longer needs CPR. Once emergency medical help arrives, they will take over the care of the victim.