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. In 2008, the AHA revised its guidelines to include hands-only CPR, a version that uses only chest compressions.

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. When performed correctly, CPR can double or even triple a person's chance of survival.

According to the AHA, you should call 911 before beginning CPR on someone who is unresponsive. Ideally, have someone else call 911 while you begin CPR. 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 two minutes (about five cycles of chest compressions and rescue breathing), and then call 911.

The AHA says the next step is to begin chest compressions, pressing hard and fast in the center of the victim's chest. Aim for 100 compressions a minute and continue until the person revives or help arrives. Give rescue breaths if you are trained in conventional CPR (chest compressions plus mouth-to-mouth breaths) and you are confident in your ability to give this type of help. 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 enable non-medical people to assist in the rescue of someone in VF 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. The AED gives verbal prompts to guide the rescuer through the resuscitation process.

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.