You see someone collapse. They may be having a heart attack. What should you do?

When in doubt, call 911 and then immediately begin chest compressions. Keep it simple.

The American Heart Association recently issued new guidelines for practicing CPR (cardiopulmonary resuscitation), the technique of keeping a heart attack victim alive until their heart can be restarted. The new guidelines prioritize keeping blood flowing by compressing the victim's chest and de-emphasize the role of mouth-to-mouth resuscitation.

The best way to keep the body and brain supplied with oxygen is for a bystander to repeatedly press down on the center of the victim's chest, taking over for the heart and pumping blood throughout the body. Until the heart can be restarted, the bystander is acting as the victim's heart.

The victim's blood still contains oxygen, but the heart is no longer pumping it to the body's tissues. The best way to keep the body and brain supplied with oxygen is for a bystander to repeatedly press down on the center of the victim's chest, taking over for the heart and pumping blood throughout the body. Until the heart can be restarted, the bystander is acting as the victim's heart.

A video called Hands-Only CPR shows how to perform chest compressions. This video is part of the American Heart Association's online CPR teaching program.

The chest should be compressed to a depth of about two inches (1.5 inches in children), which means you'll have to press hard. It's much more common for a bystander to press too softly than it is to injure the victim by pressing too hard. Strive for about 100 compressions a minute and make sure to let the chest come all the way back up between compressions. Pressing down pumps blood through the body. Releasing the pressure allows the heart to fill with blood between compressions. Both steps are equally important, which is why you can't lean on the chest between compressions. Continue the compressions until trained help arrives.

It's important to call 911 before beginning compressions. Studies have shown that the most important factor in surviving a heart attack is how quickly the heart is re-started by a shock from a defibrillator. While there are defibrillators in some public places, usually they must be brought to the victim by emergency personnel, which means calling 911. If another bystander is with you, they can try to locate a nearby defibrillator.

Bystander CPR saves lives. The best way to learn it is by taking a training course. CPR training courses are taught by the American Heart Association, as well as many local fire departments and other organizations. If you can't find one in your area, the AHA provides a toll free phone number, 1-877-AHA-4CPR, for assistance in finding one.

In the absence of training, try to keep calm, call 911 and then begin chest compressions.

The American Heart Association has now established a link to more information about the new guidelines and how to find a CPR training course near you.

The new CPR guidelines for bystanders and health professionals were published in the November 2, 2010 supplementary issue of the journal Circulation as a series of articles. These articles are freely available.