What exactly is it?

Your heart pumps blood to all parts of your body - if it stops, this means your whole body will not get the blood flow and oxygen it needs. Your heart may stop if you have a disease of the heart, but it may also stop if you have other medical conditions that have weakened the heart. CPR is an emergency, life-support procedure that can be done if your heart stops. It involves another person performing chest compressions -- which are fast, hard pushes on your chest -- to try to keep the heart pumping to all parts of the body. If done correctly, chest compressions are done by pushing down on the chest bone by 2 inches, 100 times a minute. Sometimes, it may also involve using a machine to deliver an electric shock to the heart to try to restart it. On average, CPR lasts 15-30 minutes. If the heart is restarted, patients are usually unresponsive (in a coma) for several days, and need to be on the breathing machine (ventilator) in the Intensive Care Unit (ICU) of the hospital.