Contrary to popular belief, sudden cardiac arrest (SCA) is not the same thing as a heart attack. SCA occurs when abnormal rhythms disrupt the electrical impulses of the heart, which cause it to abruptly stop pumping. On the other hand, a heart attack (myocardial infarction) is the final stage of heart disease, a condition that slows blood flow over time. Both medical events require immediate attention; however, the prognosis for SCA is much more grim. Only about 5 percent of people who experience sudden cardiac arrest survive, while many heart attack sufferers can expect to recover [sources: Red Cross; American Heart Association].
Despite the varying prognoses of these heart conditions, CPR does help play a role in survival by extending the window of time a person can be kept alive before they receive additional medical attention, such as defibrillation and emergency surgery. In addition, other sudden medical events -- such as near drowning, carbon monoxide poisoning or an electrical shock -- can lead to loss of heart or lung functioning that requires CPR.
If you witness someone collapse suddenly, or if you come across a person who appears lifeless, tap him or her on the shoulder and ask if they are OK. If they don't respond, you should immediately call for emergency responders and then try to resuscitate the unconscious per using CPR. If it's a baby that appears to be in distress, stroke the infant to see if he or she responds to touch; but never shake a baby to try to get a response.
On the next page, we'll look at the different levels of CPR.