If someone suddenly went into cardiac arrest, would you know what to do ?
1) Shake and shout to determine if the person is unresponsive. If they are, do step #2.
2) Call 911. Put your phone on speaker and talk to the dispatcher while you begin step #3.
3) Press hard and fast in the center of the victim's chest. Compress the chest at least two inches. Compress 100-120 times a minute. Keep doing this until professional rescuers arrive and can take over for you.
4) Apply an AED (automated external defibrillator) as soon as one becomes available.
These step give the person the best chance of survival.
Original source can be found here.