To continue from your point, do CPR until you become exhausted. CPR is very physically demanding you should be cracking ribs. For anyone interested the order of operations is, call 911, determine if you need to do CPR, call for bystanders to take over chest comprehensions when you get exhausted if possible, then begin CPR for as long as you can maintain it. The most important thing is to call 911, chest comprehensions don't bring people back from the dead, it gives the patient time for the paramedics to arrive.