If there is a strong/decent pulse rescue breathing is very succesful. I had a patient that I ventilated for 45 min (stuck in traffic) expecting her to go into cardiac arrest any minute. I had defib on her and medic on stand by. I went to visit her at the end of my shift (6 hours later) and she was wide awake and smiling.

The only problem with rescue breathing is that if head is positioned the wrong way you will be inflating stomach instead of lungs. When gastric inflation reaches max patient will throw up on you and that's a posible choking hazard. To avoid that just watch for even chest rise and if you you see stomach inflating reposition the head.

Matt