When I learned CPR decades ago and was the caption of the first aid squad at AT&T Red Hill facility, the A-B-C (Airway-Breathing-Circulation) in this order was stressed. Not any more! It is now CAB (Circulation-Airway-Breathing). Furthermore, mouth-to-moth is not required any more, especially, in bystander CPR. Just the chest compression at the rate of 90 to 100 per minute will do. Most effective depth of compressions =2 inches. 2 inches results in 100% resuscitation, and 1.5 inches results in 15% resuscitation.