CPR and the use of an AED are the only proven ways to help someone survive cardiac arrest. Sometimes, you may hear a cracking sound when you do CPR, do not be alarmed. The sound is caused by cartilage of ribs cracking but the damage is not serious. 

The risk of delaying CPR or not doing CPR is far greater than the risk of a broken rib. AEDs are medical devices that are designed to be used by laypersons. They can be safely used by anyone as long as they follow the audio and visual instructions.