Hands-Only CPR is taught to be used by a bystander when a teen or adult experiences out-of-hospital, witnessed Sudden Cardiac Arrest to move oxygenated blood through the body to major organs until, ideally, the AED device arrives or the ambulance arrives.
Studies have shown that Hands-Only CPR is just as effective as traditional CPR in these cases for a number of reasons.
Basic Life Support (BLS) CPR teaches checking for a pulse, opening the airway, and rescue breathing. When you perform chest-compression during Hands-Only CPR, there is still some air ventilation happening. So, unless you are expertly trained, breathing will simply waste time and make chest-compression less effective by taking so many useless breaks between compression. The introduction of Hands-Only CPR also doubled the likelihood that a bystander would perform CPR at all, since most people did not want to do the mouth-to-mouth portion.