From my perspective as a former first aider and current intensive care specialist (with various certs) the Casualties lungs will already contain 21%O2 (pretty much), so breathing into them will replace that with about 18%O2- ie nae much improvement !
In adults, the primary cause of cardiac arrest is often heart attack (MI). This disrupts the normal electrical activity and causes an abnormal rhythm (often but not always Ventricular Fibrilation). Shocking the heart (defibrillation) actually STOPS the heart, allowing the natural pacemaker to regain control of the natural rhythm, and this is the GOLD STANDARD! Breathing into a casualties chest initially will not add anything to the equation, and thus is counter productive, delaying the more important defibrillation.(hence the recommended go for help then start CPR).
The is different in children as the major cause of cardiac arrest in kids is hypoxia (low oxygen levels)- thus the recommendation of 1 min CPR to try and restart things before going for help.
Obviously, if there is more than one of you, then common sense dictates that someone goes for help, and the other resuscitates regardless of the situation (personal safety excluded).
As an ever-learning practitioner, I'm more than happy to be corrected on any aspect of the above!
ATB
KP