So a couple of things, in order: every minute from arrest that no CPR is being done, chances of survival decrease by about 10%. If bystander compression-only CPR has been started, there's about 8 minutes supply of sufficiently oxygenated blood (and while compressions aren't ventilations, they do still encourage some small oxygen exchange). Even when we arrive, gaining advanced airway access or ventilation comes secondary to compressions (our county gives a limited window of 10-15 seconds to pause compressions to intubate, but with Glidescopes it's often possible to intubate through compressions).
