well it will be breathing under pressure
Oh and my take on this is...
you have two things.
1. the required partial pressure to allow oxygen transfer across the cellular wall in the lungs. Its not binary but a none linear curve.
2. The up take of oxygen in the blood and transfer to the organs.
Pressure breathing only increases the partial pressure and puts you up the curve for number 1. Smokers etc will require higher to get the same transfer.