Unless absolutely, positively certain, always assume that old films are available somewhere. Don't ignore the lateral film. It can often clarify the presence or absence of lower lobe disease. The chest x-ray never lies. Ask for help.
Help is always available.
PaO 2 is a sensitive and non-specific indicator of the lungs' ability to exchange gases with the atmosphere. FIO 2 is the same at all altitudes. Normal PaO 2 decreases with age. The body does not store oxygen. Supplemental O 2 is a drug.