| Cause of hypoxaemia | PAO2 | PA–aO2 | PaO2 response to increased FIO2 |
| Diffusion limitation | Normal | Increased | Improved |
| Hypoventilation | Reduced | Normal | Improved |
| Decreased PIO2 | Reduced | Normal | Improved |
| Low V′A/Q′ | Reduced locally | Increased | Improved |
| Shunt | Reduced locally | Increased | Minimal improvement |
PAO2: alveolar partial pressure of O2; PA–aO2: alveolar–arterial O2 tension difference; PaO2: arterial partial pressure of O2; FIO2: inspiratory O2 fraction; PIO2: partial pressure of O2 in inspired gas; V′A/Q′: ratio of alveolar ventilation to perfusion.