Pathophysiology | PAO2 | PA–aO2 | Correctable with 100% oxygen | Possible causes |
Diffusion limitation | Normal | Increased | Yes | Emphysema, interstitial lung disease, pulmonary oedema |
Hypoventilation | Reduced | Normal | Yes | CNS depression, sedatives, neuromuscular disease |
Decreased PIO2 | Reduced | Normal | Yes | High altitude |
VʹA/Qʹ mismatch | Reduced locally | Increased | Yes | Increased: mucus plugging, chronic bronchitis, pulmonary oedema Decreased: pulmonary embolism, emphysema |
Shunt | Reduced locally | Increased | Minimal improvement | Alveolar: pulmonary oedema, pneumonia, atelectasis Cardiac: Eisenmenger syndrome Vascular: pulmonary AV shunt Other: hepatopulmonary syndrome |
PIO2: inspired oxygen tension; PAO2: alveolar oxygen tension; PaO2: arterial oxygen tension; PA–aO2: alveolar–arterial oxygen tension difference; AV: arteriovenous; CNS: central nervous system.