Table 3

Pathophysiological mechanisms and related causes of hypoxaemia

PathophysiologyPAO2PA–aO2Correctable with 100% oxygenPossible causes
Diffusion limitationNormalIncreasedYesEmphysema, interstitial lung disease, pulmonary oedema
HypoventilationReducedNormalYesCNS depression, sedatives, neuromuscular disease
Decreased PIO2ReducedNormalYesHigh altitude
VʹA/Qʹ mismatchReduced locallyIncreasedYesIncreased: mucus plugging, chronic bronchitis, pulmonary oedema
Decreased: pulmonary embolism, emphysema
ShuntReduced locallyIncreasedMinimal improvementAlveolar: 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.