TABLE 1 Separation of physiological causes of hypoxaemia

Cause of hypoxaemia	Typical example	Arterial P_CO₂	P_A−aO₂ difference	Arterial P_O₂ on 100% O₂ and Q_S/Q_T	V′_O₂/Q′ ratio
Low inspired P_O₂	Altitude	↓	Normal	Normal (for altitude)	Normal or ↓
Hypoventilation	Narcotic overdose	↑	Normal	Normal	Normal
V′_A/Q′ inequality	Most lung diseases	↑ or normal or ↓	↑	Normal	Normal
Diffusion limitation	Exercise at altitude; interstitial fibrosis	Normal	↑	Normal	Normal
Shunts	Acute lung injury	↑ or normal or ↓	↑	P_O₂ below normal; Q_S/Q_T increased	↑ or normal or ↓
Extrapulmonary: high V′_O₂/Q′ ratio (with lung disease)	Heart failure with pulmonary oedema or lung disease	↑ or normal or ↓	↑	Normal (if no shunt)	↑
Cautionary notes		Highly dependent on individual ventilatory responsiveness	Accuracy requires R (V′_CO₂/V′_O₂) to be known	Potentially large errors if P_v¯O₂ is unknown; risk of measurement error in arterial P_O₂	Requires V′_O₂ and Q′ to be measured

P_CO₂: carbon dioxide partial pressure; P_O₂: oxygen partial pressure; P_A−aO₂: alveolar−arterial P_O₂ difference; Q_S/Q_T: shunt; V′_O₂: oxygen uptake; Q′: perfusion; V′_A: alveolar ventilation; V′_CO₂: carbon dioxide elimination; P_v¯O₂: pulmonary arterial P_O₂.