Blood gases and oxygenation

The normal range for PaO2 is 11.3-13.3 kPa. A partial pressure less than 8 kPa defines respiratory failure.

Arterial hypoxaemia is commonly due to pulmonary disease:

• poor gas transfer:
  • type I respiratory failure
  • PaCO2 is typically low
• hypoventilation:
  • type II respiratory failure
  • PaCO2 is typically raised

Arterial hypoxaemia may occur when the lungs are normal:

• low inspired partial pressure of oxygen e.g. altitude
• ventilation-perfusion mismatch
• right to left shunts:
  • congenital heart disease
  • arteriovenous malformation of the pulmonary vessels

Measurement of the alveolar-arterial (A-a) gradient is useful in hypoxic patients and may help to distinguish hypoxaemia due to pulmonary disease from other causes of hypoxaemia.