Physiology of Hypoxia

Hypoxia

• Defined as a lack of oxygen at the tissue level
• Different from anoxia, which is a complete absence of oxygen in the tissues

Types of Hypoxia

• Classified into four groups:

Hypoxic Hypoxia

• Characterized by low arterial pO2 despite normal or elevated oxygen carrying capacity of blood and rate of blood flow to tissues
• Causes:
  • Low pO2 of inspired air
  • Decreased pulmonary ventilation
  • Defect in exchange of gases
  • Venous arterial shunts (Right to left shunt)
  • Ventilation-perfusion imbalance (including increased physiological dead space and physiological shunt)

Anaemic Hypoxia

• Characterized by normal arterial pO2 but reduced amount of haemoglobin available to carry oxygen
• Causes:
  • Anaemia
  • Haemorrhage
  • Conversion of haemoglobin to some abnormal form (e.g. met haemoglobin, carboxyhemoglobin)

Stagnant (Ischaemic) Hypoxia

• Blood flow to the tissue is too low to deliver adequate oxygen despite normal arterial pO2 and haemoglobin concentration
• Causes:
  • General slowing of the circulation (e.g. heart failure, shock)
  • Local slowing (e.g. vasoconstriction, cold, arterial wall spasm)
  • Haemorrhage

Histotoxic Hypoxia

• Amount of oxygen delivered to the tissues is adequate, but tissues cannot make use of the oxygen due to toxic agents inhibiting oxidative enzyme activity
• Cause:
  • Cyanide poisoning, which damages the enzyme cytochrome oxidase

Effects of Hypoxia

• Can lead to impairment of judgment, and other clinical features, depending on the degree of hypoxia