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

This quiz covers the definition, types, and characteristics of hypoxia, including hypoxic, anaemic, stagnant, and histotoxic hypoxia.