Causes of Hypoxaemia

• Causes of Hypoxaemia:

  • Hypoventilation
  • Diffusion impairment
  • Pure shunt
  • V/Q mismatch
• Hypoventilation:

  • decrease in respiratory rate and tidal volume
  • so decrease in volume of fresh gas reaching alveoli
  • Always leads to increase in CO2
  • The hypoxaemia is abolished by increase in FiO2
• Diffusion impairment:

  • can occur at the alveolar membrane, interstitial space or lung membrane
  • decrease in gas exchange across alveolar membrane
  • CO2 transfer relatively unaffected – why? as co2 is 20 times more soluble than o2 so it can easily diffuse across
  • Reduced by increase in FiO2 – why? causes a steeper concentration gradient
• Pure shunt:

  • there is no ventilation, but there is perfusion, generally caused by block in the alveoli, so gas exchange doesnt occur
  • CO2 often normal or low due to increased respiratory rate
  • NOT reduced by increase in FiO2 - why? as the alveolus is blocked, o2 cant reach the alveolar membrane and capillaries
• V/Q mismatch:

  • most common cause of hypoxaemia
  • Will lead to increase in CO2 limited by increased respiratory rate
  • Reduced by increase in FiO2