Ventilation Perfusion Matching (V/Q)

• Ventilation Perfusion Ratio:

  • In respiratory physiology the V/Q ratio is measurement used to describe efficiency and adequacy of matching two variables
  • V (ventilation) – the air which reaches the lungs
  • Q (perfusion) – the blood which reaches the lungs
• Clinical Relevance V/Q:

  • For efficient gas exchange there needs to be maximum coupling between ventilation and perfusion
  • Inadequacy of either V or Q will have significant impact on removal of CO2 & oxygenation of blood
  • The V/Q ratio can be measured clinically with a ventilation/perfusion scan
• shunt:

  • there is no ventilation (usually caused by a black), but there is perfusion
  • so there is no o2 to pick up
  • causes decrease in pAo2
  • V/Q ratio = 0
• V/Q Mismatch:

  • there is ventilation, but no perfusion
  • so o2 is not picked up
  • causes decrease in pAo2
  • this is alveolar dead space as there is o2, but its not being used
  • Increased V/Q ratio
• V/Q Differences in Normal Lung:

  • The lungs centred vertically around the heart
  • Part of lung is superior to heart and part is inferior
  • This has major impact on perfusion and V/Q ratio
  • There are also regional differences in ventilation
  • Lower part (dependent) of lung is better ventilated and better perfused than apex (non-dependent)