Basics of V/Q Matching

Optimal gas exchange occurs when regions of lung are ventilated in proportion to their perfusion, i.e. V/Q = 1

• Uneven distribution of ventilation and perfusion causes inefficient gas exchange:
  • Excessive ventilation causes excessive work
  • Inadequate ventilation causes inadequate gas exchange

Distribution of Ventilation

• The right lung is slightly better ventilated than the left
• In an erect patient the bases of the lung are better ventilated
  The weight of lung above compresses the lung below, improving the compliance of dependent lung whilst stretching the non-dependent lung.
  • This is only significant at low inspiratory flow rates
  • The V/Q ratio in the bases is ~0.6
  • The V/Q ratio in the apices is >3
• In a lateral position:
  • The dependent lung is better ventilated in a spontaneously breathing patient
  • The non-dependent lung is better ventilated in a ventilated patient

Distribution of Perfusion

• The pulmonary circulation is a low pressure circulation
• Gravity therefore has a substantial effect on fluid pressure
• Consequently, the distribution of blood throughout the lungs is uneven:
  • The bases perfused better than the apices
    This is affected by lung volume, with the effect:
    • Becoming more pronounced at TLC (with apical perfusion falling precipitously)
    • Reversing slightly at RV

V/Q Ratios

• The global V/Q ratio for normal resting lung is 0.9
• The global V/Q ratio improves to 1.0 during exercise

V/Q Mismatch and Etymology

• V/Q mismatch occurs when V/Q ≠ 1:
  • V/Q >1 (Dead Space)
    Ventilation in excess of perfusion.
    • However, pulmonary blood is passing ventilated alveoli and PaO₂ is normal
      
      
  • V/Q 0 to 1 (V/Q scatter)
    Perfusion in excess of ventilation.
    • Increasing in PAO₂ will increase PaO₂
    • This is commonly referred to by the general term of V/Q mismatch
      
      
  • V/Q = 0 (Shunt)
    Mixed venous blood entering the systemic circulation without being oxygenated via passage through the lungs. PaO₂ falls.

References

1. West J. Respiratory Physiology: The Essentials. 9th Edition. Lippincott Williams and Wilkins. 2011.
2. Lumb A. Nunn's Applied Respiratory Physiology. 7th Edition. Elsevier. 2010.