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.
Last updated 2017-10-05

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