Question 6

Last updated Wed, 12/11/2019 - 16:54
 Topic: Respiratory system
Pass rate
48%
Other SAQs in this paper:
Other SAQs in this topic:

Describe the effects of Ventilation/Perfusion (V/Q) inequality on the partial pressure of oxygen (PaO2) in arterial blood. 

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College Answer

Overall answers lacked sufficient detail on a core area of respiratory physiology. Answers expected included a description of V/Q ratios throughout the lungs and an explanation of how V/Q inequality lowers PaO2.

Discussion

For a more helpful set of college comments for what is an undoubtedly important and often-repeated question, the candidates are directed to Question 5 from the second paper of 2014 and  Question 6 from the first paper of 2008. Both of those asked for the effects of V/Q mismatch on both gases, but this time in 2017 the college decided to limit their question to oxygen. This, perhaps, is because the pass rate for the abovementioned questions was 8% and 0%, respectively. 

In summary:

  • V/Q ratios throughout the lung:
    • The upright lung has a V/Q gradient from top to bottom:
      • The lung bases have a low V/Q ratio (~ 0.6)
      • V/Q ratio reaches 1.0 at around the 3rd rib
      • Lung apices have a high V/Q ratio (~ 3.0)
  • The effect of changing V/Q ratio on gas exchange:
    • The lower the V/Q ratio, the closer the effluent blood composition gets to mixed venous blood, i.e. to "true" shunt.
    • The higher the V/Q ratio, the closer the effluent blood composition gets to alveolar gas.
    • The relationship between PaO2 and V/Q is steeper and more sigmoid than the relationship between  PaCO2 and V/Q.
  • The effect of low V/Q ratio on oxygenation:
    • Low V/Q values (V/Q ratios between 0 and 1) result in hypoxia
    • The hypoxia due to low V/Q ratio is reversible with increased FiO2
    • "True" shunt where V/Q = 0 does not improve with increased FiO2
  • High V/Q ratio units have excellent gas exchange but minimal blood flow
    • Only about 15% of the cardiac output circulates through lung units with a V/Q ratio of 5 and above
    • Therefore, these units cannot contribute enough oxygenated blood to compensate for the poor gas exchange occurring in low V/Q units

A good diagram to produce in such a question comes from West (1977):

effects of different VQ ratios on gas exchange

Specifically, if one were ever also asked about CO2 removal, one could use this graph to demonstrate the different effects of dropping the V/Q from 1.0 to 0.1 on the absorption of oxygen and removal of CO2:

 different effects of changing VQ ratio on oxygenation and CO2 clearance

References

West, John B. "Ventilation-perfusion relationships." American review of respiratory disease 116.5 (1977): 919-943.

Petersson, Johan, and Robb W. Glenny. "Gas exchange and ventilation–perfusion relationships in the lung." (2014): 1023-1041.

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