Question 6

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):

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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