Compare the effect on arterial blood carbon dioxide and oxygen levels of ventilation / perfusion inequalities.

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College Answer

The main points expected for a pass were:
· Range, regional pulmonary differences and gradients of V/Q ratios.
· Definitions of shunt (V/Q = 0) and dead space (V/Q = ).
· Explanation of why and how V/Q mismatch lowers arterial PaO2 (majority of pulmonary
blood flow being from basal regions, shape of haemoglobin disassociation curve).
· Explanation of why and how V/Q mismatch lowers arterial PaCO2 (majority of
pulmonary blood flow being from basal regions, predominately linear shape of CO2
disassociation curve within the physiological range of PaCO2 values).
Again, the use of illustrations would be very useful aids as part of a good answer.
Candidates often failed to frame their answer to the question that was asked and deviated to
areas not directly sought after by the question. This resulted in wasted time and opportunities
for marks.
Syllabus B1g
Reference Nunn 4th edition page 165-187

Discussion

To complain that "candidates often failed to frame their answer to the question that was asked" is to ignore that "the question that was asked" was worded in an intentionally difficult way. This awkward stem structure is a flaw of exam item design, as it distracts from the creative thinking and synthesis which one expects to be used for writing the answer. Students spend cognitive bandwidth on interpreting the question instead of thinking about V/Q relationships. Consider how much better it would have been if it asked, "Compare the effect of ventilation / perfusion inequalities on arterial blood carbon dioxide and oxygen levels". Having said this, changing to a more favourable wording in Question 5 from the second paper of 2014 did not seem to help, as the pass rate only went up from 0% to 8%. 

Anyway:

  • V/Q ratios throughout the lung:
    • The upright lung has a V/Q gradient from top to bottom:
      • V/Q =  ∞ : "dead space", where there is no perfusion
      • V/Q > 1.0 : lung apices
      • V/Q = 1.0 : ideal V/Q  ratio: midzones (around the 3rd rib)
        V/Q < 1.0 : lung bases 
      • V/Q = 0 : or "true" shunt - collapsed regions of lung
    • The range of V/Q ratios in a normal healthy young person's upright lung is around 0.6-3.0
    • This gradient virtually disappears when the subject is supine and the lung is horizontal.
  • 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
  • The effect of low V/Q ratio on CO2 removal:
    • The same change in V/Q (from 1.0 to 0.1) has a significant effect on oxygenation, but a minimal effect on CO2 removal
    • This is because the relationship of CO2 clearance to V/Q ratio is more flat and linear than the relationship of O2 uptake
  • 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

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.