Compare the effect on arterial blood carbon dioxide and oxygen levels of ventilation / perfusion inequalities.
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
Reference Nunn 4th edition page 165-187
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%.
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.