College Answer

a)  What  is  the  most  likely  diagnosis  diagnosis?  List  your  reasons  for  the diagnosis.

The most likely diagnosis is a pulmonary embolus. The reasons are as follows:

Sudden onset of hypoxemia raises a number of possibilities – mucus plugging, pneumothorax, LVF, aspiration etc. However, the ventilation data indicate preserved compliance, normal peak pressures (argue against a pneumothorax or plugging or LVF) as well as there is increased dead space, (raised A-et CO2 gradient)

Discussion

This question is identical to Question 13.4 from the first paper of 2014. It relies on the candidate to be able to quickly perform a calculation of the A-a gradient. However, even without maths, one can arrive at the conclusion that the PO2 is way too low for an inspired fraction of 60%.

One could rabbit on about the other causes, but the ventilator data is pristine. The patient has normal peak airway pressures, so nothing is blocked, and normal plateau pressures, demonstrating reasonably normal lung compliance. This is a defect of perfusion, not ventilation.

Lastly, the candidate is presented with an end-tidal CO₂ measurement, which is substantially lower than the arterial CO₂ measurement, suggesting that there is a large area of lung which is not participating in gas exchange, i.e. it is dead space.

Capnometry and the arterial-expired carbon dioxide gradient is discussed elsewhere.