A 58 year old woman ventilated in intensive care for a week following a motor vehicle accident was noted to drop her oxygen saturation suddenly, requiring an increase in FiO2 from 0.4 to 0.6.

The nursing staff have performed an arterial blood gas.



Normal Range





(7.36 – 7.44)


41 mm Hg

(36 – 44)


86 mm Hg

Ventilator data

Tidal Volume                        700 ml 
Respiratory rate                   14 
Peak pressures                     28 cm H2O
Plateau pressures                 18 cm H2O 
PEEP                                     7.5 cm H2O

SpO2                                        94% 
EtCO2                                      28 mm Hg

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

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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)


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.