Question 23.4

A 58-year-old female 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 nurse has performed an arterial blood gas analysis.


Patient Value

Adult Normal Range





7.36 – 7.44


41 mmHg (5.4 kPa)

35 – 45 (4.6 – 6.0)


86 mmHg (11.3 kPa)      

Ventilator data:

Tidal Volume            700 mL

Respiratory rate       14 breaths/min

Peak pressure           28 cmH2O

Plateau pressure       18 cmH2O

PEEP                         7.5 cmH2O

SpO2                         94%

EtCO2                       28 mmHg

What is the most likely diagnosis? Give the reasons for your diagnosis.                                                                                                                                (40% marks)

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College answer

Not available.


This question is identical to Question 13.4 from the first paper of 2014. 

There are numerous possible causes for "sudden onset hypoxemia" in a trauma patient recovering from surgery. The college practically give this one away by offering the candidate 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.

Of course, one should still go though the motions of calculating the A-a gradient.

PAO2 = 0.6 × (760 - 47) - (PaCO2 × 1.25) = 376.55;

thus, A-a = 290.55