Question 13.4

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 nursing staff has performed an arterial blood gas.


Patient Value

Normal Adult 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 pressures 28 cm H2O
Plateau pressures 18 cm H2O
PEEP 7.5 cm H2O
SpO2 94%
EtCO2 28 mmHg

What is the most likely diagnosis? Give the reasons for your diagnosis.

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College Answer

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) and there is increased dead space, (raised A-et CO2 gradient)


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