Question 6.1

The following data was obtained from a 30 year old spontaneously  breathing tachypnoeic individual. His GCS was15.



Normal Range



(7.36 –7.44)


30 mm Hg

(36 –44)


60 mm Hg


55 mm Hg


130 G/L

(130 – 150)

a)  What is the likely explanation for the above set of data?

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College Answer

a)  What is the likely explanation for the above set of data?

High altitude or breathing a gas mixture containing low FiO2, the combination of a low PAO2 and a normal A-a gradient makes these the likely possibilities.


The A-a gradient in this man is very odd. The college gives us the calculated alveolar O2 tension, and it is an oddly low number.

Furthemore, the A-a gradient is only 5, which makes one think that there is no gas exchange deficit.

So, lets leave the patient out of this and turn instead to blame the atmosphere.

Lets consider that this person's alveolar O2 partial pressure should be about 112 at 760mmHg:

PAO2 = 0.21 × (760 - 47) - (PaCO2 × 1.25) = 112.2

Irrespective of the actual alveolar O2 tension, the fraction of O2 should still be 21% if the patient is breathing room air, is not on Mars, and is not somehow actively secreting oxygen.

So, the patient is breathing a gas mixture with a decreased FiO2, for whatever reason.

One can calculate that if 60mmHg is in fact 21% of the total gas mixture, then the atmospheric pressure must be something like 285mmHg, equivalent to the pressure expected at an altitude of about 7.5km above sea level.


Peacock, Andrew J. "ABC of oxygen: oxygen at high altitude." BMJ: British Medical Journal 317.7165 (1998): 1063.