Tough question on alveolar-arterial oxygen gradient.

[Ruban]

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Hi all,
I have a question regarding alveolar arterial oxygen gradient.

Given: A patient is on a ventilator receiving 40% FiO2. Her PaO2 is 200 mmHg. Her PaCO2 is 30 mm Hg. The patient does NOT have an R-L shunt or fibrosis of the lung.

My calculation: PAO2 = FiO2*(760-47) - (PaCO2/0.8)
Where 47 is the water vapor pressure and 769 is atmospheric pressure.
So I get: PAO2 = 0.4(713) - (30/0.8) = 247.7 mm Hg

Yet, the patient's PaCO2 is only 200 mmHg. This is a gradient of 47.7 mm Hg, which is way high.

Can someone explain how this is possible? Why is there an A-a gradient in the absence of shunt or fibrosis?

Thanks!
-R

 

[Doctortobee]

PE maybe

 

[Doc Ivy]

Actually, I was thinking pulmonary embolus as well... or any other type of embolus for that matter (air, fat, amniotic, though quite unlikely)

 

[izz-ay]

Anything that causes a V/Q mismatch can cause an elevated A-a gradient, namely Fibrosing mediastinitis, R-L shunt (both of which you say your patient doesn't have), or effusions, vasculitis, hypoplasia of a pulmonary artery (usually evident very early in life), a pulmonary neoplasm, or an embolus.

hope that helps.