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Second year medical student asking:

A 40 year old woman is admitted to the ICU with acute respiratory distress syndrome due to a life threatening case of pneumonia.

Her respiratory values are as follow BEFORE treatment:

pH = 7,36

PCO2 = 48 mmHg

PO2 = 70 mmHg

She is treated with 100% oxygen and the values after treatment are:

pH = 7,35

PCO2 = 52 mmHg

PO2 = 145 mmHg

I am now asked to calculate her alveolar-arterial gradient and thereafter explain whether this number is indicative of a Va/Q mismatch.

I calculate her A-a gradient to be 21 mmHg. An expected A-a gradient would be roughly 20 mmHg for her age.

This seems normal? I tried with an external calculator as well and the values are normal too?

http://www.mdcalc.com/a-a-o2-gradient/

Why is she admitted to the ICU if her A-a gradient is completely normal and then treated so that her A-a gradient goes severely down?

I think I'm misunderstanding the point of it all?

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