explanation for normal A-a gradient (rapid review)

[badasshairday]

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I'm sorta confused here. Goljan talks about how there can be hypoxemia with a normal A-a gradient and lists them on page 276 (3rd ed. top of the page 276 4. abc) (2nd ed. pages 286-287 4.abc).

He states that dysfunction in the muscles of respiration ie. paralyzed diaphragm, ALS, would have a normal A-a gradient. How is this possible?

 

[turkeyjerky]

I'm sorta confused here. Goljan talks about how there can be hypoxemia with a normal A-a gradient and lists them on page 276 (3rd ed. top of the page 276 4. abc) (2nd ed. pages 286-287 4.abc).

He states that dysfunction in the muscles of respiration ie. paralyzed diaphragm, ALS, would have a normal A-a gradient. How is this possible?

Because in those syndromes, the lung parenchyma is fine and dandy. The alveolar PO2 and the arterial P02 are equal (or at least with a <10-15 mm gradient). That is, in those syndromes, there is no shunting or V/Q mismatching (at least not more than normal) present to cause an increased A-a gradient.

So what's the mechanism of the hypoxemia then? Simple--hypoventilation.

 

[ChiDO]

In hypoventilation you have increase in CO2 and decrease in O2, thus no change in the A-a gradient.

 

[badasshairday]

Thanks for the responses. 👍