Shunt and AA gradient

[lemonade90]

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Why does a shunt result in a greater A-a gradient? Is the assumption that only part of the lung is impaired in terms of ventilation? If the entire lung was impaired and you had a 100% shunt, then you wouldn't have any ventilation and so the Aa gradient should be non-existent.

Why is a shunt considered a perfusion limited situation? Isn't the ventilation the limiting problem?

 

[reedsposer22]

my guess would be becuase the blood being shunted back is already got oxygen, and so no ventilation occurs? IDK, all i know is a high A-a gradient means that there is a problem of 02 diffusing across the alveolar membrane....normal is around 5-10.

 

[Munchen]

I think it has to do with the partial pressures. The partial pressure of oxygen in the remaining alveoli (even if there was only one alveoli left) is always going to be close to a hundred percent but there's not enough oxygen to oxygenate all the blood that's flowing in. In other words, equilibration is reached early along the length of the pulmonary capillary which means that the blood flowing slightly later cannot get oxygenated. This causes the resultant hypoxemia with an increased A-a gradient. This is how I made sense of it. I could be wrong.

 

[DrDJShik]

Because the blood is being shunted away from one part to another part of the lung, it has essentially eliminated that "road" to perfusion. Now you basically have the same volume of blood trying to oxygenate in less lung. So unless velocity of blood speeds up (which I do not believe it does, only pressure increases aka pulm. HTN), your lungs will be limited in how much overall perfusion takes place, therefore decreasing PaO2 and widening the A-a gap.

You're right to say the ventilation problem causes this effect and you would naturally have an obstructive V/Q mismatch along with it.