A-a gradient

[Breezee]

What shortcut do you use to calculate the A-a gradient? I'm pretty sure no one is using the alveolar gas equation intraop to determine PAO2. Anyway to figure it out without an ABG?

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[huktonfonix]

What shortcut do you use to calculate the A-a gradient? I'm pretty sure no one is using the alveolar gas equation intraop to determine PAO2. Anyway to figure it out without an ABG?

why not? its not that difficult to determine PAO2. You generally cant figure out PaO2 without an ABG although you can guess from the SaO2 if its not on the flat part of the curve.

 

[Licoricestick]

What shortcut do you use to calculate the A-a gradient? I'm pretty sure no one is using the alveolar gas equation intraop to determine PAO2. Anyway to figure it out without an ABG?

No shortcut - PAO2 is easily calculated from alveolar gas equation.

If I need to calculate the A-a gradient then that probably suggests that the patient needs an ABG anyway (unless the boss wants you to do it for fun without an ABG...then you're pretty much guessing, maybe a good guess, but still a guess).

 

[Breezee]

I was pretty sure that it could not be done without an ABG. However, sometimes I'll have an attending come into a case and say, "the patient has a wide A-a gradient." They look at the ABG and say it like in 2 seconds. What are they looking at? I know they aren't calculating that equation in their head that fast.

 

[huktonfonix]

I was pretty sure that it could not be done without an ABG. However, sometimes I'll have an attending come into a case and say, "the patient has a wide A-a gradient." They look at the ABG and say it like in 2 seconds. What are they looking at? I know they aren't calculating that equation in their head that fast.

Assuming youre at a reasonable altitude, your PAO2 is gonna be pretty predictable at a given FIO2. If you have a PaO2 of 150 at 100 FIO2 then I can almost guarantee your gradient is gonna be wide, unless your barometric pressure is somewhere around 300.

 

[Idiopathic]

yeah all it takes is an ABG, but it cant be done without an ABG. although you could guess if someone is saturating poorly on 100% FiO2 for instance, although many factors could be to blame there.

 

[deleted50478]

I get a rough estimate by taking the FIO2 times 600.

eg. If you are on 100% O2, your PaO2 should be around 600.
If you are on 50% O2, your PaO2 should be around 300.
If you are on room air, your PaO2 should be around 125.

The gradient is the calculated number minus the PaO2 on the ABG.
It's not exact, but puts you in the right ballpark very quickly.

 

[IN2B8R]

A similar estimate to that which is listed below is the following rule: for each 10% increase in the FiO2, you can expect a 50 Torr increase in the PaO2--similar to what is written below.

I get a rough estimate by taking the FIO2 times 600.

eg. If you are on 100% O2, your PaO2 should be around 600.
If you are on 50% O2, your PaO2 should be around 300.
If you are on room air, your PaO2 should be around 125.

The gradient is the calculated number minus the PaO2 on the ABG.
It's not exact, but puts you in the right ballpark very quickly.

 

[Jeff05]

it's important to understand that A-a gradient's clinical significance depends on Fi02. on room air it should be less than 12. on 100% less than 350.

estimated PaO2 on room air is 102-age/3
also, PaO2 = approx FiO2x5

using the above and being able to quickly estimate the PAO2 from the alveolar gas equation one can work out the A-a gradient (for what that's worth...)
top causes of increased A-a gradient are V/Q mismatch (think of all the causes) and it's extreme - shunt.

consequently, the A-a gradient may be used to calculate shunt.
on 100% FiO2: % shunt = (A-a)/20

 

[rsgillmd]

...
consequently, the A-a gradient may be used to calculate shunt.
on 100% FiO2: % shunt = (A-a)/20

This is the only time I calculated the A-a gradient -- when an attending asked me to calculate the shunt of a patient.

Otherwise I don't see much point in doing the calculation. I look at the PaO2 and using the FiO2 x 5 rule, I say is it a large difference or around what I would expect.

The absolute number itself is meaningless to me.

 

[Breezee]

Appreciate the responses. I, too, agree that the actual number is worthless vs. knowing what the cause of wide gradient is.