ARDS: PaO2 FiO2 Ratio is less than 200 mmHg! - USMLE Forums
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Old 04-25-2011
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Lungs ARDS: PaO2 FiO2 Ratio is less than 200 mmHg!

I just read in FA that in ARDS
one of the diagnostic criteria is
pao2:fio2 should be equal or less than 200 mm of Hg

What is 200?? and how they reach to this figure?
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Old 04-25-2011
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Arrow Here's how they got the number

FiO2 is the fraction of inspired Oxygen in a gas mixture.
In normal air it's 0.21

PaO2 is the arterial Oxygen pressure and it's normally 80-100 mmHg.

So when you divide PaO2 by FiO2 80-100/0.2 then you should get 400-500 in normal circumstances.

If this number is less than 200, it means you are giving lots of inspired Oxygen yet you still have low arterial Oxygenation, means not much diffusion, means the lung is gone, means ARDS!

By the way, if the number is between 200-300, they don't call it ARDS, they call it Acute Lung Injury (ALI)
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Old 12-07-2013
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And how about in the case when I'm getting 100% O2? And let's say that the PaO2=100 (in its best):
It would be 100/1= 100
Doesn't that mean that everybody in this world is suffering from ARDS?
So confusing!

Quote:
Originally Posted by laithbv View Post
FiO2 is the fraction of inspired Oxygen in a gas mixture.
In normal air it's 0.21

PaO2 is the arterial Oxygen pressure and it's normally 80-100 mmHg.

So when you divide PaO2 by FiO2 80-100/0.2 then you should get 400-500 in normal circumstances.

If this number is less than 200, it means you are giving lots of inspired Oxygen yet you still have low arterial Oxygenation, means not much diffusion, means the lung is gone, means ARDS!

By the way, if the number is between 200-300, they don't call it ARDS, they call it Acute Lung Injury (ALI)
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Old 12-09-2013
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Quote:
Originally Posted by ammour444 View Post
And how about in the case when I'm getting 100% O2? And let's say that the PaO2=100 (in its best):
It would be 100/1= 100
Doesn't that mean that everybody in this world is suffering from ARDS?
So confusing!

Actually the FiO2 in regular atmosphere is inspired O2 (only 21% of our atmosphere is O2 remember that most of our Air is Nitrogen based)....Yes, though a normal regular non diseased human being is inspiring 100% O2 from the air he breathes, but it is only 21% of all the air we breathe so even if you only utilize 100% of the O2, you are still only utilizing (21% of all air breathed)....meaning you're diffusing only in A FRACTION of all the air going into your lungs (only 21% of it is O2)

so normal breathing = PaO2/FiO2 ratio = (>90mmHg)/.21 = >430 at all times

In pathologic processes however, like ARDS, remember there is shunting to change the PaO2 due to inflammatory reaction in the alveoli and inc capillary permeability which causes protein leaks.....all of this equals an INCREASED A-a gradient.....so as a general rule that I follow (for USMLE) is that if the PaO2 is less than 50 mmHg I look for potential causes of ARDS (42mmHg if you want to get technical PaO2 of <42 mmHg gives you an PaO2/FiO2 of <200 on normal room air with FiO2 of .21)

hope this helps clarify the situation.
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Old 12-09-2013
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Quote:
Originally Posted by ammour444 View Post
And how about in the case when I'm getting 100% O2? And let's say that the PaO2=100 (in its best):
It would be 100/1= 100
Doesn't that mean that everybody in this world is suffering from ARDS?
So confusing!

to get an fio2 about 90% means that patient is intubated/// other egamples are simple face mask -fio2 60% , nasal cannulae about 50% ..
in sea level fio2 is 21%
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Old 12-10-2013
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Quote:
Originally Posted by Dr_Kal_El View Post

hope this helps clarify the situation.
Thanks man.. I got it now
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