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Old 04-29-2011
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Lungs A-a gradient concept

Can someone explain to me the concept and formula of the A-a gradient
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Old 04-30-2011
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OVERVIEW
A-a gradient is the difference between alveolar oxygen pressure and arterial oxygen pressure. so A-a gradient = PAO2 - PaO2
In optimal conditions (no pathology) this should equal zero because equilibrium is always reached between alveolar space and capillary's blood
but no one is perfect! so we accept a 5-10 difference between them.

FORMULA
A-a gradient = PAO2 - PaO2
PAO2 = FiO2 x (Patm - H2O pressure) - PaCO2/0.8
at sea level:
FiO2 = fraction of inspired O2 = 0.21
and Patm = Atmospheric Pressure = 760 mmHg
and H2O pressure is 47 mmHg
so PAO2 = 0.21 X ( 760 - 47) - 40/0.8 = 100
and since PaO2 = 100
so A-a gradient = 100-100 = zero

SIGNIFICANCE
is to figure out the cause of hypoxia
if A-a gradient is normal so diffusion is intact. the problem is outside the body (decrease atmospheric pressure (Patm) or decrease FiO2)
if A-a gradient is HIGH then there's probably an increase in dead space or shunting of blood is taking place.

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Old 04-30-2011
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THANKS FOR GREAT EXPLANATION

I AM CONFUSED BECAUSE THESE FORMULA IN KAPLAN NOTE INTERNAL MEDICIN STEP 2 CK page 286

PAO2 - PaO2 gradient = 150 - (1.25 X PaCO2) - PaO2

do you have explanation for this formula
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Old 04-30-2011
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Quote:
Originally Posted by kemoo1985 View Post
THANKS FOR GREAT EXPLANATION

I AM CONFUSED BECAUSE THESE FORMULA IN KAPLAN NOTE INTERNAL MEDICIN STEP 2 CK page 286

PAO2 - PaO2 gradient = 150 - (1.25 X PaCO2) - PaO2

do you have explanation for this formula
It's the same thing, assuming sea level, right?
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Old 04-30-2011
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It looks like the same formula just with PAO2 substituted with the alveolar gas equation.
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Physiology-, Respiratory-

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