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#1
04-29-2011
 USMLE Forums Guru Steps History: 1+CK+CS Posts: 378 Threads: 161 Thanked 103 Times in 52 Posts Reputation: 117

Can someone explain to me the concept and formula of the A-a gradient

#2
04-30-2011
 USMLE Forums Scout Steps History: 1+CK+CS Posts: 37 Threads: 4 Thanked 37 Times in 14 Posts Reputation: 47

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.

 The above post was thanked by: curiousmind (07-23-2012), DoktorNo (05-04-2015), excellence (12-12-2013), kemoo (04-30-2011), madeha shahid (04-02-2012), mayankkaushal (08-15-2013), Mondoshawan (04-30-2011), nakeya52 (05-07-2014), nevillenic (05-02-2011), prataptetali (04-30-2011), shaan175 (07-25-2012), vienna4u (06-01-2011)
#3
04-30-2011
 USMLE Forums Guru Steps History: 1+CK+CS Posts: 378 Threads: 161 Thanked 103 Times in 52 Posts Reputation: 117

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

#4
04-30-2011
 USMLE Forums Master Steps History: 1+CK+CS Posts: 590 Threads: 31 Thanked 1,233 Times in 411 Posts Reputation: 1257

Quote:
 Originally Posted by kemoo1985 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?
#5
04-30-2011
 USMLE Forums Scout Steps History: Not yet Posts: 46 Threads: 3 Thanked 34 Times in 20 Posts Reputation: 44

It looks like the same formula just with PAO2 substituted with the alveolar gas equation.

 Tags Physiology-, Respiratory-

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