This is another classic formula.
- It is still used a lot, but new studies
indicate that there may be other methods that work
- It predicts the degree of shunt by
comparing the partial pressure of O2 in the (A)
alveoli to that in the (a) artery.
- The difference between them gives us an
idea how well the oxygen is moving from the alveoli to
the arterial blood.
(PAO2 - PaO2)
- The PaO2 is
obtained from the ABG
- The PAO2 is
obtained from the Alveolar Gas equation:
- Alveolar Gas equation:
- Daltons law says the total
pressure of gases in a mixture is equal to the
sum of the partial pressures of the constituent
- 2 + 2 + 2 = 6 (easy
- Now work that backwards. If we
know the total and all but one of the
constituents, we can find the partial pressure of
the last one.
- 6 - 2 - 2 = 2 (still
- We start with:
- the pressure of the
inspired air (760 mm Hg at sea level),
- subtract water vapor
pressure (47 mm Hg), and
- PaCO2 (value from your
- We also have to take into
account how much oxygen was in the
inhaled air, (FiO2) and a
"respiratory quotient" ( 0.8 or
if patient is on 100% FiO2, it's 1.0)
- Translated into the
language of mathematics it looks like
PAO2 = ( 760 - 47 ) x FiO2 -
PaCO2 /0.8 (or 1)
Normal A-a gradient is 20 in a
health young person.
- In this formula, the same values as A-a
gradient are divided, rather than subtracted.
/ FiO2 Ratio or "P/F" Ratio
- Another much friendlier method ( because it doesn't use the
alveolar gas equation) used to predict shunt.
- Just like the name says, PaO2 is divided by FiO2
- Normal is 286; lower indicates a shunt.
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