So now, we need to figure out what
these alveolar PO2s and CO2s are.
Unfortunately, when you do an
arterial blood gas measurement,
you do not get an alveolar
But we need to know
this to be able to go
through a differential
diagnosis of hypoxemias.
For CO2, it’s rather easy because you can
substitute your PaCO2 for your PACO2.
So these are interchangeable numbers.
For O2, you cannot do this and so
we have to eventually calculate it.
Fortunately, we have the
alveolar gas equation,
that allows for us to
calculate this parameter.
Lucky for us, we can usually simplify
this formula into this reduced equation.
So let’s go through this reduced
equation in a little bit more detail.
PAO2, meaning alveolar O2,
is going to be equal to your
inspired O2, which is PIO2,
minus 1.2 times your PaCO2,
which is your arterial partial
pressure of carbon dioxide.
And then we add in the number 2.
This will give us the
millimeters value for PACO2.
You might, “Well, how in the world did
you get this 1.2 number and this 2?”
These are things that
we can reduce from that
above equation that’s a
little bit more complex.
So this works pretty much on
most standard conditions.
We don’t have to use
the derived formula.