Alveolar Gas Equation – Hypoxemia and Hypercapnia

00:00 Okay.

00:01 So now, we need to figure out what these alveolar PO2s and CO2s are.

00:10 Unfortunately, when you do an arterial blood gas measurement, you do not get an alveolar gas concentration.

00:19 But we need to know this to be able to go through a differential diagnosis of hypoxemias.

00:24 For CO2, it’s rather easy because you can substitute your PaCO2 for your PACO2.

00:35 So these are interchangeable numbers.

00:38 For O2, you cannot do this and so we have to eventually calculate it.

00:47 Fortunately, we have the alveolar gas equation, that allows for us to calculate this parameter.

00:52 Lucky for us, we can usually simplify this formula into this reduced equation.

01:00 So let’s go through this reduced equation in a little bit more detail.

01:04 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.

01:23 And then we add in the number 2.

01:27 This will give us the millimeters value for PACO2.

01:34 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.

01:44 So this works pretty much on most standard conditions.

01:49 We don’t have to use the derived formula.