Alveolar Gas Equation – Hypoxemia and Hypercapnia

by Thad Wilson, PhD

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    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.

    About the Lecture

    The lecture Alveolar Gas Equation – Hypoxemia and Hypercapnia by Thad Wilson, PhD is from the course Respiratory Physiology.

    Author of lecture Alveolar Gas Equation – Hypoxemia and Hypercapnia

     Thad Wilson, PhD

    Thad Wilson, PhD

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