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.

    Included Quiz Questions

    1. 110 mmHg
    2. 100 mmHg
    3. 90 mmHg
    4. 80 mmHg
    5. 120 mmHg

    Author of lecture Alveolar Gas Equation – Hypoxemia and Hypercapnia

     Thad Wilson, PhD

    Thad Wilson, PhD

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