Practice Calculation – Hypoxemia and Hypercapnia

by Thad Wilson, PhD

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    00:01 Alright. So knowing that this is our alveolar gas equation, let's go through one example together before we move on.

    00:09 So let's say there's a 30 year old patient female patient who's recovering from an operation in which she had an arterial line placed in her radial artery.

    00:18 Why this is important is because if we have radial artery access we can yield or get an arterial blood gas and that's we did.

    00:27 An ABG yielded the following data. A PaO2 of 92.

    00:33 PaCO2 of 40, a bicarb of 24 and a pH of 7.4.

    00:40 We need to know what the current ambient conditions are.

    00:44 And in this case we have a barometric pressure of 747.

    00:50 Okay. Now that we have the scenario down, let's go through how you would go talk through the calculation.

    00:56 The first thing to do is we need to calculate the PIO2.

    01:00 That's the partial pressure of inspired O2.

    01:04 For this we take the barometric pressure minus the water vapor pressure.

    01:08 In the case, we have a barometric pressure of 747 and the water vapor pressure of fully saturated air is 47 mmHg.

    01:17 Subtract those two together, you should yield 700.

    01:21 Now we need to take that number and multiply it by the fraction of gas O2 in the air and that's 21%.

    01:30 So if we go through 700 times 21% that should yield 147.

    01:36 So that's the first part of the equation for alveolar partial pressure of O2.

    01:42 The second part involves the carbon dioxide concentration.

    01:47 So here we need to look at the arterial blood gas and find the PaCO2 which is 40.

    01:54 We take 40 times 1.2 and that should yield 48. So now we simply subtract 147 minus 48 and that should yield 99.

    02:05 Then we have to add back in that correction factor which is two and that would yield 101.

    02:12 So, the PAO2 is 101. In our particular case, we want to know what the A-a gradient is.

    02:23 So the first capital A is 101.

    02:26 Now what we're going to do is subtract from the case with the small AO2 is.

    02:33 So we now look for PaO2 and that's 92. So you subtract 101.

    02:40 From 92 that should yield 9. So our A-a O2 gradient is nine which is normal.

    02:49 We want to have a number that's below 15.

    02:53 When you go through a lot of different pathologies such as a right to left shunt and their ventilation to perfusion inequality that A-a gradient will be quite a bit larger.

    03:05 Sometimes it gets in numbers of 60, 70, even 100 A-a gradient.

    03:12 But, we'll get to that in a few slides.

    03:14 Now we know how to calculate an alveolar to arterial O2 gradient.

    About the Lecture

    The lecture Practice Calculation – Hypoxemia and Hypercapnia by Thad Wilson, PhD is from the course Respiratory Physiology.

    Included Quiz Questions

    1. 11 mm Hg
    2. 13 mm Hg
    3. 9 mm Hg
    4. 8.2 mm Hg
    5. 6 mm Hg

    Author of lecture Practice Calculation – Hypoxemia and Hypercapnia

     Thad Wilson, PhD

    Thad Wilson, PhD

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