Normal Range Ratio PaO₂/FiO₂ (Nursing)

by Rhonda Lawes, PhD, RN

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    00:01 So, how do we do this normal range? How do they figure out what normal is for PaO2 divided by FiO2? Well, that's exactly what we do.

    00:11 So if we figured, PaO2 divided by FiO2, that ratio, a normal range is 300-500 millimeters of mercury.

    00:20 Okay, I know that might not make perfect sense now but I'm gonna show you how we come up with that, but for now, I want you to think about, we just divide the PaO2, that's a number we would get from ABGs and we divide that by FiO2, that's the amount of oxygen that the patient is breathing in.

    00:39 If they don't have anything on, no nasal cannula, no mask, nothing else, they're breathing in 21%. So, let's work through an example.

    00:49 PaO2 divided by FiO2 is how we figure the PaO2/FiO2 ratio.

    00:55 So we take the patient's PaO2 and we divide it by, let's say room air.

    01:01 Now, you'll next you'll see me replace those numbers.

    01:04 So let's say, you drew ABGs on me, my PaO2 is 98, well, you're welcome, I know, it's pretty impressive.

    01:12 Hell, I don't know what it really is but it's probably pretty close to that.

    01:16 So, my PaO2 is 98, and I'm gonna divide it by the amount of fractional inspired oxygen I'm currently on.

    01:23 Right, 21% because we know that's the average FiO2 of room air.

    01:30 Okay, so, 98 divided by 21% we've got that next frame there, 98 divided by 0.21 equals 466.7 millimeters of mercury.

    01:43 So if these we're my numbers, my PaO2/FiO2 ratio would be 466.7, is that normal? Sure is, kind of in the high-end, right, because normal range is 300-500.

    01:59 What do you do next? You look at me, does it make sense? Now, I mean look at me in the video but if I was your client, if I was your patient, you'll always make sure that the lab work lines up with what you're seeing.

    02:13 If there's discrepancies or something that doesn't look right, keep asking why, why, why? Was the sample not right? Is something changed in the patient as we drew the lab? That's what we're always looking for, those clinical cues to keep our patient safe.

    02:29 Now let's take a patient whose PaO2 is 80. Ooh, okay.

    02:34 So, are they normal or not normal? Well, see if you can write in what is the normal range for PaO2? Good, it's 80-100. So, let's say that this PaO2 is lower, right, before it was high like 98, now it's 80, but it's still within normal range.

    02:52 Amount of oxygen or fraction of inspired oxygen the patient is on, 21%, so let's do the math.

    02:59 80 divided by 0.21 equals 380.9 millimeters of mercury. Is that normal? Sure is.

    03:10 The normal range is 300-500, so that kind of shows you where we're coming from, right, you can see where that range comes from.

    03:18 Now, whether my PaO2 was 80 or my PaO2 was 98, I still came up with a PaO2/FiO2 ratio that was within normal range.

    03:29 When you assess me, I don't appear to be in any respiratory distress, I'm not having mentation changes, I'm not restless, I'm not really working to breath, so this all adds up.

    03:41 Now, we're gonna apply what we've been learning so you can understand what experienced nurses already know.

    03:48 Look at the PaO2/FiO2 ratio.

    03:51 We know it's PaO2 divided by FiO2, that's the amount of air or oxygen that they're on.

    03:59 So, the patient's PaO2 here is normal, but look at the amount of oxygen that they're on, 80%.

    04:07 Hey, this is significantly different than what we've looked at in the other examples.

    04:11 Those patients were on room air, this one is on 80%, much higher than room air at 21%.

    04:20 You know we can only go up to a 100% so it's taking a lot of oxygen to get us that normal PaO2.

    04:29 So right away you've got some clues based on your understanding that things aren't okay with this patient, but let me show you how we quantify it.

    04:39 This patient is in significant respiratory problems, so we do the math and we come up with a PaO2/FiO2, whoa, so low we put it in pink, right? Its 100. We know the normal range is 300-500, this patient is 100.

    04:59 So, I'm gonna walk through some examples of what could possibly be going on that a patient when have a PaO2/FiO2 this low.

    05:09 Okay, you know, lab values have to be applied to their patients, so let's understand what's clinically likely going on with this patient.

    05:17 Well, my guess would be acute respiratory distress syndrome.

    About the Lecture

    The lecture Normal Range Ratio PaO₂/FiO₂ (Nursing) by Rhonda Lawes, PhD, RN is from the course Acute Respiratory Distress Syndrome (ARDS) (Nursing).

    Included Quiz Questions

    1. 313.3 mmHg
    2. 466.7 mmHg
    3. 357.1 mmHg
    4. 438.1 mmHg
    1. 95 mmHg
    2. 90 mmHg
    3. 106 mmHg
    4. 110 mmHg

    Author of lecture Normal Range Ratio PaO₂/FiO₂ (Nursing)

     Rhonda Lawes, PhD, RN

    Rhonda Lawes, PhD, RN

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