V/Q Ratio (Nursing)

by Rhonda Lawes

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    00:01 So, when we talk about the V/Q ratio, we're talking about a ratio that divides.

    00:07 It's a calculated number or ratio. So, you've got air divided by blood or ventilation divided by perfusion.

    00:16 So, for now, go ahead and write V on the top and Q on the bottom.

    00:21 That'll just help you start to register that V stands for ventilation and Q stands for perfusion.

    00:28 We'll talk a little bit more about that later.

    00:30 So, this represents this ratio is a calculation that's done and it represents the amount of air that reaches your alveoli and the amount of blood flow in the capillaries in your lungs.

    00:42 That's what a V/Q ratio tells us. So, we know what the normal is.

    00:46 Now, if it's elevated or if it's decreased, that helps us identify where the problem is with our patient.

    00:52 So, if the V/Q ratio is calculated by dividing the ventilation by perfusion, why is it V/Q and not V/P for perfusion? Well, that's a really fair question.

    01:06 I wondered that myself and that's why I researched it to find out for you.

    01:10 See, V/P actually would be better if you ask me but you'll most often see V/Q or V/Q scan.

    01:18 It's not the perfect name but, you know, it just kind of stuck much like one of those really bad nicknames you got as a little kid. Let me explain where it came from.

    01:26 V is for ventilation, that's the symbol for measuring a volume of gas. We got that, that makes sense.

    01:32 It's the Q equals perfusion, well, this is why.

    01:37 Because Q is the symbol for a volume of blood flow rate.

    01:40 It just is. So, the name kind of stuck for V/Q scans, even though, it's not completely accurate.

    01:47 That's what we're gonna work with, alright? So, you'll hear this ordered as a V/Q scan.

    01:52 That's a test or procedure that a patient goes through to actually come up with that measurement.

    01:58 So, a ventilation/perfusion scan or a V/Q scan is a scan of the lungs that uses inhaled gas and injected dye.

    02:07 Okay, so, you're gonna have to go somewhere for this procedure.

    02:10 The patient will be transported to the place for the procedure.

    02:12 It will involve inhaled gas through a mask and injected radioactive dye. Okay? You do it in nuclear medicine. So, you have to go there for the test. Two things, what are they? Good, an inhaled gas and an injected dye. Now, I actually had one of these done.

    02:31 Didn't think it was gonna be a big deal but it ended up being pretty stressful for me.

    02:36 The mask felt like I was being smothered.

    02:39 I didn't mind the dye so much but the mask really gave me this strange sensation.

    02:44 So, I got kind of panicky toward the end of the mask part and I looked over at the technician afterwards and I said, "What was that?" She said, "Oh, yeah, that's pretty common." I was like, "You know, a heads up would've been nice so I knew what to expect when you go through that." So, if you have a patient that's gonna go down for a ventilation/perfusion scan or a V/Q scan, let them know, "Hey, some parts of this test can be a little uncomfortable.

    03:13 You'll be okay but let the technician know if it feels just beyond what you can handle." Okay, so, one of the scan measures how well the air flows through the lungs and one of the scan evaluates where the blood is flowing in the lungs.

    03:29 Well, you can imagine, it's the smother mask, I really should stop calling it that, it's the mask with the gas. That's one that's gonna measure the air flowing through the lungs.

    03:40 The radioactive dye is gonna help you figure out where the blood is flowing in the lungs.

    03:45 So, let's look at this formula that you're gonna be almost tired of by the time we're done today but V divided by Q. That stands for the ventilation divided by the perfusion.

    03:56 The ventilation is controlled by the lungs. We're talking about blood perfusion on the bottom.

    04:01 Well, we know the air is received at 4 liters a minute by the alveoli and the capillaries, 5 liters a minute for the blood. So, the V/Q ratio is calculated by dividing whatever your patient's liters per minute are for the air and whatever your patient's liters per minute of blood are.

    04:23 Now, remember, it's the mask that's gonna tell us what your patient's actual number is for ventilation and it's the radioactive dye that's injected that's gonna tell us about their perfusion but 4 over 5 represents what is normal. In a perfect world, it would be 4 divided by 5.

    04:42 So, 4 divided by 5 is 0.8. That is the normal average V/Q ratio, 0.8. Now, how do we get that? Well, we just divided the number 4 by the number 5.

    04:56 So, you can either do that longhand or check it with your calculator but it's gonna come up to be 0.8 is the normal V/Q ratio.

    05:05 Okay, so, we know the 4 divided by 5, we end up with 0.8.

    05:10 That's the normal V/Q ratio. That tells me this person has enough air coming in and good blood supply and flow going by to take oxygenated blood to my cells where I need it.

    About the Lecture

    The lecture V/Q Ratio (Nursing) by Rhonda Lawes is from the course Respiratory Assessment (Nursing).

    Included Quiz Questions

    1. V/Q ratio
    2. SaO2 (pulse oximetry)
    3. End-tidal CO2 (EtCO2)
    4. Ejection fraction
    1. Q = perfusion
    2. R = perfusion
    3. V = ventilation
    4. Q = ventilation
    1. Ventilation/perfusion (VQ) scan
    2. Computed tomography (CT) scan
    3. Magnetic resonance imaging (MRI)
    4. Chest X-ray

    Author of lecture V/Q Ratio (Nursing)

     Rhonda Lawes

    Rhonda Lawes

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