Diffusional Impairment – Hypoxemia and Hypercapnia

by Thad Wilson, PhD

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    00:00 Another way to cause a hypoxemia is via diffusional impairment and that is seen upwards on this particular diagram by not letting O2 get from the lungs into the blood.

    00:14 And this means there’s inadequate amount of gas exchange that’s occurring across that blood gas barrier.

    00:23 So let’s go through this in a little bit more detail because it’s an important process and oftentimes pathologies result because of diffusional impairments.

    00:33 So normally, you have diffusion that occurs because you have to get the O2 from the lung to the plasma, to the red blood cells.

    00:42 Diffusion is occurring between both of these places, between the lung and the plasma and the plasma and the red blood cell.

    00:50 Then you need to of course get that particular O2 bound to hemoglobin on your red blood cell or erythrocyte.

    01:01 There is a way to calculate this and that is taking into account these variables.

    01:07 We need to know what the partial pressure of O2 is and we need to know how far it needs to diffuse.

    01:15 So this is oftentimes denoted by this theoretical equation in which the diffusion of a particular gas is related to the surface area available divided by the thickness or the distance it needs to travel times a coefficient.

    01:32 And this coefficient is specific to each gas such as O2.

    01:37 It is its solubility divided by the square root of its molecular weight.

    01:42 So if we’re dealing with any gas, we can pretty much ignore that factor because it’ll be the same no matter what particular – If we’re talking about oxygen, it will always be the same diffusion coefficient.

    01:54 The other variable that comes very important is this pressure differential.

    01:59 You need to know what P1 minus P2 is.

    02:02 That would be the partial pressure in the lung versus the plasma and the partial pressure between the plasma and the red blood cell.

    02:13 To better understand diffusional impairments, we need to compare and contrast a diffusion-limited gas versus a perfusion-limited gas.

    02:21 To do that, we’re going to use these particular figures.

    02:25 Along the X-axis is going to be the capillary length from the beginning of the capillary to the end of the capillary.

    02:33 Along the Y-axis is going to be the partial pressure within the capillary.

    02:40 The very top dashed line is the partial pressure in the alveolar space.

    02:47 So if we take a gas like carbon monoxide, you can see that it never quite gets to the area of the alveolar space.

    02:57 Let’s contrast that with nitrous oxide, which is an anesthetic gas.

    03:03 Here, if we look at the beginning of the capillary to the end of the capillary, nitrous oxide is rapidly diffused across the membrane so it matches the pulmonary alveolar gas tension.

    03:21 This allows for there to be quick diffusion across the particular lung tissue.

    03:28 So a perfusion-limited substance has very fast diffusion and a diffusion-limited substance is very slow to diffuse across the length of the pulmonary capillary.

    03:41 Now, let’s use our two gasses that we deal with in physiology, O2 and CO2.

    03:50 So here is the same type of graph with the beginning of the capillary through the end of the capillary.

    03:55 We have the partial pressure of the gas, in this case, O2.

    03:59 And then we have a dashed line along the top that is the partial pressure of O2 in the alveolar space.

    04:08 Normally, you have a fairly quick equilibration of the gas in the pulmonary capillary for what is in the alveolar capillary meaning that it diffused across quite well.

    04:24 If we take a diffusional-limited person such as someone that has interstitial fibrosis, you can see that they have trouble getting their gas from the lungs into the pulmonary capillary.

    04:37 They never reach alveolar gas concentrations.

    04:42 This also occurs if you lower the PO2 in the alveolar gas.

    04:48 Normally, you’ll take a little bit longer for you to get to that equilibration phase so you get all of the gas from the alveolar space into the capillary.

    04:58 And a fibrotic person has even lower amounts of diffusion that occurs.

    05:05 So if you’re not able to reach up to the PAO2, a diffusion-limitation has occurred.

    05:13 The next type of hypoxemia is something called a right to left shunt.

    05:17 And for this, it is blood that’s travelling from the right side of the heart to the left side of the heart without undergoing oxygenation.

    05:26 This can be seen in this diagram where some of the venous blood is bypassing the lungs and going directly to the arterial circulation.

    05:36 And of course, if you don’t get oxygenated, you’re going to be dumping deoxygenated blood into the oxygenated blood.

    05:43 And that’s going to lower your PaO2.

    05:49 Now, there is some natural nonpathological right to left shunt.

    05:56 And that occurred from the physiological shunt.

    05:59 Remember that was the draining of the blood from the thebesian veins as well as the bronchial circulation, but that’s perfectly normal.

    06:07 But again that should be less than 15 millimeters of mercury.

    06:12 Pathological right to left shunts occur because the severity has increased.

    06:19 But it’s going to be very dependent upon the amount of cardiac output that is shunted.

    06:24 So if you have a small hole in the septum of your heart that allowed some blood to pass from the right ventricle to the left ventricle without going to the lungs, that is a great example of a right to left shunt.

    06:40 Because some blood went from the right side of the heart to the left side of the heart without going to the lungs.

    06:47 That sometimes does happen especially for infants and they will require some surgery to help fix that particular shunt problem.

    06:59 The final type of hypoxemia that we’re going to go through is one called a ventilation to perfusion inequality.

    07:07 And some people will refer to this as ventilation to perfusion mismatch.

    07:12 Now, I’m going to go through this in two forms.

    07:15 One is in the upright lung and one is in the pathology.

    07:18 The upright lung allows us to have a very good example of this process in physiology.

    07:24 And then you can simply apply it to a pathophysiological state.

    07:35 So in our ventilation to perfusion inequality diagram, here I’ve simplified the lungs into three different alveoli or air sacs.

    About the Lecture

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

    Included Quiz Questions

    1. 15 mmHg
    2. 10 mmHg
    3. 5 mmHg
    4. 12mmHg
    5. 7 mmHg
    1. Increased surface area
    2. Increased diffusional distance
    3. A lower partial pressure gradient
    4. Lower gas solubility
    5. Pulmonary fibrosis
    1. Some venous blood bypasses the pulmonary circulation, which decreases PaO2.
    2. Some arterial blood bypasses the bronchial circulation, causing a decrease in Pa02.
    3. Some venous blood bypasses the pulmonary circulation causing an increase in PaCO2.
    4. It allows more blood to travel through the pulmonary circulation, causing a dilution in PaO2.
    5. It allows more blood to be channeled through under-ventilated areas of the lungs, causing a decrease in PaO2.
    1. An increase in cardiac output
    2. An increase in PA02 above 100 mmHg
    3. A decrease in blood flow through the lungs
    4. A decrease in PA02 to 50 mmHg
    5. An increase in blood flow through a right-to-left shunt

    Author of lecture Diffusional Impairment – Hypoxemia and Hypercapnia

     Thad Wilson, PhD

    Thad Wilson, PhD

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