Diffusional Impairment – Hypoxemia and Hypercapnia

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.