Airway Resistance – Breathing and Lung Mechanics

00:02 Now, what involves the kind of resistance to airflow? Airflow resistance is based upon a specific law of tubes.

00:14 In this Poiseuille law, which is resistance is eight times the length of the tube times a factor, which accounts for the air viscosity divided by pi times radius raised to the fourth power.

00:31 This law of rigid tubes allows us to calculate a resistance.

00:36 This is a theoretical equation so it’s not one we’re going to plug numbers into.

00:42 But let’s kind of tease through it so we know what are our most important factors.

00:48 If you look at this kind of graph here, where you have airway generation on the X-axis and you have resistance along the Y-axis.

00:57 Hopefully, you can appreciate that there is a change in the amount of resistance as you increase the number of airways.

01:07 This is an important process.

01:09 This means that as you add airways to the system, you decrease it’s overall resistance.

01:19 Another factor that affects airway resistance, is pulling on the air sacs themselves.

01:25 So as you pull on an air sac, you make it what? Larger.

01:31 As you make it larger, you’re increasing the radius.

01:36 And therefore, you’re decreasing the amount of resistance.

01:40 So the effective radial traction as you pull on something allows an airway to dilate so it has lower resistance.

01:51 So parallel circuits, radial traction, both decrease resistance in the airways.

02:01 The other factor that we need to think about is that the diameter or the radius of the airway is what? The most important aspect in this particular law of rigid tubes.

02:13 If you decrease the diameter, you increase the resistance.

02:20 And therefore less air will be able to move into the lungs.

02:27 This can be observed in this classical pulmonary function test.

02:32 So this is someone’s normal pulmonary function test.

02:36 So I will walk you through or actually you through this particular diagram.

02:41 We have time on the X-axis and we have volume on the Y-axis.

02:47 So what the green line is showing you are inhalations and exhalations.

02:51 So in the example, it’s inhale, exhale.

02:55 Inhale, exhale.

02:58 Inhale, exhale.

03:00 Maximal deep breath in and then a maximal breath out.

03:08 And you keep breathing out until you get all the air out of your lungs.

03:14 And what we’re looking for here is a volume change overtime.

03:20 If you can get air out rapidly, then you have less resistance in your airways.

03:27 Let’s contrast this to someone who has an obstructive lung disease.

03:32 So in this case, you still have time on the axis and volume on the Y.

03:38 And you’re going to do this.

03:39 Normal breath in, normal breath out.

03:42 Normal in, normal out.

03:45 Normal in, normal out.

03:46 Maximal deep breath in.

03:48 And now when you breathe out, it doesn’t go out as fast.

03:57 And I’ll do that until I get as much air as I possibly can out.

04:01 So they’re both trying at the same amount of effort to breathe the air out.

04:06 It just doesn’t come out as rapidly.

04:09 And that is because there’s more resistance in those individuals with an obstructive disorder, airways.

04:16 Let me give you some clinical examples.

04:20 An example of this particular response and you can see the two pulmonary function tests, is let’s say you have a foreign object trapped in your airway.

04:29 Asthma is a great example of an increase in resistance.

04:33 Because what happens there is there is a bronchial constriction.

04:37 In cystic fibrosis, there’s increase mucus production and therefore, the airway is a lower diameter or lesser diameter.

04:46 Chronic bronchitis and emphysema, you also can get increases in resistance.

04:52 But this, instead of having something within the airway, it occurs oftentimes because in emphysema’s case, because you have dynamic compression of the airways.

05:02 So with dynamic compression of the airways, we have a normal lung over here on the left.

05:09 This is going to be during a forced expiration type of maneuver.

05:14 So you’re generating a lot of pleural pressure.

05:18 In this case, it goes all the way up to 20 centimeters of water.

05:25 Now, you’ll notice that the value within the alveoli or this air sac is a +35.

05:34 The reason that occurs is because there is both the ability for this 20 to press in on the airway as well as that airway naturally wants to recoil.

05:47 And you add those two together and you get this 35 cm of water.

05:53 This yields a 15 differential between what’s in the alveolar space and what is in the pleural space.

06:01 If there’s more pressure in the airway or the alveolus, it will still stay inflate.

06:08 As you travel up the tube to zero, which is the mouth, you can see that there’s drop in pressure.

06:14 And this simply occurs because as you’re travelling through a tube, you’ll lose energy because of resistance.

06:20 But you’ll notice in this example, you still have a +5 differential between airway pressure and pleural pressure.

06:28 So the airway stays open.

06:30 We’ll contrast that to someone who has emphysema.

06:34 In emphysema, there is destruction of some of the elastic fibers.

06:39 Meaning that the alveoli no longer wants to collapse in the same way.

06:45 This then, if a person does the same expiratory manuever of a +20 cm of water, you only are going to add 5 to that instead of 15 like you did in the normal condition.

06:58 It still maintains that the alveoli will be open because it has a higher pressure within the alveolus than it does in the pleural space.

07:07 But now, as you travel down that tube, you’re still going to be losing energy because of resistance of the tube.

07:15 So if you drop to somewhere like 15 millimeters of mercury, now you’re at a spot in which pleural pressure is greater than airway pressure.

07:26 And this causes a compression of the airways.

07:30 And as you decrease the diameter of the airway, you increase its resistance.

07:36 And so this is a great example of how emphysema increases airway resistance through the dynamic compression of airways.