airway resistance
Flow in tubesSite of resistance in the bronchial treeIncreased airway resistanceRadial traction

You really need to understand the difference between resistance and compliance



Overview

Increased airway resistance makes it harder to breath and may generate turbulent flow that can be heard as a wheeze with a stethoscope.  It is a feature of many respiratory diseases and results from a change in the diameter of the conducting airways.  Only a small change in radius is required to increase airway resistance.  The radius of the airways can be reduced by factors such as bronchoconstriction, mucus secretion and inflammation (oedema).  Increased resistance may not be the only cause of difficulty in breathing; compliance plays an important role too.


Flow in tubes

Try breathing through a drinking straw and see how long you think you could keep that up.  It’s by no means impossible, but it’s harder to breathe though such a small hole, for one thing,  It’s made even harder because the straw is long.  Every centimetre of length adds more resistance to the problem (if you’re unconvinced, try cutting another straw down to only a few centimetres and comparing the two.)  Resistance (R) to flow in any tube is a property of the length of the tube (l), its radius (r) and the viscosity (n) of the material flowing though it:

airway resistance
Figure 1: Air always moves from high pressure to low pressure. Flow through a tube connecting two air pressures depends on the resistance (R) to flow, which is a property of the length (l) of the tube, its radius (r) and the viscosity of air (n).

You be the judge of whether you need to memorise that equation in detail.  Since only the radius changes in the airways (air doesn’t change very much and the length of the airways doesn’t change minute to minute), it’s simpler to lump all of the constants together (k):

resist_eq2.png

Now the relationship between resistance and radius becomes clearer: if radius is merely halved, resistance will increase by 16 fold.  If radius is reduced to a quarter, resistance will increase 256 times!

Resistance varies in proportion to the radius to the power of 4.

Site of airway resistance in the bronchial tree

You’d think that the small airways would contribute the most to airways resistance, but this is not the case.  The highest resistance is in the trachea and larger bronchi, dropping away after about 7 branching generations (Figure 2).  There are many smaller airways, but because they get shorter with each branching generation, their contribution to airways resistance is more trivial. Furthermore, there are so many of them in parallel compared to the larger airways that they have a low combined resistance.

resist_fig2.png

Figure 2: Plot of the degree of airway resistance (the units don’t matter for our purposes here) versus the generation of branching of the airway tree. The first seven branches make up the most resistance while the remainder contribute much less. (Adapted from JB West, Respiratory physiology – the essentials (5th Ed.))


Increased airway resistance

In many lung diseases airway resistance is greater than normal.  Don’t be fooled into thinking that this is the case in all cases where breathing is difficult though  because changes in compliance make breathing difficult too.  Any process that decreases the diameter of the airways will increase airways resistance, including:

•    Contraction of airway smooth muscle (eg in asthma)
•    Swelling (oedema) of the mucosa (eg in asthma, but also many lung infections)
•    Permanent structural changes to the airways (COPD and asthma in particular)
•    Increased mucus production (many airway diseases)
•    Airway compression during forced expiration (COPD again; see below)

An increase in airway reistance is revealed by spirometric testing, in which the decreased rate of airflow is measured.


Radial traction and airway resistance

The elastic recoil in the alveoli surounding airways contributes to keeping aiways patent by pulling outwards in all directions (Figure 3).  When alveoli are degraded in emphysema, they lose their elastin content and as the disease progresses they break down completely.  This has two important effects.  Firstly, one a local scale the loss of alveoli leads to loss of radial traction keeping the airways open; their reduced diameter may increase airway resistance.  Secondly, since total lung recoil is reduced expiration is more difficult and patients with emphysema need to force air out of their lungs using forced expiration (i.e. using acessory muscles). The increased intrapleural pressure that results from this more likely to collapse smaller airways.  Combined, these effects make it difficult for a patient with emphysema to empty their lungs and they sometimes can't do so adequately.  This is known as gas (or air) trapping and results in hyperinflation of the lungs. 

resist_fig3.png
Figure 3: The elastic recoil in the alveoli surrounding airways helps to hold them open. This is known as radial traction. In emphysema, the structure of the alveoli is disrupted and the impact of radial traction is markedly reduced. Indeed, in such patients, the increased pressure required to exhale (due to lack of recoil) may tend to collapse the airways, increasing resistance and trapping air.


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