3 - Lung Mechanics & Function Testing

Question 1

What has to happen for breathing to be successful?

Answer 1

The lungs and thorax must move (due to action of diaphragm etc) and gas exchange must take place.

Question 2

What is pleural fluid?

Answer 2

A thin layer of fluid between the visceral (attached directly to the lungs) and parietal (attached to the thoracic cavity; both are mesoderm derivatives) layers.

Question 3

What are the functions of pleural fluid?

Answer 3

Ensure the lungs fill the thoracic cavity and that they change volume as the thorax does.

Question 4

What is a pneumothorax?

Answer 4

Where the integrity of the pleural seal is lost (i.e. a hole in the pleura) - the lungs will tend to collapse if this occurs.

Question 5

Breathing is about the balance of forces. How do the lungs, thoracic cage and diaphragm affect the balance?

Answer 5

Lungs pull in and up, Thoracic cage pulls out, Diaphragm (passive stretch - contracts on inspiration) pulls down.

Question 6

At resting expiratory level what is the disturbance in the balance of the forces?

Answer 6

There is no disturbance - forces are balanced. If disturbed - e.g. in the process of breathing the lungs will return to this state.

Question 7

What structures are involved in quiet inspiration? Is it active or passive?

Answer 7

It is an active process. The Diaphragm (contraction) and Intercostals

Question 8

What structures are involved in quiet expiration? Is it active or passive?

Answer 8

It is a passive process. Nothing involved - muscles involved in inspiration will relax.

Question 9

When quiet breathing, what process expends the most energy? Can this change?

Answer 9

The stretching of the lungs. This may not be the case if the diaphragm struggles to move into the abdomen (e.g. pregnancy, obesity)

Question 10

What structures are involved in forced expiration? Is it active or passive? How is inspiration that follows it affected?

Answer 10

It is an active process - must breathe out beyond resting expiratory level. This requires force which is exerted by abdominal muscles. Inspiration following a forced expiration will be passive up to the resting expiratory level.

Question 11

What is lung compliance?

Answer 11

A measure of the stretchiness of the lungs. This can be calculated through (change in volume / change in pressue).

Question 12

What is the lung specific compliance? Why is this important?

Answer 12

Due to the fact compliance will be different depending on the starting lung capacity.

i.e. (volume change / pressure change) / starting volume of lungs.

Question 13

If the lungs have a high compliance what will their stretchiness be like?

Answer 13

Great. They will have a low resistance to stretching.

Question 14

What are the two main contributing factors to lung elasticity?

Answer 14

Surface Tension,
Surfactant.

Question 15

What is surface tension? How does lung elasticity affect it?

Answer 15

Surface tension is the interactions between molecules at the surface of a liquid. This makes the surface resist stretching.

Question 16

If the lungs have high surface tension what will their stretchiness be like?

Answer 16

Reduced. A high surface tension will mean there is an increased resistance to stretching.

Question 17

What are surfactants? How do they affect surface tension?

Answer 17

They are a mixture of detergents that reduce surface tension by disrupting the interactions between surface molecules.

Question 18

Molecularly, what does surfactant consist of?

Answer 18

A mix of phospholipids and proteins. The hydrophilic ends of these molecules lies in the alveolar fluid with the hydrophobic ends projecting into the alveolar gas. These molecules therefore float on the surface of the lining fluid disrupting interaction between surface molecules.

Question 19

In the lungs what is surfactant produced by?

Answer 19

Type 2 alveolar cells.

Question 20

Does surfactant always affect surface tension the same way?

Answer 20

No it depends on how inflated the lungs are.

Deflated: reduces surface tension (therefore increasing compliance)

Fully inflated: does not affect surface tension

This means that breathing is easier initially and as the breaths get larger in volume, they become more difficult.

Question 21

What is hysteresis?

Answer 21

The energy needed to stretch a film of surfactant.is not all recovered when the film recoils. Energy is lost - being at its greatest when tidal volume is maximal - little breaths are better.

Question 22

What are bubbles?

Answer 22

They are formed when a film of fluid, surrounding gas, shrinks compressing gas until there is an equilibrium between tension and pressure.

Question 23

What is Laplace’s Law and its relevance to bubbles?

Answer 23

Pressure = 2 x surface tension / radius

Big bubbles have a low pressure; small bubbles have a high pressure.

Question 24

What is the ‘law of bubbles’? How does this apply to Laplace’s Law?

Answer 24

If bubbles are connected in series, air will flow from high pressure to low pressure, i.e. small bubbles to big bubbles. Therefore big bubbles eat small bubbles.

Question 25

Why does this law of bubbles not apply to the lungs?

Answer 25

Pressure = 2 x surface tension / radius

Surfactant affects the surface tension (more surfactant reduces surface tension) - it is less effective as alveoli get bigger, so in large alveoli surface tension and radius will be higher - pressure stays high.
Large alveoli will not eat small alveoli!

If large alveoli (low pressure) ate small alveoli (high pressure) then the lungs could not exist.

Question 26

What is respiratory distress syndrome?

Answer 26

It occurs in babies born prematurely, where too little surfactant is produced - decreasing compliance and there are few, large alveoli.

(Large bubbles = ineffective surfactant; change is in Small bubbles = ineffective surfactant. The law of bubbles applies and large alveoli eat small alveoli).

Question 27

What does Poiseulles’ law imply about air movement in airways?

Answer 27

Small radius = high flow resistance.

Therefore in small tubes (most parts of the airways) there is a very high resistance and gas exchange would be difficult.

Question 28

Why does Poiseulles’ law not impact movement of air as much as you would expect?

Answer 28

There are a lot of small airways in the airways… but they are connected in PARALLEL.

Air resistance is then highest in the trachea and lowest in the smallest airways (terminal bronchioles) - breathing is easy.

Question 29

Why is forced expiration still difficult however?

Answer 29

When the lung is compressed (against the thoracic wall) the small airways narrow and resistance increases dramatically - trapping air in the alveoli.

Also decreased compliance of the lung when it is fully inflated (due to less effective surfactant - higher surface tension).

Question 30

What happens to resistance in the airways in Obstructive Airway Disease?

Answer 30

In Obstructive Airway Disease (asthma, chronic bronchitis) where the small airways are narrowed, resistance increases much earlier in expiration - making breathing out very difficult.

Question 31

What does lung function testing assess?

Answer 31

Mechanical condition of the lungs,
Resistance of airways,
Gas exchange at alveolar membrane.

Question 32

How is lung function assessed non-invasively?

Answer 32

It may be inferred from measurement of volumes, pressures/flows and composition at the mouth.

Question 33

What is vital capacity - what affects it?

Answer 33

The difference in volume between maximum inspiration and maximum expiration.

Maximum inspiration: compliance of lungs; force of inspiratory muscles.

Maximum expiration: increasing airway resistance as airways are compressed.

Question 34

How can we predict vital capacity?

Answer 34

From tables (nomograms) which use age, gender and height to estimate.

Question 35

How can we measure forced vital capacity (FVC)? In what form are these results normally presented?

Answer 35

Single breath spirometry: subject undertakes forced inspiration before expiring as far and fast as possibly through a spirometer.

Vitalograph trace a plot of volume expired vs time.

Question 36

What is FVC normally?

Answer 36

~5L, will vary though so check nomograms.

Question 37

What is FEV_1.0? What is it affected by?

Answer 37

The volume expired in the first second.

The resistance in the airways - less if airways are narrowed.

Question 38

What is the FEV_1.0/FVC ratio normally? What is the significance of this ratio?

Answer 38

>70%.

If <70% this may be due to an obstructive deficit (asthma, chronic bronchitis).

If >70% patient may still have lung problems if FVC is impaired - restrictive deficit (lung fibrosis).

Question 39

Why do restrictive airway diseases cause FEV_1.0/FVC to be >70%?

Answer 39

If lungs are difficult to fill (stiff, weak accessory muscles, problem with chest wall) they will start less full (impaired maximum inspiration).

FVC will be reduced, but FEV_1.0 will be normal - air exits the airways as normal.

Question 40

Why do obstructive airway diseases cause FEV1.0/FVC to be <70%?

Answer 40

If airways narrow lungs are still easy to fill. However resistance increases in expiration.

Air comes out more slowly - FEV_1.0 impaired - but FVC can still be normal (albeit slower to reach).

Question 41

What are flow volume curves? How can we produce these? What are their benefits over vitalographs?

Answer 41

A measure of volume expired against flow rate derived from a vitalograph trace.

They are a more sensitive indicator of airway narrowing and can discriminate between large and small airway narrowing.

Question 42

On a flow volume curve when will flow rate be at its maximum? What is this also known as?

Answer 42

Maximum flow rate when lungs are full: little air expired; the airways are stretched so that resistance is at a minimum.

Peak expiratory flow volume (PEFR).

Question 43

What will happen to a flow volume curve when lungs are compressed (e.g. chronic bronchitis)?

Answer 43

More air is expired; airways narrow as resistance increases. Thus flow rate volume will fall.

Flow rate volume will fall more if the airways are narrower to start with.

Question 44

What devices do asthmatics frequently use to help them manage their condition?

Answer 44

A peak flow - measuring peak flow expiratory rate.
It is a cheap device but insensitive.

Question 45

How can you measure residual volume? How is this done?

Answer 45

Helium dilution (not possible by spirometry). Helium is not present in air and not soluble in blood.

Patient starts at functional residual capacity (volume in lungs at end of quiet expiration).
Breathes in known [Helium].
Breathe out and measure concentration of expired air (how much [Helium] has been reduced by air already in lungs).

Question 46

What is nitrogen washout?

Answer 46

It measures serial dead space and can be used to measure ventilation-perfusion matching.

Subject breathes in pure O₂,
Breathes out via meter measuring % Nitrogen,
Initially only O₂ expired,
Then mixture of O₂ and air (incluing Nitrogen) from alveoli,
Volume expired at ‘transition’ = serial dead space.

Question 47

What is diffusion conductance? How do you measure it?

Answer 47

How easily gas diffuses from alveolar air to blood.

CO is used because the high affinity-binding to Hb means it will not dissociate: no partial pressure in mixed venous blood.