Lung mechanics******

Question 1

what is obstructive and restrictive respiratory problems?

Answer 1

Obstructive: The flow of air into and out of the lung is obstructed
Lungs are operating at higher volumes

Restrictive: Inflation/deflation of the lung or chest wall is restricted.
Lungs are operating at lower volumes.

Question 2

Give examples of respiratory problems that are obstructive

Answer 2

Obstructive:
Chronic causes:
COPD
 Emphysema
 Bronchitis

Acute causes:
Asthma

Restrictive:
Pulmonary causes:
Lung fibrosis
Interstitial lung disease

Extrapulmonary causes
Obesity
Neuromuscular disease

Question 3

How does total lung capacity change in obstructive and restrictive disease and what is the effect of vital capacity, and what happens to the residual volume ?

Answer 3

Obstructive - increases
Restrictive - decreases

Vital capacity decreases in both.

Residual voume is greater in obstructive and lower in restrictive.

Question 4

How does the change in volume per unit pressure change as you move further from functional residual capacity?

Answer 4

Change in volume per unit pressure DECREASES as you move further from FRC.

Question 5

What is transrespiratory pressure?

Answer 5

The pressure between the inside of the airways and the outside atmosphere

Question 6

What does the tension between the 2 layers depend on

Answer 6

The two layers will increase or decrease depending on whether there is a pulling force (end of inspiration/start of expiration) or a pushing force( end of expiration/ start of inspiration)

Question 7

How does the pleural pressure change from FRC to a) end of tidal inspiration and b) end of tidal expiration?

Answer 7

End of tidal inspiration - more negative than FRC
End of tidal expiration - FRC (lungs passively recoil inwards to return the pleural pressure to FRC)
You can only get a more positive intrapleural pressure than FRC if you do FORCED expiration.

Question 8

why is it easy for us to maintain tidal breathing at rest ?

Answer 8

A small change in pressure will cause a large change in volume.

Question 9

What is the significance of the shape of the curve with regards to ease of tidal breathing?

Answer 9

Close to FRC, you get a large change in volume per unit pressure so we can relatively easily inspire and expire in tidal breathing. The further we deviate from FRC, the more difficult it becomes to increase the volume.

Question 10

How does the volume-pressure curve change in obstructive and restrictive disease?

Answer 10

Obstructive - moves up

Restrictive - moves down

Question 11

what is happening during FRC?

Answer 11

At FRC the lungs are pulling in and the chest wall is pushing out the same amount so there is negative pressure in the middle because the pleural layers are being pulled in both directions and hence there is negative pressure (a little vacuum).

Question 12

what does it mean if the transrespiratory pressure is zero and when does this happen?

Answer 12

There is no NET movement of air at FRC.

At the end of tidal inspiration there is also no air movement.

Question 13

Describe negative pressure breathing referring to the alveolar pressure and volume

Answer 13

The respiratory muscles work to expand the chest wall and increase the capacity of the thoracic cavity so we create a negative pressure inside which makes the air flow in.
The respiratory muscles work to decrease intrathoracic pressure and so the alveoli are pulled open thus increasing their volume and decreasing their pressure (generating negative pressure). This creates a pressure gradient between the alveoli and the atmosphere so air is drawn into the alveoli to re-establish the initial pressure.

Question 14

Define compliance.

Answer 14

The tendency to distort under pressure

Question 15

Define elastance.

Answer 15

The tendency of something to recoil to its original volume.

Question 16

In terms of change in volume per unit pressure, how is the ease of inflation of the lung different to deflation a?

Answer 16

Inflation is more difficult that deflation (requires more pressure per unit change in volume) because the lungs have a natural tendency to recoil.

Question 17

How is the ease of inflating and deflating a fluid-filled lung different to an air-filled lung?

Answer 17

It is much easier to inflate a fluid-filled lung - it is more compliant. The air-filled lung has a surfactant-air interface which has surface tension and hence decreases compliance. The fluid-filled lung has a fluid-water interface, which doesn’t have surface tension so the lung is more compliant.

Question 18

What type of cell produces surfactant?

Answer 18

Type II Pneumocyte

Question 19

What is the role of surfactant?

Answer 19

It breaks up the water molecules, reduces surface tension and hence prevents alveolar collapse.

Question 20

What are the effects of surfactant on surface tension and compliance?

Answer 20

It reduces the surface tension and hence increases the compliance of the lungs

Question 21

what is conductance?

Answer 21

How well the airways will conduct and allow air to pass through.

Question 22

Why doesn’t the resistance of the airways increase as you go down the generations?

Answer 22

Because flow also decreases.

Question 23

When does peak resistance occur?

Answer 23

Around the 4th generation

Question 24

How do conductance and resistance change with increasing lung volume? Why does this happen?

Answer 24

Conductance increases and resistance decreases with increasing volume because when the lungs inflate, the airways dilate (i.e. the radius increases).

Question 25

Describe the phenomenon of interdependence with regards to alveoli.

Answer 25

The function of any single alveolus is dependent on the function of the alveoli around it.

Question 26

Describe what is considered a ‘stress point’ in obstructive and restrictive disease.

Answer 26

In a lung with obstructive and restrictive disease, there will be a border between normal lung tissue and lung tissue that is either resistance to stretch (restrictive) or wants to stretch too much (obstructive) because they’ve had ore of the tissue broken down and its structural integrity is a lot worse. There will be a connection point between these two tissues which is a real focal point for damage.

Question 27

what is found at the connection point?

Answer 27

Elastic tissue and structural tissue like collagen which could be subject to damage

Question 28

Explain why forced expiration can lead to the collapsing of airways.

Answer 28

Forced expiration leads to a dramatic increase in pleural pressure. There is a gradient of pressure between the alveoli and the atmosphere. The sudden increase in pleural pressure could mean that at a point along the collapsible tubes, the pleural pressure exceed the internal pressure of the airways and hence the airways collapse.

Question 29

Describe the shape and position of the flow-volume loop in obstructive disease.

Answer 29

The loop is displaced to the left and the second half of the expiratory line is indented.
The indentation is suggestive of small airway collapse

Question 30

Describe the shape and position of the flow-volume loop in restrictive disease.

Answer 30

The loop is narrow and displaced slightly right.

Question 31

Describe the shape of the flow-volume loop in a) variable extrathoracic obstruction, b) variable intrathoracic obstruction and c) fixed obstruction.

Answer 31

Variable extrathoracic obstruction - blunting of inspiratory curve
Variable intrathoracic obstruction - blunting of expiratory curve
Fixed obstruction - blunting of both curves