Lung Mechanics

Question 1

How does TLC change in obstructive and restrictive disease?

Answer 1

Obstructive - increases

Restrictive - decreases

Question 2

How does residual volume change in obstructive and restrictive disease?

Answer 2

Obstructive - increases

Restrictive - decreases

Question 3

How does the change in volume per unit pressure change as you move further from FRC?

Answer 3

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

Question 4

What is transrespiratory pressure?

Answer 4

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

Question 5

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

Answer 5

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 6

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

Answer 6

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 7

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

Answer 7

Obstructive - moves up

Restrictive - moves down

Question 8

Describe negative pressure breathing referring to the alveolar pressure and volume.

Answer 8

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 9

Define compliance.

Answer 9

The willingness of a material to distend under pressure.

Question 10

Define elastance.

Answer 10

The tendency of something to recoil to its original volume.

Question 11

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

Answer 11

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

Question 12

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

Answer 12

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 13

What type of cell produces surfactant?

Answer 13

Type II Pneumocyte

Question 14

What is the role of surfactant?

Answer 14

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

Question 15

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

Answer 15

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

Question 16

What is conductance?

Answer 16

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

Question 17

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

Answer 17

Because flow also decreases

Question 18

When does peak resistance occur?

Answer 18

Around the 4th generation

Question 19

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

Answer 19

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

Question 20

Describe the phenomenon of interdependence with regards to alveoli.

Answer 20

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

Question 21

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

Answer 21

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)

Question 22

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

Answer 22

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 23

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

Answer 23

The loop is displaced to the left and the second half of the expiratory line is indented.

Question 24

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

Answer 24

The loop is narrow and displaced slightly right.

Question 25

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

Answer 25

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