Lung mechanics

Question 1

Normal vs Obstructive respiratory disorder changes

Answer 1

Lower IRV (Inspiratory Reserve Volume)
Lower TV (tidal volume)
Lower ERV (expiratory reserve volume)
Higher RV (Residual volume) (bronchoconstriction and elastic properties of parenchyma has deteriorated)

The flow of air into and out of the lung is obstructed

Question 2

Chronic causes of obstructive disease

Answer 2

COPD
Emphysema
Bronchitis

Question 3

Acute causes of obstructive disease

Answer 3

Asthma

Question 4

Normal vs Restrictive changes

Answer 4

Lower IRV (Inspiratory reserve volume)
Lower TV (Tidal volume)
Lower ERV (Expiratory reserve volume)
Much lower RV (residual volume)

Lungs are operating at lower volumes

Question 5

Pulmonary causes of restrictive disease

Answer 5

Lung fibrosis

Interstitial lung disease

Question 6

Extrapulmonary causes of restrictive disease

Answer 6

Obesity (extrathoracic)

Neuromuscular disease

Question 7

Transrespiratory system pressure graph + explanation
slide 10, lecture 12
Normal shape?
At middle?
Down the curve?
Up the curve?

Answer 7

Normal= Sigmoid shaped
At middle, mechanical forces are at equilibrium
Down the curve, after a normal tidal expiration then more breathing out, trying to create a positive pressure inside the chest (ribcage closing, stomach muscles contracting, air being compressed to push it out which pushes the pleura together= increases pleural pressure.
Up the curve, puts the pleural space under more tension (outward force)

Question 8

Effect on transrespiratory system pressure graph with:
Obstructive
Restrictive
(slide 11, lecture 12)

Answer 8

Restrictive= squashed down but also stretched out because it takes much more effort to move the air in (+out) because the chest wall is less compliant
Obstructive= operating at higher volumes but the tissue is more compliant
Height: Width ratio changing

Question 9

Link between volume change, flow rate, p(alv) and p(pl) over time- draw graphs
(slide 12, lecture 12)

Answer 9

Last graph= pleural pressure relationship, if you take the first curve and you superimpose it on the bottom one you get a deficit between the two lines
At the red dots, if you subtract one line from the other its 0
In the middle, the lines have the greatest difference
If you subtract the dotted line from the solid line you get a negative value on the left, positive value on the right

This is why you get the shapes on the middle two graphs (relationship between the intrapleural pressure and the changes in volume) (all linked)

Question 10

COMPLIANCE
Definition
Equation

Answer 10

The tendency to distort under pressure
𝑪𝒐𝒎𝒑𝒍𝒊𝒂𝒏𝒄𝒆= ∆𝑽/∆𝑷
V= vol, p= pressure

Question 11

ELASTANCE
Definition
Equation

Answer 11

The tendency to recoil to its original volume

𝑬𝒍𝒂𝒔𝒕𝒂𝒏𝒄𝒆= ∆𝑷/∆𝑽

Question 12

Condomn compared to balloon (Pressure- Volume)graph of compliance and elastance
(slide 13, lecture 12)

Answer 12

Condomn has a greater compliance than a balloon

Elastance is not elasticity, it’s the opposite of compliance (balloon has more elastance than condomn)

Question 13

COPD lung change in structural properties of lung

Answer 13

COPD lung becomes more compliant which is not good because you want recoil properties/ tendency to resist change)

Question 14

Effect on transrespiratory system pressure graph with:
Fluid filled lungs compared to air filled lungs during inflation and deflation
(slide 14, lecture 12)

Answer 14

-

Question 15

Why are fluid filled lungs are more compliant than airfilled lungs?
Effect on alveoli?
Solution?
Positive effect?

Answer 15

Air-water interface exhibits surface tension
Fluid-water interface does not
No opposing upwards force for balance= lots of upward pressure without any opposing forces collapsing the lung= bad because it would take a lot of pressure to push air into the alveolus
But because surfactant is secreted by Type II pneumocytes (80% polar phospholipids, 10% non-polar lipids, 10% protein), it breaks up the surface tension, help reduce the collapsing pressure and prevent all the air to go to larger areas (Law of Laplace)

Prevents collapse of small alveoli
Increases compliance (by reducing surface tension)
Reduces the ‘work of breathing’

Question 16

Resistance and conductance
Graph of number of airways, resistance, airway generation

Graph of conductance, lung volume, resistance

(slide 18, lecture 12)

Answer 16

First graph
No. of airways grows exponentially (bifocates): resistance increases as the tube decreases but the pressure is lower (just like in capillaries) because there are much more of them which means a higher cumulative cross sectional area
This means that resistance peaks at generation 4
Airways behave in a similar way to the smooth muscle in the aorta which distends under pressure to turn an intermittent pressure into a more stable pulsatile pressure- see second graph

Question 17

Flow in collapsible tubes

slide 19, lecture 12 for visuals

Answer 17

Also applicable in every part of the body with a collapsible tube
At rest the pressure in the lungs is 0= no airflow but intrapleural pressure is -5 (because of recoil of chest and lung)
Mid- inspiration: pull the chest and lung apart= more tension in intrapleural space= pressure gradient for air to flow in
End inspiration: everything 0 from outside to inside but greater pressure in pleura
Transmural pressures= chest+ lung, but if you just look at the lung,
This is why our large airways are supported with cartilage

Question 18

SBA: How would compliance and resistance be affected by chronic obstructive pulmonary disease?
A: ↑ compliance and ↑ resistance
B:↑ compliance and ↓ resistance
C:↓ compliance and ↑ resistance
D:↓ compliance and ↓ resistance

Answer 18

A
Without looking at the answers available, think of the pathophysiology of COPD. Bronchitis causes a narrowing of the airways and hypersecretion of mucus, which INCREASES RESISTANCE. Also, emphysema breaks down the connective and elastic tissue that give the lungs their structure, so they’d become more floppy and have INCREASED COMPLIANCE.

Question 19

SBA: What is the most likely reason for under-reading FVC in a patient with COPD?

Answer 19

A
The key in the question here is ‘under-reading’ and not ‘getting a low reading’. Of course you know that responses B, C and D all contribute to reduced FVC, however only A: Poor technique could cause the measurement to be lower than the true value.

Question 20

Assertion: Resistance to airflow increases as the airways get narrower
Reason: Poiseuille’s Law states that as radius of tube is halved, resistance increases 16-fold

Answer 20

Assertion: This is FALSE – resistance peaks at generation 4 then enormously decreases
Reason: This is TRUE – however the bigger factor here is the exponential increase in airways number, which is associated with a large net increase in cumulative cross sectional area.

Question 21

Q: Move from A to B, how will Palv, Patm and Ppl change?
(slide 24, lecture 12)
A: Palv= decrease, Patm= no change, Ppl= decrease
B: Palv= decrease, Patm= increase, Ppl= increase
C: Palv= increase, Patm= increase, Ppl= decrease
D: Palv= increase, Patm= no change, Ppl= increase

Answer 21

D

As A to B is a maximal expiration, the expiratory muscles will contract, pushing the pleural layers together (increasing pleural pressure) and compressing the lung tissue (increased alveolar pressure). Atmospheric pressure is always 0 cmH2O in an open circuit.