Lung Mechanics

Question 1

During inspiration what are the 2 main sources of resistance that respiratory muscles work to over come?

Answer 1

• Elastic resistance of the lung (the resistance to expanding the lung)
• Air way resistance to airflow (along the tube)

Question 2

What 2 factors contribute to elastic resistance?

Answer 2

• The presence of elastic lung tissues

- And the presence of air liquid interface that increases surface tension.

Question 3

What determines airway resistance?

Answer 3

One thing is blockages, as when we breath in air it has laminar flow (flows smoothly in 1 direction) but if it hits any blockages/ sharp edge/ branch point, then the laminar flow is disturbed and becomes turbulent which significantly increases resistance whilst causing vibrations aka Wheeze. The same occurs when there’s high velocity airflow down a narrow point/tube which will also result in wheeze.

A wheeze can disappear when the blockages are more significant/ more narrow so they can’t generate any high velocity.

Question 4

What is Poiseuille’s law and why is it significant for air way resistance?

Answer 4

Flow= the pressure gradient x pie x r⁴
÷8 x viscosity x length.

The ⁴ is significant because if you half the radius then resistance will increase x16. so small change in radius = big change to resistance.

Question 5

Where are the main sites of airway resistance?

Answer 5

Whole respiratory system: Nose, pharynx, larynx.

within lungs; Medium sized Bronchi

Question 6

List factors causing variations in airway resistance, within the airways.

Answer 6

• Bronchiole smooth muscle tone. (high smooth muscle tone will reduce radius)
• inflammation of epithelium/glands (increase diameter)
• secretions e.g. mucus via CF, bronchiectasis.

Question 7

What else can affect airway resistance in the lung. (Which pressures e.g. Intrapleural, alveolar, barometric, etc)

Answer 7

(+) Ppl will cause compression of the airways.

Question 8

What factors can cause bronchoconstriction.

Answer 8

• Allergic asthma, you get activation of white blood cells called Mast cells, eosinophil these produce inflammatory factors; Histamine, Prostaglandins, leukotrienes etc. these bind to receptors on the smooth muscle cells activating it causing it to constrict.
• Cholinergic receptors (M3 receptors) if they’re activated they’ll also cause bronchoconstriction.
• Excitatory Non adrenergic non cholinergic (NANC) nerves, mediated by SP & neurokinins

Question 9

What factors can cause bronchodilation.

Answer 9

• CO₂
• Inhibitory NANC
  NO(nitrogen oxide) & VIP being the mediators that inhibit the NANC nerves.
• Drugs that work through the β₂ receptors. β adrenergic agonists such as Salbutamol and adrenaline. These relax the smooth muscle, often used in the management of allergenic asthma.

Question 10

What is dynamic compression of the airways?

Answer 10

For a normal healthy adult this only occurs during forced expiration, this is because expiratory accessory muscles contract, reducing the volume of the lung, which increases the PA, and as the PA is higher in the intrapleural space than in the lung, the lung is under pressure to collapse. but doesn’t due to cartilage in larger airways but in some small airways they do collapse (due to negative distending PA).

This happens more readily if you narrower airways to begin with usually in asthmatic & COPD patients.

The airways most likely to collapse are around generation 3 or 4 because that’s where the pressure within the airways falls below Ppl during forced expiration.

Question 11

What does dynamic
compression of the airways lead to?

(Explain effort independent flow as a result of dynamic airways compression)

Answer 11

So in normal lungs it limits expiratory airflow at low lung volumes, because no matter how much extra effort you put in trying to force air out quicker, you can’t because what you’re actually doing is increasing Ppl, leading to more small airway collapses. (this is aka Effort independent flow)

Question 12

What does RAW stand for?

Answer 12

Airway resistance

Question 13

What occurs in lungs with high RAW?

Answer 13

Causes marked limitation of airflow resulting in; Slow expiration, low peak expiratory flow, air trapping and expiratory wheeze.
(Due to dynamic compression).

Question 14

Define Lung compliance?

Answer 14

Change in lung volume
÷ change in pressure gradient.
(alveolar P (PA) - Ppl)

Question 15

What is elastic resistance?

Answer 15

essentially = stiffness and is measured in compliance.

Question 16

How would you measure lung compliance?

Answer 16

PA (alveolar pa) is difficult to measure but when there’s no airflow its 0(=PB).
to record Ppl you need a oesophageal balloon. goes u[p their nose and the balloon part is in the oesophagus behind the lung and can indirectly measure the Ppl when measure with a spirometer.

As the breath in from RV –> TLC at each step they hold their breath, measurements of volume inspired and Ppl are recorded and used to generate a: Static pressure - volume loop.

Question 17

For a healthy adult when is lung compliance highest in a static pressure loop?
(at high lung volume, low lung volume, FRC, VT etc)

Answer 17

Highest at normal Tidal volume /VT.

lowest at high & low lung volumes.

Question 18

Factors affecting lung compliance.

Answer 18

• Lung fibrosis (scared = stiff) = low compliance

- Emphysema (destruction tissue = floppy) high compliance (too high)

Question 19

What is the role of surfactant?

Answer 19

• To reduce surface tension of alveolar lining fluid. Because the fat protein molecules interfere/ get in-between the water molecules.
• Also help stabilise smaller alveoli and reduces the elastic work of breathing
• The phospholipids have an insoluble fatty acid end and a hydrophilic end, so they float on the surface of alveolar lining fluid.
• Reduces surface tension in proportion to surface concentration.
• VIMP They’re present in the alveolus but quite spread out, so when they alveolus shrinks, the surface concentration of surfactant rises, therefore surface tension falls. In other words it helps oppose the collapsing in pressure.

Question 20

What is surfactant and what produces it?

Answer 20

Produced by: type 2 alveolar cells

made up of phospholipids and surfactant proteins

Question 21

What is the LaPlace equation?

what’s it’s significance?

Answer 21

P= 2T÷R
P= pressure
T= surface tension
R= radius

(so larger radius = lower surface tension, less prone to collapse in terms of alveoli)

Question 22

What are the effects of surfactant?

Answer 22

• Increase compliance
  (Bc less likely the alveoli will collapse, so easier to expand the lung as its more likely to stay open)
• Reduce tendency for alveoli to collapse: especially effective in small alveoli where surface concentration is high. (and small alveoli are more likely to collapse)
• Reduce tendency to suck fluid into alveoli (transudation)

Question 23

What happens when a person doesn’t have enough surfactant?

+ Give an E.G.

Answer 23

• Neonatal Respiratory Disease Syndrome
  : Occurs in premature babies due to inadequate surfactant production. Have stiff not compliant lungs, so higher breathing rate and their alveoli are prone to collapse.

Question 24

How else can alveoli resist collapse?

Answer 24

Joining alveoli help each other resist collapse.

just bc they’re joined together

Question 25

Abnormalities of lung compliance occur when what 2 things are abnormal?

Answer 25

• Lung tissue

- Alveolar lining fluid

Question 26

What do we use simple lung function tests to distinguish?

Answer 26

If their lungs are either:
- Healthy
- Suffering from obstructive lung disease
(Asthma/COPD/Chronic bronchitis)
- Suffering from restrictive lung disease
(Lung fibrosis/respiratory muscle weakness/Phrenic nerve damage)

Question 27

Is Asthma an obstructive or restrictive lung disease and why?

Answer 27

Obstructive, this is because:
- Airways are narrowed due to bronchoconstriction because the smooth muscle is active.

• There is also mucosal oedema (build up of fluid in the mucosal layer lining the bronchi tubes),
  the alveoli are fine.

Both of these obstruct airways, but this is reversible with the help of drugs.

Question 28

Is Chronic bronchitis an obstructive or restrictive lung disease and why?

Answer 28

Obstructive because;

• The walls are damaged due to hypertrophy
• There’s a build up of mucus, so the tubes are narrowed.
• Alveoli are fine.

Question 29

Is Emphysema an obstructive or restrictive lung disease and why?

Answer 29

Obstructive because;

• Destruction of alveoli and elastic tissue
• This causes issues with gas exchange and airway narrowing.
• Because the elastic tissue normally holds open the airways, so because they’re destroyed the airways are more collapsible
  Which contributes to the obstruction.

Question 30

Is Lung fibrosis an obstructive or restrictive lung disease and why?

Answer 30

Restrictive because;

• The airways are unaffected.
• Alveoli are stiff
• Muscles are fine

Question 31

Is Respiratory muscle weakness an obstructive or restrictive lung disease and why?

Answer 31

Restrictive because;

• Haven’t got the respiratory muscle strength to expand the lung.
• Airways and alveoli are fine.

Question 32

Which ways can you measure lung function?

Answer 32

• Indirect assessment of airway resistance using Forced expiration manoeuvres e.g. Via Peak flow meter.
• Diagnostic tests via spirometry, measuring FEV1, and FVC.

Question 33

Define FEV1

Answer 33

Forced expiratory volume in 1 second.

Question 34

Define FVC

Answer 34

Forced vital capacity.

Question 35

What is a healthy FEV1:FVC ratio?

Answer 35

FEV1 should be above 75% of FVC.

Question 36

What effect would a obstructive disease have on FVC and FEV1, and the ratio?

Answer 36

• FEV1: larger decrease
• FVC: decrease
• Ratio: decrease

Question 37

What effect would a restrictive disease have on FVC and FEV1, and the ratio?

Answer 37

• FEV1: decrease
• FVC: decrease
  -Ratio: normal
  (because both values decrease equally, as airways are fine)

Question 38

What can cause a barrel chest?

Answer 38

Emphysema, because of the reduced lung recoil, this increases FRC.

(google to see pics)