Lung mechanics

Question 1

Draw and label a flow volume loop (respiratory flow envelope, vital capacity, ERV, IRV, tidal volume, expiration, inspiration)

Answer 1

diagram from notes

Question 2

What is the effect of mild obstructive disease on the flow volume loop and give an example

Answer 2

Displaced to the left
Indented exhalation curve (coving)

COPD: lungs become larger to move the curve up, gradient is lower, smaller airways -> lower flow rates -> coving

Question 3

What is the effect of severe obstructive disease on the flow volume loop and give an example

Answer 3

Shorter curve
Displaced to the left
Indented exhalation curve

decreased vital capacity
Coving
Peak expiratory flow rate decreases

Question 4

What is the effect of restrictive disease on the flow volume loop

Answer 4

Displaced to the right
Narrower curve

Less access to air
Peak is lower or unchanged (filling problem)

Question 5

What is the effect of variable intra thoracic obstruction on the flow volume loop

Answer 5

Blunted expiratory curve

Otherwise normal

Question 6

What is the effect of fixed airway obstruction on the flow volume loop

Answer 6

Blunted inspiratory curve
Blunted expiratory curve
Otherwise normal

Question 7

Describe obstructive lung diseases

Answer 7

Flow of air into and out of the lung is obstructed
Lungs operate at higher volumes -> higher residual volume
Lower IRV, TV and ERV

Question 8

What are the causes of obstructive diseases

Answer 8

chronic: COPD, emphysema, bronchitis

Acute: asthma

Question 9

Describe restrictive lung diseases

Answer 9

Inflation/deflation of the lung or chest wall is restricted
Lungs are operating at lower volumes (IRV, TV, ERV)
residual volume greatly decreased

Question 10

What are the causes of restrictive lung diseases

Answer 10

Pulmonary: lung fibrosis, interstitial lung disease

Extrapulmonary: Obesity, neuromuscular disease

Question 11

Describe the volume / transmural pressure graph

Answer 11

Sigmoid shape

Lots of effort required to move to either side of the curve

Question 12

What is the effect of restrictive and obstructive disease on volume/ transmural pressure graph

Answer 12

Obstructive increases gradient and shifts the curve upwards

Restrictive decreases gradient and shifts the curve downwards

Question 13

What is surface tension in the lungs

Answer 13

No opposing upwards force caused by air-fluid interfaces

Filling the lungs with fluid instead of air makes the lungs more compliant (less ST)

Question 14

What causes surface tension in the lungs

Answer 14

Surfactant release (lipids) from type II pneumocystis on to the surface of the alveoli -> reduces surface tension
Polar phospholipids, non-polar lipids, protein

Question 15

What is the role of surfactant

Answer 15

Prevents the collapse of small alveoli
Increases compliance by reducing surface tension
Reduces the work of breathing

Question 16

What is compliance

Answer 16

Tendency to distort under pressure

Question 17

How is compliance calculated

Answer 17

Volume / pressure (gradient)

Question 18

How is compliance affected by COPD

Answer 18

More compliant, less elastic

Question 19

What is elastance

Answer 19

The tendency to recoil to its original volume

Question 20

How is elastane calculated

Answer 20

Pressure / volume

Question 21

How does the number of airways vary with airway generations

Answer 21

Exponential increase in cross-sectional area from generation 9 onwards

Question 22

How does the resistance vary with airway generations

Answer 22

Increases then decreases after generation 4 (inversely proportional to the fourth power of the radius)

Question 23

How does conductance vary with an increase in lung volume

Answer 23

Increases with lung volume

Question 24

How does resistance vary with an increase in lung volume

Answer 24

Resistance decreases

Airways are not rigid and they dilate as lung volume increases

Question 25

Describe airflow in a tube pre-inspiration

Answer 25

No airflow (therefore the volume of air = functional residual capacity)
Pressure in the airways and lungs is 0 (no gradient)
Recoil of the lung means intrapleural pressure is - 5

Question 26

Describe airflow in a tube during inspiration

Answer 26

Increase in tension in the intrapleural space -> pressure = -9
Pressure gradient established
Overall airway transmural pressure = -2 - (-8) = +6
Positive -> airway is open and air flows in

Question 27

Describe airflow in a tube at the end of inspiration

Answer 27

The intrapleural pressure is -8 due to strain

Question 28

Describe airflow in a tube during hard expiration

Answer 28

Transmural pressure = +20 - 22 = -2
Negative
Airway becomes occluded if not supported by cartilage