Session 3 - Mechanics Of Breathing + Lung Function Testing

Question 1

How does the fluid in the pleural space allow the lungs to expand?

Answer 1

• Surface tension of fluid holds two layers of pleura together
• Holds the lungs to the thoracic wall
• Hence when thoracic cage expands lungs expand with it

Question 2

What is the relationship between surface tension and compliance?

Answer 2

An increase in surface tension causes a decrease in compliance

Question 3

How does surfactant decrease surface tension?

Answer 3

• Molecules float on the surface of the fluid
• disrupt interactions between surface molecules
• hence surface tension is reduced

Question 4

How does surfactant prevent alveoli from collapsing into one another?

Answer 4

• Alveoli form interconnecting set of bubbles
• large alveoli should ‘eat’ smaller ones
• however as alveoli expand the surface tension in the walls increases
• as surfactant is less effective
• so pressure stays high and the smaller alveoli are not ‘eaten’ by bigger ones

Question 5

What does Poiselle’s law say about radius of a tube and resistance?

Answer 5

Resistance of a tube increases sharply with a falling radius

Question 6

Why is resistance kept relatively low in the small airways despite a small radius of the tubes?

Answer 6

They are connected in parallel

Question 7

In what general circumstances may vital capacity be reduced?

Answer 7

• If lungs are not filled normally in inspiration
• If lungs are not emptied normally in expiration
• or if both of the above are occurring

Question 8

What is Forced Vital Capacity?

Answer 8

Maximum volume of air that can be expired from full lungs

Question 9

What is Forced expiratory volume in 1 second?

Answer 9

The volume expired in first second of forced expiration from full lungs

Question 10

How will FVC and FEV1 be effected by a restrictive deficit?

Answer 10

FVC will decrease

FEV1 will stay the same

Question 11

How will FVC and FEV1 be effected by an obstructive deficit?

Answer 11

FVC will stay the same

FEV1 will decrease

Question 12

What are flow volume curves?

Answer 12

A graph of volume expired against flow rate which is derived from a vitalograph trace

Question 13

What is the CO transfer factor used to measure?

Answer 13

It measures the diffusion capacity of the lungs

Question 14

Why in CO transfer factor do we give the patient a mixture which contains:

i) Only a small amount of CO
ii) A fraction of helium

Answer 14

i) CO is toxic

ii) The small amount of helium allows us to estimate total lung volume

Question 15

What does the nitrogen washout test allow us to measure?

Answer 15

The anatomical dead space volume

Question 16

Describe the process of the nitrogen washout test

Answer 16

• Patient takes inspiration of 100% oxygen
• Air in dead space is 100% oxygen, in alveolar there is a mix
• Patient exhales and nitrogen % is measured
• Initially nitrogen % is 0% but rises as alveolar air is exhaled
• graph plotted of expired volume against nitrogen % and volume of dead space can be calculated