Session 3 - Mechanics of breathing

Question 1

Define a pneumothorax

Answer 1

air leaking into pleural seal, separating pleural fluid, and collapsing lungs

Question 2

In which direction do the lungs pull compared to the thoracic cage? What is the equilbrium between the two known as?

Answer 2

• Lungs pull in and up
• Thoracic cage pulls out and diaphragm pulls down
• Eventually equilibrium is reached, this is the resting expiratory level which the system will bounce back to when disturbed.

Question 3

Define compliance of the lungs

Answer 3

Stretchiness of the lungs

Question 4

How is compliance measured?

Answer 4

Volume change in lungs per unit of pressure change

Question 5

Why are little breaths easy but big breaths difficult?

Answer 5

Compliance of the lungs is reduced by the surface tension of lining fluid – the interface created between air and water exhibits surface tension, which is the amount of force you have to put in to make the interface larger.

However, the surface tension of the lungs is much less than expected at low lung volumes:

• This is due to the action of surfactants which act by disrupting interactions between surface molecules.
• Produced by type 2 alveolar cells
• Since surfactant only reduces surface tension when lungs are deflated, little breaths are easy but big breaths are hard

Question 6

Explain using hysteresis why shallower breaths are more favourable than bigger ones

Answer 6

• The energy put into stretching a film of surfactant is not all recovered when the film recoils
• The loss is greatest when the tidal volume is maximal (during biggest breaths)
• Therefore small breaths lead to higher relative recovery of energy put into stretching the lungs.

Question 7

State laplace’s law and how the lung gets around it.

Answer 7

Pressure is decreased in bigger radiuses.

To prevent big bubbles with low pressure collapsing little bubbles with high pressure, surfactant produced:

• As alveoli get bigger, surface tension in walls increases as surfactant is less effective
• Therefore pressure stays high and little alveoli do not collapse as a result.

Question 8

What is respiratory distress syndrome and how would you treat it?

Answer 8

• Occurs when babies born prematurely and have too little surfactant
• Therefore, lungs very stiff and breathing and gas exchange compromised
• If baby suspected of being born prematurely, treated by pumping mother with steroids to get the lungs to develop quicker.

Question 9

Where is air resistance at its highest in the resp tract and why?

Answer 9

Airway resistance in the normal lung is highest in the trachea and lowest in the small airways. This is due to all the small airways being connected in parallel.

Question 10

How is airway resistance changed during forced expiration?

Answer 10

• During forced expiration lung is compressed and small airways narrowed
• Resistance increases dramatically and air is trapped in the alveoli

Question 11

What dos the spirometer assess?

Answer 11

Vital capacity

Question 12

How do you perform a spirometer test?

Answer 12

• Subject fills lungs from atmosphere
• Breathes out as far and fast as possible through a rapid responding spirometer

Question 13

Define Forced vital capacity and FEV1.

Answer 13

Forced vital capacity – The maximum volume that can be expired from full lungs, usually 5l in adults.

FEV1.0 – Volume expired in first second, affected by how quickly air flow slows down. Less if airways are narrowed. Usually >70% of FVC.

Question 14

Explain what a restrictive deficit of the lungs is and how it would appear on a spirometer trace (vitalograph trace)

Answer 14

• Lungs are difficult to fill and therefore start less full, decreased FVC
• But air comes out normally so FEV1.0 still >70% of FVC.

Question 15

Explain what an obstructive deficit of the lungs is and how it would appear on a spirometer trace.

Answer 15

• Airways narrowed but lungs still easy to fill. Resistance will increase in expiration, therefore air comes out more slowly.
• FVC normal, but FEV1.0 reduced.

Question 16

How would a mild or severe obstruction of the airways be shown on a flow volume loop?

Answer 16

Mild - scalloping of curve

Severe - Scalloping and reduction of PEFR

Question 17

Explain the process of helium dilution and what it is used to measure.

Answer 17

used to find residual volume

1. Helium insoluble in blood.
2. A known concentration of helium is added to the apparatus in a closed loop (not open to patient).
3. Loop then opened to patient and patient asked to breathe.
4. Volume of system is now expanded by an amount equal to the volume of the subject’s lungs and airways, decreasing concentration of helium in closed system.
5. This is used to calculate the functional residual capacity of the lungs (vol of He added/final He Conc – vol of He added/initial He conc). Air at end of passive expiration.
6. Residual volume = FRC – Expiratory reserve volume
7. ERV calculated using the spirometer and therefore we can calculate the RV.

Question 18

Explain how you would assess diffusion conductance

Answer 18

Measured using CO, and seeing how easily CO crosses alveolar air to the blood.

1. Tracer gas and CO mixture held in the lungs for about 10 seconds.
2. Amount of CO in exhaled 1L of alveolar air is compared to amount of CO in inhaled 1L to determine the diffusion conductance.

Question 19

What is the nitrogen washout curve used for and how does it work?

Answer 19

Used to measure serial dead space.

1. Patient takes breath of 100% oxygen
2. Patient breaths out through valve measuring nitrogen content
3. Dead space determined from curve by drawing vertical line down the middle of the curve