S2: Mechanics of Breathing II

Question 1

How can lung function be investigated?

Answer 1

• Lung volumes
• Ventilation
• Lung compliance
• Air flow

Question 2

What is lung volume?

Answer 2

Total volume of air an individual can breathe in/out

Question 3

Factors affecting lung volume

Answer 3

Large FVC (forced vital capacity) :

• Taller individuals
• Men
• Healthy BMI
• Healthy lung tissue

Small FVC:

• Shorter individuals
• Women
• Obese
• Restrictive lung disease - fibrosis and emphysema

Question 4

What is ventilation?

What is the equation for ventilation?

Answer 4

What volume of fresh air reaches respiratory surfaces over a given time.

• Ventilation depends on volume (depth) and rate of breathing

V. = Vt x f

Vt = tidal volume (mL) the volume of air inhaled in each breath

f= frequency

Question 5

What does alveolar air contain?

Answer 5

Alveolar air contains a mixture of fresh and stale air

Question 6

What is alveolar ventilation?

Equation

Answer 6

Alveolar ventilation corrects for the volume of inspired air which doesn’t take part in gas exchange (due to the dead space)

V.A = (VT - VD) x f

V.A = Alveolar minute volume (mL) the total volume of fresh air entering the alveoli across all breaths over one minute
VT - VD = The volume of fresh air entering the alveoli in each fresh breath
VD = Dead space volume (mL) the volume of air remaining in the respiratory system at the end of expiration

Question 7

What is Lung Compliance?

Answer 7

How much force is required to overcome the recoil of the lungs.

A more compliant lung needs less pressure to force open. If lungs are stiff and hard (low compliance), a certain amount of pressure may not be enough to change volume

Question 8

What is Transpulmonary Pressure?

Answer 8

Difference with alveoli and pleural cavity which equals amount of force generated to pull the lungs

Question 9

Equation of lung compliance

Answer 9

Lung compliance is the relationship between transpulmonary pressure and lung volume

Compliance (CL) = Change Volume/ Change Pressure

Question 10

What is Static and Dynamic Compliance?

Answer 10

Static compliance = incrementally increasing lung volume of person (parts of breath increasing volume)
Dynamic compliance = Inhalation and Exhalation (one big breath taking in all the volume all at once)

A graph is plotted with lung volume against intrapleural pressure and the gradient is worked out for the compliance

Question 11

Factors affecting lung compliance

Answer 11

1. Chest wall mechanics- scoliosis muscular dystrophy + obesity decrease CL
2. Alveolar surface tension- NRDS (surfactant) increase CL
3. Elastin fibres- Fibrosis decreases CL + COPD increases CL

Question 12

How does emphysema and fibrosis affect compliance?

Answer 12

Emphysema:

• Elastin degradation increases compliance
• Lungs are hard to recoil back into normal shape

Fibrosis:

• Fibrosis causes scarring and deposition of collagen
• Stiff lung
• Decreases compliance

Question 13

What is Airflow?

Answer 13

At what rate can air be moved between the lungs and atmosphere

Question 14

What is FEV1, FVC and 100 x FEV1/FVC?

Answer 14

FEV1 = Forced expiratory volume in 1 second
FVC= Forced vital capacity

100 x FEV1/FVC = % of total lung capacity of an individual can exhale in the first second (<80% is indicative of obstructive airways disease)

Question 15

What is Ohms law?

Answer 15

Airflow (V) = Change Pressure (P)/Resistance (R)

More resistance= less airflow
Unless pressure gradient is increased to compensate

Question 16

What is the Hagen-Poiseulle Equation?

Answer 16

Resistance (R) = 1/r4

As an airways radius decreases, the resistance increases (and the airflow decreases) dramatically

Question 17

What is airflow proportional to?

Answer 17

Airflow is proportional to the size of the airway lumen

• Hagen Poiseulle equation

Increased luminal area decreases resistance and increases air flow

Question 18

Compare healthy airway and asthmatic airway

Answer 18

Asthmatic airway:

• Contraction of smooth muscle
• Excess mucus secretion
• Oedema/swelling

The overall effect decreases luminal area compared to healthy airway. This increases airway resistance which decreases airflow.

Question 19

How does airway resistance affect airflow?

Answer 19

Obstruction changes the pattern of airflow producing turbulent flow due to increasing resistance. The wheezing sound is caused by the vibration of air due to the turbulent flow.

Question 20

Give an example of airway obstruction

Answer 20

· Loss of airway patency due to degradation of structure can cause airway obstruction
· Patency=state of being open
Healthy: elastin in surrounding alveoli provides radial traction to splint bronchioles against positive Palv

COPD: without radial traction, bronchioles collapse. There is therefore obstruction

Question 21

Name two things that measure level of airway obstruction

Answer 21

• Peak flow

- Spirometry

Question 22

FEV1/FVC cut off for obstructive and restrictive

Answer 22

Obstructive:

• FEV1/FVC < 70%
  e. g. Asthma increasing resistance

Restrictive:

• FEV1/FVC > 80%
  e. g. Fibrosis decreasing compliance

Question 23

What is the airflow like in upper and lower airway?

Answer 23

· Upper and lower airway contribute 50% each to total airway resistance
· Upper airway- lots of turbulent flow
. Lower airway- flow becomes laminar

Question 24

Sites of resistance within the respiratory tree

Answer 24

With increasing branching, airways get more numerous and shorter and contribute less to resistance.

e.g. segmental bronchi