Work of Ventilation

Question 1

What is Work of breathing?

Answer 1

• Work is required to move the lung and chest wall
• W = P x ΔV

Question 2

What happens during normal, quiet breathing?

Answer 2

• Inspiratory muscles do all the work
• Expiration is passive
• Elastic work: 70%
• Non-elastic work: 30%

Question 3

What increases oxygen cost in breathing?

2-5% normally, 50L/min

Answer 3

• Exercise
• Disease

Question 4

What is minute volume/ventilation?

L/min

Answer 4

• Respiratory rate per minute x Tidal volume

Question 5

What is increased with an increase in tidal volume?

Answer 5

• Elastic work

Question 6

What is increased with an increase with respiratory rate?

Answer 6

• Flow and resistive work

(non-elastic work)

Question 7

What are the optimal values of RR and tidal volume?

Answer 7

• RR: 15/min
• TV: 500ml

Question 8

In which lung diseases do you get an increase in elastic resistance?

Answer 8

• Restrictive

Question 9

In which lung diseases do you get an increase in airway resistance?

Answer 9

• Obstructive

Question 10

What is Compliance?

Answer 10

• The measure of ease with which the lungs can be stretched or inflated
• CL = ΔV / ΔP
• Normal adult lung compliance = 0.1-0.4
• Total respiratory system compliance < compliance of lung/chest alone

Question 11

What causes increased compliance?

Answer 11

• Loss of elastin fibres
• Emphysema + aging

decreased elastance

Question 12

What causes decreased compliance?

Answer 12

• Scoliosis, ankylosing spondylitis
• Pulmonary fibrosis

increased elastance

Question 13

What is elastance?

Answer 13

• Inversely related to compliance
• elastic recoil force

Question 13

What contributes to elastic work?

Answer 13

1. Tissue elasticity: energy needed to deform elastic tissue
2. Surface tension: minimises SA of alveoli

Question 14

How is surface tension alleviated?

Answer 14

• Surfactant
• Increases compliance
• prevents collapse of alveoli

Question 15

What is surfactant?

Answer 15

• 10% surfactant proteins, 90% lipid
• synthesized by type II pneumocytes
• stored in lamellar bodies

Question 16

How does the law of Laplace relate to alveolar surface tension?

absence of surfactant*

Answer 16

• pressure in smaller alveoli is higher
• so air from small alveoli would enter big alveoli
• causes small alveoli to collapse, however with surfactant this would not happen!

Question 17

What is the surface tension lowering effect of surfactant?

Answer 17

• Surfactant hydrophobic chains face alveolar air
• Hydrophillic ends face alveolar fluid
• Works more at low volumes (small alveoli, expiration)

Question 18

How is RDS treated?

Answer 18

• corticosteroids given to mother prior delivery
• O2 delivery
• Survanta

Question 19

We know that surfactant prevents alveolar collapse, but what else does?

Answer 19

• Alveoli interdependence
• surrounding alveoli pull outwards to keep middle alveoli open

Question 20

What is non-elastic work?

Answer 20

• Resistance to air flow

Question 21

What components determine airway resistance?

Answer 21

1. Diameter of airway + distance air must travel
2. Flow type (laminar vs turbulent)

Question 22

How is airway diameter linked to flow resistance?

Answer 22

• Hagen-Poiseuille Law
• If airway radius is reduced 50%, resistance increases 16-fold

Question 23

Since a reduced airway radius causes more resistance, how do the bronchioles adapt?

Answer 23

• Large number of bronchioles
• Compensates, so overall airway widens
• Tracheal resistance higher than bronchioles

increase in cross-sectional area as you go down respiratory tract

Question 24

How does lung volume affect resistance?

Answer 24

• Increased lung volume → airway expands → less resistance
• Decreased lung volume → airways contract → more resistance

Question 25

What are the determinants of airway diameter?

Answer 25

Trachea and bronchi
- cartilage rings

Small bronchi and bronchioles
- no cartilage
- smooth muscle tone
- radial traction of elastic tissue
- small diameter
- mucus, thickness of mucosa, submucosa

all the factors of small bronchi cause significant disease

Question 26

How is smooth muscle tone controlled extrinsically?

Answer 26

• Catecholamines cause broncodilation, β2-receptors
• PNS causes bronchoconstriction via Ach
• NANC release dilators like NO and constrictors like neurokinin A

NANC = non adrenergic non-cholinergic

Question 27

How is smooth muscle tone controlled intrinsically?

Answer 27

• Mast cell degranulation → bronchoconstriction
• ↓CO2 = bronchconstriction
• ↑CO2 = bronchodilation

Question 28

What controls mucus secretions?

Answer 28

• PNS
• decreased in atropine
• increased in bronchitis

Question 29

What type of flow increases air flow resistance?

Answer 29

• Turbulent

Question 30

What is Reynolds number?

flow

Answer 30

• Dimensionless number to determine whether airflow is turbulent or laminar

turbulence occurs if Reynold number > 2000

Question 31

What causes turbulence?

Answer 31

• High velocities
• Large diameters
• usually upper airways; Empty nose syndrome

hence no turbulence in bronchioles since both parameters are low

Question 32

What are the sites of airway resistance in the lungs?

Answer 32

• 1/2 total resistance in nose, pharynx, larynx
• below larynx 80% in trachea and main bronchi
• less than 20% in bronchioles

Question 33

What is Chronic Bronchitis?

COPD

Answer 33

• Excess production of mucous

Question 34

What is emphysema?

COPD

Answer 34

• Loss of elastic tissue
• lung elastase inhibited by α1–anti-trypsin
• 1/4000 genetic deficiency

Question 35

What is pulmonary fibrosis?

Answer 35

• Excess fibrous connective tissue in lungs

Question 36

How are obstructive diseases classified?

Answer 36

• Increased flow-resistive work (non-elastic)
• Compliance normal, but increased in emphysema
• Decreases FEV1/FVC ratio

Question 37

How are restrictive diseases classified?

Answer 37

• Increased elastic work
• decreased compliance
• FEV1/FVC ratio normal or increased