30 - Respiratory Mechanics

Question 1

Muscles of inspiration

Answer 1

• Inspiration:
o Diaphragm
 Main muscle (75% of change in intrathoracic volume)
o External Intercostal
 Normally contribute very little but used when difficulty in breathing is experienced = dyspnea sign
o Scalene and SCM
 Raise 1st and 2nd ribs
 Used when difficulty in breathing is experienced = dyspnea sign

Question 2

Muscles of expiration

Answer 2

• Expiration:
o Internal Intercostal
 Normally contribute very little but used when difficulty in breathing is experienced = dyspnea sign
o Abdominal Wall muscles
 Most important
 Forces diaphragm up
o Passive process typically. Due to relaxation of diaphragm and intrinsic recoil of lung tissue.

Question 3

• Pleural pressure:

Answer 3

o Pressure of pleural fluid
o Normally a negative pressure.
o At the beginning of inspiration, pleural pressure is the amount of suction needed to hold the lungs open.
o During inspiration, the chest wall expands, pulling the lungs outward creating a larger negative pressure.
o Expiration events are reversed.

Question 4

• Alveolar pressure:

Answer 4

o Pressure of air inside alveoli.
o During inspiration this pressure drops to about -1 cm (below atm pressure).
o During expiration it raises to about 1 cm (above atm pressure).

Question 5

• Transpulmonary pressure:

Answer 5

o Difference between alveolar and pleural pressure.
o Measure of the elastic forces in the lungs that tend to collapse lungs at each instant of respiration (recoil pressure)

Question 6

• Pressure gradient for normal respiration

Answer 6

o As soon as lowering alveolar pressure below atmospheric pressure has surpassed the amount of force needed to counteract airflow resistance by the airway, inspiration of air can happen.
o And vice versa for expiration.

Question 7

3) Describe the sequence of events in normal breathing

Inspiration

Answer 7

Inspiration:
o Brain initiates inspiratory effort
o Nerves carry message to muscles
o Diaphragm contracts
o Thoracic volume ↑ with chest wall expansion
o Pleural pressure ↓
o Transmural pressure gradient ↑
o Alveoli expand increasing alveoli elastic recoil
o Alveolar pressure ↓(establishing a pressure gradient)
o Air flows into alveoli until pressure is equal to atmospheric pressure

Question 8

3) Describe the sequence of events in normal breathing

Expiration

Answer 8

Expiration:
o Brain ceases inspiratory command
o Muscles relax
o Thoracic volume ↓
o Pleural pressure ↑
o Transmural pressure gradient ↓
o Alveolar elastic recoil return alveoli to pre-inspiratory volumes
o Alveolar pressure ↑ (establishing pressure gradient)
o Air flows out of alveoli until pressure is equal to atmospheric pressure

Question 9

• Lung Compliance

Answer 9

o Lung compliance is the extent to which the lungs will expand for each unit increase in transpulmonary pressure
o Lung compliance reflects the distensibility of the lung
 The lungs distend easily at low lung volume
 At high lung volume, there is an increase in transpulmonary pressure, which results in a small increase in volume
o Compliance is the inverse of elastic recoil *****

Question 10

• Hook’s Law

Answer 10

o With an elastic structure, an ↑ in length or volume varies directly with an ↑ in the force or pressure until the elastic limit is reached

Question 11

Determinants of compliance

Answer 11

o Elastic force of lung (elastin or collagen) and elastic forces of surface tension

Question 12

5) Understand the hysteresis between the inspiratory and expiratory curves in a pressure-volume curve

Answer 12

• The hysteresis is due to surface tension at liquid-air interface of air-filled lung – Liquid molecules are stronger than the force between liquid-air
• Inspiration:
o Liquid molecules closest together will need to break intermolecular forces to inflate lung
• Expiration:
o There is no need to break intermolecular forces
• Compliance curves are determined by intrinsic compliance of lung and surface tension at liquid-air interface

Question 13

6) Understand the concept of combined lung and chest compliance

Answer 13

• (1/ Total compliance ) = (1/lung compliance) + (1/ chest wall compliance)
o Functional residual capacity is the equilibrium between the two compliances
o When volume is less than FRC…
 Chest wall expanding force is greater
o When volume is greater than FRC…
 Lung collapsing force is greater
o When volume is extremely higher than FRC…
 Both lung and chest wall want to collapse

Question 14

7) Understand the role and mechanism of action of surfactant

Answer 14

• Synthesized by type II cells, surfactant decreases surface tension preventing atelectasis following expiration. Its main component is lecithin.

• Mechanism:
o Align on alveolar surface (hydrophobic portions attracted to each other and hydrophilic repelled)
o Intermolecular forces between surfactant molecules break up attracting forces between liquid molecules lining alveoli
o This decreases surface tension and collapsing pressure

Law of Laplace

Question 15

• Law of LaPlace:

Answer 15

o P = 2T/r
o P: pressure tending to collapse alveolus
o T: alveolar surface tension
o r: alveolar radius

Question 16

8) Know the use of lecithin/sphingomyelin ratio in clinical settings

Answer 16

• Ratio use in amniotic fluid as measure of fetal lung maturity.
• Fetal lung synthesizes mostly sphingomyelin
• In 3rd trimester, type II cells convert stored glycogen  FA  dipalmitoyllecithin (lecithin detectable in 30-32nd week)
• L/S ratio less than 2 is a potential RDS
• L/S ratio = 1.5 than 80% chance RDS

Question 17

9) Explain the pathophysiology of Infantile Respiratory Distress Syndrome (IRDS)

Answer 17

• Newborns with ineffective lecithin at high risk
• Glucocorticoids could be used during pregnancy to stimulate fetal lung growth
• IRDS patients need both glucocorticoid and + end expiratory pressure (PEEP)

Question 18

10) Understand the mechanisms of changes in lung compliance in emphysema

Answer 18

Emphysema (↑ lung compliance)
o Loss of elastic fibers in lungs which increases compliance (inverse relationship).
o This correlates with a steeper slope in the pressure-volume curve.
o Collapsing force for lungs is decreased.
o This results in a new, higher FRC (volume is added to lungs to increase collapsing force).
o Patients breath at ↑ lung volumes and have barrel-shaped chest.

Question 19

10) Understand the mechanisms of changes in lung compliance in pulmonary fibrosis

Answer 19

Lung Fibrosis (↓ lung compliance)
o Stiffening of lung tissue.
o This will decrease the slope of the pressure-volume curve.
o At original FRC, the tendency for lungs to collapse is greater than tendency for them to expand.
o This means we have to lower the FRC.
o Also occurs in kyphosis and pectus excavatum.

Question 20

11) Know the key factors that determine resistance to airflow in the lungs. Understand the mechanism in relevant examples of altered airway resistance.

Answer 20

R = (8 n l) / pi x r^4

R = airway resistance
n = viscosity of air
l = length of airway
r = radius of airway

• Radius of airway is most influential factor determining airflow resistance.