16- Lung Mechanics

Question 1

Differentiate the determinants of ventilation between fluid- and air-filled lungs

Answer 1

Ventiltion is caused by a pressure gradient between the atmosphere and alveoli, caused either by external positive pressure or internal negative pressure
•Inflation and deflation require less pressure in fluid-filled lungs due to elimination of the air-water interface

Question 2

Explain the pressure-volume relationship for inspiration and expiration in a healthy individual and patients with obstructive/restrictive lung disease

Answer 2

The pressure volume curve is sigmoid-shaped; large changes in volume per unit pressure at middle volumes and small changes in volume per unit pressure towards RV and TLC
•Obstructive diseases operate at higher volumes, with greater changes per unit change in pressure
•Restrictive diseases operate at lower volumes and involve considerably higher pressures to inflate lungs (especially at high volumes)

Question 3

Define elastance and compliance and explain the relationship between them

Answer 3

• Compliance is the tendency to expand on inflation dV/dP

* Elastance is the tendency to recoil to original volume dP/dV

Question 4

Explain the role of surfactant in ventilation and lung structure

Answer 4

• Surface tension of pulmonary fluid is a contributor to the recoil of the lung (and resistance to expansion)
• Surfactant helps to reduce the surface tension and increase compliance of lung

Question 5

Explain the concept of airway resistance in the lung

Answer 5

• Resistance is highest in the medium-sized airways

* Airway resistance is lower at higher volumes because the airways also dilate

Question 6

Explain how flow in collapsible tubes influences pulmonary function

Answer 6

• Excessive intrapleural pressures can collapse bronchial structures
• Largest airways supported by cartilage, smaller airway collapse has fewer effects

Question 7

What are the causes of restrictive and obstructive disorders

Answer 7

Obstructive Chronic causes:
COPD
Emphysema
Bronchitis
 Acute causes:
Asthma
Restrictive:
Pulmonary causes:
Lung fibrosis
Interstitial lung disease

Extrapulmonary causes
Obesity
Neuromuscular disease

Question 8

What happens to the lung volumes in obstructive

Answer 8

RV increases

TV, IRV,ERV decrease

Question 9

What happens to lung volumes in restriction

Answer 9

RV,TV,IRV,ERV decrease

Question 10

What are 3 functions of surfactant

Answer 10

Prevents collapse of small alveoli - law of Laplace P= T/r
•Increases compliance (by reducing surface tension)
•Reduces the ‘work of breathing’

Question 11

What is the composition of pulmonary surfactant

Answer 11

“polar phospholipids (80%), neutral lipids (10%) and protein (10%”