Consequences of Disruption or Destruction of the Alveolar-Capillary Membrane

Question 1

Give an overview of Diffusion of Gases in the Lungs?

Answer 1

• occurs across the alveoar-capillary membrane
  
  • -> 200-300 million alveoli, covered by a capillary network
• The A-C membrane is composed of:
  
  1. Layer of Surfactant [reduces surface tension and prevents the alveoli from collapsing]
  2. Type 1 alveolar cell
  3. Basement membrane
  4. Vascular endothelial cell
• The A-C membrane:
  
  • thin (0.5 microns)
  • large surface area (50 - 100 square metres)
  • alveolar volume (~3-6L)
  • capillary volume (80mL) - greater if there is an increase cardiac output
  • -> this makes the membrane ideally suited for gas exchange because of the large SA and thinness
• Diffusion of gas in the lungs is a passive, non-energy requiring process driven by differences in partial pressures of gases on either side of the A-C Membrane

Question 2

What are some Processes/Diseases that can disrupt the AC Membrane?

Answer 2

–> Inflammation, Infection, Fibrosis, Emphysema, Fluid, Cancer etc

The likely phsyiological effects of the disruption of the AC membrane include:

• abnormal gas exchange
• abnormal lung mechanics
• pulmonary vascular complications

Question 3

How is rate of diffusion calculated?

Answer 3

• Rate of diffusion is determined by Fick’s Law

V(rate) α (A.D. (P1 – P2)) / T

Question 4

What are some features of diffusion of oxygen?

Answer 4

• Diffusion of oxygen is very fast
  PCO2=PAO2 in 0.25 seconds
• There is a diffusion limitation of oxygen transfer:
  • -> Only occurs at rest if the A-C membrane is grossly abnormal
  • ->Does occur with less severe disease during exercise

Question 5

What are some features of diffusion of CO2?

Answer 5

• Diffusion of CO2 is similar to O2, but 20 times faster
• Diffusion limitation for CO2 only occurs with very severe abnormalities of the A -C membrane
• In general, an elevated PaCO2 is due to inadequate alveolar ventilation (VA)

Question 6

What are some characteristics of Abnormal PaO2 and PaCO2?

Answer 6

Low PaO2 if:  
	- low PiO2    
	- low ventilation    
	- abnormal gas exchange*  
		low V/Q  
		shunt  
		diffusion impairment  
		(*wide A-a gradient for oxygen )
High PaCO2 if:    
	- low ventilation

Question 7

What are some mechanical effects of Abnormal PaO2 and PaCO2?

Answer 7

• If there is a restriction around the chest, the individual expreiences:
  
  • -> Breathlessness
  • -> Increased work of breathing
  • -> Reduced lung volumes
  • -> Altered pattern of breathing
  • -> Reduced maximum ventilation

Question 8

Explain the features of increased sensation of breathing?

Answer 8

Increased Sensation of Breathing:
Increased sensation of breathing is described by people as breathlessness
If it seems appropriate, it is accepted as a sensation
If it appears inappropriate, it is perceived as a symptom

Question 9

What are the features of increased respiratory muscle effort?

Answer 9

Increased Respiratory Muscle Effort:
Diseases of the alveolar-capillary membrane can increase the work of breathing, because the inspiratory muscles need to generate higher pressures to overcome the stiffness (reduced compliance) of the lungs (elastic WOB).
- There are several important consequences
1. Recruitment of accessory muscles of inspiration (scalene and sternomastoid muscles)
2. Increased oxygen consumption by respiratory muscles
3. Risk of respiratory muscle fatigue, if the airway obstruction is severe

Question 10

What are some consequences of stiff lungs?

Answer 10

• If the lungs are stiff (reduced compliance), all lung volumes will be reduced which will cause:
  
  • -> reduced FVC
  • -> reduced FEV1
  • -> normal FEV1/FVC

Question 11

What are the elastic properties of the lungs?

Answer 11

Compliance of the lungs

1. Tissue composition
2. Surface tension in alveoli (reduced by surfactant)

Question 12

What is meant by an altered pattern of breathing?

Answer 12

Altered Pattern of Breathing:

• For any minute ventilation (VE), a person will adopt a pattern of breathing that minimises the WOB.
• In general, people with stiff lungs take rapid, shallow breaths

Question 13

What occurs with reduced maximum ventilation?

Answer 13

Reduced Maximum Ventilation:

• MV is reduced in restrictive lung disease (ie 35 x FEV1)
• However, exercise is more often limited by hypoxia +/- pulmonary hypertension, which both worsen with exercise