79 - The Physiological Consequences of Disruption or Destruction of the Alveolar Capillary Membrane Flashcards
Number of alveoli in lungs
~200-300 million
Interstitium in lungs
Space between alveolar membranes and capillaries
What is the alveolar-capillary membrane composed of? 1 2 3 4
Layer of surfactant
Type I alveolar cells (forms alveolar membrane)
Basement membrane
Vascular endothelial cell
Features of A-C membrane 1 2 3 4
Thin (0.5 microns)
Large surface area (50-100M2)
Alveolar volume is ~3-6L
Capillary volume is ~80L (increases is increased cardiac output)
Examples of diseases/processes that can disrupt A-C membrane 1 2 3 4 5 6
Acute H1N1 (pandemic influenza) Emphysema Lymphangitis carcinomatosis (infiltrating cancer) Drug-induced pneumonitis Idiopathic pulmonary fibrosis Tuberculosis
Pneumonitis
Inflammation of alveoli (also called alveolitis).
Pneumonia is a subtype of pneumonitis (implies an infectious aetiology)
Likely physiological effects of disrupting A-C membrane
1
2
3
Abnormal gas exchange
Abnormal lung mechanics
Pulmonary vascular complications
What affects partial pressures of gasses in the alveoli?
Ventilation of alveoli
Equation that describes rate of diffusion of a gas
Fick’s law
Fick’s law
Rate of ventilation is proportional to (surface area x gas constant x (difference in partial pressures)) / Thickness of membrane
Amount of time that an average RBC spends in contact with alveolar membrane at rest
~0.75 second
Amount of time that an average RBC spends in contact with alveolar membrane at rest
~0.25 second
Rough amount of time required for RBC Hb molecules to become oxygenated
~0.25 second
What doesn’t limit gas exchange in a normal person?
Diffusion (only takes ~0.25 second to oxygenate Hb in an RBC)
When is O2 transfer diffusion-limited at rest?
Only when there is a gross abnormality of the AC membrane (EG: thickening).
With exercise, can become apparent with less-severe disease).
Can desaturate O2 with exercise
Effect of thickened A-C membrane on O2 saturation
Takes a longer time to saturate RBC Hb
Difference in diffusion between CO2 and O2
The same, except CO2 diffusion is 20 times faster.
V/Q mismatch of CO2
Doesn’t really affect CO2.
If hypercapnic, is from poor alveolar ventilation, not from poor diffusion, as CO2 diffuses so well
What can cause low PaO2
Low PiO2 (low inspired oxygen, EG: altitude) Low ventilation Abnormal gas exchange (low V/Q, shunt, diffusion impairment)
What does abnormal gas exchange for O2 mean?
Wide A/a gradient for O2
What can cause high PaCO2?
Low ventilation
Effect of low ventilation
Not breathing in enough O2, not breathing out enough CO2.
Causes of low PaO2 with normal A-a gradient
Low PiO2 and low ventilation
Most common cause of abnormal gas exchange
V/Q mismatch.
Particularly units with inadequate ventilation for the amount of perfusion (amount of blood going through segment). Extreme form is shunts.
When is diffusion impairment most obvious?
During exercise
Mechanical effects of restrictive lung disease
Like having a tight band around your chest (lungs stiffer, harder to inflate).
–> breathless, increased work of breathing, reduced lung volumes, altered patterns of breathing
Why do alveolar-capillary membrane diseases increase work of breathing ?
Because the inspiratory muscles need to generate higher
pressures to overcome the stiffness (reduced compliance) of the lungs (elastic work of breathing)
Important consequences of increased WOB
1
2
3
- Recruitment of accessory muscles of inspiration (scalene and sternomastoid muscles)
- Increased oxygen consumption by respiratory muscles
- Risk of respiratory muscle fatigue, if the airway obstruction is severe
Effect on lung volumes of stiff lungs
All lung volumes reduced
Effect on FEV1 and FVC of stiff lungs
Decreased FEV1, FVC.
Normal FEV1/FVC ratio
Effect on FEV1 and FVC of narrowed airways
Decreased FEV1, normal FVC.
Decreased FEV1/FVC raito
FER
Forced expiratory ratio (FEV1/FVC)
Only constrictive lung disease without stiff lungs
Emphysema
Components of lungs leading to compliance
1
2
1) Tissue composition (elastic, fibrous)
2) Surface tension in alveoli. Reduced by surfactant.
Example of a disease leading to reduced surfactant
Premature baby hyaline membrane disease
What determines elastic properties of the lungs
Lung compliance
Lung resistance in emphysema
Decreased, because of destruction of elastic tissue,
Emphysematous lungs hyperinflate
Breathing pattern of those with restrictive lung disease
Prefer rapid, short breaths (to try to maintain alveolar ventilation)
Breathing pattern of those with airflow obstruction
Prefer fewer, longer, deeper breaths (because difficult to get air through airways, so try to do this less)
Effect of restrictive lung disease on maximum lung volume
Decreases
What often limits exercise capacity of those with restrictive lung disease
Hypoxia with or without pulmonary hypertension
Gas exchange and mechanical effects of restrictive lung disease 1 2 3 4 5 6
- Increased sensation of breathing
- Increased elastic WOB
- Reduced lung volumes
- Altered pattern of breathing
- Reduced maximum ventilation
- Abnormal gas exchange, which
worsens with exercise
