RP - Static Lung Mechanics Flashcards
Define the different lung volumes:
Tidal volume - volume of air moving in or out the lung in one breath
Inspiratory Reserve Volume (IRV) - additional volume of air which can be inhaled after a quiet inhalation
Expiratory reserve volume - additional air that can be exhaled after a quiet expiration
Residual volume - air remaining in lungs after maximal exhalation
Vital capacity - sum of the tidal volume, inspiratory and expiratory reserve volumes
Total lung capacity - some of the residual and vital capacity
Define the different lung capacity equations:
Vital capacity = TV + IRV + ERV
Total Lung Capacity = VC + RV
Functional Residual Capacity = ERV + RV
Inspiratory Capacity = TV + IRV
Minute volume = tidal volume x respiration rate
Define compliance and elastance
Compliance:
- lung distensibility; the ability of the lung to stretch in response to pressure changes
- High compliance → lungs expand easily
- Formula: C = ΔV/ ΔPC
(change in volume per unit pressure)
Elastance:
- The recoil force of the lungs opposing expansion
- formula: E = 1/ C
- high elastance = stiff lungs
- low elastance = floppy lungs
Describe factors affecting lung compliance:
- Elastic Properties of Lung Tissue
- Surface Tension of Alveoli
Elastic Properties of Lung Tissue:
- Elastin & collagen fibres determine stretchability
- Fibrosis → Increased collagen → ↓ compliance
- Emphysema → Destruction of elastin → ↑ compliance
Surface tension of alveoli:
- Water molecules on alveolar surface exert inward force → reducing compliance
- Pulmonary surfactant (produced by Type II pneumocytes) reduces surface tension → increases compliance
- Surfactant deficiency (cells must be moist to facilitate gas exchange and cells will die) ↓ compliance
- elastic fibres in alveolar wall contribute to surface tension
Describe factors affecting lung compliance:
- lung volume
- chest wall compliance
- age + disease
Lung volumes:
- at low lung volumes: Alveoli are collapsed, hard to expand → low compliance
- At high lung volumes: Lungs are stretched tight → low compliance
Chest Wall Compliance:
- Chest wall also contributes to total respiratory compliance
- Conditions affecting thoracic cage stiffness reduce compliance
- functional residual capacity is decreased
- Decreased chest wall compliance increases work of breathing and reduces total compliance of the respiratory system
Age + Disease:
- Aging = loss of elastic tissue which increases compliance
- fibrosis reduces compliance
- emphysema increases compliance
Explain pressure/volume curves for:
- normal lung
- isolated lung
Normal lung:
- sigmoidal shaped
- At low lung volumes: Alveoli are collapsed → hard to expand (low compliance
- At mid lung volumes: Most compliant (easiest to inflate)
- At high lung volumes: Lungs are stretched → less compliant
Isolated lung:
- Since an isolated lung does not have the chest wall to resist inflation, the overall compliance might appear higher
- small increases in pressure could cause a relatively larger increase in volume compared to a normal lung
- hysteresis seen in the normal lung during inflation and deflation may be reduced in the isolated lung because factors like surfactant production (which affects alveolar surface tension) are likely altered when the lung is isolated
Explain pressure/volume curves for:
- Saline-Inflated Isolated Lung
- Premature Newborn Lung
Saline:
- curve shifts left (higher compliance)
- saline eliminates surface tension forces → compliance is much higher
Premature:
- curve shifts to the right (low compliance)
- lack of surfactant causes high surface tension making lung inflation more difficult
- Requires higher pressures to achieve the same lung volume
What are Approximate Normal Values for Lung Volumes and Pressures ?
Vital capacity = 4-6 L
Tidal volume = 0.5 L
Functional residual capacity = 2-3 L
IRV = 3L
ERV = 1 to 1.5 L
Residual volume = 1 to 1.5L
Total lung capacity = 6L
Describe the Effects of Changed Compliance on Work of Breathing:
Low Compliance (e.g., Fibrosis, NRDS)
- Stiff lungs → hard to inhale.
- Breathing needs more effort and energy
- Patients take small, rapid breaths to reduce work.
High Compliance (e.g., Emphysema)
- Floppy lungs → easy to inhale, hard to exhale.
- Loss of elastic recoil → air traps inside lungs.
- Breathing out requires effort (exhaling becomes the problem)
- Patients adopt slow, deep breathing to reduce work
Describe the pressure volume graph key features:
X-axis = Transpulmonary Pressure (cmH₂O) → Force needed to expand the lung.
Y-axis = Lung Volume (% Vital Capacity) → How much air the lungs hold at that pressure.
Slope of the curve = Compliance
Steeper slope → Higher compliance (lungs expand easily)
Flatter slope → Lower compliance (lungs resist expansion)
Normal Lung (Middle Curve)
S-Shaped curve: moderate slope in mid-range, flatter at extremes.
Good balance between elasticity and compliance.
🟢 Emphysema (High Compliance, Left-Shifted & Steep Slope)
Steeper slope → Lungs expand too easily (loss of elastic recoil).
Less pressure needed to inflate lungs → More air trapping.
Curve shifts left (lungs overinflate easily).
🔴 Fibrosis (Low Compliance, Right-Shifted & Flattened Slope)
Flatter slope → Stiff lungs require high pressure to inflate.
More effort to expand the lungs → Higher Work of Breathing (WOB).
Curve shifts right (lungs resist inflation).
A steep slope at the far right of the graph suggests:
✅ High compliance (lungs expand easily).
✅ However, if far right, it means more pressure is needed → May indicate overinflation in severe emphysema.
✅ If the curve is flat at the far right, it would indicate low compliance (fibrosis).
Define Hysteresis
Hysteresis is the difference between inflation/deflation curves, due to changing surfactant availability and delayed alveolar recruitment
