2018A Question 13
Define and describe LUNG compliance. Describe the difference between static and dynamic compliance.
Examiner Report
49% of candidates achieved a pass in this question.
To score well on this question both halves had to be addressed. Candidates who did well generally began with an accurate definition of compliance, usual units and values, and then went on to elaborate upon lung compliance including its determinants. Accurately drawn diagrams often aided the descriptions being given, but were not a requirement to do well. In the second half of the question attention had to turn to static and dynamic compliance. Descriptions of these and how each were measured led neatly into a discussion of some of the important causes of time dependence.
Model Answer
Structure:
- Graphs x 2
- Static compliance
- Determinants of static compliance
- Dynamic compliance
- Difference between static and dynamic compliance: I.e. hysteresis
- Causes of hysteresis: Airflow resistance, time-dependence
Graphs
Static Compliance
| Factor | Details |
|---|---|
| Definition | - when airflow has stopped |
| Normal | - 200mL/cmH2O |
| Measurement in spontaneous ventilation | - Inhale known volume from FRC, pause with open glottis - Distal oesophageal manometer for intrathoracic pressure ≈ intrapleural pressure - ∆P = distal oesophageal pressure – mouth pressure - (Pmouth ≈ Poesophageal) |
| Measurement in mechanical ventilation | - Volume control ventilation (fixed inspiratory flow rate) - ∆P = Plateau pressure – PEEP |
Determinants of Static Compliance
| Factor | Details |
|---|---|
| Intrinsic elasticity | - ↓ Intrinsic elasticity → ↓ Inward recoil: - Elderly/smoking → Emphysema → ↑ LC - Fibrosis → ↓ LC - Pulmonary oedema → ↓ LC |
| Surfactant | - Amphipathic - Reduces surface tension at air-water interface - (LaPlace’s law: Surface tension (ST) = pressure x radius / 4) - ↑ Compliance, ↓ alveolar collapse - Deficiency → ↓ LC (prematurity, SP-B or SP-C deficiency) |
| Absolute lung size | - LaPlace’s law: ↑ Size → ↑ Alveolar radius → ↓ ST → ↑ LC - Adult LC: 100mL/cmH2O > Neonate LC: 1.5-6mL/cmH2O - Male > Female - Taller > shorter |
| Relative lung volume | - High: Surfactant spread out → ↑ ST → ↓ LC - Low: ↓ Radius → ↑ ST, Alveolar collapse → ↓ LC - e.g. Pregnancy, obesity - Max compliance at FRC |
| Gravity | - Basal compression → ↓ Alveolar volume at FRC → ↑ Basal LC - Apical traction → ↑ Alveolar volume at FRC → ↓ Apical LC |
| Posture | - Supine: ↓ LC - Dorsal lung compressed by ventral lung/mediastinum/abdo viscera - Awake: Compression + → Dorsal LC > ventral LC - Under GA: Compression +++ → Ventral LC > dorsal LC - Prone: ↑ LC - Lung/mediastinum/abdo viscera supported by sternum and ribs - ↑ Uniformity of intrapleural pressure / volume / compliance - Overall ↑ FRC and ↑ LC (esp. if abdomen free) |
| Pulmonary blood volume | - Congestion → ↓ LC - (e.g. Heart failure, supine posture) |
Dynamic Compliance
| Factor | Details |
|---|---|
| Definition | - during airflow |
| Normal | - 50-100mL/cmH2O - i.e. Much less than static compliance |
| Measurement in spontaneous ventilation | - Cannot measure directly - Pmouth ≠ Palveolar due to airway resistance |
| Measurement in mechanical ventilation | - Volume controlled ventilation - |
Difference Between Static And Dynamic Compliance
| Difference | Details |
|---|---|
| Hysteresis | The lag in a property of a system behind changes in the factor determining that property |
| Manifestations | Lung volume lags behind changes in airway pressure - For a given lung volume, TPP during inspiration > TPP during expiration |
Causes of Hysteresis
Resistance to airflow:
| Factor | Details |
|---|---|
| Equations | - Laminar: - Turbulent: |
| Resistance ↑ with: | - ↓ Airway radius (most important) - ↓ Absolute lung size (e.g. Neonate cf. adult) - Relative lung volume (e.g. Diaphragm displacement in pregnancy) - Intraluminal obstruction (e.g. Mucus) - Luminal obstruction (bronchoconstriction, swelling) - Extraluminal obstruction (e.g. Dynamic airways compression) - ↑ Viscosity: e.g. ↑ Temp - ↑ Length |
Time-dependence:
| Factor | Details |
|---|---|
| Surfactant changes | - Lag in even spread → Lag in equilibration of surface tension between alveoli - For a given lung volume: Surface tension in inspiration > surface tension in expiration - *most important factor* |
| Stress relaxation | - Due to viscoelasticity of collagen |
| Pendelluft | - Distribution of air from fast-τ to slow- τ lung units at end inspiration and early expiration - τ= resistance x compliance - Fast unit: Low resistance, low compliance - Slow unit: High resistance, high compliance - e.g. Bronchoconstriction → ↑ Resistance → ↑ τ |
| Recruitment | - Recruitment of collapsed alveoli during inspiration - Quasi Starling resistor - e.g. Collapse after thoracic surgery |