2020B Question 03
Describe the factors that affect respiratory system compliance.
Examiner Report
The main domains assessed in this question were:
- Definition of compliance in context of respiratory physiology
- Composition of compliance of respiratory system – chest wall and lung
- Different types of compliance – static and dynamic
- Factors that increase and decrease:
- Chest wall compliance
- Lung compliance
With appropriate explanation illustrating understanding of the topic.
Extra credit was given for:
- Description of measurement method difference between static and dynamic compliances
- Explanation of hysteresis
- Correctly giving examples related to “slow” and “fast” alveoli and how they affect static and dynamic compliance
- Effect of anaesthesia on respiratory compliance
Common issues were:
- Incorrect value and unit for respiratory compliance
- Compliance graphs (pressure-volume loop) were often used, but often incorrect
- Poor understanding of static/dynamic compliance and factors that affect them
- Factors affecting chest wall compliance were often missed
- Describing examples without indicating which direction they affect the compliance
- Misconception about relationship between some physiological/pathological state and the respiratory compliance (e.g. Pregnancy, small airway disease)
Model Answer
Structure:
- Intro and definitions
- Graphs
- Determinants of static lung compliance
- Determinants of hysteresis
- Determinants of chest wall compliance
Intro and Definitions
| Component | Detail |
|---|---|
| Compliance | - Key determinant of work of breathing |
| Static compliance | - ∆V/∆P at cessation of airflow |
| Dynamic compliance | - ∆V/∆P during airflow - Less than static compliance (due to hysteresis) |
| Specific compliance | - (∆V/∆P) / FRC - i.e. Indexed for absolute lung size |
| Normal values | - Lung compliance (LC): 200mL/cmH2O - Chest wall compliance (CWC): 200mL/cmH2O - Total: 1/200 + 1/200 = 1/100 → 100mL/cmH2O |
| Recoil tendency | - Lung Inward (~5cmH2O at FRC) - Chest wall outward (~5cmH2O at FRC) |
Graphs
Determinants of Static Lung Compliance
| Component | Detail |
|---|---|
| Intrinsic elasticity | - ↓ Intrinsic elasticity → ↓ Inward recoil → ↓ LC - Elderly/smoking → Emphysema → ↑ LC - Interstitial fibrosis → ↓ LC - Pulmonary oedema → ↓ LC |
| Surfactant | - (LaPlace’s law: ) - Amphipathic - Reduces surface tension at air-water interface - ↑ 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 → Basal > ↓ 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) |
Determinants of Hysteresis
i.e. Reasons for the difference between static and dynamic compliance
| Component | Detail |
|---|---|
| Resistance to airflow | - Laminar: - Turbulent: |
| Time-dependent Pulmonary Elastoid Behavior | - Surfactant Changes (Lag in equilibration of surface tension between alveoli) - Stress Relaxation (of viscoelastic lung tissue e.g. Collagen) - Pendelluft (distribution of air between regions with different time constants - Re-opening of collapsed alveoli (quasi-Starling resistor) |
Determinants of Chest Wall Compliance
| Component | Detail |
|---|---|
| Intrinsic Elasticity | - ↓ Intrinsic elasticity → ↓ Outward recoil → ↓ CWC - Neonate (cartilaginous ribs) - Elderly (ossified costal cartilage) - Distortion of chest wall → ↓ CWC - Pregnancy - Kyphoscoliosis - Diaphragm and intercostal relaxation → ↓ CWC - Anaesthesia - Paralysis |
| Relative Lung Volume | - ↓ Volume → ↑ Outward recoil → ↑ CWC - Pregnancy - Obesity - Supine posture |
| External Compression | - Compress diaphragm and/or chest wall → ↓ CWC - Obesity - Pregnancy - Posture: Supine > lateral decubitus > erect |