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
  (Prone: ↑ CWC if abdomen free)


Last updated 2021-08-23

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