2020B03 Describe the factors that affect respiratory system compliance.



·         Intro and definitions

·         Graphs

·         Determinants of static lung compliance

·         Determinants of hysteresis

·         Determinants of chest wall compliance


Intro and definitions:


·   ∆volume (V)/∆transpulmonary pressure (P)

·   Key determinant of work of breathing

Static compliance

·   ∆V/∆P at cessation of airflow

Dynamic compliance

·   ∆V/∆P during airflow

·   Less than static compliance (i.e. 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

·   (in parallel, hence additive inverses)

Recoil tendency

·   Lung inward (~5cmH2O at FRC)

·   Chest wall outward (~5cmH2O at FRC)



Combined compliance curves

Regional differences


Determinants of static lung compliance:

Intrinsic elasticity

·   Intrinsic elasticity -> ↓inward recoil -> ↓LC

o Elderly/smoking -> emphysema -> ↑LC

o Interstitial fibrosis -> ↓LC

o Pulmonary oedema -> ↓LC


·   (LaPlace’s law: surface tension = pressure x radius / 4)

·   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

o Adult LC: 100mL/cmH2O > Neonate LC: 1.5-6mL/cmH2O

o Male > female

o Taller > shorter

Relative lung volume

·   High: surfactant spread out -> ↑ST -> ↓ LC

·   Low: ↓ radius -> ↑ST, alveolar collapse -> ↓LC

o e.g. pregnancy, obesity

·   Max compliance at FRC


·   Basal compression -> ↓alveolar volume at FRC -> basal LC

·   Apical traction -> ↑alveolar volume at FRC -> basal > apical LC


·   Supine: ↓LC

o Dorsal lung compressed by ventral lung/mediastinum/abdo viscera

o Awake: compression + -> dorsal LC > ventral LC

o Under GA: compression +++ -> ventral LC > dorsal LC

·   Prone: ↑LC

o Lung/mediastinum/abdo viscera supported by sternum and ribs

o ↑Uniformity of intrapleural pressure / volume / compliance

o 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

Resistance to airflow

·   Laminar: R = (8 x length x viscosity) / (π x radius4)

·   Turbulent: (P1-P2) (length x density) / (radius5)

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:

Intrinsic elasticity

·   Intrinsic elasticity -> ↓outward recoil -> ↓CWC

o Neonate (cartilaginous ribs)

o Elderly (ossified costal cartilage)

·   Distortion of chest wall -> ↓CWC

o Pregnancy

o Kyphoscoliosis

·   Diaphragm and intercostal relaxation -> ↓CWC

o Anaesthesia

o Paralysis

Relative lung volume

·   ↓Volume -> ↑outward recoil -> ↑CWC

o Pregnancy

o Obesity

o Supine posture

 (note external compression predominates in the above cases)

External compression

·   Compress diaphragm and/or chest wall -> ↓CWC

o Obesity

o Pregnancy

o Posture: supine > lateral decubitus > erect
(Prone: ↑CWC if abdomen free)



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