2020B03 Describe the factors that affect respiratory system compliance.
List:
· Intro and definitions
· Graphs
· Determinants of static lung compliance
· Determinants of hysteresis
· Determinants of chest wall compliance
Intro and definitions:
|
Compliance |
· ∆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) |
Graphs:
|
Combined compliance curves |
|
|
Regional differences |
|
Determinants of static lung compliance:
|
Intrinsic elasticity |
· Elderly/smoking -> emphysema -> ↓inward recoil -> ↑LC · Interstitial fibrosis -> ↓LC · Pulmonary oedema -> ↓LC |
|
Surfactant |
· LaPlace’s law: Pressure (P) = 2 x surface tension (T) / radius (r) o Fluid surfaces acquire least surface area due to surface tension o Alveoli tend to collapse · Role of surfacant o Amphipathic molecule o Reduces surface tension at air-water interface o Becomes more concentrated (= more effective) at low alveolar volume o -> ↓alveolar collapse, ↑compliance · Factors increasing surface tension: o Alveolar oedema -> surfactant dilution o Surfactant deficiency (e.g. prematurity, SP-B or SP-C deficiency) |
|
Absolute lung size |
· Reasons: o ↑Alveolar number -> ↑LC o ↑Alveolar size -> ↑LC (↑radius -> ↓pressure – LaPlace) · Examples: o Adult > Neonate (100 cf. 1.5-6mL/cmH2O) o Male > Female o Tall > Short · N.B. compliance constant if indexed to size (i.e. compliance/FRC) |
|
Relative lung volume |
· High: surfactant spread out -> ↑T -> ↓ LC · Low: ↓ radius -> ↑T, alveolar collapse -> ↓LC o 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 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 |
Feedback welcome at ketaminenightmares@gmail.com