2018A13 Define and describe LUNG compliance. Describe the difference between static and dynamic compliance.



·         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






Static compliance


·   ∆volume/∆transpulmonary pressure (TPP) when airflow has stopped


·   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:

Intrinsic elasticity

·   ↓Intrinsic elasticity -> ↓inward recoil:

o Elderly/smoking -> emphysema -> ↑LC

o Fibrosis -> ↓LC

o Pulmonary oedema -> ↓LC


·   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

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 -> ↓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

·   Overall ↑FRC and ↑LC (esp. if abdomen free)

Pulmonary blood volume

·   Congestion -> ↓ LC

·   (e.g. heart failure, supine posture)


Dynamic compliance (DC):


·   ∆volume/∆transpulmonary pressure during airflow


·   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

·   ∆V/(peak inspiratory pressure – PEEP)


Difference between static and dynamic compliance: i.e. hysteresis


·   The lag in a property of a system behind changes in the factor determining that property


·   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:


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

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


↑Resistance if:

·   ↓Airway radius
(*most important factor since raised to power of 4 or 5*)

o ↓Absolute lung size (e.g. neonate cf. adult)

o Relative lung volume (e.g. diaphragm displacement in pregnancy)

o Intraluminal obstruction (e.g. mucus)

o Luminal obstruction (bronchoconstriction, swelling)

o Extraluminal obstruction (e.g. dynamic airways compression)

·   ↑Viscosity: e.g. ↑temp

·   ↑Length



1) 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*

2) Stress relaxation

·   Due to viscoelasticity of collagen

3) 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

4) Recruitment

·   Recruitment of collapsed alveoli during inspiration

·   Quasi Starling resistor

·   e.g. collapse after thoracic surgery



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