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

 

List:

·       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

 

Graphs:

               

 

 

Static compliance

Definition

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

Normal

·  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

·  Elderly/smoking -> emphysema -> ↓inward recoil -> ↑LC

·  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

·  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 -> ↑ST -> ↓ LC

·  Low: ↓ radius -> ↑ST, 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 -> ↓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

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

Pulmonary blood volume

·  Congestion -> ↓ LC

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

 

Dynamic compliance (DC):

Definition

·  ∆volume/∆transpulmonary pressure during airflow

Normal

·  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

Definition

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

Manifestations

·  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:

Equations

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

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

Determinants:

↑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

 

Time-dependence:

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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