Pulmonary Compliance

Pulmonary Compliance


PULMONARY COMPLIANCE:

  • “Compliance” – Change in volume per unit change in pressure.
  • Extend of lung expansion per unit rise in trans-pulmonary pressure.
  • Lung compliance (CL) – 
  • Measure of the elastic properties (Distensibility/stretchability) of lung.
  • Hence, measure of Total Lung Capacity (TLC). 

Formula:

  • Measured by,
  • Compliance = Lung volume / Trans-pulmonary pressure.
  • {Trans-pulmonary pressure = Alveolar pressure – Intra-pleural pressure}
  • Total compliance of both lungs in normal human adult.
  • 0.2 L/cm (200ml/cm) water
  • Compliance of respiratory system (combined lung & thorax).
  • Exactly half of compliance of lung alone – 110 ml/cm H20.

VOLUME – PRESSURE CURVE (Vcurve):

  • Plotting changes in lung volume against changes in transpulmonary pressure.
  • Curve slope determines compliance.

High compliance lungs:

  • Curve shifting upward & left with steep slopes.
  • Emphysema

Low complaint lungs:

  • Curve shifting downward & right.
  • Pulmonary congestion and interstitial pulmonary fibrosis.

Variations in compliance:

Increased compliance:

  • Emphysema (COPD)
  • Old age
  • Bronchial asthma

Decreased compliance:

  • Pulmonary interstitial fibrosis
  • Pulmonary hypertension
  • Pulmonary congestion 
  • Pulmonary edema
  • Reduced surfactant (Pre-mature infants)
  • Interstitial lung disease
  • Alveolar atelectasis
  • De-formalities of thorax (kyphosis and scoliosis)
  • Pleural effusion
  • Pneumo/hydro/hemothorax

Factors affecting compliance:

Lung volume:

  • Smaller lungs = smaller compliance
  • Person with one lung means half compliance of normal.

Surface tension:

  • More surface tension = Less compliance

Inflation & deflation:

  • Comparatively more during Inflation than deflation.

Types of compliance measurements:

  • Static compliance:
  • Compliance of lungs at rest.
  • Reflects elasticity of lung (distensibility)
  • Unaffected by airway resistance, because of nil airflow. 
  • Measured without considering effect of different phases of respiration.
  • Values reflect lung compliance alone.

Variations in static compliance:

  • Increased static compliance:
  • Emphysema 
  • Due to destruction of elastic tissues in alveolar walls 

Decreased static compliance:

  • Bronchospasm with hyperinflation
  • Atelectasis
  • Pneumonia 
  • Cardiogenic pulmonary edema
  • ARDS

Unchanged:

  • Bronchospasm without hyperinflation.

Dynamic compliance:

  • Compliance of lungs during air flow.
  • Affected by both elasticity of lung & airway resistance
  • Values reflect lung compliance along with airway resistance

Variations in dynamic compliance:

  • Increased airway resistance reduces both static & dynamic compliance in most condition except Emphysema.
  • Emphysema (COPD) – Increased static & reduced dynamic compliance

Comparison between static & dynamic compliance:

  • Increased static & decreased dynamic suggest
  • Emphysema
  • Reducing static & dynamic suggest:
  • Stiffer lungs (Pulmonary edema) 
  • Reduced dynamic & stable static suggest:
  • Obstructed airway

Pulmonary compliance in various conditions:

Emphysema:

  • Condition under COPD with increased pulmonary compliance

Mechanism:

  • Highly compliant lungs.
  • Due to poor elastic recoil.
  • Damaged elastic tissue due to over-stretched/chronic over-inflation.
  • Associated with increased static compliance & decreased dynamic compliance.
  • Pressure-volume curve is shifted upward & to left
  • Extreme difficulty in exhalation with unaffected inflation.

Relationship bt. Surfactant & pulmonary compliance:

  • Secreted by type-II pneumocytes of alveolar epithelium.
  • Main surface tension lowering phospholipid in surfactant,
  • Dipalmitoyl-phosphatidylcholine (DPPC) (dipalmitoyl-lecithin)

Mechanism of action:

  • Surfactant breaks surface tensional force of water molecules within alveoli

Functions:

  • Acts on alveoli
  • Reduces surface tension of fluid-lined alveoli
  • Alveolar stabilization
  • Thus, increases lung compliance

Effects of surfactant deficiency:

  • Decreased compliance
  • Hyaline membrane formation

Important metrics:

  • Normal respiratory compliance:
  • Total compliance of both lungs together in normal adult human – 200 ml/cm water (0.2 L/cm)
  • Compliance of respiratory system (combined lung & thorax) 110 ml/cm water
Exam Question
 

PULMONARY COMPLIANCE

  • Lung compliance (CL) – Measure of the elastic properties (Distensibility/stretchability) of lung
  • Total compliance of both lungs in normal human adult
  • 0.2 L/cm (200ml/cm) water
  • Compliance of respiratory system (combined lung & thorax), 
  • Exactly half of compliance of lung alone – 110 ml/cm H20.

Volume-pressure curve (Vcurve):

High compliance lungs:

  • Curve shifting upward & left with steep slopes.
  • Emphysema

Low complaint lungs: 

  • Curve shifting downward & right.
  • Pulmonary congestion & interstitial pulmonary fibrosis.

Variations in compliance:

Increased compliance:

  • Emphysema (COPD)
  • Old age

Decreased compliance:

  • Pulmonary interstitial fibrosis
  • Pulmonary congestion
  • Reduced surfactant (Pre-mature infants)

Factors affecting compliance:

  • Surface tension:
  • More surface tension = Less compliance

Variations in static compliance:

  • Increased static compliance:
  • Emphysema
  • Due to destruction of elastic tissues in alveolar walls 

Decreased static compliance:

  • ARDS

Emphysema (COPD) –

  • Increased pulmonary compliance
  • Pressure-volume curve is shifted upward & to left.

Relationship bt. Surfactant & pulmonary compliance:

  • Secreted by type-II pneumocytes of the alveolar epithelium
  • Main surface tension lowering phospholipid in surfactant,
  • Dipalmitoyl-phosphatidylcholine (DPPC) (dipalmitoyl-lecithin)

Mechanism of action:

  • Breaking water molecule inside alveoli and its surface tension subsequently

Functions:

  • Acts on alveoli
  • Alveolar stabilization
  • Reduction in alveolar surface tension.
  • Increases lung compliance

Effects of surfactant deficiency:

  • Decreased compliance
  • Hyaline membrane formation

Important metrics:

  • Total compliance of both lungs together in the normal adult human – 200 ml/cm water (0.2 L/cm)
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