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 (Vp curve):

• 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