Resistance definition etc

• Source of Resistance:

  • The major source of resistance to the movement of air is the friction encountered between air and the airway walls
  • In general terms the smaller the airway the greater the resistance
• • In health airways resistance decreases as lung volume increases because the airways distend as the lungs inflate
  • Wider airways have lower resistance
• • Airway calibre may be reduced by pathology inside the lumen of airway or pathology outside the airway causing compression
• Consequences of Airway Resistance:

  1. decrease in Airway calibre leads to
  2. increase in Airways resistance leads to
  3. decrease in Air flow and Air flow limitation leads to
  4. increase in Work of breathing leads to
  5. Breathlessness
• Clinical Signs of Airflow Limitation:

  • Increased work of breathing
  • Accessory muscle use
  • Breathlessness
  • Wheeze
  • Fatigue
  • Reduced alveolar ventilation
  • Reduced PaO2
  • Increased PaCO2
• Signs of increased airflow limitation is always more apparent in expiration first - Why?

  • Expiration is passive recoil of the lung and is therefore already associated with shortening and narrowing of the airways further decreasing airway calibre