Respiratory mechanics and work of breathing

Compliance & Recoil - Lungs Get Stretchy

• Compliance (C): A measure of how easily the lungs and chest wall can be stretched.

  • Formula: $C = \Delta V / \Delta P$ (change in volume per unit pressure).
  • ↑ Compliance: Lungs are "floppy" and easy to inflate (e.g., emphysema, aging).
  • ↓ Compliance: Lungs are "stiff" and difficult to inflate (e.g., fibrosis, ARDS, pulmonary edema).

• Elastic Recoil: The tendency of the lungs to return to their resting state after being stretched during inspiration.

  • Generated by elastin fibers and alveolar surface tension.

⭐ At Functional Residual Capacity (FRC), the inward pull of the lungs is perfectly balanced by the outward spring of the chest wall. The respiratory system's compliance is highest at FRC.

Airway Resistance - The Daily Grind

• Resistance (R) to airflow is determined by Poiseuille's Law: $R \propto \frac{\eta L}{r^4}$. Airway radius ($r$) is the most powerful determinant.
• Key Factors Influencing Resistance:
  • Airway Caliber:
    • Bronchoconstriction (↑R): Parasympathetic stimulation (ACh), histamine, leukotrienes.
    • Bronchodilation (↓R): Sympathetic stimulation (β2-agonists), ↑$CO_2$.
  • Lung Volume: Resistance is lowest at high lung volumes (radial traction pulls airways open) and highest at low volumes.

⭐ The highest point of airway resistance is not in the smallest airways (due to their huge total cross-sectional area) but in the medium-sized bronchi.

Work of Breathing - The Energy Bill

• Total work of breathing is the energy spent to overcome two forces: elastic recoil and airway resistance. It is represented by the area on a pressure-volume loop.
• Elastic Work: Overcomes lung & chest wall stiffness (recoil). Depends on compliance. ↑ in fibrosis.
• Resistive Work: Overcomes friction of gas flow in airways. Depends on airway resistance. ↑ in asthma/COPD.

⭐ Clinical Pearl: To minimize work, patients adapt breathing patterns.

• Fibrosis (High Elastic Work): Fast, shallow breaths.
• Asthma/COPD (High Resistive Work): Slow, deep breaths.

Pressure-Volume Loops - Spirometry's Story

• Graphs lung volume (Y-axis) vs. intrapleural pressure (X-axis) through one breath, creating a loop.
• The total area of the loop represents the work of breathing.
• Compliance is the slope of the loop.
  • ↑ in emphysema (taller loop, shifted left).
  • ↓ in fibrosis (shorter loop, shifted right).
• Airway resistance determines the width of the loop.
  • ↑ in asthma/COPD (wider loop).

⭐ Obstructive disease loops are shifted left (higher compliance/volume) and are wider (↑ resistance). Restrictive disease loops are shifted right (lower compliance/volume) and are narrower but steeper (↑ work for volume gain).

• Compliance is the change in lung volume per unit change in pressure; it is ↑ in emphysema and ↓ in fibrosis.
• Elastance, the reciprocal of compliance, is the tendency of the lung to recoil.
• Pulmonary surfactant (dipalmitoylphosphatidylcholine) ↓ surface tension, preventing atelectasis and increasing compliance.
• The major site of airway resistance is the medium-sized bronchi, not the terminal bronchioles.
• Work of breathing is minimized by slow, deep breaths in obstructive disease and rapid, shallow breaths in restrictive disease.