Flow-volume loops

FV Loops - The Basic Breathprint

• X-axis: Lung Volume (L) from Total Lung Capacity (TLC) to Residual Volume (RV).
• Y-axis: Airflow (L/s). Expiration is positive (top), inspiration is negative (bottom).
• Forced Vital Capacity (FVC): Total volume exhaled, measured on the X-axis.
• Peak Expiratory Flow (PEF): Highest speed of exhalation, the peak of the loop.
• Inspiration is a symmetric, saddle-shaped curve.
• Expiration has a rapid peak followed by a linear decline.

⭐ The initial part of forced expiration is effort-dependent (PEF), while the later part is effort-independent, determined by airway compression and elastic recoil.

Disease Patterns - Scoops vs. Skinnies

• Obstructive Pattern (“Scoop”)

  • Appearance: Concave, "scooped-out" expiratory limb.
  • Mechanism: Air trapping and prolonged expiration due to airway narrowing (e.g., bronchoconstriction, mucus).
  • Key Parameter: ↓ FEV₁/FVC ratio (< 0.7).
  • Volumes: FVC often ↓, but TLC is characteristically ↑ or normal.
  • Examples: COPD, asthma, bronchiectasis.

• Restrictive Pattern (“Skinny”)

  • Appearance: Narrow, "witch's hat" shape; morphologically normal but smaller.
  • Mechanism: Reduced lung compliance and capacity prevents full inflation.
  • Key Parameter: Normal or ↑ FEV₁/FVC ratio.
  • Volumes: ↓ FVC, ↓ TLC. All lung volumes are reduced proportionally.
  • Examples: Interstitial lung disease (fibrosis), sarcoidosis, neuromuscular weakness.

⭐ The FEV₁/FVC ratio is the cornerstone for differentiation. An FEV₁/FVC < 70% is the defining feature of an obstructive defect, whereas it remains normal or elevated in restrictive disease because both values decrease proportionally.

Upper Airway - The Tricky Tubes

• Obstruction in the large airways (pharynx, larynx, trachea) distorts flow-volume loops characteristically. Differentiated by location relative to the thoracic inlet.

• Fixed Obstruction

  • Stenosis is constant regardless of pressure (e.g., tracheal stenosis, goiter).
  • Causes ↓ flow during both inspiration and expiration.
  • Loop appears flattened or "blunted" on top and bottom.

• Variable Obstruction

  • Stenosis changes with transmural pressure during breathing.
  • Extrathoracic: (e.g., vocal cord paralysis) -> flattened inspiratory loop.
  • Intrathoracic: (e.g., tracheomalacia) -> flattened expiratory loop.

⭐ Extrathoracic obstruction worsens on inspiration because negative intratracheal pressure narrows the airway. Intrathoracic obstruction worsens on forced expiration as positive pleural pressure compresses the trachea.

High‑Yield Points - ⚡ Biggest Takeaways

• Obstructive diseases (e.g., COPD) show a "scooped-out" expiratory curve, with ↓ peak flow and ↑ residual volume (RV).
• Restrictive diseases (e.g., fibrosis) have a "witch's hat" shape with ↓ TLC and FVC, but a normal or ↑ FEV1/FVC ratio.
• Fixed upper airway obstruction demonstrates flattening of both the inspiratory and expiratory loops.
• Variable extrathoracic obstruction (e.g., vocal cord paralysis) selectively flattens the inspiratory loop.
• Variable intrathoracic obstruction (e.g., tracheomalacia) selectively flattens the expiratory loop.