Pulmonary function testing interpretation

PFT Fundamentals - The Lung Numbers Game

• Spirometry: Measures airflow.
  • FVC (Forced Vital Capacity): Total air exhaled.
  • FEV1 (Forced Expiratory Volume in 1s): Air exhaled in the first second.
  • FEV1/FVC Ratio: Key differentiator. Normal is > 0.7.
• Lung Volumes: Measured by plethysmography or dilution.
  • TLC (Total Lung Capacity): Total air in lungs after max inspiration.
  • RV (Residual Volume): Air remaining after max expiration.
• Diffusing Capacity (DLCO): Measures gas exchange efficiency (CO uptake).
  • ↓ in emphysema, ILD, pulmonary embolism.
  • Normal in asthma, chronic bronchitis, chest wall restriction.

⭐ The FEV1/FVC ratio is the primary value used to distinguish obstructive ($< 0.7$) from restrictive ($
ormalsize \geq 0.7$) lung disease patterns.

The PFT Algorithm - Obstructive vs. Restrictive

The primary decision point is the FEV1/FVC ratio. This value determines the initial branch into either an obstructive or a restrictive/normal pattern.

⭐ In isolated asthma, DLCO is often normal or even ↑ high due to increased pulmonary capillary blood volume and more uniform ventilation-perfusion matching. This helps differentiate it from COPD, where DLCO is typically ↓ low due to emphysematous destruction.

Obstructive Patterns - The Airway Squeeze

• Hallmark: ↓ Ratio of $FEV_1/FVC$ < 0.7, indicating airflow limitation. Air trapping often leads to an ↑ Total Lung Capacity (TLC).

• Flow-Volume Loop: Shows a characteristic "scooped-out" or coved expiratory curve.

• Bronchodilator Challenge: Differentiates asthma from COPD.

  • Asthma (Reversible): Significant response with ↑ FEV₁ by >12% AND >200 mL.
  • COPD (Irreversible): Little to no change post-bronchodilator.

⭐ A decreased DLCO (Diffusing Capacity for Carbon Monoxide) points towards emphysema-predominant COPD due to alveolar destruction. DLCO is typically normal or ↑ in asthma.

Restrictive Patterns & DLCO - The Stiff Lung

• Hallmark: Total Lung Capacity (TLC) <80% of predicted. The FEV₁/FVC ratio is normal or ↑ (>0.7).
• Diffusing Capacity (DLCO): The key test to differentiate intrinsic vs. extrinsic causes.
  • ↓ DLCO (Intrinsic/Parenchymal): Gas exchange surface is damaged. Includes Interstitial Lung Diseases (ILDs).
    • Examples: Idiopathic Pulmonary Fibrosis (IPF), sarcoidosis, asbestosis.
  • Normal DLCO (Extrinsic): Lungs are healthy but compressed from the outside.
    • Examples: Obesity, kyphoscoliosis, neuromuscular weakness (e.g., ALS, Myasthenia Gravis).

• 📌 Mnemonic (PAINT): Causes of restrictive disease: Pleural, Alveolar, Interstitial, Neuromuscular, Thoracic wall.

⭐ In asbestosis, look for pleural plaques and lower lobe predominance. The FEV₁/FVC ratio is often normal or even elevated as elastic recoil increases.

High‑Yield Points - ⚡ Biggest Takeaways

• The FEV1/FVC ratio is the key differentiator: < 0.7 indicates obstructive disease.
• Asthma is distinguished by bronchodilator reversibility (> 12% ↑ in FEV1), unlike COPD.
• Restrictive patterns show a normal or high FEV1/FVC ratio but a decreased Total Lung Capacity (TLC).
• DLCO is low in both emphysema and interstitial lung disease (ILD).
• DLCO is typically normal in chronic bronchitis and asthma.
• Flow-volume loops: "scooped-out" for obstruction, tall and narrow for restriction.