Pulmonary Function Testing

PFT Fundamentals & Spirometry - Breath Basics & Blowouts

• PFTs (Pulmonary Function Tests): Non-invasive tests assessing lung health; diagnose, monitor disease & treatment.
• Spirometry: Core PFT measuring airflow & lung volumes during forced expiration.
  • Key Parameters Measured:
    • FVC (Forced Vital Capacity): Max air exhaled forcefully post-max inhalation.
    • $FEV_1$ (Forced Expiratory Volume in 1s): Volume exhaled in 1st second of forced blow.
    • $FEV_1/FVC$ Ratio: Key diagnostic ratio; $FEV_1$ as % of FVC.
    • PEFR (Peak Expiratory Flow Rate): Max flow achieved during forced expiration.
  • Procedure: Max inspiration → forceful, fast, complete expiration ('blowout').
  • Indications: Dyspnea, chronic cough, wheeze, pre-op eval.
  • Contraindications (Relative): Recent MI (<1 wk), active pneumothorax, ongoing hemoptysis, recent major surgery (eye/thoracic/abdominal).

⭐ The FEV1/FVC ratio is crucial; a value < 0.7 or below the lower limit of normal (LLN) typically indicates an obstructive lung defect.

Lung Volumes & Capacities - Lung Size Matters

• Volumes:
  • Tidal Volume (TV): Normal breath (~500 mL).
  • Inspiratory Reserve Volume (IRV): Max air inhaled > TV.
  • Expiratory Reserve Volume (ERV): Max air exhaled < TV.
  • Residual Volume (RV): Air post-max exhalation; not by spirometry.
• Capacities:
  • Inspiratory Capacity (IC) = $TV + IRV$.
  • Functional Residual Capacity (FRC) = $ERV + RV$; air at rest.
  • Vital Capacity (VC) = $TV + IRV + ERV$; max usable volume.
  • Total Lung Capacity (TLC) = $VC + RV$.
• RV/FRC/TLC Measurement: Helium dilution, N₂ washout, Body Plethysmography.

⭐ Body plethysmography is the gold standard for measuring Residual Volume (RV) and Total Lung Capacity (TLC) as it measures all gas in the thorax, including trapped gas.

• Clinical: ↓TLC in restrictive; ↑RV, ↑FRC, ↑TLC in obstructive (hyperinflation).

Diffusing Capacity (DLCO) - Gas Gateway Test

• Measures CO transfer from alveoli to RBCs; reflects alveolar-capillary integrity.
• Method: Single breath-hold of CO & tracer gas.
• $DLCO = VA \times KCO$. $KCO = DLCO/VA$ (transfer coefficient).
• Normal: ~25 mL/min/mmHg (correct for Hb).
• ↓ DLCO:
  • Emphysema (↓ alveolar surface)
  • ILD, edema (↑ membrane thickness)
  • PE, PAH (↓ capillary blood volume)
  • Anemia
• ↑ DLCO: Pulmonary hemorrhage, polycythemia, L-R shunt (early), asthma (some).

⭐ DLCO is characteristically decreased in emphysema (due to alveolar destruction) and interstitial lung diseases (due to thickened alveolar-capillary membrane), but typically normal in chronic bronchitis or asthma (unless severe air trapping affects VA).

PFT Patterns in Disease - Disease Detective Puffs

• Initial Step: Assess FEV1/FVC ratio.
  • Obstructive Pattern: FEV1/FVC < 0.7; FEV1 ↓.
    • Lung Volumes: Typically ↑TLC, ↑RV (hyperinflation).
    • Asthma: Significant reversibility (FEV1 or FVC ↑ ≥12% AND ≥200mL post-BD). DLCO Normal/↑.
    • COPD (Emphysema): DLCO ↓.
    • COPD (Chronic Bronchitis): DLCO Normal.
    • Flow-Volume Loop: "Scooped-out" expiratory limb.
  • Restrictive Pattern: FEV1/FVC Normal or ↑; FVC ↓; TLC < 80% predicted.
    • Lung Volumes: ↓TLC, ↓RV, ↓FVC.
    • Parenchymal (e.g., ILD): DLCO ↓.
    • Extraparenchymal (e.g., neuromuscular, chest wall): DLCO Normal.
    • Flow-Volume Loop: "Witch's hat" (tall, narrow).
• Mixed Pattern: ↓FEV1/FVC, ↓FVC, ↓TLC.

⭐ A significant bronchodilator response (e.g., increase in FEV1 or FVC by ≥ 12% AND ≥ 200 mL) after salbutamol is characteristic of asthma, though not exclusive to it.

High‑Yield Points - ⚡ Biggest Takeaways

• Spirometry is crucial: FEV1/FVC ratio differentiates obstructive (<0.7) from restrictive (normal/↑) patterns.
• Obstructive diseases (Asthma, COPD) show ↓ FEV1, often with ↑ TLC and RV.
• Restrictive diseases (ILDs) show ↓ TLC, ↓ FVC, ↓ FEV1 but normal/↑ FEV1/FVC ratio.
• DLCO is ↓ in emphysema & ILDs; normal in asthma & pure chronic bronchitis.
• Significant bronchodilator reversibility (>12% & 200ml ↑ FEV1/FVC) indicates asthma.
• Flow-volume loops reveal upper airway obstruction patterns.