Pulmonary Function Tests (PFTs)

Introduction

• PFTs (Pulmonary Function Tests): Not a single test but a series of small tests.
• Purpose: Distinguish between obstructive and restrictive lung diseases, and identify specific conditions like asthma or COPD.

Obtain Lung Volumes

• Lung Volumes Measurement: Often done via plethysmography.
• Obstructive Lung Diseases:
  • Characteristics: Hyperinflated lungs, high compliance, dynamic hyperinflation, air trapping.
  • Key Volumes:
    • Residual Volume (RV): High.
    • Expiratory Reserve Volume (ERV): High.
    • Functional Residual Capacity (FRC): High (RV + ERV).
    • Inspiratory Volumes: Reduced.
• Restrictive Lung Diseases:
  • Characteristics: Fibrotic, stiff lungs with increased elasticity.
  • Key Volumes:
    • Residual Volume (RV): Low.
    • Expiratory Reserve Volume (ERV): Low.
    • Functional Residual Capacity (FRC): Reduced (RV + ERV).
    • Inspiratory Volumes: Reduced.
• Total Lung Capacity (TLC):
  • Obstructive: High.
  • Restrictive: Low.

Flow-Volume Loops

• Normal Patient: Black curve, forced vital capacity (FVC).
• Obstructive Lung Diseases:
  • Curve: Shifts left.
  • FVC: Reduced.
  • Residual Volume (RV): Increased.
  • Total Lung Capacity (TLC): Increased.
• Restrictive Lung Diseases:
  • Curve: Shifts right.
  • FVC: Significantly reduced.
  • Residual Volume (RV): Reduced.
  • Total Lung Capacity (TLC): Reduced.

Spirometry (Forced Vital Capacity)

• Measurement: Deep breath in and forceful breath out.
• Key Metrics:
  • FEV1 (Forced Expiratory Volume in 1 second):
    • Obstructive: Significantly reduced.
    • Restrictive: Slightly reduced.
  • FVC (Forced Vital Capacity):
    • Obstructive: Reduced but not as much as restrictive.
    • Restrictive: Significantly reduced.
  • FEV1/FVC Ratio:
    • Obstructive: < 70%.
    • Restrictive: > 70%.

Bronchodilator Test

• Purpose: Differentiate between COPD and asthma.
• Procedure: Administer short-acting beta-2 agonist.
  • Asthma: FEV1 increase ≥ 12% suggests reversibility.
  • COPD: FEV1 increase < 12% suggests irreversibility.
• Methacholine Challenge:
  • Inducible airway test to further confirm asthma.
  • FEV1 decrease ≥ 20% suggests inducibility.

DLCO Testing (Diffusion Limitation of Carbon Monoxide)

• Purpose: Measure gas diffusion across alveoli.
• Observation in Obstructive Lung Diseases:
  • Emphysema: Decreased surface area, DLCO reduced.
  • Asthma & Chronic Bronchitis: Generally normal DLCO.
• Observation in Restrictive Lung Diseases:
  • Intrinsic (e.g., interstitial lung disease): Thickened respiratory membrane, DLCO reduced.
  • Extrinsic: Normal DLCO.

Diagnostic Algorithm

1. Initial Spirometry: FEV1/FVC ratio.
   • < 70%: Obstructive.

   • 70%: Restrictive or normal.

2. Lung Volumes & Flow-Volume Loops:
   • Obstructive: Increased TLC, RV, FRC; left-shifted curve.
   • Restrictive: Decreased TLC, RV, FRC; right-shifted curve.
3. Bronchodilator Test:
   • FEV1 increase ≥ 12%: Asthma.
   • FEV1 increase < 12%: COPD.
4. Methacholine Challenge: Confirm inducibility for asthma.
5. DLCO Testing:
   • Normal or increased: Asthma or extrinsic restrictive.
   • Reduced: Emphysema or intrinsic restrictive.

Conclusion

• Approach: Use spirometry, flow-volume loops, bronchodilator tests, and DLCO testing to distinguish between obstructive and restrictive lung diseases.
• Objectives: Accurately diagnose and differentiate between conditions like asthma, COPD, and interstitial lung diseases.
• Reminder: Review and practice the algorithm to master the diagnostic steps.

Remember: Consistent review of these concepts and practical application is key to proficiency.