Lecture: Pulmonary Function Tests (PFTs)

Introduction

• Topic: Pulmonary Function Tests (PFTs)
• Purpose: Differentiate between obstructive and restrictive lung diseases (e.g., Asthma, COPD).

PFT Overview

• Components: Not a single test but a series of smaller tests.

Lung Volumes

• Obstructive Lung Disease:

  • Characteristics: Hyperinflated, compliant lungs with dynamic hyperinflation and air trapping.
  • Measurement: High residual volumes, high expiratory reserve volumes (ERV), and increased functional residual capacity (FRC).
  • Impact: Reduced inspiratory volumes.

• Restrictive Lung Disease:

  • Characteristics: Stiff, fibrotic lungs with low compliance.
  • Measurement: Low reserve volumes, low ERV, reduced FRC.
  • Impact: Reduced ability for deep breaths.

Total Lung Capacity

• Obstructive: Large total lung capacity due to high FRC.
• Restrictive: Small total lung capacity.

Flow Volume Loops

• Normal: Defined FVC, residual volume, and total lung capacity.
• Obstructive: Shift to the left, increased residual volume, increased total lung capacity.
• Restrictive: Shift to the right, decreased FVC, decreased residual volume, reduced total lung capacity.

Forced Spirometry

• FEV1: Volume expired at one second.
  • Obstructive: Significant reduction in FEV1.
  • Restrictive: Slight reduction in FEV1.
• FVC: Force vital capacity.
  • Obstructive: Usually preserved.
  • Restrictive: Significantly reduced.
• FEV1/FVC Ratio:
  • Obstructive: Ratio <70%.
  • Restrictive: Ratio ≥ 70%.

Bronchodilator Test

• Objective: Differentiate between Asthma (reversible) and COPD (irreversible).
• Procedure: Administer short-acting beta 2 agonist; check for FEV1 improvement.
  • Asthma: FEV1 increase >12%.
  • COPD: FEV1 increase ≤12%.

Methacholine Challenge

• Purpose: Test inducibility of airway restriction.
• Asthma: FEV1 drop ≥20% indicates inducibility.

DLCO Testing

• Purpose: Assess diffusion capacity of carbon monoxide.
• Obstructive:
  • Emphysema: Decreased DLCO due to reduced surface area.
  • Chronic Bronchitis: Normal DLCO.
• Restrictive:
  • Intrinsic: Decreased DLCO due to thickened respiratory membrane.
  • Extrinsic: Normal DLCO.

Diagnostic Approach to PFTs

• Obstructive Lung Disease:

  • FEV1/FVC <70%, increased TLC, RV, FRC.
  • Use bronchodilator test to differentiate asthma from COPD.
  • DLCO to distinguish emphysema (low DLCO) from chronic bronchitis (normal DLCO).

• Restrictive Lung Disease:

  • FEV1/FVC ≥ 70%, decreased TLC, RV, FRC.
  • Use DLCO to differentiate:
    • Low DLCO: Intrinsic lung disease.
    • Normal DLCO: Extrinsic lung disease.

Conclusion

• Comprehensive understanding of PFTs aids in accurate diagnosis of lung diseases.