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Understanding Pulmonary Function Tests

Feb 20, 2025

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.