What is a pulmonary function test (PFT) and how is it used to diagnose and manage respiratory disease?

Pulmonary Function Testing: A Comprehensive Guide

Pulmonary function tests (PFTs) are essential diagnostic tools that provide objective measurements of lung function, helping clinicians diagnose respiratory conditions, monitor disease progression, and evaluate treatment response by assessing airflow, lung volumes, and gas exchange capacity. These standardized tests are crucial for the management of respiratory diseases and provide quantifiable data that complements clinical assessment.

Core Components of Pulmonary Function Testing

1. Spirometry

• Definition: Measures airflow and volumes during forced breathing maneuvers

• Key Measurements:

  • Forced Vital Capacity (FVC): Maximum volume of air exhaled forcefully after maximum inhalation
  • Forced Expiratory Volume in 1 second (FEV1): Volume exhaled in the first second
  • FEV1/FVC ratio: Key indicator for obstructive vs. restrictive patterns
  • Forced Expiratory Flow 25-75% (FEF25-75): Mid-expiratory flow rate

• Patterns:

  • Obstructive pattern: Reduced FEV1, normal/reduced FVC, reduced FEV1/FVC ratio (<70-80% in adults, <90% in children) 1
  • Restrictive pattern: Reduced FVC, normal/high FEV1/FVC ratio
  • Mixed pattern: Features of both obstruction and restriction

• Special Considerations for Children: In preschool children (2-6 years), FEV0.75 or FEV0.5 may be more appropriate than FEV1 due to their inability to maintain flow limitation for a full second 2

2. Lung Volumes and Capacities

• Measurements:

  • Total Lung Capacity (TLC)
  • Residual Volume (RV)
  • Functional Residual Capacity (FRC)
  • Vital Capacity (VC)

• Measurement Methods:

  • Body plethysmography (gold standard)
  • Gas dilution techniques (nitrogen washout, helium dilution)

3. Diffusion Capacity

• Definition: Measures gas transfer across the alveolar-capillary membrane
• Key Measurement: Diffusing capacity of the lung for carbon monoxide (DLCO)
• Clinical Significance: Reduced in conditions affecting the alveolar-capillary interface (emphysema, pulmonary fibrosis, pulmonary vascular disease)

Clinical Applications

1. Diagnosis of Respiratory Conditions

• Obstructive Lung Diseases:

  • Asthma: Characterized by bronchodilator reversibility (increase in FEV1 ≥12% and ≥200ml from baseline) 1
  • COPD: Fixed airflow limitation with limited bronchodilator response

• Restrictive Lung Diseases:

  • Interstitial lung disease
  • Chest wall disorders
  • Neuromuscular weakness

2. Disease Monitoring

• Asthma Management:

  • Well-controlled asthma may show normal baseline spirometry
  • Acute exacerbations show more severe obstruction with greater bronchodilator response 1
  • Normal spirometry between exacerbations does not exclude asthma diagnosis

• Interstitial Lung Disease (ILD) Monitoring:

  • For patients with systemic autoimmune rheumatic diseases (SARDs), PFTs should be performed every 3-6 months during the first year after ILD diagnosis, then less frequently once stable 2
  • Includes spirometry, lung volumes, and diffusion capacity measurements

3. Bronchodilator Response Testing

• Procedure: Spirometry performed before and after administration of a bronchodilator
• Interpretation: Positive response defined as increase in FEV1 ≥12% and ≥200ml from baseline
• Clinical Value: Helps distinguish asthma from other obstructive lung diseases 1

4. Bronchial Challenge Testing

• Types: Methacholine, histamine, mannitol, exercise
• Purpose: Assesses airway hyperresponsiveness
• Interpretation: Positive test supports asthma diagnosis when spirometry is normal but symptoms suggest asthma

Special Considerations

1. Pediatric PFTs

• Age-Specific Approaches:

  • Preschool children (2-6 years): Modified techniques including tidal breathing, interrupter technique, forced oscillation, and multiple-breath washout 2
  • Reference data from older subjects should not be extrapolated to children under 6 years 2

• Quality Control:

  • Requires specially trained technicians with skills in working with young children
  • Alternative quality control criteria may be needed as international standards derived from adults may not be applicable 2

2. Monitoring Specific Conditions

• Sjögren's Syndrome:

  • Baseline chest radiograph and full PFTs recommended for all patients 2
  • PFTs should include spirometry, lung volumes, and diffusion capacity 2

• Systemic Autoimmune Rheumatic Diseases (SARDs):

  • Regular monitoring with PFTs recommended for patients with diagnosed ILD
  • Frequency: Every 3-6 months in first year for inflammatory myopathies and systemic sclerosis; every 3-12 months for RA, Sjögren's disease, and mixed connective tissue disease 2

Interpretation Principles

1. Compare to Reference Values:

   • Results should be expressed as z-scores (multiples of standard deviation from the mean) rather than percentages of predicted values 2
   • Consider age, sex, height, ethnicity when selecting reference values

2. Pattern Recognition:

   • Obstructive: Reduced FEV1/FVC ratio
   • Restrictive: Reduced FVC with normal/increased FEV1/FVC ratio
   • Mixed: Features of both patterns
   • Flow-volume loop analysis: Obstructive patterns show concavity in the expiratory portion 1

3. Assess Severity:

   • Asthma classification based on lung function 1:
     • Mild: PEF or FEV1 ≥80% predicted
     • Moderate: PEF or FEV1 60-80% predicted
     • Severe: PEF or FEV1 ≤60% predicted

Common Pitfalls and Limitations

1. Technical Issues:

   • Poor patient effort or understanding
   • Equipment calibration problems
   • Improper technique

2. Interpretive Challenges:

   • Normal PFTs do not exclude respiratory disease (especially in early stages)
   • PEF is inferior to FEV1 for diagnosis as it lacks accurate reference values 1
   • Patients with asthma may have poor perception of their degree of airflow limitation 1

3. Special Populations:

   • In patients with Raynaud phenomenon or systemic sclerosis, oxygen saturation measurement should use ear or forehead monitors rather than finger probes due to poor finger perfusion 2
   • Some patients with systemic sclerosis may require pediatric mouthpieces due to decreased oral aperture 2

Conclusion

Pulmonary function tests are invaluable tools in respiratory medicine, providing objective data for diagnosis, disease monitoring, and treatment evaluation. The selection and interpretation of specific tests should be guided by the clinical question, patient characteristics, and established guidelines. Regular monitoring with PFTs is particularly important in chronic respiratory conditions and systemic diseases with pulmonary manifestations.