What is the typical lung pattern on Pulmonary Function Tests (PFTs) for asthma?

Typical Lung Pattern on Pulmonary Function Tests (PFTs) for Asthma

The classic lung pattern on PFTs in asthma is an obstructive pattern with significant bronchodilator reversibility, characterized by reduced FEV1, normal or reduced FVC, and a reduced FEV1/FVC ratio that improves after bronchodilator administration. 1

Key Spirometric Findings in Asthma

• Obstructive Pattern:

  • Reduced FEV1 (Forced Expiratory Volume in 1 second)
  • Normal or reduced FVC (Forced Vital Capacity)
  • Reduced FEV1/FVC ratio (<70-80% in adults, <90% in children) 1
  • Flow-volume loop showing concavity of the expiratory portion 1

• Bronchodilator Reversibility:

  • Increase in FEV1 ≥12% and ≥200ml from baseline after bronchodilator administration 1, 2
  • This reversibility is a hallmark feature that helps distinguish asthma from other obstructive lung diseases like COPD

• Lung Volumes:

  • Total Lung Capacity (TLC): Normal or increased
  • Residual Volume (RV): Increased due to air trapping
  • Functional Residual Capacity (FRC): Increased 1

Variability in PFT Findings Based on Asthma Severity

The American Thoracic Society classifies asthma severity based on lung function as follows 3:

Asthma Classification	Lung Function
Mild Intermittent	PEF or FEV1: ≥80% of predicted function
Mild Persistent	PEF or FEV1: ≥80% of predicted function
Moderate Persistent	PEF or FEV1: 60-80% of predicted function
Severe Persistent	PEF or FEV1: ≤60% of predicted function

Additional PFT Characteristics in Asthma

• Airway Hyperresponsiveness:

  • Positive bronchial challenge tests (methacholine, histamine, mannitol) demonstrate increased sensitivity of airways to inhaled stimuli 1
  • This is considered a cardinal feature of asthma when spirometry is normal between exacerbations

• Airway Resistance:

  • Increased airway resistance that decreases after bronchodilator administration 1
  • Can be measured via body plethysmography or forced oscillation technique

• PEF Variability:

  • Diurnal PEF variability >20% is characteristic of persistent airway obstruction 3
  • However, PEF is inferior to FEV1 for diagnosis as it lacks accurate reference values and may underestimate airway obstruction 3

Important Clinical Considerations

• Normal Baseline Spirometry:

  • Well-controlled asthma may have normal baseline spirometry 1
  • Normal spirometry does not exclude asthma, as some patients may have normal PFTs between exacerbations 1

• Exacerbations:

  • Acute exacerbations are characterized by more severe obstruction with greater bronchodilator response 1
  • Severe exacerbations are associated with a more rapid decline in lung function over time 4

• Severe/Refractory Asthma:

  • May show persistent airway obstruction (FEV1 < 80% predicted) despite treatment 3
  • May demonstrate less bronchodilator reversibility despite significant obstruction 1
  • Fixed airflow limitation can be defined as a postbronchodilator FEV1 of < 80% predicted (with reduced FEV1/FVC) after a course of oral corticosteroids 3

• Differential Diagnosis:

  • Unlike COPD, asthma typically shows significant bronchodilator reversibility 2
  • In asthma-COPD overlap, spirometry will show both reversibility after bronchodilators (consistent with asthma) and persistent baseline airflow limitation (characteristic of COPD) 2

Remember that PFT findings should be interpreted in the context of the patient's clinical presentation, as the pattern can vary depending on disease severity, control status, and whether the patient is experiencing an exacerbation at the time of testing.