What are the expected pulmonary function test (PFT) findings in a patient with asthma?

Expected Pulmonary Function Test Findings in Asthma

The classic PFT finding 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.

Spirometry Findings

Pre-bronchodilator Measurements

• FEV1: Reduced (severity depends on asthma control)
• FVC: Normal or reduced
• FEV1/FVC ratio: Reduced (<70-80% in adults, <90% in children) 1
• Flow-volume loop: Shows concavity of the expiratory portion 1

Post-bronchodilator Measurements

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

Lung Volumes

• Total Lung Capacity (TLC): Normal or increased
• Residual Volume (RV): Increased due to air trapping
• Functional Residual Capacity (FRC): Increased 1
• These changes reflect hyperinflation resulting from air trapping

Airway Resistance

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

Airway Hyperresponsiveness

• Positive bronchial challenge tests (methacholine, histamine, mannitol)
• Demonstrates increased sensitivity of airways to inhaled stimuli
• Considered a cardinal feature of asthma 1

Peak Expiratory Flow (PEF)

• Reduced values, especially during exacerbations
• Significant diurnal variation (>10% in adults, >13% in children) 1
• May show wide day-to-day variability 1

Special Considerations

Pattern Variations

1. Well-controlled asthma: May have normal baseline spirometry
2. Acute exacerbation: More severe obstruction with greater bronchodilator response
3. Severe persistent asthma: May show less bronchodilator reversibility despite significant obstruction

Important Caveats

• Normal spirometry doesn't exclude asthma: Some patients may have normal PFTs between exacerbations 1
• Perception of symptoms: Patients with asthma may have poor perception of their degree of airflow limitation 1
• Small airway involvement: Even with normal spirometry, there may be evidence of small airway dysfunction 1

Clinical Applications

Diagnostic Value

• Objective confirmation of variable airflow limitation is essential for asthma diagnosis 1
• Spirometry is more reliable than peak flow measurements for diagnosis 1
• In children, bronchodilator responsiveness testing provides better diagnostic accuracy than baseline measurements alone 1

Monitoring Disease

• Serial PFTs help assess:
  • Response to therapy
  • Disease progression
  • Exacerbation risk (low FEV1 is a strong predictor of future exacerbations) 1

By understanding these characteristic PFT findings, clinicians can more accurately diagnose asthma, assess its severity, and monitor response to treatment.