How is spirometry interpreted in an asthma patient?

Spirometry Interpretation in Asthma Patients

Spirometry is an essential diagnostic tool for asthma that should be performed in all patients aged 5-16 years with suspected asthma, with interpretation focusing on airflow obstruction patterns and bronchodilator reversibility. 1

Key Spirometric Parameters in Asthma

• FEV1 (Forced Expiratory Volume in 1 second): Represents the volume of air expired in the first second of forced expiration 1
• FVC (Forced Vital Capacity): Total volume of air expired from start to end of forced expiration maneuver 1
• FEV1/FVC ratio: Key indicator of airflow obstruction when reduced 1
• Bronchodilator response (BDR): Change in lung function following bronchodilator administration 1

Interpretation Algorithm

Step 1: Assess Baseline Spirometry

• FEV1 < Lower Limit of Normal (LLN) or < 80% predicted and/or FEV1/FVC < LLN or < 80% suggests airflow obstruction supportive of asthma diagnosis 1
• Normal spirometry does not exclude asthma due to the variable nature of the condition 1
• Use LLN rather than fixed cutoffs when possible (fixed cutoffs may falsely identify obstruction in 6.4% of children) 1

Step 2: Evaluate Bronchodilator Response

• Administer 400 μg of short-acting β2-agonist 1
• An increase in FEV1 ≥ 12% and/or ≥ 200 mL is diagnostic of asthma 1
• Negative BDR (< 12%) does not exclude asthma 1

Step 3: If Initial Tests Inconclusive

• Consider peak expiratory flow rate (PEFR) variability testing 1
  • Variability > 10% in adults and > 13% in children suggests asthma 2
• Perform bronchial challenge testing (methacholine or exercise) 1
  • PC20 < 8 mg/mL in methacholine challenge considered positive 3
  • Fall in FEV1 > 10% in exercise challenge suggests exercise-induced asthma 3

Common Spirometric Patterns in Asthma

• Classic obstructive pattern: Reduced FEV1/FVC ratio with normal or reduced FVC 4
• Normal spirometry: Common in mild asthma when patient is asymptomatic at testing time 5
• Mixed pattern: Both obstructive and restrictive features may be present 6
• Reduced PEFR: Often seen even when FEV1/FVC ratio is normal 4

Interpreting Bronchodilator Response

• Response magnitude correlates with asthma severity - greater reversibility often indicates more severe underlying inflammation 7
• In IOS (Impulse Oscillometry), resistance at 5 Hz (R5) correlates with FEV1 changes - a 1 unit change in %FEV1 corresponds to approximately 2.5% change in %R5 7

Important Caveats and Pitfalls

• One-time spirometry has low sensitivity (0.12-0.52) but moderate to high specificity (0.72-0.93) for asthma diagnosis 1
• Serial measurements may be required due to the variable nature of asthma 1
• Spirometry results may be normal between exacerbations 1
• Proper technique is critical - poor effort can lead to false results 2
• Fixed FEV1/FVC ratio cutoffs may lead to false positives in older adults and false negatives in younger adults 1
• Consider differential diagnoses when spirometry is normal but symptoms persist 3

Special Considerations

• In children aged 5-16 years, spirometry should be part of the initial diagnostic workup 1
• Female asthmatic patients may show more significant impairment in FVC, FEV1, FEV1/FVC, and MVV compared to males 4
• In smokers with intermittent respiratory symptoms, normal post-bronchodilator spirometry rules out COPD 5
• Exhaled nitric oxide (FeNO) testing complements spirometry - FeNO ≥ 25 ppb supports asthma diagnosis when spirometry is normal 1