How to interpret normal spirometry with air trapping on the flow volume loop?

Interpreting Normal Spirometry with Air Trapping on Flow Volume Loop

Air trapping on flow-volume loops despite normal spirometry values is most commonly associated with small airway disease, particularly early or mild obstructive airway diseases that standard spirometric parameters may not detect. 1

Understanding Air Trapping on Flow Volume Loop

Air trapping manifests on the flow-volume loop as:

• A concave appearance toward the end of the expiratory manoeuvre, particularly when FEV1 and FVC are normal but the expiratory flow appears diminished 1
• Normal TLC (Total Lung Capacity) with increased RV (Residual Volume) and potentially increased RV/TLC ratio 1
• Low FEF75 (forced expiratory flow at 75% of FVC) despite normal FEV1/FVC ratio 1

Diagnostic Approach

Step 1: Verify Technical Adequacy

• Ensure the pattern is reproducible across multiple flow-volume loops (not just a single maneuver) 2
• Confirm patient performed maximal sustained effort during testing 1

Step 2: Consider Additional Testing

• Perform slow vital capacity (SVC) measurement to get a more accurate estimate of FEV1/VC ratio 1

  • FVC may underestimate VC in the presence of increased small airway collapsibility 1
  • 20% of patients with preserved FEV1/FVC ratio may show low FEV1/SVC ratio, indicating mild airflow obstruction 1

• Measure lung volumes to assess for:

  • Normal TLC with increased RV (suggestive of air trapping) 1
  • Increased RV/TLC ratio (indicative of hyperinflation) 1

Step 3: Bronchodilator Testing

• Administer bronchodilator and repeat spirometry 1
• Significant improvement in FEV1, FVC, or both suggests reversible airflow obstruction 1
• Volume responses (FVC improvement) may occur without flow responses (FEV1 improvement) in small airway disease 1

Clinical Significance and Differential Diagnosis

Air trapping with normal spirometry is most commonly associated with:

• Bronchiolitis obliterans - most frequent finding in patients with normal inspiratory scans but air trapping on expiratory imaging 3
• Asthma (particularly mild or early disease) 3
• Chronic bronchitis 3
• Patchy collapse of small airways early in exhalation 1
• Early COPD not yet detectable by standard spirometric criteria 1

Important Considerations

• Air trapping may be the earliest manifestation of obstructive lung disease before conventional spirometric parameters become abnormal 1, 3
• The pattern is more common in patients <60 years old and those with BMI >30 kg/m² 1
• Flow decay analysis (measuring the exponential decay in airflow during forced exhalation) may help quantify dynamic airway resistance even when standard spirometric values appear normal 4
• In patients with strong clinical suspicion of obstructive disease but normal FEV1/FVC ratio, consider FEV1/SVC ratio which may be more sensitive for detecting mild airflow obstruction 1

Pitfalls to Avoid

• Don't dismiss air trapping when standard spirometric values are normal - it may represent early small airway disease 1, 3
• Don't confuse this pattern with poor patient effort - true air trapping will be consistent across multiple maneuvers 2
• Don't rely solely on FEV1/FVC ratio when suspecting early obstructive disease - FEV1/SVC may be more sensitive 1
• Don't overlook the expiratory portion of the flow-volume loop - the concave pattern is a key diagnostic feature 1, 4