Interpreting Normal Spirometry with Air Trapping on Flow Volume Loop

Understanding 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, 2
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 is associated with air trapping 1, 3
Low FEF75 (forced expiratory flow at 75% of FVC) despite normal FEV1/FVC ratio is a characteristic of air trapping 1

Confirm patient performed maximal sustained effort during testing to ensure accurate results 1
Perform slow vital capacity (SVC) measurement to get a more accurate estimate of FEV1/VC ratio, as FVC may underestimate VC in the presence of increased small airway collapsibility 1, 4
20% of patients with preserved FEV1/FVC ratio may show low FEV1/SVC ratio, indicating mild airflow obstruction 4
Measure lung volumes to assess for normal TLC with increased RV, suggestive of air trapping, and increased RV/TLC ratio, indicative of hyperinflation 1

Patchy collapse of small airways early in exhalation is associated with air trapping 1
Early COPD not yet detectable by standard spirometric criteria may present with air trapping 4

Air trapping may be the earliest manifestation of obstructive lung disease before conventional spirometric parameters become abnormal 1
The pattern is more common in patients <60 years old and those with BMI >30 kg/m² 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 4