Can Asthma Cause Air Trapping?
Yes, asthma definitively causes air trapping, which is a characteristic pathophysiologic feature of the disease resulting from small airway obstruction, inflammation, and mucus plugging. 1, 2
Mechanisms of Air Trapping in Asthma
Air trapping occurs through multiple interconnected pathways in asthma:
Bronchoconstriction and mucus plugging create widespread but variable airflow obstruction, with edema, mucus hypersecretion, and formation of inspissated mucus plugs limiting airflow particularly during expiration 1, 2
Small airway inflammation is a major contributor, as the inflammatory process evident in large airways also occurs extensively in distal airways, creating a significant site of airway obstruction even when lung function appears normal 3
Incomplete expiration results from these obstructive changes, leading to breath stacking and buildup of positive end-expiratory pressure (auto-PEEP), particularly evident during acute exacerbations 1
Clinical Evidence and Severity Correlation
Air trapping is particularly prominent in severe asthma and represents a characteristic feature that distinguishes severe from nonsevere disease:
Research demonstrates that severe asthma exhibits prominent air trapping (reduced FVC) across the entire range of airflow limitation, while nonsevere asthma does not show this pattern even with significant obstruction 4
Air trapping is associated with increases in total lung capacity (TLC) and functional residual capacity, reflecting hyperinflation 4
High-resolution CT imaging confirms that distal airways are a major site of obstruction in asthma patients and may play a significant role in airway hyperresponsiveness 3
Even in mild-to-moderate uncontrolled asthma, approximately 43% of patients (25 out of 58 studied) demonstrate abnormal air trapping on quantitative CT assessment 5
Reversibility and Treatment Implications
Air trapping in asthma is partially but often incompletely reversible:
After maximal bronchodilation, severe asthma shows residual reduction in FEV₁, with incomplete reversal of obstruction being a predictor of severe disease 4
Inhaled corticosteroids decrease air trapping regardless of particle size, with the reduction serving as an outcome measure related to disease control 5
When baseline FEV₁ is <60% predicted, an increase in FVC (reflecting reduced air trapping) accounts for most of the reversal in FEV₁ after bronchodilation 4
Clinical Recognition
Key indicators of air trapping in asthma include:
Hyperinflation on physical examination with or without wheeze in patients with chronic asthma 1
Reduced FVC disproportionate to the degree of airflow limitation on spirometry 4
Elevated residual volume to total lung capacity ratio (RV/TLC) on full pulmonary function testing 4
Quantitative findings on high-resolution CT showing increased lung attenuation differences between inspiration and expiration 5
Critical Pitfall
The presence of air trapping indicates small airway involvement and suggests more severe disease pathology. Clinicians should not assume that normal-appearing spirometry excludes significant air trapping, as functional bronchoscopic studies reveal high peripheral airflow resistance even when lung function appears normal 3. This underscores the importance of comprehensive pulmonary function testing including lung volumes in patients with persistent symptoms despite apparently adequate spirometric values.