What Causes Air Trapping?
Air trapping is caused by obstruction of the small airways, resulting in abnormal retention of air in the lungs that fails to empty properly during exhalation. 1, 2
Primary Mechanisms
Lobular air trapping is usually caused by obstruction of the supplying bronchioles and is indicative of small airways disease. 1 The European Respiratory Society identifies that air trapping occurs through small airway luminal narrowing caused by inflammation and remodeling, leading to increased airway resistance and reduced expiratory flow. 2
The pathophysiology involves:
- Complete or partial airway obstruction from small airway pathologies that prevents normal air egress during expiration 3
- Small airway inflammation and remodeling that narrows the bronchiolar lumen and increases resistance 2
- Premature airway closure during expiration, particularly in diseases with loss of elastic recoil 2
Disease-Specific Causes
COPD
In COPD patients, air trapping results from a combination of mechanisms 1:
- Small airway narrowing and inflammation
- Loss of elastic recoil in emphysema
- Premature airway closure during expiration 2
- The increased ventilatory demand in flow-limited patients during exercise gives rise to air trapping and dynamic hyperinflation 1
Hypersensitivity Pneumonitis
Bronchiolar obstruction manifests with several features in fibrotic HP, including mosaic attenuation and lobular air trapping. 1 The combination of lung fibrosis and signs of bronchiolar obstruction is highly suggestive of fibrotic HP. 1
Asthma
Air trapping reflects small airway obstruction in asthma and can be assessed quantitatively by high-resolution computed tomography. 4 In mild-to-moderate asthma, air trapping may represent a marker of disease control. 4
Other Causes
When air trapping is found in isolation (without bronchiectasis or interstitial lung disease), the most common causes include 5:
- Chronic bronchitis
- Asthma
- Bronchiolitis obliterans
- Unspecified small airways disease
When found with bronchiectasis, nontuberculous mycobacteria, cystic fibrosis, idiopathic bronchiectasis, and transplant-related bronchiolitis obliterans are common causes. 5
When found with interstitial lung disease, sarcoidosis, hypersensitivity pneumonitis, or unspecified interstitial lung disease are the most common causes. 5
Clinical Significance
Air trapping has significant clinical implications, including association with lung function impairment in patients with small airways disease. 2 The European Respiratory Society indicates that air trapping contributes to dyspnea, especially during exertion, as gas trapping worsens with exercise due to hyperventilation-related reduction in expiratory time. 2
In asthmatic patients and patients with COPD, lung function impairment is strongly associated with air trapping. 6 A 10% increase in the RV/TLC ratio (representing air trapping) increases the risk of moderate to severe COPD exacerbation by 35% and severe exacerbation by 36%. 7
Important Caveats
- Areas of air trapping can be seen in normal patients, and some degree of heterogeneity of lung attenuation is expected at expiratory imaging. 1
- Air trapping is a nonspecific finding that can reflect small airway alterations of variable cause and severity, and is also found in non-HP interstitial lung diseases, especially connective tissue disease-associated ILD and sarcoidosis. 1
- Perfusional differences due to underlying vascular diseases could also result in mosaic attenuation appearances, and hence inspiratory and full expiratory phase CT studies are required to accurately diagnose true air trapping. 3