What Does Air Trapping on CT Mean?
Air trapping on CT refers to geographic areas of abnormally decreased lung attenuation (darker areas) that fail to increase in density on expiratory imaging, indicating abnormal retention of air due to small airways disease or obstruction. 1
Definition and Technical Characteristics
Air trapping appears as sharply demarcated areas of decreased lung density on expiratory CT scans that remain hyperlucent (darker) when the rest of the lung normally increases in density during exhalation. This finding specifically requires expiratory phase imaging to be accurately diagnosed—the term "air trapping" is reserved exclusively for expiratory CT findings, while "mosaic attenuation" describes similar patterns on inspiratory images 1.
What It Indicates Pathophysiologically
Air trapping results from:
- Complete or partial obstruction of small airways (bronchioles), preventing normal air egress during expiration 2
- Less commonly, perfusional differences from underlying vascular disease 2
The obstruction occurs at the level of small airways and reflects small airways disease, which can result from inflammation, fibrosis, or bronchiolar narrowing 1, 2.
Clinical Significance and Disease Associations
When air trapping is found on CT, the differential diagnosis depends critically on associated imaging findings 3:
Air Trapping WITH Bronchiectasis (38% of cases)
Most commonly indicates:
- Nontuberculous mycobacterial infection
- Cystic fibrosis
- Idiopathic bronchiectasis
- Transplant-related bronchiolitis obliterans 3
Air Trapping WITH Interstitial Lung Disease (31% of cases)
Most commonly indicates:
- Hypersensitivity pneumonitis (HP) - particularly important
- Sarcoidosis
- Unspecified interstitial lung disease 3
For HP specifically, the combination of centrilobular nodules and air-trapping has a positive predictive value of 100% and negative predictive value of 98% 1. The "three-density sign" (areas of normal density, ground-glass opacity, and decreased attenuation from air-trapping within the same lobe) is highly specific for HP 1.
Air Trapping ALONE (29% of cases)
Most commonly indicates:
- Chronic bronchitis
- Asthma
- Bronchiolitis obliterans
- Unspecified small airways disease
- Less commonly: vasculitis, diffuse idiopathic neuroendocrine cell hyperplasia (DIPNECH) 3
When air trapping occurs with normal inspiratory CT, the most likely diagnoses are bronchiolitis obliterans and asthma 4.
Important Caveats
Some degree of air trapping can be seen in healthy patients 1, 2. This is a critical pitfall—not all air trapping is pathologic. The clinical context, extent of air trapping, and associated findings determine significance.
Functional Correlation
Air trapping correlates with:
- Reduced FEV1 (forced expiratory volume in 1 second)
- Reduced FEV1/FVC ratio
- Airflow obstruction on pulmonary function tests 5, 6
Quantitative measurements (pixel indices) correlate well with pulmonary function test results and can identify patients with more severe disease 5. Air trapping is independently associated with accelerated lung function decline over time 6.
Diagnostic Approach
To determine the cause of air trapping:
- Obtain both inspiratory AND expiratory CT images (essential for accurate diagnosis) 2
- Assess for associated findings:
- Bronchiectasis present? → Consider infectious/post-infectious causes
- Interstitial changes present? → Consider HP, sarcoidosis
- Centrilobular nodules? → Strongly suggests HP
- Tree-in-bud opacities? → Suggests infection
- Isolated finding? → Consider asthma, chronic bronchitis, bronchiolitis obliterans
- Integrate with clinical history: exposure history (for HP), smoking history, infection history, transplant status 1, 3