• Etymology
  The term “air trapping” originates from the inability of air to exit the lungs or specific lung regions during exhalation, leading to radiologically visible findings of hyperinflation.
• AKA
  Air retention; Regional hyperinflation.
• What is it?
  Air trapping refers to the retention of air in the lungs due to partial or complete obstruction of the small airways. It is a hallmark of obstructive pulmonary diseases.

• Characterized by:
  • Persistent inflation of lung parenchyma during exhalation.
  • Discrepant aeration between inspiratory and expiratory phases on imaging.
• Anatomically affecting:
  • Primarily involves the small airways (e.g., bronchioles).
  • Secondary pulmonary lobules, often seen in mosaic attenuation patterns.
• Causes include:
  • Most Common Causes:
    • Asthma.
    • Chronic Obstructive Pulmonary Disease (COPD).
  • Other Causes include:
    • Infection: Bronchiolitis (e.g., post-viral bronchiolitis), including childhood infections leading to Swyer-James Syndrome (post-infectious obliterative bronchiolitis).
    • Inflammation: Hypersensitivity pneumonitis, obliterative bronchiolitis.
    • Neoplasm: Endobronchial tumors causing obstruction.
    • Mechanical: Foreign body aspiration, airway stenosis, or ball-valve effect at the segmental or subsegmental level, where partial obstruction permits inhalation but restricts exhalation. Causes of the ball-valve effect include:
      • Endobronchial tumors (e.g., carcinoid tumors, metastases).
      • Mucous plugging (e.g., in asthma or allergic bronchopulmonary aspergillosis).
      • Foreign body aspiration (e.g., food particles, small toys).
      • Airway stenosis or scarring (e.g., post-inflammatory or post-surgical).
    • Trauma: Airway injury causing scarring or obstruction.
    • Iatrogenic: Post-surgical changes, radiation fibrosis.
    • Inherited and Congenital:
      • Congenital Lobar Emphysema (CLE): CLE is characterized by overexpansion of a lung lobe due to structural abnormalities, such as defective cartilage in the bronchial walls, leading to airway collapse during expiration and resultant air trapping.
      • Congenital Hypoplasia of Airways: Involves underdeveloped bronchial structures, leading to inadequate airway caliber and regional air trapping. This may cause localized hyperinflation and mosaic attenuation patterns on imaging.
      • Alpha-1 Antitrypsin Deficiency: A genetic disorder that results in a lack of protective alpha-1 antitrypsin enzyme, leading to early destruction of alveolar walls (panacinar emphysema). This structural loss causes widespread air trapping and hyperinflation, predominantly in the lower lobes.
    • Idiopathic: Idiopathic bronchiolitis.
• Pathophysiology:
  • Air trapping occurs when the small airways fail to empty completely during exhalation due to obstruction or decreased elastic recoil. This leads to regional hyperinflation and ventilation-perfusion mismatch.
• Histopathology:
  • Findings include inflammation, fibrosis, or destruction of small airways and alveoli depending on the underlying cause.
• Imaging Radiology:
  • Applied Anatomy to CT:
    • Parts: Involves secondary pulmonary lobules and bronchioles.
    • Size: Variable; may affect isolated lobules or large regions.
    • Shape: Hyperlucent regions with preserved vascularity.
    • Position: Commonly involves the lower lobes in obstructive diseases but may vary. The involvement can range from a single secondary pulmonary lobule in focal diseases like obliterative bronchiolitis, to subsegmental or segmental regions in conditions such as hypersensitivity pneumonitis. Larger lobar or whole-lung involvement can occur in diffuse obstructive pathologies like severe COPD or asthma, illustrating the variability in disease distribution.
    • Character: Mosaic attenuation with areas of hypodensity.
    • Time: More apparent on expiratory phase imaging.
  • CXR:
    • Subtle findings; localized hyperinflation or flattening of the diaphragm.
  • CT:
    • Expiratory phase:  High-resolution CT is used to clarify the etiology of mosaic attenuation. In the expiratory phase, patients with air trapping show persistence of the mosaic attenuation, with hyperlucent areas surrounded by normal lung capable of air expiration, visible as ground-glass opacities.
    • Inspiratory phase: Mosaic attenuation pattern.
    • May show bronchial wall thickening, air trapping lobules, or emphysematous changes.
  • PET-CT:
    • May be used for associated lesions or inflammation; not primary for air trapping.
  • PFTs:
    • Pulmonary Function Tests (PFTs) are crucial in evaluating air trapping by measuring lung volumes and airflow. Reduced expiratory flow rates and increased residual volume (RV) or total lung capacity (TLC) confirm obstructive physiology.
• Differential Diagnosis:
  • Emphysema.
  • Bronchiolitis.
  • Pulmonary vascular disease (e.g., chronic thromboembolic pulmonary hypertension).
• Recommendations:
  • Perform inspiratory and expiratory phase CT imaging for confirmation.
  • Consider pulmonary function tests (PFTs) for functional correlation.
  • Further evaluation with bronchoscopy if airway obstruction is suspected.
• Key Points and Pearls:
  • Inspiratory-expiratory CT imaging is essential to diagnose air trapping.
  • Mosaic attenuation is a key radiological finding, with hyperlucent areas on expiratory imaging.
  • Differentiate air trapping from emphysema using vascularity: air-trapped areas retain vascular markings, while emphysema does not.
• Parallels with Human Endeavors:
  • Air trapping mirrors the concept of bottlenecks in systems where flow is restricted but not halted, such as traffic jams or supply chain disruptions.
  • It also symbolizes imbalance—where input (inhalation) is unregulated compared to output (exhalation), leading to inefficiency and strain.

CXR Air Trapping Superior Segment LLL and
Small Subsegment in the RLL above the Diaphragm

CT Air Trapping Superior Segment LLL and
Small Post Segment in the RLL above the Diaphragm

The pathogenesis involves airway obstruction or collapse during expiration, preventing air from escaping the affected parts of the lungs, resulting in hyperinflation and difficulty with ventilation. Over time, this can impair lung function, leading to symptoms such as shortness of breath, wheezing, and reduced exercise tolerance. Air trapping is typically diagnosed through imaging, where it appears as areas of hyperlucency on a chest X-ray or CT scan, particularly in expiratory views. Pulmonary function tests (PFTs) may also show a decreased expiratory flow rate, further confirming the presence of obstructive processes.

Small Airway Obstruction

Bronchiolitis and Mosaic Attenuation

 

Hypersensitivity Pneumonitis
Mosaic Attenuation with Persistence on Expiration  = Air Trapping

CXR Air Trapping LUL and
Compressive Atelectasis of the Lingula

 

Inferior Lingula Air Trapping

 

 

• It is commonly associated with obstructive lung diseases such as asthma, chronic obstructive pulmonary disease (COPD), and bronchiolitis. The pathogenesis involves airway obstruction or collapse during expiration, preventing air from escaping the affected parts of the lungs, resulting in hyperinflation and difficulty with ventilation. Over time, this can impair lung function, leading to symptoms such as shortness of breath, wheezing, and reduced exercise tolerance. Air trapping is typically diagnosed through imaging, where it appears as areas of hyperlucency on a chest X-ray or CT scan, particularly in expiratory views. Pulmonary function
  tests (PFTs) may also show a decreased expiratory flow rate, further confirming the presence of obstructive processes.
• is an imaging and physiologic  term to
• retained air in a part or parts of the lung
• more easily identified during expiration
• caused by
  • obstruction
    • often small airway disease
      • chronic bronchitis
      • asthma
      • Hypersensitivity pneumonitis
      • sarcoidosis
      • bronchiolitis
      • cystic fibrosis/bronchiectasis
      • ILD
      • obesity
  • abnormality in lung compliance
  • sometimes seen in normal people
    • 50% of CT scans

 

• 

Mosaic Attenuation Caused by Obstruction of Small Airways 

Medium Sized Airways and Smaller Airways are Filled with Mucus in a patient with COPD – Note Centrilobular Impaction of Mucus

 

Medium Sized Airways and Smaller Airways are Filled with Mucus – Note Centrilobular Impaction of Mucus

• • Mosaic attenuation is an
    • imaging pattern
      • variable lung attenuation
      • results in a heterogeneous appearance of the parenchyma.
      • sometimes it is caused by air trapping
      • sometimes by perfusion abnormalities
      • sometimes normal

Lobar Air Trapping Due to Mucus Impaction

Note Realative Lucency of the RLL compared to the LLL

Social and Societal Equivalents: Trapped individuals or groups in oppressive societal systems, such as slavery or systemic inequality, reflect the concept of air trapping. Solutions often require systemic changes, akin to addressing underlying airway obstructions.

Art: “The Scream” by Edvard Munch captures the concept of being trapped in an overwhelming emotion or state.

Sculpture: Auguste Rodin’s “The Thinker” evokes a sense of mental entrapment or deep introspection.

 

Music: “Bohemian Rhapsody” by Queen, particularly the lyrics “I’m just a poor boy, I need no sympathy,” reflects the feeling of being trapped by fate or circumstances.

Bohemian Rhapsody by Queen

Dance: Interpretive dance styles often explore the theme of entrapment, using restricted movement to symbolize struggle.

Literature: Franz Kafka’s “The Metamorphosis” reflects being physically and emotionally trapped in an altered existence.

Quotes by Famous People:

“None are more hopelessly enslaved than those who falsely believe they are free.” – Johann Wolfgang von Goethe.

“We are all prisoners, but some of us are in cells with windows and some without.” – Khalil Gibran.

Poetry: Emily Dickinson’s poem “Hope is the thing with feathers” captures the contrast between entrapment and the resilience of hope.

Architecture: Labyrinthine structures, such as the Minotaur’s labyrinth in Greek mythology or modern-day prisons, embody the physical manifestation of being trapped.

Links and References

Fleischner Society

air trapping

Pathophysiology.—Air trapping is retention of air in the lung distal to an obstruction (usually partial).

CT scans.—Air trapping is seen on end-expiration CT scans as parenchymal areas with less than normal increase in attenuation and lack of volume reduction. Comparison between inspiratory and expiratory CT scans can be helpful when air trapping is subtle or diffuse (^,11,^,12) (^,Fig 4). Differentiation from areas of decreased attenuation resulting from hypoperfusion as a consequence of an occlusive vascular disorder (eg, chronic thromboembolism) may be problematic (^,13), but other findings of airways versus vascular disease are usually present. (See also mosaic attenuation pattern.)