• Etymology

  • Derived from the word air, referring to gaseous content, and trapping, indicating the retention or inability to expel air.

  AKA

  • Air retention
  • Trapped lung air

  Definition

  • What is it? Air trapping refers to the abnormal retention of air in the lungs or specific lung regions during expiration, often visible on imaging as areas of reduced lung density that do not empty fully.
  • Caused by:
    • Obstructive airway diseases such as asthma, chronic obstructive pulmonary disease (COPD), and bronchiolitis.
    • Small airway obstruction from inflammation, fibrosis, or foreign bodies.
    • Mechanical factors like airway collapse or external compression.
  • Resulting in:
    • Structural changes: Hyperinflated lung regions, mosaic attenuation on imaging, or lobar volume changes.
    • Pathophysiology: Airflow obstruction prevents complete expiration, leading to localized or generalized air retention.
    • Pathology: Hyperinflation and trapping of air in alveoli due to partial airway obstruction.
  • Diagnosis:
    • Clinical: Symptoms such as wheezing, dyspnea, or exercise intolerance, often varying by underlying condition.
    • Radiology: Detected on expiratory CT as areas of low attenuation relative to normal lung parenchyma. May also appear as hyperlucent regions on chest X-ray.
    • Labs: Generally non-specific but may include arterial blood gas abnormalities in severe cases.
  • Treatment: Management focuses on addressing the underlying cause, such as bronchodilators for obstructive airway diseases, anti-inflammatory treatments, or removal of mechanical obstructions.

  Radiology

  • CXR
    • Findings: Hyperlucent lung regions, flattened diaphragms, or increased retrosternal airspace in severe cases.
    • Associated Findings: Hyperinflation or signs of associated airway disease (e.g., bronchial wall thickening).
  • CT
    • Parts: Segmental or lobar regions of reduced attenuation, often seen during expiratory imaging.
    • Size: Variable, from small localized areas to extensive lobar or whole-lung involvement.
    • Shape: Irregular or lobular regions of low attenuation.
    • Position: Often distributed in dependent lung regions but varies by disease.
    • Character: Persistent low attenuation on expiratory CT compared to inspiratory scans.
    • Time: Chronic in obstructive diseases, transient in reversible conditions.
    • Associated Findings: Mosaic attenuation, bronchial wall thickening, or evidence of airway narrowing.
  • Other Imaging Modalities
    • MRI may demonstrate areas of low signal intensity corresponding to trapped air but is less commonly used.
    • Ventilation-perfusion (V/Q) scans may show perfusion defects without corresponding ventilation abnormalities.

    • Pulmonary function tests (PFTs) in air trapping often demonstrate an obstructive pattern, with decreased FEV1, a reduced FEV1/FVC ratio, and elevated residual volume (RV) or total lung capacity (TLC).

  Key Points and Pearls

  • Air trapping is a hallmark of obstructive airway diseases and is best assessed on expiratory CT.
  • Mosaic attenuation on imaging may help distinguish air trapping from vascular causes of hypoperfusion.
  • The extent and distribution of air trapping can guide diagnosis and management of underlying conditions.
  • Early recognition of small airway disease through imaging can prevent progression to advanced obstructive lung diseases.

  

Air Trapping
Artistically rendered depiction of air trapping in the upper lung fields characteristic of emphysema
Ashley Davidoff MD art TheCommonVein.net

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

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.)