Air trapping

Air trapping or gas trapping is a pattern seen on expiratory CT and is caused by the relative retention of gas in lung distal to one or more obstructed airways ¹⁴. Expiration allows normal lung to deflate with consequent increased attenuation but air-trapping prevents deflation so that volume and low attenuation persist in areas that are well demarcated by lobular boundaries.

Terminology

Although not in common usage, the term gas trapping is more accurate ¹⁴.

Epidemiology

Air-trapping of limited extent is common in normal individuals, occurring in ~50% of CT thorax examinations ⁶.

Clinical presentation

Mild (<25% parenchyma) air trapping may be asymptomatic or clinically insignificant ⁶.

Pathology

It is usually defined pathophysiologically as the abnormal retention of air within the lung distal to a complete or partial airway obstruction. Decreased ventilation may cause reflex decreased perfusion.

Etiology

The presence of air trapping can arise from a number of causes (the mnemonic HSBC can be used to help remember these) but usually suggests airway disease (often small airways disease). Air trapping can occur in isolation, or in association with bronchiectasis, interstitial lung disease, or rarely tree-in-bud opacity, which can help narrow the etiology ^3,6:

• in isolation

  • chronic bronchitis

  • asthma

  • obliterative bronchiolitis

• with bronchiectasis

  • non-tuberculous mycobacterial infection

  • cystic fibrosis

  • idiopathic bronchiectasis

• with interstitial lung disease

  • sarcoidosis

  • hypersensitivity pneumonitis

• high BMI / obesity ^12,13 

  • maybe due to reduced chest wall compliance prompting a rapid, shallow breathing pattern 

Other uncommon conditions include ^5,6: 

• pulmonary tumourlets

  • neuroendocrine cell proliferation spectrum

  • diffuse idiopathic pulmonary neuroendocrine cell hyperplasia (DIPNECH) ⁷  

• scoliosis: especially when severe

• vasculitides

  • granulomatosis with polyangiitis ¹⁰

  • eosinophilic granulomatosis with polyangiitis ¹⁰

Radiographic features

Plain radiograph

The CXR is typically normal unless there is marked unilateral involvement causing unilateral hypertransradiancy. In this case an end-expiratory CXR will demonstrate maintained volume and radiolucency on the affected side and mediastinal shift towards the normal side.

CT

Air-trapping refers to the well-defined mosaic pattern of varying attenuation seen on expiratory CT caused by obstruction of airways.

The areas of air trapping vary in size depending on the site of airway obstruction ranging from lobular involvement to involvement of an entire lung if the main bronchus is partially occluded.

Reduced perfusion in the affected area is due to reflex vasoconstriction.

The concurrent presence or absence of bronchiectasis or interstitial lung disease may provide clues to the etiology ¹⁰.

Differential diagnosis

• mosaic attenuation due to vascular occlusion disease (e.g. chronic pulmonary embolism) is poorly demarcated

• emphysema.