Air trapping

Air trapping in chest imaging refers to retention of excess gas (“air”) in all or part of the lung, especially during expiration, either as a result of complete or partial airway obstruction or as a result of local abnormalities in pulmonary compliance. It may also sometimes be observed in normal individuals ³.

Terminology

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

Epidemiology

Air trapping is common, occurring in ~50% of CT thorax examinations ⁶.

Clinical presentation

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

Pathology

CausesAetiology

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 aetiology ^3,6:

• in isolation
  • chronic bronchitis
  • asthma
  • bronchiolitis obliterans
  • chronic bronchitis
• with bronchiectasis
  • non-tuberculous mycobacterial infection
  • cystic fibrosis
  • idiopathic bronchiectasis
• with interstitial lung disease
  • sarcoidosis
  • hypersensitivity pneumonitis

Other uncommon conditions include ^5,6: 

• pulmonary tumour letstumourlets: 
  • 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

CT

Air trapping is a descriptor used in lung CT seen as a decreased attenuation of pulmonary parenchyma, especially manifested as a less than normal increase in attenuation during expiratory acquisition. This appearance must be differentiated from the decreased attenuation of hypoperfusion secondary to locally increased pulmonary arterial resistance ¹.

The concurrent presence of absence or bronchiectasis and interstitial lung disease may be useful to narrow the differential possibilities ¹⁰.