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 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.

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

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.