Air trapping

Air trapping is the retention of excess gas in lung distal to one or more obstructed airways. Subnormal reduction in volume and subnormal increase in attenuation on end-expiratory CT are diagnostic findings and the affected areas are typically sharply demarcated. Reactive vasoconstriction is often apparent ¹⁴.

Terminology

Although not in common usage, the term gas trapping is more accurate ^ref14.

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.

Aetiology

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

  • 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 can only be applied to expiratory CT appearances of subnormal attenuation of pulmonary parenchyma ¹⁴. This appearance must be differentiated from the decreased attenuation of hypoperfusion secondary to vascular occlusion in CTEPH which may be less well demarcated. The concurrent presence of absence or bronchiectasis and interstitial lung disease may provide clues to the diagnosis ¹⁰.