Air trapping

Air trapping in chest imaging refers tois the retention of excess gas (“air”)in lung distal to one or more obstructed airways. Subnormal reduction in all or part ofvolume and subnormal increase in attenuation on end-expiratory CT are diagnostic findings and the lung, especially during expiration, either as a result of complete or partial airway obstruction or as a result of local abnormalities in pulmonary complianceaffected areas are typically sharply demarcated. It may also sometimes be observed in normal individualsReactive vasoconstriction is often apparent ³¹⁴.

Terminology

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

Epidemiology

Air trapping-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 can be used to help remember these) but but usually suggests airway disease (often small airways disease). Air Air trapping can occur in isolation, or in association with bronchiectasis, interstitial lung disease, or 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 especially when severe

• vasculitides

  • granulomatosis with polyangiitis¹⁰

  • eosinophilic granulomatosis with polyangiitis ¹⁰

Radiographic features

CT

Air trapping is a descriptor used in lungcan only be applied to expiratory CT seen as a decreasedappearances of subnormal attenuation of pulmonary parenchyma, especially manifested as a less than normal increase in attenuation during expiratory acquisition¹⁴. This appearance must be differentiated from the decreased attenuation of hypoperfusion secondary to locally increased pulmonary arterial resistance ¹vascular occlusion in CTEPH which may be less well demarcated.

The concurrent presence of absence or bronchiectasis and interstitial lung disease may be usefulprovide clues to narrow the differential possibilitiesdiagnosis ¹⁰.