Air trapping

Last revised by Yuranga Weerakkody on 4 May 2023

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.

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

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

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

It is usually defined pathophysiologically as the abnormal retention of air within the lung distal to a complete or partial airway obstruction.

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:

Other uncommon conditions include 5,6

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.

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Cases and figures

  • Case 1: focal air trapping in bronchial atresia
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  • Case 2: due to rheumatoid arthritis
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  • Case 3: post transplant bronchiolotis obliterans
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