Air trapping

Last revised by Liz Silverstone on 14 Mar 2024

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.

Although not in common usage, the term gas trapping is more accurate 14.

Air-trapping of limited extent is common in normal individuals, 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 of subnormal attenuation of pulmonary parenchyma on expiratory CT 14. This appearance must be differentiated from the decreased attenuation of hypoperfusion secondary to CTEPH which may be less well demarcated. The concurrent presence or absence of bronchiectasis and interstitial lung disease may provide clues to the etiology 10.

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