Air bronchogram

Last revised by Ian Bickle on 22 Jul 2024

An air bronchogram occurs when endobronchial air is visible against a background of increased lung opacity. Expulsion of gas from the parenchyma is partial or complete and can be due to atelectasis and/or replacement by fluid, inflammatory cells, blood, tumour or interstitial thickening. The persistence of gas in the bronchi implies patency of proximal airways 6.

Air bronchograms can be seen with several processes:

Air bronchograms that persist for weeks despite appropriate antimicrobial therapy should raise the suspicion of a neoplastic process. CT and guided biopsy may be helpful in such cases.

Air bronchograms appear as branching low attenuating (lucent) bronchi surrounded by high attenuating (dense) consolidated/opacified alveoli. Note that in a normal lung, the gas-filled bronchi are usually indistinguishable from the surrounding gas-filled alevoli.

Air bronchograms are more commonly seen in consolidation than atelectasis, although their presence does not exclude atelectasis.

Sonographic air bronchograms arise as a secondary consequence of an extreme perturbation of the air-fluid relationship in the lung parenchyma, in which fluid-filled alveoli act as an excellent acoustic medium and allow visualisation of the lung parenchyma. Arborising tubular structures representing the bronchial tree may be visualised which, when patent, appear to contain punctiform-to-linear foci. These structures may remain fixed in position throughout the respiratory cycle or be observed to propagate distally and proximally with inspiration and expiration, respectively. This distinction is important for determining the aetiology of the underlying pathology 4:

  • dynamic air bronchograms move centrifugally with respiration

    • represent fluid mixed with air inside larger bronchi, which are in continuity with the gas inspired by the patient

    • indicates a non-retractile consolidation, ruling out resorption atelectasis

    • the specificity of 94% and a positive predictive value of 97% for pneumonia as the cause of the consolidation

  • static air bronchograms lack detectable movement 

    • indicate isolated, trapped air, consistent with resorptive atelectasis

Alveolar filling with fluid or cells contrasts with gas in the airways resulting in an air bronchogram.

Consolidation may be acute, or chronic and progressive depending on the cause. Pneumonia is typically acute, resolving within six weeks, however failure to resolve may indicate chronic progressive disease such as adenocarcinoma or lymphoma.

The term air bronchogram was coined by Ben Felson (1913-1988) 5.

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