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, tumor or interstitial thickening. The persistence of gas in the bronchi implies patency of proximal airways 6.
Air bronchograms can be seen with several processes:
pulmonary edema: especially with alveolar edema 3
non-obstructive atelectasis
severe interstitial lung disease
neoplasms: adenocarcinoma; pulmonary lymphoma
normal expiration
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.
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Radiographic features
Plain radiograph
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.
Ultrasound
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 visualization of the lung parenchyma. Arborising tubular structures representing the bronchial tree may be visualized 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 etiology of the underlying pathology 4:
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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
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static air bronchograms lack detectable movement
indicate isolated, trapped air, consistent with resorptive atelectasis
CT
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.
History and etymology
The term air bronchogram was coined by Ben Felson (1913-1988) 5.