What is the air bronchogram (radiologic finding) sign?

What is the Air Bronchogram Sign?

The air bronchogram sign is a radiologic finding where air-filled bronchi remain visible as branching lucent structures within areas of consolidated or opacified lung tissue, indicating that the airways are patent while the surrounding alveoli are filled with fluid, pus, blood, cells, or other material. 1

Radiologic Appearance

On Chest Radiography

• Appears as branching radiolucent (dark) tubular structures representing air-filled bronchi surrounded by consolidated lung tissue 1
• Unilateral air bronchograms have the best predictive value for pneumonia among all radiographic signs in ICU patients 1
• A single air bronchogram has 96% specificity for pneumonia when present 1

On CT Imaging

• Visualized as air-containing bronchi within areas of consolidation or ground-glass opacity 1
• Can appear as patchy consolidation with air bronchograms, particularly in organizing pneumonia where they typically occur in subpleural locations 1
• In COVID-19 pneumonia, sign of intra-bronchial air-bronchogram appears during the ultra-early stage within 1-2 weeks of exposure 1

On Lung Ultrasound

• Two distinct patterns exist: 1

  • Static air bronchogram: Non-moving hyperechoic linear structures within consolidated lung, seen in both atelectasis and pneumonia 1, 2
  • Dynamic (linear-arborescent) air bronchogram: Hyperechoic structures that move synchronously with tidal ventilation and mimic anatomical airway branching patterns 1

• Dynamic air bronchograms are highly specific for pneumonia (community-acquired or ventilator-associated, depending on context) and indicate patent main airways 1, 2

• Appear as strong linear echoes with characteristic air artifacts in 92.3% of patients with pneumonia 3

Clinical Significance

Indicates Patent Airways

• The presence of air bronchograms confirms that the bronchi remain open and patent while surrounding alveolar spaces are filled 1
• Dynamic air bronchograms on ultrasound specifically indicate patent main airways and help differentiate pneumonia from atelectasis 1, 2

Diagnostic Value by Pathology

Pneumonia (Most Common):

• Air bronchograms are characteristic of alveolar filling processes where infection fills alveoli while bronchi remain open 1
• In rapid progression stage of COVID-19 pneumonia (3-7 days after symptom onset), CT shows fused large-scale consolidation with air-bronchogram inside 1

Other Conditions:

• Alveolar proteinosis and bronchioloalveolar carcinoma: Classic alveolar filling processes 4
• Interstitial diseases: Can occur due to compressive atelectasis causing tissue crowding around open airways, or obstructive pneumonia in conditions like sarcoidosis and lymphoma 4
• Mixed processes: Pneumocystis carinii pneumonia and late-stage hemosiderosis may show air bronchograms when alveolar filling masks underlying interstitial disease 4

Distinguishing Features on CT

• Shape and lumen characteristics of bronchi with air bronchogram sign can differentiate lung cancer, tuberculosis, and pneumonia 5
• Length of involved bronchus and ratio to lesion length help distinguish lung cancer from tuberculosis and pneumonia 5
• In bronchioloalveolar carcinoma, air bronchograms can occur even in non-secretory tumors, not just through mucus-filled alveoli 6

Common Pitfalls

• Do not assume all air bronchograms indicate pneumonia: They can occur in atelectasis, pulmonary edema, ARDS, and various tumors 4, 6
• On ultrasound, distinguish static from dynamic air bronchograms: Only dynamic air bronchograms are highly specific for pneumonia rather than atelectasis 1, 2
• Poor quality portable chest radiographs may miss air bronchograms; CT is more sensitive for detection 1
• Air bronchograms alone do not exclude interstitial disease: Mixed alveolar-interstitial processes can produce this sign 4