Air Bronchogram
On this page
Direct answer
An air bronchogram is an air-filled, patent bronchus rendered visible because the surrounding alveoli have been filled by fluid or exudate; it is the signature of alveolar (air-space) disease rather than interstitial disease. Consolidation is pathological air-space opacification, and because the bronchi remain open there is classically no volume loss. Its causes — lobar pneumonia, pulmonary oedema, ARDS and pulmonary haemorrhage — are separated by distribution and by the clinical setting rather than by the shadow alone.
What you must remember
- Air bronchogram equals alveolar filling with a patent airway: pneumonic consolidation, pulmonary oedema, ARDS, pulmonary haemorrhage and alveolitis; it argues against pleural fluid and against complete collapse with absorbed air.
- The silhouette sign (Felson) localises disease: loss of the right heart border means middle lobe disease, loss of the left heart border means lingular disease, and loss of a sharp hemidiaphragm means lower lobe disease — an opacity that obliterates a border touches that structure.
- Consolidation shows no mediastinal shift or fissure displacement (no volume loss); collapse shows volume loss with fissure deviation, crowded vessels and shift of the mediastinum towards the lesion.
- Lobar pneumonia is homogeneous and confined by fissures; bronchopneumonia is patchy and multifocal; central bat-wing consolidation that spares the periphery suggests pulmonary oedema.
- Interstitial pattern is different in kind: reticular or reticulonodular opacities, Kerley B lines (short, peripheral, basal linear opacities) and ground-glass haze rather than confluent air-space shadowing.
- Cavitation within consolidation narrows the cause to tuberculosis, staphylococcal or klebsiella pneumonia, anaerobic abscess or infarct — a favourite follow-on question.
- A unilateral white-out must be interpreted with the mediastinum: shifted away suggests effusion, shifted towards suggests collapse, and central (unshifted) suggests consolidation or post-pneumonectomy with a thoracotomy clue.
Common confusion
The classic confusion is between consolidation and collapse: both produce opacity, but only collapse loses volume, and an air bronchogram within the opacity favours consolidation with a patent bronchus. Students also overcall interstitial markings as consolidation; Kerley lines, fine reticulation and ground glass belong to the interstitial compartment, where air bronchograms are characteristically absent. Finally, an air bronchogram is not specific for infection — oedema and haemorrhage produce it too, so the vignette's temperature, orthopnoea or haemoptysis does the discriminating, not the radiograph.
Exam-focused takeaway
NEET-PG frames this topic as three question types: the mechanism stem (air bronchogram indicates patent bronchi with alveolar filling), the silhouette stem (which lobe is involved when a named border is lost), and the white-out stem (effusion, collapse or consolidation decided by mediastinal position). Kerley B lines as a one-liner for pulmonary oedema and cavitation within consolidation as a one-liner for tuberculosis or staphylococcal pneumonia recur constantly. Learn the compartment logic — alveolar versus interstitial — as the first sorting step, and every subsequent option in these stems falls into place.
Frequently asked questions
What does an air bronchogram indicate?
Alveolar (air-space) filling with a patent airway — classically consolidation, and also pulmonary oedema, ARDS or haemorrhage; it is not a feature of pure interstitial disease.
How does the silhouette sign localise a lesion?
An opacity that obliterates the border of a structure must be in contact with it: right heart border loss means middle lobe, left heart border loss means lingula, and diaphragmatic loss means lower lobe.
How are consolidation and collapse distinguished radiographically?
By volume: consolidation preserves lung volume with no fissure or mediastinal shift, while collapse shows fissure deviation, crowding of markings and mediastinal shift towards the opaque side.
What are Kerley B lines?
Short, horizontal, peripheral basal linear opacities representing thickened interlobular septa, classically of pulmonary oedema and also of lymphangitis carcinomatosa.
Which consolidations cavitate?
Tuberculosis, staphylococcal and klebsiella pneumonia, anaerobic lung abscess and pulmonary infarction — cavitation within consolidation should always prompt this differential.
How is a unilateral white-out approached?
By the mediastinum: shifted away indicates a large effusion, shifted towards indicates collapse, and an unshifted mediastinum suggests consolidation, ARDS of one lung or post-pneumonectomy change.