Lung ultrasound: A-lines, B-lines and lung sliding
- Finding
Lung ultrasound reads patterns of artefacts at the pleura to tell normal lung from air leak, fluid or wet lung.
What it is
Aerated lung blocks ultrasound, so lung ultrasound reads the pleural line and the artefacts it makes. With the probe across two ribs, the bright pleural line lies just below the rib shadows. In normal lung it shimmers with breathing (lung sliding) and throws regular horizontal copies of itself (A-lines). B-lines are vertical bright lines starting at the pleura and running to the bottom of the screen; several in one space mean fluid or thickening in the interstitium. Free air between the pleural layers stops sliding and B-lines, and the point where normal lung touches the air pocket (the lung point) confirms pneumothorax. Fluid and consolidation, which let sound through, are seen directly. It is more sensitive than a supine chest X-ray for pneumothorax and effusion.
What it looks like
- Normal: pleural line sliding with breathing plus A-lines; in M-mode the seashore sign (waves above, sand below).
- Pneumothorax: no sliding, no B-lines, A-lines only; in M-mode the barcode (stratosphere) sign of flat lines throughout.
- Lung point: where sliding lung meets the still air pocket; it confirms pneumothorax.
- B-lines: laser-like vertical lines from the pleura to the bottom of the screen that move with sliding and erase A-lines; three or more per space is abnormal.
- Pleural effusion: black space above the diaphragm, with the spine visible above the diaphragm (spine sign) and lung floating in it.
- Consolidation: lung that looks like liver (tissue-like), often with bright air bronchograms and a ragged deep edge (shred sign).
Why it looks that way
- Air rises: in a supine patient a pneumothorax gathers at the front of the chest, so scan the anterior chest first.
- A-lines are reverberations between the probe and the very reflective pleura; they are present in normal lung and in pneumothorax.
- B-lines arise when thickened, fluid-filled interlobular septa sit against aerated lung.
- Fluid and consolidated lung transmit sound, so they show as real structures instead of artefacts.
Differential and pitfalls
- Seeing sliding, B-lines or a lung pulse under the probe rules out pneumothorax at that spot.
- Absent sliding is not specific: right mainstem intubation, apnoea, pleural adhesions, previous pleurodesis and severe ARDS also stop it.
- Many B-lines on both sides suggest pulmonary oedema or interstitial syndrome; patchy or one-sided B-lines with consolidation suggest pneumonia.
- Look at the sides and bases too: effusions and consolidation gather in the dependent, posterior lung, not at the front.
- Subcutaneous emphysema blocks the view and can create false lines.
- Tension pneumothorax is a clinical diagnosis: do not delay decompression for a scan.
Next step
- Suspected pneumothorax without sliding: search for the lung point; if found, treat as pneumothorax according to size and stability.
- Unstable patient with absent sliding and clinical tension: immediate needle or finger decompression, then a chest drain.
- Effusion seen: mark the site or drain under ultrasound guidance if indicated, and send fluid studies.
- Diffuse bilateral B-lines: assess heart function and volume status (focused cardiac scan, IVC) and treat the cause.
- Uncertain findings: chest X-ray, or chest CT if the patient is stable; ask the radiologist.
Test yourself
Think of the answer, then tap the question to see it.
What does the M-mode barcode (stratosphere) sign indicate?
Absence of lung sliding: flat horizontal lines throughout, seen in pneumothorax but also in apnoea, adhesions or mainstem intubation.
Which lung ultrasound sign confirms pneumothorax?
The lung point, where sliding lung meets the motionless air pocket during breathing.
What are B-lines and what do many of them suggest?
Vertical bright lines from the pleura to the screen bottom that move with sliding; three or more per space suggest interstitial fluid, as in pulmonary oedema.
Why scan the anterior chest first for pneumothorax in a supine patient?
Free air rises to the least dependent part of the chest, which in a supine patient is the front.
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Connections
Educational content; it does not replace a radiologist's report and is not a clinical instruction. Each study is ordered on the patient's assessment and your institution's protocols.
Sources: Thoracic and Lung Ultrasound (StatPearls (NCBI Bookshelf), 2023); Acute Pneumothorax Evaluation and Treatment (StatPearls (NCBI Bookshelf), 2025).
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Representative model, for educational purposes only.