Curriculum · Clinical Ultrasound: Multisystem Diagnostic & Point-of-Care Sonography
Lung & Pleural Ultrasound and the BLUE Protocol
Reading the lung by its artifacts: pleural line, A-lines, lung sliding (seashore vs barcode), B-lines and interstitial syndrome, the lung point (pneumothorax), pleural effusion — assembled into the BLUE protocol for acute dyspnea.
~40 min · level: advanced · POCUS draft — pending clinical review
Learning objectives
- Identify the pleural line, A-lines, and lung sliding (and seashore vs barcode/stratosphere on M-mode).
- Define a B-line and the spectrum from normal to interstitial syndrome, with quantification.
- Recognize pneumothorax (absent sliding, absent B-lines, lung point) and a pleural effusion.
- Apply the BLUE-protocol profiles to the patient with acute respiratory failure.
The normal lung is full of air, which reflects and reverberates sound — so lung ultrasound is read almost entirely from artifacts (this is the Artifacts course applied clinically). The diagnostic currency is the balance of air vs fluid at the pleural surface.
| Sign | Appearance | Meaning |
|---|---|---|
Pleural line | Bright horizontal line below the ribs | The visceral–parietal pleural interface |
A-lines | Horizontal lines repeating at pleural-line depth | Reverberation artifact → air below pleura (normal OR pneumothorax) |
Lung sliding | Shimmering 'ants marching' at pleura | Pleurae sliding → rules out pneumothorax AT that spot |
Seashore sign | M-mode: waves over sand | Normal sliding present |
Barcode / stratosphere | M-mode: all horizontal lines | Absent sliding (pneumothorax, etc.) |
B-lines | Vertical, laser-like, to screen bottom | Interstitial fluid/thickening (≥ 3 per field = pathologic) |
Lung point | Alternating sliding/no-sliding | Boundary of a pneumothorax (highly specific) |
A discrete vertical, hyperechoic, laser-like artifact that arises from the pleural line, erases A-lines, extends to the bottom of the screen without fading, and moves with lung sliding. (Anything failing these — fading, not from pleura, not moving — is a mimic, e.g., a comet tail or Z-line.)
The next clip shows multiple B-lines arising from the pleural line — the pattern of interstitial syndrome. Slow the playback to 0.5× and watch them sweep with sliding; toggle the labels to check yourself.
Reveal findings
- ≥ 3 B-lines in a single intercostal scan = a positive region; multiple positive regions bilaterally = diffuse interstitial syndrome.
- Diffuse, bilateral, symmetric B-lines → cardiogenic pulmonary edema (or diffuse interstitial disease); focal B-lines → pneumonia/contusion/infarct.
- B-lines erase A-lines because fluid in the subpleural interstitium changes the acoustic mismatch — fewer pure-air reverberations.
Several vertical, laser-like artifacts arise from the pleural line, reach the bottom of the screen without fading, and move with lung sliding. The most appropriate interpretation is:
Pneumothorax abolishes lung sliding and B-lines (only A-lines remain), but absent sliding is non-specific (mainstem intubation, apnea, pleural adhesions, ARDS). The pathognomonic confirmation is the lung point — the exact location where a collapsed lung intermittently re-contacts the chest wall, so sliding/no-sliding alternate. The clip below demonstrates it.
Reveal findings
- Lung sliding present on one side of the field and absent on the other, alternating with the respiratory cycle = the lung point.
- The lung point marks the boundary of the pleural air collection; its position helps gauge pneumothorax size.
- On M-mode you would see the seashore sign transition to a barcode sign at the lung point.
You find a spot where lung sliding is present, then absent, alternating with breathing. This 'lung point' is:
| Profile (anterior) | Lung sliding | Add-on | Most likely |
|---|---|---|---|
B-profile (bilateral B-lines) | Present | — | Pulmonary edema |
A-profile (bilateral A-lines) | Present |
| Pulmonary embolism |
A-profile | Present | No DVT, + PLAPS | Pneumonia |
A-profile (nude) | Present | No DVT, no PLAPS | COPD / asthma |
A′-profile | Absent + lung point | — | Pneumothorax |
B′ / A-B / C-profile | Variable | Consolidation/asymmetry | Pneumonia |
Worked example — acute dyspnea
A dyspneic patient has bilateral anterior B-lines with preserved lung sliding and a plethoric IVC. What is the leading diagnosis and the matching BLUE profile?
Bilateral B-lines with sliding = the B-profile, which maps to cardiogenic pulmonary edema; the plethoric IVC (high right-atrial pressure) supports a cardiac/volume-overloaded etiology. Next steps integrate cardiac views (LV function) and clinical context.
Key takeaways
- A true B-line is a vertical, hyperechoic, laser-like artifact that arises from the pleural line, erases A-lines, reaches the bottom of the screen without fading, and moves with lung sliding; mimics like comet tails and Z-lines fade or fall short.
- Three or more B-lines in a single intercostal field define a positive region for interstitial syndrome, and bilateral diffuse positivity points to cardiogenic pulmonary edema while focal B-lines suggest pneumonia, contusion, or infarct.
- The presence of lung sliding or B-lines at a spot excludes pneumothorax there; absent sliding (barcode/stratosphere on M-mode) is non-specific, so the lung point is the highly specific confirmation of pneumothorax.
- BLUE-protocol anterior profiles guide acute respiratory failure: B-profile (bilateral B-lines, sliding) = pulmonary edema, A-profile + DVT = PE, A-profile + PLAPS = pneumonia, nude A-profile = COPD/asthma, and A-prime (absent sliding + lung point) = pneumothorax.
- The BLUE protocol reaches about 90.5% accuracy for the cause of acute respiratory failure and lung ultrasound outperforms supine chest radiography for pneumothorax, effusion, and interstitial syndrome.
Check your understanding
Registry-style items with worked rationales.
1On M-mode, the 'seashore sign' indicates:recall
2Which single finding is the most specific for pneumothorax?analysis
3Three or more B-lines in a single intercostal field most directly indicate:application
4Bilateral anterior A-lines with preserved lung sliding, no DVT, and no PLAPS (the 'nude A-profile') most suggests:analysis
Go deeper — trusted free resources
Hand-picked, verified links to authoritative open resources. Opens in a new tab.
Walks through Lichtenstein's BLUE protocol decision tree, scanning points, and the lung profiles used to diagnose causes of acute respiratory failure.
Defines the B-line comet-tail artifact and its diagnostic criteria for interstitial syndrome and pulmonary edema, contrasted with normal A-lines.
Emergency-medicine reference covering lung sliding, A-lines, B-lines, pneumothorax, lung point, and pleural effusion with image-based scanning technique.
Clinical breakdown of the sonographic signs of pneumothorax including absent lung sliding, the stratosphere sign, and the pathognomonic lung point.
References
- Lichtenstein DA, Mezière GA. Relevance of lung ultrasound in the diagnosis of acute respiratory failure: the BLUE protocol. Chest. 2008;134(1):117-125.
- Volpicelli G, Elbarbary M, Blaivas M, et al. International evidence-based recommendations for point-of-care lung ultrasound. Intensive Care Med. 2012;38(4):577-591.
- American College of Emergency Physicians. Ultrasound Guidelines: Emergency, Point-of-care, and Clinical Ultrasound Guidelines in Medicine. 2016/2023.
- Moore CL, Copel JA. Point-of-care ultrasonography. N Engl J Med. 2011;364(8):749-757.