POCUS and lung ultrasound: A-lines, B-lines, lung sliding, effusion and the BLUE protocol
What lung POCUS is, how to read A-lines, B-lines, lung sliding and pleural effusion, how the BLUE protocol profiles work, including their limits, and how to document the exam.
LaudárioUltrasound reports in practice
This article is part of the guide Abdominal Ultrasound: A Guide to the Findings That Actually Change Management.
POCUS (point-of-care ultrasound) is ultrasound performed at the bedside to answer a focused clinical question; in the lung it relies on a few findings: lung sliding, A-lines, B-lines, consolidation and effusion. The exam does not replace complete diagnostic ultrasound or CT, and the record must state which question was asked and what was seen. For trauma, see our text on e-FAST, which we do not repeat here.
Lung POCUS: technique
- Convex or linear transducer; the microconvex probe is used in rapid emergency protocols.
- For cardiogenic edema, the 2023 consensus suggests a center frequency of 5 to 6 MHz, without harmonic imaging.
- Focus on the pleural line, identified between the shadows of two ribs (bat sign).
- Scan the largest area the patient's condition allows and record the regions assessed.
Lung sliding
This is the back-and-forth movement of the pleural line with breathing, and it shows the pleurae are in contact at that point: when present, it rules out pneumothorax at that site. On M-mode, the normal pattern is the seashore sign; without sliding, the stratosphere sign appears.
Absent sliding alone does not establish pneumothorax. Diagnosis combines abolished sliding with absent signs of aeration, such as B-lines, and the lung point allows confirmation with high certainty.
A-lines and B-lines on ultrasound
- A-lines: horizontal artifacts, parallel and equidistant to the pleural line. They indicate gas below the pleura and appear in both normal lung and pneumothorax.
- B-lines: vertical hyperechoic laser-like artifacts that arise from the pleural line, move with sliding and erase A-lines.
- Three or more B-lines between two ribs (lung rockets) correlate with interstitial syndrome.
Counting is semi-quantitative and depends on the operator and settings. The 2026 update considers scores most reliable for tracking change and does not yet recommend fixed numerical cutoffs for routine use in prognosis or treatment response. B-lines do not give the cause either: distribution (focal or bilateral, homogeneous or heterogeneous, with or without a gravitational gradient) and the clinical picture guide the reasoning.
Pleural effusion
Ultrasound separates fluid from consolidation and detects significant effusion better than supine radiography. At the posterolateral point, the quad sign is formed by the pleural line, the lung line and the rib shadows; the sinusoid sign is the inspiratory approach of that line on M-mode, and it may be missing in very viscous or septated effusions.
BLUE protocol
Created for acute respiratory failure, it assesses the upper and lower BLUE points on each side, the PLAPS (posterolateral) point and the veins, when indicated.
| Profile | Finding | Diagnosis | Sens./Spec. |
|---|---|---|---|
| A | A-lines with sliding | asthma or COPD | 89% / 97% |
| B | anterior diffuse B-lines with sliding | pulmonary edema | 97% / 95% |
| A with DVT | normal anterior profile and venous thrombosis | pulmonary embolism | 81% / 99% |
| A' with lung point | no sliding, A-lines and a lung point | pneumothorax | 81% / 100% |
| B', A/B, C or PLAPS | no sliding with B-lines, asymmetric pattern, anterior or posterior consolidation | pneumonia | 89% / 94% |
These figures come from 260 ICU patients, with 90.5% overall accuracy, after excluding uncertain and rare diagnoses. The study had no validation group, and critical reviewers expect lower real-world performance.
What the report should include
- Clinical question, context (ventilation, position) and transducer.
- Regions assessed per hemithorax and what could not be seen.
- Sliding, artifact pattern and B-line distribution.
- Effusion with side, appearance and measurement in mm; consolidation with location; BLUE profile, if done.
- Saved clips of dynamic findings, because a still image does not show sliding.
Common mistakes
- Reporting pneumothorax from absent sliding alone.
- Treating B-lines as a synonym for pulmonary edema.
- Practicing without training: the 2023 consensus strongly recommends adequate training before diagnostic use, which also applies when choosing a POCUS course with supervised practice.
See also our text on venous Doppler for suspected DVT.
How Laudário helps
The chest ultrasound template in Laudário was built for this reasoning:
- Pleura tab with sliding, lung pulse, B-line distribution, effusion and pneumothorax.
- BLUE Protocol tab by point, with PLAPS, DVT search and a profile derived automatically or chosen by the physician.
Try it for 15 days free, no credit card required.
References
Lichtenstein DA, Mezière GA. Chest 2008. Lichtenstein DA. Ann Intensive Care 2014. Volpicelli G et al. Intensive Care Med 2012. Demi L et al. J Ultrasound Med 2023. Focused update on point-of-care lung ultrasound, Intensive Care Med 2026.
Physician with an RQE specialty registration in general ultrasound. Writes on the Laudário blog about reports, classifications and standardization. Who writes and how.
Educational content for healthcare professionals. It does not replace clinical judgment or the guidelines adopted by your service. Editorial policy.