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Lung ultrasound (LUS) is an emerging bedside imaging tool that uses high-frequency sound waves to assess the lungs and pleura. It’s a safe, radiation-free, and highly reproducible technique that enables clinicians to expedite the diagnosis and monitoring of a range of presentations in critically ill patients. LUS has been shown to have a higher diagnostic accuracy than physical examination and chest radiography combined.1
This guide should be used in conjunction with the basics of ultrasound article.
Ultrasound code of conduct
- POCUS is rarely used to rule out pathology
- POCUS should be used in accordance with local hospital guidelines, which often require credentialing or accreditation
- Ensure patient consent, privacy and comfort throughout
- Document/capture your findings
- Seek help if you are unsure of any findings
- Ultrasound is a radiation-free method of imaging, so the risks to the patient are minimal
- Always use POCUS as part of a wider clinical examination
Indications
When combined with other imaging and POCUS modalities, LUS may enhance decision-making in patients with:
- Pneumothorax
- Pneumonia
- Pulmonary oedema
- Atelectasis
- Pleural effusion
- Acute respiratory distress syndrome (ARDS)
- Circulatory shock
LUS can also assist in guiding pleural procedures, such as thoracocentesis.
Lung ultrasound characteristics
Many of the findings on lung ultrasound can be explained by how ultrasound waves interact with air, interstitial fluid, and tissue. Many of the pathological findings of lung ultrasound are artefacts created by these interactions.
Air
Normal lungs are air-filled and appear grey on ultrasound. This is because air scatters sound waves off solid structures such as the pleura, creating a grey, patchy appearance underneath, that can be punctuated with artefacts, as described below.
Echoes
These occur at boundaries of differing density. In normal lungs:
- Ribs appear as hyperechoic (bright) curved lines with dark shadows beneath
- Pleura appears as a bright line and causes reverberation artefacts, producing repeating horizontal A-lines which indicate a normal air-filled lung
Interstitial fluid
When fluid or pus accumulates in the lung interstitium, ultrasound waves reverberate between the air-fluid interface, producing B-line artefacts.
These artefacts appear as bright vertical lines that originate from the pleural line, extend to the bottom of the screen, obscure normal A-lines, and move with ventilation. This shows that the lung tissue is no longer fully air-filled.
A vs B lines
- A-lines = horizontal lines
- B-lines = vertical lines
Anatomy
Understanding the borders of the lung and its relationships with surrounding anatomy will help to identify crucial landmarks when scanning.
Organ relationships
The lungs sit above the diaphragm, with the liver positioned just below the right lung and the spleen just below the left lung. In lung ultrasound, these organs help identify the diaphragm and confirm probe position, especially when scanning the lung bases or when detecting pleural effusions.
Pleura
The lungs are covered by two pleural layers, visceral (attached to the lung) and parietal (attached to the chest wall), which move against each other during breathing.
The costophrenic angles (lung bases), the junction between the diaphragm and the chest wall, are common sites for fluid collection, making them key areas to assess for pleural effusion on lung ultrasound.
The lung interstitium serves as the supportive framework of the lungs, comprising connective tissue that surrounds the airways, blood vessels, and alveoli.
Preparing the ultrasound machine
1. Turn on the ultrasound machine.
2. Select the ‘lung’ preset if it is available; otherwise, an ‘abdominal’ preset is acceptable.
3. Select the curvilinear probe and ensure it is connected to the machine.
4. Disinfect the probe with a universal disinfectant wipe.
5. Position yourself comfortably, allowing yourself to easily scan the patient with your dominant hand. For right-handed scanners, position yourself on the right side of the patient, with the machine on your left-hand side.
Introduction
Wash your hands using alcohol gel. If your hands are visibly soiled, wash them with soap and water.
Don PPE if appropriate.
Introduce yourself to the patient including your name and role.
Confirm the patient’s name and date of birth.
Briefly explain what the procedure will involve using patient-friendly language.
Example explanation
“I’d like to perform a lung ultrasound to help us assess how your lungs are working. It’s a safe and painless test that uses sound waves to create images. I’ll place some gel on your skin and move a small probe over your chest and sides. There’s no radiation involved, and it only takes a few minutes. It helps us look for things like fluid around the lungs, signs of infection, or collapsed areas.”
Gain consent to proceed with lung ultrasound.
Check if the patient has any allergies (e.g. latex or ultrasound gel)
Expose the patient entirely from the waist upwards. Female patients may keep their bra on if it does not obstruct the scan sites. A chaperone should be offered where appropriate.
Depending on the context and level of comfort, position the patient supine, upright or at 45° on the bed. Raising the patient’s arm above their head on the side being scanned helps widen the intercostal spaces, providing a better acoustic window for imaging. However, this may not always be feasible depending on the patient’s condition.
Ask if the patient has any pain before beginning.
Scanning locations
While structured methods for LUS have been developed (e.g. BLUE protocol), the number of areas to scan in the lung field varies between operator preference and clinical context.1
In this guide, we use a 6-point protocol:
- Four anterior chest points: which image the lung whilst avoiding the heart border
- Two posterolateral chest points: where the horizontal continuation of the lower anterior chest point intersects the posterior-axillary line (PAL) to image the lung bases
At each point, the curvilinear probe is held by the dominant hand, using a ‘pencil’ grip, with the probe indicator towards the patient’s head.
Performing lung ultrasound
Anterior chest points
At each anterior chest point, examine for and note the following.
A-lines
Place the probe in the intercostal space at the anterior chest points, and aim to achieve the ‘batwing’ sign.
The ‘batwing sign’ refers to the appearance of the rib shadows and pleural line, resembling the wings of a bat, when the probe is placed longitudinally on the chest wall. This sign helps confirm correct probe placement.
Start with a depth of between 9 and 11 cm, and in a normal lung, A-lines should be visible.
Lung sliding
Reduce the depth to approximately 5 cm and bring the pleural line into the centre of the image; this will allow for maximum definition.
During normal ventilation, a minute layer of pleural fluid allows the visceral and parietal pleura to ‘slide’ against each other, producing a characteristic ‘marching of ants’ or lung sliding appearance.
Any pathology that separates the pleural layers or prevents normal ventilation, such as pneumothorax or airway obstruction, will stop the movement of the parietal and visceral pleura, which will be evident as an absence of lung sliding.
In some cases, a region of lung sliding may be adjacent to a region of no lung sliding, described as a lung point or the exact boundary of the pneumothorax.
If ‘lung sliding’ is not completely visible, this can be confirmed using M-mode, which tracks a single scan line over a short period of time. Skin, subcutaneous tissue, and muscle above the pleural line are typically stationary during normal ventilation, while the pleura and the tissue below the lung move, creating the seashore sign.
With any pathology affecting lung sliding, the pleural line and lung tissue will also appear stationary, creating the stratosphere or barcode sign.
B-lines
A normal anterior chest point on lung ultrasound should have two or fewer B-lines in a rib space; any more than this is considered pathological. One particular pattern, characterised by more than two B-lines in more than two rib spaces on both lungs, is known as interstitial syndrome. This syndrome is associated with ARDS, pulmonary oedema, and viral pneumonitis.
When pathological B-lines become confluent, they are called lung rockets and suggest more severe interstitial syndrome. Multiple, widespread lung rockets are equivalent to a ground-glass appearance on a chest CT scan, such as in ARDS.
As lung involvement worsens, B-lines can become so numerous that they merge, indicating increasing fluid or tissue density. When more infective tissue begins to replace the air-filled lung, the shred sign appears, which is an irregular, broken-up pleural line indicating the transition between aerated and consolidated lung tissue.
With further progression, the lung may become completely airless, appearing solid on ultrasound, a stage known as hepatisation or a tissue-like sign, because the lung tissue resembles liver tissue.
Posterolateral chest points
Scanning the posterolateral chest points is most useful for identifying pleural effusions or lung consolidation at the lung bases and costophrenic angles. Compared to chest X-ray, lung ultrasound is more sensitive and specific in identifying small amounts of pleural fluid, in particular between the lung and chest wall at the anterior chest points.2
A normal lung base features the costophrenic angle sitting above the diaphragm, with the abdominal organs (liver and spleen) acting as scanning landmarks. It is normal for the costophrenic angle to appear ‘tissue-filled’ due to a mirror image artefact. If pleural effusion is present, this artefact is replaced with anechoic fluid (appearing black).
Whilst visual inspection for pleural fluid in the lung bases is most useful, there are a variety of signs, taught at advanced levels, that can assist in decision making.
A quick start guide
1. Have a clinical question in mind (e.g. Does this patient have a pneumothorax?)
2. Turn on the ultrasound machine
3. Choose a curvilinear/linear probe, the ‘lung’ preset and position your patient (and yourself) comfortably
4. Apply lots of ultrasound gel
5. Optimise your image by manipulating depth and gain
6. Identify the ‘batwing’ sign at each anterior chest point
7. Assess for the presence of lung sliding, A and B-lines at all anterior chest points
8. Perform a visual inspection for pleural fluid at the posterolateral points and be aware of more specific signs.
9. Follow the ultrasound code of conduct
Reviewer
Dr Segun Olusanya
Consultant in Intensive Care Medicine
Editor
Dr Jamie Scriven
References
- Lichtenstein DA, Mezière GA. Relevance of lung ultrasound in the diagnosis of acute respiratory failure: the BLUE protocol. Chest. 2008. Available from: [LINK].
- Jarman RD, McDermott C, Colclough A, et al. EFSUMB Clinical Practice Guidelines for Point-of-Care Ultrasound: Part One (Common Heart and Pulmonary Applications) LONG VERSION. Ultraschall in der Medizin. 2022. Available from: [LINK].
Image references
- Figure 1. AbsurdWordPreferred. Anatomical Model PNG. Adapted by Geeky Medics. License: [CC BY 3.0].
- Figure 2. Cancer Research UK uploader. Diagram of the lung showing the pleura. License: [CC BY-SA 4.0].
- Figure 10. Peter Gutierrez. The Pocus Atlas. Lung point. License: [CC BY-NC 4.0].
- Figure 12. Dr. Eric Roseman. The Pocus Atlas. Pneumothorax: M-mode: Seashore vs Barcode. Adapted by Geeky Medics. License: [CC BY-NC 4.0].
- Figure 13. Dr. Justin Bowra et al. The Pocus Atlas. B-Lines – Pulmonary Edema. Adapted by Geeky Medics. License: [CC BY-NC 4.0].
- Figure 13. Peter Gutierrez. The Pocus Atlas. Confluence of B-Lines. Adapted by Geeky Medics. License: [CC BY-NC 4.0].
- Figure 13. Aaron Inouye. The Pocus Atlas. Classic Findings in Pneumonia. Adapted by Geeky Medics. License: [CC BY-NC 4.0].
- Figure 13. Dr. Sathya Subramaniam. The Pocus Atlas. Lung Hepatization in Pneumonia. Adapted by Geeky Medics. License: [CC BY-NC 4.0].
- Figure 16. Dimitri Livshits et al. The Pocus Atlas. Pleural Effusion with Compressed Lung and Spine Sign. License: [CC BY-NC 4.0].
UltraLearn
The UltraLearn Project is a student-led, clinician-supervised initiative whereby students across UK medical schools gain confidence and interest in the basics of POCUS. Find out how you can join the community @ultralearnpocus on Instagram, X and LinkedIn.
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