Lung POCUS Image Interpretation

Lung ultrasound has an odd property that makes it easier to learn than it first appears: aerated lung reflects almost all of the beam, so you are not really imaging the lung at all. You are reading the artefacts that different amounts of air and fluid produce at the pleural line. Once that clicks, the findings stop being pictures to memorise and become consequences of physics.

That framing matters practically. If you know what an artefact requires in order to exist, you can reason about a clip you have never seen before, and you can recognise when your own scanning technique has manufactured a finding that is not there.

If you want moving images rather than descriptions, work through POCUS interpretation cases in the app.

Start at the pleural line, always

Nearly every finding in lung ultrasound is anchored to one structure: the bright horizontal line where the probe's beam meets the pleura. Find it first, centre it, and only then interpret anything else in the image. A clip in which you cannot confidently identify the pleural line is a clip you should not report.

Three questions, in this order, get you most of the way:

1. Is the pleural line moving? The shimmer of the two pleural surfaces sliding against each other with respiration is the fundamental sign. Its presence tells you the surfaces are in contact at that point. Its absence at a given interspace has several explanations, only one of which is pneumothorax.
2. What comes off it vertically? Vertical artefacts extending from the pleural line into the far field indicate an interface between air and something denser in the subpleural space. Their number and distribution is the main quantitative finding in lung ultrasound.
3. What comes off it horizontally? Repeating horizontal lines at intervals matching the pleura-to-skin distance are reverberation artefact from a fully aerated lung, and their presence is broadly reassuring for that interspace.

Note the phrase "at that point" and "at that interspace". Lung ultrasound is regional. A finding describes the small window under your probe, not the lung as a whole, which is why the number of windows you scan determines what your scan can claim.

The four patterns worth knowing well

Consensus recommendations describe a small number of recognisable patterns that cover most of the acute respiratory presentations you will meet. Learn these four properly rather than a longer list superficially.

Aerated pattern

Sliding pleura with horizontal reverberation artefact and few or no vertical artefacts. This is the appearance of normally aerated lung at that interspace.

Interstitial pattern

Multiple vertical artefacts arising from the pleural line, increasing in number as the underlying process worsens. The clinically decisive feature is distribution. A symmetrical, bilateral, gravity-dependent pattern suggests a diffuse process; patchy areas of vertical artefact adjacent to normal-looking areas suggest a focal one. The pattern itself does not name the cause — the distribution plus the history does.

Consolidated pattern

Loss of the normal air interface, with tissue-like appearance in the subpleural space, sometimes with visible bright specks moving within it. Often accompanied by adjacent fluid.

Absent-sliding pattern

Loss of pleural movement with only horizontal artefact. This is the pattern that raises the possibility of air in the pleural space, and it is also the pattern most often produced by technique problems — the probe on a rib, the wrong depth, the patient holding their breath, or an intercostal muscle mistaken for pleura. Confirm it in more than one window before you believe it, and correlate it with the patient.

How many windows, and where

A scan protocol is a claim about coverage. Published approaches divide each hemithorax into zones — commonly anterior, lateral and posterolateral, upper and lower — and the number you scan should follow the question you are asking.

If the question is "is there a large volume of air anteriorly in a supine, deteriorating patient", a small number of anterior windows may be enough to change management immediately. If the question is "does this breathless patient have a diffuse or a focal process", you need enough windows on both sides to establish distribution, and a two-window scan cannot answer it.

Whatever protocol you use, record which zones you scanned. A documented six-zone scan reported as negative means something specific. "Lung ultrasound normal" means very little, because the reader cannot tell whether you looked at the area where the finding would have been.

Errors that manufacture findings

Most false findings in lung ultrasound come from acquisition rather than interpretation. These are the ones worth actively checking for.

• Depth set too shallow or too deep. Too shallow and vertical artefacts are truncated so you undercount them; too deep and the pleural line becomes a thin bright band with little detail.
• Probe on a rib. Rib shadow removes pleural movement in that part of the image and readily imitates absent sliding. Angle so that you have pleura between two rib shadows.
• Excess gain. Turning gain up until the image looks "clearer" generates noise that resembles vertical artefact. Set gain so that fluid-filled structures are genuinely black.
• Too short a clip. Sliding is a movement finding. A one-second clip in a patient with shallow respiration can miss it entirely. Record several respiratory cycles.
• Scanning only where it is easy. The anterior chest is comfortable to reach in a supine patient. Gravity-dependent findings collect posteriorly, so a comfortable scan is frequently an incomplete one.
• Subcutaneous air. Air in the soft tissues obscures everything deeper and produces an unreadable study, not a negative one. Say so rather than reporting a normal scan.

Interpretation is the scan plus the patient

The same interstitial pattern can appear in several very different clinical problems. Lung ultrasound does not distinguish between them on its own; it constrains the differential, and the history, examination, observations and other bedside findings do the rest.

This is why practising on isolated clips has limited value beyond the first few hours of learning. The skill being tested at the bedside is the combination: does this pattern, in this distribution, in this patient, with this trajectory, change what I do in the next ten minutes?

It is also why scope discipline matters. Point-of-care ultrasound is performed and interpreted by the clinician managing the patient, for a focused question, within a defined level of training and local governance. It does not replace formal imaging, formal reporting, or the process of getting a scan reviewed by someone with more experience. Where your training and local policy do not cover a question, the honest answer is that your scan cannot answer it.

A practice routine that builds real competence

Three habits, in order of return on effort.

Scan normal chests deliberately. Most of what makes an abnormal finding obvious is a strong internal model of normal. Scan colleagues, with consent, until the aerated pattern and pleural movement are instantly recognisable at every zone.

Commit to an interpretation before revealing the answer. Write down zone-by-zone description, image quality, and conclusion. Compare your reasoning, not just your verdict.

Practise on clips of variable quality. Teaching libraries curate towards clear examples. Real studies are frequently mediocre, and recognising an uninterpretable clip is one of the more valuable things you can learn. Cases that pair an average clip with a decision are better preparation than a gallery of perfect ones.