Scan longitudinally across two ribs. The rib shadows are the bat's wings and the pleural line, about 0.5 cm below the rib line in adults, is its body. Always find the bat sign first, so you know you are looking at the pleura and not at a rib or a fascial plane.

The pleural line is the bright line where parietal and visceral pleura meet. Air in the lung beyond it reflects almost all of the beam.

A-lines are horizontal, bright, evenly spaced reverberation artefacts: the beam bounces between the pleura and the probe face, so each A-line sits at a whole multiple of the skin-to-pleura distance. A-lines mean air lies below the pleura: normal aerated lung, or free air in a pneumothorax. A-lines by themselves cannot tell the two apart; sliding can.

Discrete, laser-like, vertical, hyperechoic reverberation artefacts that:

• arise from the pleural line,
• extend to the bottom of the screen without fading,
• erase A-lines where they cross, and move with sliding when sliding is present.

This is the conventional description, not a hardware-independent definition. Depth, gain, frequency and processing change their appearance; B-lines may persist when sliding is absent (Demi 2023).

They reflect altered air–tissue interfaces near the lung surface: oedema, inflammation, fibrosis and loss of aeration can all produce them. A few isolated basal B-lines can be normal. The traditional 8-region method calls a longitudinal intercostal view with ≥3 B-lines positive; this is a protocol threshold, not a universal diagnosis.

Z-lines (and short I-lines) are short, ill-defined and fade within a few centimetres; they do not erase A-lines and are not B-lines. Older papers, including Lichtenstein 1997 and 1999, use 'comet-tail artefact' to mean the B-line itself. E-lines come from subcutaneous emphysema and start above the pleural line. A pleural line that cannot be seen means you cannot interpret B-lines.

• High-frequency linear (about 5–13 MHz), depth 4–6 cm: the best view of the pleural line, sliding, lung point and small subpleural consolidations. The usual choice in theatre and after procedures.
• Curvilinear or microconvex (about 2–6 MHz), depth 12–18 cm: B-lines across the whole field, consolidation, effusion and the PLAPS point. A good all-round lung probe; Lichtenstein uses a microconvex for lung, heart and veins.
• Phased array (cardiac): usable when it is the only probe on the machine, but its narrow near field shows the pleura poorly.

Choose a lung preset if the machine has one. Otherwise turn off tissue harmonics, compound imaging and heavy smoothing or persistence filters, because they suppress or blur artefacts. Set the focus at the pleural line.

Longitudinal (probe marker towards the head) across two ribs to find the bat sign. Then, if needed, rotate along the intercostal space to see more pleura.

Place both hands on the chest, thumbs excluded. The upper hand's little finger touches the lower edge of the clavicle, and the fingertips reach the midline.

• Upper BLUE point: between the bases of the middle and ring fingers of the upper hand.
• Lower BLUE point: middle of the palm of the lower hand.
• Phrenic line: the lower edge of the lower hand, roughly where the diaphragm sits.
• PLAPS point (posterolateral alveolar and/or pleural syndrome): where a horizontal line from the lower BLUE point meets the posterior axillary line. Scan as far posteriorly as the bed allows.

• 8 regions (Volpicelli 2012): anterior and lateral, upper and lower, on each side. Used to call interstitial syndrome.
• 12 regions: adds posterior upper and lower on each side, bounded by the parasternal, anterior axillary and posterior axillary lines. Used for the LUS score.

Air rises and fluid falls. Look for pneumothorax at the anterior points (non-dependent in a supine patient) and for effusion and consolidation posteriorly at the bases.

• Lung sliding: a shimmer, like ants marching along the pleural line, as the visceral pleura moves against the parietal pleura with each breath.
• A-lines, with 2 or fewer B-lines.

Seashore sign. Put the M-mode cursor through the pleural line between two ribs. Above the pleura, the static chest wall makes flat horizontal lines (the waves). Below it, the moving lung makes a granular, sandy pattern (the beach).

Barcode (stratosphere) sign. Horizontal lines above and below the pleural line, so no sandy texture. It means there is no movement at the pleural line. This fits pneumothorax but is not proof of it.

When the lung is not ventilated but still lies against the chest wall, the heartbeat moves the pleural line in time with the ECG. A lung pulse shows the two pleural layers are touching, so there is no pneumothorax under the probe. Typical settings: apnoea, endobronchial intubation, complete atelectasis (Lichtenstein 2003).

• Present: no pneumothorax at that point.
• Absent, with a lung pulse or B-lines: no pneumothorax at that point (the lung touches the wall but is not moving).
• Absent, with neither: pneumothorax is possible; look for a lung point.

• Lung sliding. In Lichtenstein's ICU series (1995), preserved sliding ruled out pneumothorax at the point scanned (NPV 100%). Absent sliding was sensitive (95.3%) but less specific (91.1% in that cohort, and 78% in a CT-controlled series of occult pneumothorax in critically ill patients, including trauma; Lichtenstein 2005).
• B-lines. They arise from the visceral pleura, so they cannot be present where free air separates the two layers (Lichtenstein 1999).
• Lung pulse.
• Consolidation reaching the pleura, or an effusion with lung visible floating in it (sinusoid sign), at that point.

Beware hydropneumothorax: air sits anteriorly above the fluid level (look for a 'hydro-point'). In trauma, see eFAST for haemothorax and traumatic pneumothorax.

Absent sliding, A-lines only (the A'-profile anteriorly), barcode on M-mode. Sensitive, but not specific: see the pitfalls slide.

At the edge of the pneumothorax, the lung intermittently touches the chest wall during inspiration. In one view you see sliding (seashore) alternating with no sliding (barcode) at the same spot. In M-mode this is a seashore-barcode alternation in time with breathing. The original study reported 100% specificity and about 66% sensitivity (Lichtenstein 2000). A large pneumothorax may have no visible lung point. Bullae, adhesions and interfaces near the heart or diaphragm can mimic it: confirm the anatomy and clinical context.

Size. A more lateral lung point can suggest greater extent in a supine patient. It does not provide a reliable volume, identify tension, or apply uniformly to loculated/postoperative pneumothoraces. No lung point is not a size measurement.

• Positive region: ≥3 B-lines in a longitudinal scan between two ribs.
• Diffuse interstitial syndrome: ≥2 positive regions on each side, using the 8-region scan.
• Aeration: confluent vertical artefacts suggest greater loss of aeration. Counts depend on probe, settings and view width; use a consistent protocol for serial scans.

Pneumonia or pneumonitis, atelectasis, contusion, pulmonary infarction, pleural disease, neoplasia.

• Cardiogenic pulmonary oedema
• ARDS
• Interstitial pneumonia or pneumonitis, including viral
• Diffuse parenchymal lung disease, such as pulmonary fibrosis

Anterior lung rockets on both sides with lung sliding. For haemodynamic (cardiogenic) pulmonary oedema it had a sensitivity of 97% and a specificity of 95% (Lichtenstein 2008).

B-lines respond quickly to diuresis or dialysis and appear early when lung water rises. Serial changes can help assess decongestion, but neither their presence nor absence measures fluid responsiveness.

See focused cardiac ultrasound: a dilated, poorly contracting LV, a dilated IVC with little respiratory variation, or significant valve disease support a cardiogenic cause. Preserved LV ejection fraction does not exclude cardiogenic oedema: raised filling pressure, acute valve disease and HFpEF remain possible. Consider ARDS, pneumonitis or fibrosis alongside the cardiac findings.

When alveoli fill with fluid, pus, blood or cells, or collapse, ultrasound can image tissue-like lung if it reaches the pleural surface. Disease hidden behind aerated lung can be missed.

• Tissue-like sign (hepatisation). Lung with the echotexture of liver.
• Shred (fractal) sign. The deep border, where consolidated lung meets aerated lung, is ragged and irregular. A lobar consolidation that reaches the whole lobe has no shred border and looks like a smooth, solid organ.
• Ultrasound detected alveolar consolidation with about 90% sensitivity and 98% specificity against CT in ICU patients (Lichtenstein 2004).

• Dynamic: the bright spots move outwards with inspiration, so the airway is patent and air is moving into it. Favours pneumonia over resorptive atelectasis (specificity 94%, PPV 97%; Lichtenstein 2009).
• Static: trapped air that does not move. Seen in atelectasis and some pneumonias. Dynamic bronchograms were only 61% sensitive in the original study: their absence does not exclude pneumonia.
• Fluid bronchograms (anechoic tubes, no colour flow) suggest an obstructed airway, as in post-obstructive pneumonia.

• Static or absent air bronchograms
• Signs of volume loss: raised hemidiaphragm, mediastinal shift, crowded parallel bronchograms
• Lung pulse with absent sliding when the whole lung is collapsed (as after endobronchial intubation)
• Compressive atelectasis floats in a large effusion (the 'jellyfish' or flapping lung).

Under general anaesthesia, dependent atelectasis appears within minutes of induction. It shows as posterior B-lines, then small subpleural and then larger consolidations.

Supine patient: posterior axillary line at the base (PLAPS point), probe marker towards the head, aiming slightly upwards. Find diaphragm, liver or spleen, and lung.

• Anechoic (or echogenic) space between the parietal pleura and the lung, bounded below by the diaphragm.
• Spine sign. Normally air above the diaphragm hides the vertebral bodies. Fluid or consolidated lung can transmit sound so the thoracic spine remains visible above the diaphragm. The spine sign alone does not prove an effusion.
• Loss of the mirror image and of the curtain sign. Normally the aerated lung reflects the liver or spleen above the diaphragm and slides down like a curtain on inspiration; fluid abolishes both.
• Quad sign (static): a space bordered by the pleural line, the two rib shadows and the lung line (visceral pleura) beneath.
• Sinusoid sign (M-mode): the lung line moves towards the pleural line on inspiration and away on expiration, making a sine wave. It supports mobile fluid; it does not establish a safe drainage route. It does not tell you transudate from exudate.
• Ultrasound shows small effusions that an erect CXR misses; an erect PA film usually needs about 200 mL or more to blunt the costophrenic angle (a lateral film about 50 mL); a supine film may miss much more.

• Anechoic: either a transudate or an exudate.
• Complex septated, complex non-septated or homogeneously echogenic fluid favours an exudate, blood or pus; appearances do not replace sampling when indicated.
• Septations or loculations: think empyema or haemothorax; may need an image-guided drain at a chosen spot.

Original method: ventilated adults, supine with the trunk raised 15°. At the posterior axillary line, use a transverse view perpendicular to the body axis; measure the maximum basal interpleural separation at end-expiration, in mm (Sep).

The original mean prediction error was 158 ± 161 mL; this is not an individual ±160 mL confidence interval. The study excluded empyema, haemothorax, prior lung surgery, thoracic deformity and diaphragm disease. Do not apply it to loculated collections or use volume alone to decide drainage.

Drain for a clinical indication, not a volume estimate. Ultrasound must identify the chest wall, diaphragm, lung and abdominal organs in the actual procedure position, immediately before the procedure. Check the window throughout respiration. A gap ≥15 mm was an entry criterion in one study, not a universal safe-depth rule. A trained operator must plan the path and depth; if there is no safe window, seek image-guided help (BTS 2023).

• A-profile: A-lines on both sides with sliding.
• A'-profile: A-lines without sliding.
• B-profile: anterior B-lines on both sides (lung rockets) with sliding.
• B'-profile: B-lines on both sides without sliding.
• A/B-profile: B-lines on one side, A-lines on the other.
• C-profile: anterior consolidation.
• PLAPS: consolidation and/or effusion at the posterolateral point.

• B-profile → haemodynamic (cardiogenic) pulmonary oedema.
• B'-profile, A/B-profile or C-profile → pneumonia.
• A'-profile + lung point → pneumothorax. A'-profile without a lung point → no conclusion; needs other imaging.
• A-profile → scan the leg veins (see DVT): DVT → pulmonary embolism; no DVT, PLAPS present → pneumonia (A-no-V-PLAPS); no DVT, no PLAPS ('nude profile') → COPD or asthma exacerbation.

Reported accuracy (Lichtenstein 2008), for example: B-profile for pulmonary oedema, sensitivity 97% and specificity 95%; A-profile plus DVT for PE, sensitivity 81% and specificity 99%.

The original FALLS framework (Lichtenstein 2013) assessed obstructive and cardiogenic causes of shock, then followed an A-profile during fluid administration; new B-lines were a stop signal. It is a historical diagnostic framework, not a validated universal resuscitation endpoint.

1. Define the perfusion problem and assess the heart. Absent sliding alone does not diagnose tension pneumothorax; an enlarged RV alone does not diagnose PE.
2. Use a reversible dynamic test, such as PLR with contemporaneous stroke-volume or cardiac-output measurement, when appropriate.
3. If fluid is indicated, reassess flow, perfusion and congestion after a small, measured intervention. Stop if benefit is absent or congestion worsens.

A fluid-responsive patient may still not need fluid. See flow and fluid responsiveness.

New diffuse B-lines during treatment raise concern for deteriorating lung tolerance. They do not identify the cause of shock, measure cardiac output or prove a hydrostatic mechanism. Pre-existing ARDS, fibrosis and pneumonia make this harder to interpret. Do not deliberately give fluid until B-lines appear.

• 0: normal aeration. A-lines, or ≤2 isolated B-lines.
• 1: moderate loss of aeration. ≥3 well-separated B-lines.
• 2: severe loss of aeration. Coalescent B-lines.
• 3: complete loss of aeration. Consolidation (tissue-like).

Total: 0 (normal) to 36 (worst). Some groups use a modified version: score 1 when B-lines cover less than 50% of the pleural line and 2 when they cover more, or add points for subpleural consolidations. State which version you use, and use the same one every time.

• Recruitment and PEEP (Bouhemad 2011, ARDS). The reaeration score after a PEEP change tracked recruited volume measured by pressure-volume curves: +8 or more corresponded to >600 mL recruited. Lung ultrasound cannot see hyperinflation, so it must not be the only tool for setting PEEP.
• Weaning (Soummer 2012). After a successful spontaneous breathing trial, an LUS score below about 13 at the end of the trial predicted successful extubation; above about 17 predicted post-extubation distress. In between is a grey zone. Later studies gave varying cut-offs, so treat these as a guide, not a rule.
• Following progress in ARDS, pneumonia or atelectasis day to day; under anaesthesia, measuring atelectasis before and after a recruitment manoeuvre.

The score measures aeration. It does not tell you why aeration is lost.

The intubated lung slides. The non-ventilated lung shows no sliding but a lung pulse, followed later by absorption atelectasis. To check: put a linear probe at the upper BLUE point on each side during ventilation, and look for sliding on both sides.

In one single-centre study of 42 patients, ultrasound (cuff and tracheal views plus bilateral sliding) separated tracheal from bronchial intubation with 93% sensitivity and 96% specificity, against 66% and 59% for auscultation (Ramsingh 2016). It does not replace capnography or bronchoscopy. If sliding is unilateral with contralateral lung pulse, check for mainstem migration or obstruction and correct the cause; a fixed withdrawal distance is not appropriate for every patient. Use the same sign as an adjunct to confirm lung isolation; fibreoptic bronchoscopy remains the standard for double-lumen tube or blocker position.

Posterior B-lines and dependent subpleural consolidations develop soon after induction, particularly with high FiO2, obesity, laparoscopy and steep Trendelenburg. A pilot study suggested LUS can track perioperative atelectasis (Monastesse 2017). Use these signs to show the effect of a recruitment manoeuvre or a PEEP change, and to assess hypoxia in recovery.

Subclavian or internal jugular line, supraclavicular or infraclavicular brachial plexus block, paravertebral or intercostal block, thoracic surgery or laparoscopy.

• Ideally scan the anterior BLUE points on that side before and after the procedure.
• Sliding or B-lines on the anterior chest after the procedure rule out a pneumothorax at those points, in the supine patient where air would collect.
• Lung findings do not confirm catheter-tip position. Use the appropriate line-position assessment separately (see vascular access).

Run a quick checklist: tube position (bilateral sliding), pneumothorax, new B-lines (fluid overload, TRALI, negative-pressure pulmonary oedema), atelectasis or consolidation, effusion.

Two domes on a central tendon, supplied by the phrenic nerve (C3–C5). On inspiration the muscle shortens and thickens, and the dome descends towards the abdomen. Laterally, the muscle lies flat against the inner chest wall below the lung edge: the zone of apposition, where you measure thickness (next slide). The right dome is easy to see through the liver; the left needs the spleen, a smaller window, often with gas from the stomach in the way.

• Probe: curvilinear or phased array (2–5 MHz). Patient supine or semi-recumbent; use the same position each time, because the values change with posture.
• Right: subcostal, mid-clavicular or anterior axillary line, aimed cranially and medially through the liver to the back of the dome. Left: subcostal or low intercostal, anterior to mid-axillary line, through the spleen.
• Find the bright curved line of the dome in B-mode, then place the M-mode cursor as close to perpendicular to the dome as you can. An off-axis cursor biases the measurement; align with the direction of excursion rather than applying Doppler angle correction.
• Measure from the baseline at end-expiration to the peak of inspiration. Average three breaths. On M-mode, normal inspiration moves the line towards the probe (upwards on screen).
• Three manoeuvres: quiet breathing, voluntary sniff (a short sharp sniff through the nose, mouth closed) and deep breathing.

Use sex-specific values. These were healthy volunteers breathing spontaneously; under positive-pressure ventilation the ventilator moves the diaphragm passively, so excursion then says little about the muscle. Assess excursion during spontaneous breathing (or a spontaneous breathing trial).

A paralysed hemidiaphragm is pulled upwards (cranially) by the negative pressure in the chest when the other side contracts. On M-mode the line moves away from the probe during inspiration. A sniff makes this paradoxical movement obvious; quiet breathing may only show small or absent movement. Clear paradoxical movement on sniff, with little or no caudal movement on quiet and deep breaths, indicates paralysis. A small paradoxical flicker on sniff can occur in healthy people, and an oblique cursor or a weak sniff can mimic it, so compare with the other side or a pre-block baseline. A common block-study definition uses voluntary sniff excursion against the pre-block baseline: >75% reduction or paradoxical movement is complete paresis, and 25–75% is partial (Petrar 2015). These are study definitions, not universal thresholds across all manoeuvres.

• Linear probe (high frequency, 7–15 MHz), longitudinal, in the 8th–10th intercostal space between the anterior and mid-axillary lines, usually 0.5–2 cm below the costophrenic angle (the lung edge).
• The diaphragm is the hypoechoic layer between two bright lines: the pleura above and the peritoneum below, often with a thin bright line in the middle. Measure the muscle between the bright pleural and peritoneal interfaces, using the same caliper convention each time; small placement errors materially affect the fraction (EXODUS 2022).
• Measure in B-mode or M-mode at end-expiration (Tdi,ee) and at end-inspiration (Tdi,ei); average at least three breaths. Mark the skin so that daily scans are in the same place.

Thickening fraction reflects inspiratory thickening and can be assessed during assisted ventilation, but depends on effort, loading and support. Little thickening during fully controlled ventilation may reflect absent neural drive, not intrinsic weakness.

• Normal thickness at end-expiration: lower limit about 1.5 mm (Boon 2013, 150 healthy adults). With maximal inspiration, <20% thickening is abnormal in the published reference method. It supports further assessment for weakness/paralysis; it is not a stand-alone diagnosis.
• Dysfunction at the switch to pressure support: TFdi below 29% identified diaphragm weakness against phrenic nerve stimulation (sensitivity 85%, specificity 88%; Dubé 2017). In a similar study, 25.8% was the best threshold to predict SBT failure (Dres 2018).
• Weaning: TFdi ≥30% during an SBT or PSV trial predicted extubation success (sensitivity 88%, specificity 71%, PPV 91%, NPV 63%; DiNino 2014, 63 patients). Pooled data are less impressive and heterogeneous (Llamas-Álvarez 2017), and a 191-patient multicentre study found that neither TFdi nor excursion predicted extubation failure (Vivier 2019). Use it to explain a failed SBT, not as a gate to extubation.
• Effort while ventilated: a TFdi of 15–30% (similar to quiet breathing) in the first 3 days went with the shortest duration of ventilation (Goligher 2018). Very low TFdi suggests over-assistance; high values suggest excessive effort.

Thresholds vary with the mode, the level of support, the breath (quiet or maximal) and how the membranes are measured. No single cut-off is agreed.

A fall in end-expiratory thickness of more than 10% from baseline is the usual research definition of atrophy (Goligher 2015, 2018); the EXODUS 2022 consensus adopted the same >10% cut-off. Atrophy developed in 41% of ventilated patients by a median of day 4 and was associated with a lower daily probability of liberation from ventilation and longer ventilation (Goligher 2018). Thinning tracks with low inspiratory effort and thickening with excessive effort, but these observational associations do not establish a TFdi target for ventilator adjustment (Goligher 2015, 2018).

• After cardiac surgery: phrenic nerve injury (topical ice slush, hypothermia, dissection near the nerve) causes a raised hemidiaphragm and difficult weaning. Reported incidence varies widely with era and definition. Look for absent or paradoxical movement on the sniff test and a thin muscle that does not thicken.
• Weaning failure: one part of the picture alongside the heart (diastolic dysfunction), lung (LUS score) and pleura (effusion).