Lung ultrasound: pleura, A/B-lines, effusion, pneumothorax
Why examiners ask this
Lung ultrasound answers the hypoxemia and hypotension questions faster than a chest radiograph and at the bedside: is there a pneumothorax, pulmonary edema, an effusion, a consolidation, or a paralyzed diaphragm. Pleural effusion, pneumothorax and pulmonary edema are named echo diagnoses, and lung, pleura, diaphragm and A/B-lines are named acquisition targets; lung M-mode is in scope.1 The examiner tests whether I can read artifacts correctly and act on them. It stresses application of knowledge and judgment.
The 60-second answer
I start with a linear probe in the sagittal plane over an intercostal space, marker to the head, so two ribs and their shadows frame the pleural line, the "bat sign."2 I ask three questions there. Is the pleura sliding? Sliding excludes pneumothorax at that point. What lines lie beneath? Horizontal A-lines are reverberations of a normal or air-filled pleura; vertical B-lines that run from the pleura to the bottom of the screen and erase the A-lines mean fluid in the interstitium. Then I move to the posterolateral base with a curvilinear or phased-array probe, the right choice for anything deeper than the pleura,3 and look above the diaphragm for an anechoic effusion or tissue-like consolidation. I scan the anterior chest first for pneumothorax, because air rises, and the bases for fluid, because fluid falls.
Decision points and the "why"
| Finding | How it looks | What it means |
|---|---|---|
| Lung sliding | Shimmer along the pleural line; M-mode "seashore" | Pleural layers apposed; no pneumothorax at this point |
| Absent sliding | Static pleura; M-mode "barcode" or stratosphere sign | Pneumothorax, mainstem intubation, apnea, ARDS adhesions, pleurodesis or bullae2 |
| Lung point | Sliding and non-sliding meet and move with breathing | Pneumothorax confirmed; pathognomonic2 |
| Lung pulse | Pleura pulses with the heartbeat but does not slide | Lung is apposed but not ventilated: mainstem intubation or atelectasis2 |
| A-lines | Horizontal echoes at multiples of pleural depth | Normal aerated lung, or pneumothorax if no sliding |
| B-lines | Comet tails from the pleural line, laser-like, move with sliding, erase A-lines | ≥3 between two ribs ("lung rockets"): interstitial syndrome, pulmonary edema, ARDS, contusion2 |
| Effusion | Space above the diaphragm, quad and sinusoid signs, "spine sign" | Pleural fluid; the vertebrae are visible through it;3 hemothorax and empyema may be echoic |
| Consolidation | Liver-like tissue with air bronchograms | Pneumonia or atelectasis; dynamic bronchograms favor pneumonia, a lung pulse favors atelectasis2 |
- Absent sliding is not pneumothorax — it is consistent with it. I look for a lung pulse, or any B-line, which rules it out at that spot, and for a lung point, which rules it in.2 That separates a dropped lung from a tube in the right mainstem.
- Bilateral anterior B-lines — diffuse anterior lung rockets with preserved sliding (the BLUE-protocol B-profile) point to cardiogenic pulmonary edema; patchy B-lines with spared areas and pleural-line changes favor ARDS.2 I pair them with a cardiac view: a poor LV and a plethoric IVC mean diuresis and inotropy; normal LV function points to ARDS, TRALI or aspiration.
- Effusion size — the maximal distance between the lung and the chest wall at the base, in millimeters, times 20 gives an approximate volume in milliliters in a supine patient. A large effusion with hypoxemia supports drainage, which ultrasound can guide.
- Diaphragm — I watch excursion with a curvilinear probe below the costal margin. Paradoxical cranial motion on a sniff means hemidiaphragmatic paralysis, the expected consequence of an interscalene block.
The complication they will inject
A patient becomes hypoxemic and hypotensive with rising airway pressure after a subclavian line. The loop shows no sliding in the left anterior chest and A-lines only. I say: "Absent sliding with A-lines on the left; I need a lung point to confirm, but in this patient I treat tension pneumothorax now." Then the action: 100% oxygen, tell the surgeon, needle decompression in the fourth or fifth intercostal space just anterior to the midaxillary line, or the second space midclavicular line, then a chest tube. I do not delay decompression for a lung point in a crashing patient. The quieter variant shows absent left sliding with a lung pulse and a tube recently advanced; that is a right mainstem intubation, and the answer is to withdraw the tube and confirm bilateral sliding.
Pitfalls that cost points
- Calling absent sliding a pneumothorax without considering mainstem intubation.
- Counting one or two B-lines as pathology; isolated B-lines are normal at the bases.
- Scanning for pneumothorax at the base and for effusion at the apex.
- Leaving the depth too deep to see the pleura clearly with a linear probe.
- Waiting for imaging confirmation before decompressing a tension pneumothorax.
Rapid-why
Q: Why do A-lines appear? A: Ultrasound reverberates between the probe and the highly reflective pleura-air interface, producing copies at multiples of the pleural depth.
Q: Why are B-lines a sign of fluid? A: Thickened, fluid-filled interlobular septa next to air create a reverberating comet-tail that the beam cannot pass.
Q: Why is the lung point so specific? A: It is the exact edge where the collapsed lung touches the chest wall, which only exists in a pneumothorax.
Q: Why look anterior for pneumothorax in a supine patient? A: Free air rises to the least dependent part of the chest, which is anterior and medial.
Numbers to know
| Item | Value |
|---|---|
| Linear probe | Pleura only;3 6–15 MHz, depth 4–6 cm |
| Curvilinear or phased probe for bases | 1–5 MHz |
| Pathologic B-lines | ≥3 between two ribs2 |
| Effusion volume estimate | Distance (mm) × 20 = mL |
| Lung point | Pathognomonic of pneumothorax2 |
| Normal quiet diaphragm excursion | ~1–2 cm |
Links
- Pneumothorax and tension pneumothorax, ARDS and lung-protective ventilation (coming), Interscalene block and its complications, POCUS image acquisition: probe positions and Doppler sectors, aspiration
- Atlas: The 5 POCUS cardiac views
- International Evidence-Based Recommendations for Point-of-Care Lung Ultrasound (2012); SHM minimum image quality criteria for POCUS (2025)
Sources
- 1.APPLIED Exam Objective Structured Clinical Examination (OSCE) Content Outline · ABA · theaba.org
- 2.Lung ultrasound in the critically ill · Annals of Intensive Care (Lichtenstein), 2014 · ebi.ac.ukOpen-access full text (PMC3895677) via Europe PMC; B-line definition, lung point, lung pulse, BLUE-protocol.
- 3.Defining minimum image quality criteria for common diagnostic point-of-care ultrasound images: A position statement of the Society of Hospital Medicine · SHM (J Hosp Med), 2025 · ebi.ac.ukOpen-access full text (PMC12865264) via Europe PMC; Table 3, lung and pleural images.
Facts on this page were checked against these sources on Oct 14, 2026. Guidelines change; confirm doses against current institutional protocols.