Lung POCUS - Parameters and Protocols

4.1 Core parameters

Lung sliding

  • 2D/M-mode at 2nd–5th intercostal spaces.

  • Present: aerated lung in apposition.

  • Absent: search for lung point.

Lung point

  • Present plus absent sliding: pneumothorax likely.

  • Absent plus absent sliding: atelectasis, pleurodesis, pneumonectomy, apnea, technical limitation.

B-lines per intercostal space

  • 0: aerated.

  • 1: may be normal.

  • 2–3: suspicious.

  • 3 or more: interstitial or alveolar syndrome.

B-line score

  • 4 intercostal spaces per side, 8 total, or capped total 0–12 depending on protocol.

  • Count spaces with 3 or more B-lines.

Lung congestion score

  • 3 zones per side.

  • 0: A-lines or 2 or fewer B-lines.

  • 1: 3 or more discrete B-lines.

  • 2: coalescent B-lines.

  • Total 0–12.

  • One or more in multiple zones correlates with increased extravascular lung water.

Consolidation

  • Number, size, air bronchograms, lung pulse.

  • Less than 2 cm: small.

  • 2–5 cm: moderate.

  • 5 cm or greater: large.

Pleural effusion

  • Sites, maximal depth, septations, loculations.

  • Less than 1 cm: minimal.

  • 1–3 cm: clinically relevant.

  • 3 cm or greater: often drainable.

  • Quad sign and/or sinusoid sign support pleural effusion.

Pleural line

  • Number of abnormal sites.

  • Thickness.

  • Thickened or irregular pleural line suggests pleural disease.

Diaphragm excursion

  • M-mode measurement in cm.

  • 1.5 cm or greater: adequate.

  • Less than 1.5 cm: reduced.

4.2 BLUE protocol

Windows: bilateral upper anterior, bilateral lower anterior 4th/5th intercostal spaces midclavicular, plus posterior axillary PLAPS windows when relevant. A proficient operator can complete BLUE in approximately 3 minutes or less.

BLUE profiles

  • A: sliding present, A-lines, no or few B-lines, no consolidation. Normal lung, asthma, COPD, possible PE; check DVT.

  • Nude A: A-profile, no DVT, no PLAPS abnormality. Asthma/COPD exacerbation.

  • A′: A-pattern, sliding absent. Pneumothorax if lung point present; otherwise atelectasis, pleurodesis, pneumonectomy.

  • B: sliding present, 3 or more B-lines per space, bilateral. Acute pulmonary oedema; reported sensitivity approximately 97%, specificity approximately 95%.

  • B′: B-lines present, sliding absent. Pneumonia typical in BLUE; also ARDS or mixed process.

  • A/B: one A side, one B side. Pneumonia, non-homogeneous disease, ARDS.

  • C: anterior consolidation, thickened pleural line. Pneumonia, atelectasis, compression.

  • PLAPS: postero-lateral B-lines, consolidation, thickening, effusion. Alveolar or pleural pathology posteriorly.

  • A-V-PLAPS: A-profile plus free venous network plus PLAPS. Pneumonia typical in BLUE.

  • A + DVT: A-profile plus proximal DVT on POCUS. PE likely; reported sensitivity approximately 81%, specificity approximately 99%.

Critical caveat

B-lines represent alveolar-interstitial syndrome. They are not pathognomonic for cardiogenic oedema. ARDS, pneumonia, ILD, non-cardiogenic oedema, pulmonary haemorrhage, and other processes can produce B-lines.

4.3 ARDS versus pulmonary oedema

Features favouring pulmonary oedema:

  • Bilateral symmetric B-lines.

  • Preserved sliding.

  • Less consolidation.

  • Less pleural thickening.

  • PLAPS may be absent.

  • Response to diuretics.

  • VExUS grade 2–3.

  • Elevated LV filling pressures.

  • Clinical volume overload or cardiac history.

Features favouring ARDS/pneumonia:

  • Non-homogeneous, patchy B-lines.

  • Sliding may be absent.

  • Consolidation common.

  • Pleural thickening common.

  • PLAPS often present.

  • No response to diuretics.

  • VExUS grade 0–1.

  • Normal LV filling pressures.

  • Fever, leukocytosis, infection.

4.4 FALLS

  • A-profile or A/B-profile: likely low PAOP.

  • B-lines appearing or increasing: PAOP approaching approximately 18 mmHg.

  • Use to limit excessive fluid and avoid pulmonary oedema.

  • Complement IVC and cardiac POCUS.

4.5 Diagnostic performance and limitations

Performance

  • BLUE protocol in acute respiratory failure: overall correct diagnosis approximately 90.5%, with greater than 90% accuracy for major respiratory aetiologies.

  • Asthma/COPD: A-profile with preserved sliding; reported sensitivity 89%, specificity 97%.

  • Acute pulmonary oedema: diffuse anterior B-lines with sliding; reported sensitivity 97%, specificity 95%.

  • PE profile: A-profile plus DVT; reported sensitivity 81%, specificity 99%.

  • Pneumothorax: absent anterior sliding plus lung point; reported sensitivity 81%, specificity 100%.

  • Pneumonia: anterior alveolar consolidation, asymmetric interstitial pattern, abolished sliding, or posterior consolidation/effusion without anterior diffuse B-lines; reported sensitivity 89%, specificity 94%.

  • Pleural effusion: quad sign and/or sinusoid sign; reported sensitivity approximately 93%, specificity approximately 97%.

Practical timing

  • BLUE can be completed in less than 3 minutes by a proficient operator.

  • A more complete lung ultrasound may require approximately 20 minutes, depending on body habitus and protocol thoroughness.

Limitations

  • Lung ultrasound primarily detects abnormalities abutting the pleura.

  • Central parenchymal disease that does not contact the pleural line may be missed.

  • A-lines are nonspecific.

  • B-lines are nonspecific.

  • Chest radiography remains useful for lines, tubes, global thoracic anatomy, and situations where lung POCUS is technically limited.

  • CT remains superior for deep parenchymal disease, small peripheral lesions, complex pleural disease, and comprehensive staging.

  • Obesity, subcutaneous emphysema, and poor windows limit visualization.

  • Interpretation is operator-dependent.

4.6 Lung-specific pitfalls

  • A single B-line may be normal.

  • Comet-tail artifacts mimic B-lines.

  • Overcounting with incorrect probe angle.

  • B-lines are reduced in severe emphysema or pneumothorax.

  • B-lines do not distinguish ARDS from cardiogenic oedema.

  • Absent sliding does not always mean pneumothorax: atelectasis, pleurodesis, pneumonectomy, apnea, and technical limitation can cause absent sliding.

  • A-lines are normal in aerated lung; absence of A-lines does not by itself define pathology.

  • Small effusions may be missed in obesity, emphysema, or poor windows.

  • Lung ultrasound detects pleura-based pathology; central parenchymal disease may be missed.

  • Over-relying on lung ultrasound for lines and tubes.

  • Forgetting that a full lung ultrasound may take approximately 20 minutes, whereas BLUE can be completed in less than 3 minutes.