4. LUNG POCUS

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. ERS-preferred term “interstitial syndrome”; “B-pattern” preferred over “lung rockets” [3]. ERS definition: interstitial syndrome = ≥3 B-lines per intercostal space in ≥3 standard regions [3]. 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 B-lines. 1: ≥3 discrete B-lines. 2: coalescent B-lines. Total 0–12. One or more in multiple zones correlates with increased extravascular lung water. — v5 add: full 12-region aeration score 0–36 (worst pattern/region: A-lines=0, discrete B=1, coalescent B=2, consolidation=3) for serial ICU monitoring [4,5].

Consolidation — Number, size, air bronchograms, lung pulse.

Pleural effusion — Sites, maximal depth, septations, loculations. <1 cm: minimal. 1–3 cm: clinically relevant. ≥3 cm: often drainable. Quad sign and/or sinusoid sign support pleural effusion. — v5 add: safe thoracentesis requires echo-free depth ≥15 mm [3].

Pleural line — Number of abnormal sites. Thickness. Thickened/irregular pleural line suggests pleural disease.

Diaphragm excursion — M-mode, cm. ≥1.5 cm: adequate. <1.5 cm: reduced.

— v5 add: perioperative aeration loss (LUS score) inversely correlates with oxygenation (ΔPaO₂/FiO₂; Spearman r = −0.43) [6].

4.2 BLUE protocol

Windows: bilateral upper anterior, bilateral lower anterior 4th/5th ICS midclavicular, plus posterior axillary PLAPS when relevant. A proficient operator can complete BLUE in approximately 3 minutes or less [1].

BLUE profiles

A: sliding present, A-lines, no/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; else atelectasis, pleurodesis, pneumonectomy [1].

B: sliding present, ≥3 B-lines/space, bilateral. Acute pulmonary oedema; sens ≈97%, spec ≈95% [1].

B′: B-lines present, sliding absent. Pneumonia typical; also ARDS or mixed. 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/pleural pathology posteriorly.

A-V-PLAPS: A-profile + free venous network + PLAPS. Pneumonia typical.

A + DVT: A-profile + proximal DVT on POCUS. PE likely; sens ≈81%, spec ≈99% [1].

Critical caveat — B-lines represent alveolar-interstitial syndrome, not pathognomonic for cardiogenic oedema; ARDS, pneumonia, ILD, non-cardiogenic oedema, pulmonary haemorrhage, and other processes can produce B-lines [3].

4.3 ARDS versus pulmonary oedema

Favouring pulmonary oedema: Bilateral symmetric B-lines [4]. 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.

Favouring ARDS/pneumonia: Non-homogeneous, patchy B-lines [4]. 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/increasing: PAOP approaching ≈18 mmHg [2]. Use to limit excessive fluid and avoid pulmonary oedema.

— v5 add: change from A- to B-lines = endpoint of fluid administration [2]. Complement IVC and cardiac POCUS [3].

4.5 Diagnostic performance and limitations

Performance

BLUE in acute respiratory failure: overall correct diagnosis ≈90.5%, >90% accuracy for major aetiologies [1].

Asthma/COPD (A-profile, preserved sliding): sens 89%, spec 97% [1].

Acute pulmonary oedema (diffuse anterior B-lines, sliding): sens 97%, spec 95% [1].

PE (A + DVT): sens 81%, spec 99% [1].

Pneumothorax (absent anterior sliding + lung point): sens 81%, spec 100% [1]; 2014 alternate 88/100 [2].

Pneumonia (anterior consolidation / asymmetric interstitial / abolished sliding / posterior consolidation-effusion without anterior diffuse B): sens 89%, spec 94% [1].

Pleural effusion (quad and/or sinusoid): sens ≈93%, spec ≈97% [2].

Practical timing

BLUE <3 min by a proficient operator; a more complete lung US ≈20 min (habitus/protocol-dependent).

Limitations

Detects mainly pleura-abutting disease; central parenchymal disease may be missed. A-lines and B-lines are nonspecific. CXR remains useful for lines/tubes/global thoracic anatomy and technically limited cases. CT superior for deep parenchymal disease, small peripheral lesions, complex pleural disease, staging. Obesity, subcutaneous emphysema, poor windows limit visualization. Operator-dependent.

4.6 Pitfalls

  1. A single B-line may be normal.

  2. Comet-tail artifacts mimic B-lines.

  3. Overcounting with incorrect probe angle.

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

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

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

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

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

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

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

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

References

  1. Lichtenstein DA, Mezière GA. Relevance of lung ultrasound in the diagnosis of acute respiratory failure: the BLUE protocol. Chest. 2008;134(1):117-125.

  2. Lichtenstein DA. Lung ultrasound in the critically ill. Ann Intensive Car. 2014;4:1.

  3. Volpicelli G, Elbarbary M, Blaivas M, et al.; ILC-LUS for ICC-LUS. International evidence-based recommendations for point-of-care lung ultrasound. Intensive Care Med. 2012;38(4):577-591.

  4. Bouhemad B, Mongodi S, Via G, et al. Ultrasound for “lung monitoring” of ventilated patients. Anesthesiology. 2015;122:437-447.

  5. Mojoli F, Bouhemad B, Mongodi S, et al. Lung ultrasound for critically ill patients. Am J Respir Crit Care Med. 2019;199(6):701-714.

  6. Monastesse A, Girard F, Massicotte N, et al. Lung ultrasonography for the assessment of perioperative atelectasis: a pilot feasibility study. Anesth Analg. 2017;124(2):494-504.