Lung Ultrasound

Lung ultrasound is a bedside, radiation free examination of the pleura, lung surface and pleural spaces, done with a curvilinear (2 to 5 MHz), phased array or linear (7 to 12 MHz, for the pleural line) probe. Because aerated lung reflects ultrasound, the diagnosis rests on artefacts and on the visible changes in the pleural line, rather than on direct imaging of the aerated parenchyma.

The patient is examined sitting, supine or semi-recumbent, and each hemithorax is divided into zones (anterior, lateral and posterior, upper and lower; usually 6 to 8 zones for each side, or 12 for a full examination). Findings are described in the longitudinal orientation (the "bat" sign: two ribs with the pleural line between them), with B-mode and M-mode.

The examination takes 5 to 15 minutes and can be done in emergency, critical care, ward, clinic and primary care settings. It is commonly combined with a focused cardiac, inferior vena cava and lower limb venous study.

Lung ultrasound has become a first line bedside tool for dyspnoea.

• Pneumothorax: sensitivity is 80 to 95% and specificity 95 to 100% (compared with 40 to 50% sensitivity for supine chest radiograph).
• Pleural effusion: sensitivity and specificity exceed 90 to 95%.
• Cardiogenic pulmonary oedema: multiple bilateral B-lines have a sensitivity of 85 to 95% and specificity of 90 to 95%.
• Pneumonia: sensitivity is 85 to 95% and specificity 90 to 95% in adults (similar to or higher than radiograph).
• Consolidation in children: it reduces radiation exposure.

It directs thoracentesis and drain insertion and monitors response to therapy.

Indications. Acute dyspnoea of unknown cause (differentiation of pulmonary oedema, pneumonia, chronic obstructive pulmonary disease, asthma, pulmonary embolism, pneumothorax). Suspected pneumothorax (after trauma, procedure, or in a ventilated patient). Pleural effusion detection, quantification and drainage.

Pneumonia and consolidation (including in children, pregnancy and when radiograph is not appropriate). Acute heart failure and monitoring of congestion (B-line count). Acute respiratory distress syndrome and ventilated patients (monitoring of aeration and recruitment). Pulmonary infection pandemic or epidemic disease, such as viral pneumonia (patchy B-lines, subpleural consolidations).

Suspected pulmonary embolism with lower limb venous duplex. Interstitial lung disease (B-lines, irregular pleural line), subpleural masses, and chest wall lesions. Diaphragmatic function (excursion, thickening fraction) and weaning from ventilation. Neonates: respiratory distress syndrome, transient tachypnoea of the newborn, pneumothorax, meconium aspiration. Rib and sternal fractures.

Ultrasound control of thoracentesis, pleural biopsy, and drain placement. Not as a replacement for CT in complex or deep parenchymal disease.

Contraindications and precautions. None. Subcutaneous emphysema, large chest wall dressings, burns, obesity and pleural adhesions can limit views. Probe and gel are cleaned between patients for infection control (especially in respiratory infection). Findings are always correlated with the clinical picture, and a fixed scoring system and operator training improve reliability.

Preparation and technique. The probe is held perpendicular to the chest wall in a longitudinal intercostal orientation, with depth set to 10 to 14 cm.

The pleural line (a bright horizontal line 0.5 cm deeper than the rib line) is identified, and the following are assessed in every zone:

• Lung sliding: to-and-fro movement of the pleural line with breathing.
• A-lines: horizontal reverberation echoes parallel to the pleural line at regular intervals, normal aeration.
• B-lines: discrete, laser like, vertical hyperechoic artefacts arising from the pleural line, erasing A-lines, moving with sliding, reaching the bottom of the screen.
• Consolidation: tissue like pattern with air bronchograms.
• Pleural effusion: anechoic or echogenic space between parietal and visceral pleura, with the spine sign.
• The lung point.

M-mode confirms sliding (seashore sign) or its absence (barcode or stratosphere sign). The diaphragm is imaged with a low frequency probe in the subcostal or intercostal window. The patient is scanned posterolaterally in the supine position to detect dependent fluid and consolidation (posterolateral alveolar and/or pleural syndrome point, PLAPS).

Normal appearances. Smooth, thin, echogenic pleural line with visible lung sliding, A-lines on B-mode, seashore sign on M-mode, and 0 to 2 B-lines in each zone (up to 2 isolated B-lines may be normal in the dependent zones). Diaphragm excursion is clear (1.5 to 2 cm in quiet breathing, 6 to 9 cm in deep breathing in adults). There is no fluid or consolidation.

The lung point is not seen in the normal lung. Heartbeat can transmit to the pleura (lung pulse), which confirms that the pleura are apposed.

Interpretation.

• Normal aeration pattern (A profile): bilateral A-lines with lung sliding; with a venous thrombosis, suggests pulmonary embolism; with wheeze, suggests asthma or chronic obstructive pulmonary disease.
• Pulmonary oedema (cardiogenic): multiple (3 or more in a view), bilateral, diffuse, symmetrical B-lines (B profile), starting at the bases and increasing with severity.
• The count of lines gives a congestion score (more than 30 total lines in 28 zones is significant).
• Other features of pulmonary oedema: a thickened, regular pleural line, preserved sliding, and pleural effusions; B-lines fall with diuresis, dialysis, or treatment.
• Pneumonia: focal consolidation with air bronchograms (dynamic air bronchograms indicate patent bronchi, versus static in atelectasis), irregular and thickened pleural line, focal B-lines, small effusion, and decreased sliding over the consolidation; the "shred" sign is the irregular border between consolidated and aerated lung.
• Atelectasis/compression: consolidation without dynamic air bronchograms, volume loss, and a lung pulse; often with pleural effusion; re-expansion with recruitment.
• Interstitial syndrome: bilateral, diffuse B-lines with an irregular, thickened or fragmented pleural line (interstitial lung disease, fibrosis, pneumonitis, acute respiratory distress syndrome, viral pneumonia), usually asymmetrical or patchy, with spared areas ("white lung" in severe disease).
• COVID-19 and other viral pneumonias: patchy bilateral B-lines, thickened irregular pleural line, subpleural consolidations, and spared areas; staged by score.
• Pneumothorax: absent lung sliding, absent lung pulse, absent B-lines, A-lines present only (A profile with absent sliding), barcode sign on M-mode, and the lung point (specific for the diagnosis); lung point location relates to the pneumothorax size (lateral position indicates large).
• Pleural effusion: anechoic (transudate may be anechoic; exudate and haemothorax may have septations, echogenic debris, or "swirling" content), over the diaphragm.
• The atelectatic lung floats in the fluid (jellyfish or flapping sign), with a sinusoid sign on M-mode (the visceral pleura moves toward the parietal pleura with inspiration) and a quad sign.
• Volume can be estimated by measuring the distance between the lung base and diaphragm at end expiration (separation of 5 cm or more suggests over 500 mL in the supine patient).
• Empyema and complicated effusion: thick septations, loculation, echogenic fluid, and thickened pleura.
• Pulmonary embolism: wedge shaped, subpleural, hypoechoic peripheral lesions (pulmonary infarcts) with a normal A-line profile in the other zones, supported by deep vein thrombosis on venous duplex.
• Chronic obstructive pulmonary disease and asthma: A-lines with preserved sliding; hyperinflation lowers the diaphragm.
• Diaphragm dysfunction: excursion under 1 cm (or paradoxical movement) in quiet breathing, thickening fraction under 20% suggests weakness or paralysis.
• Subpleural masses, pleural thickening and plaques, and chest wall masses: irregular lesions that are fixed or move with the lung; biopsy is directed by ultrasound.
• Neonatal and paediatric lungs: respiratory distress syndrome (bilateral coalescent B-lines, "white lung," absent A-lines, pleural line abnormalities), transient tachypnoea (double lung point: a distinct line between a dense B-line area at the bases and a normal upper region), meconium aspiration (patchy consolidations), and pneumothorax.
• Lung ultrasound scores: aeration score by zone (0 normal, 1 B-lines, 2 coalescent B-lines, 3 consolidation) for monitoring acute respiratory distress syndrome and ventilator weaning.
• Rib fractures: cortical break or step, adjacent haematoma and local pleural effusion.

Critical values and findings needing immediate action.

• Pneumothorax with haemodynamic compromise or in a ventilated patient: immediate decompression and chest drain; do not wait for a chest radiograph.
• Large pleural effusion causing respiratory distress or haemothorax: therapeutic drainage under ultrasound control.
• Empyema (septated, echogenic effusion with sepsis): drainage and antibiotics, with consideration of fibrinolytics or surgery.
• Acute pulmonary oedema (B profile with shock or distress): treatment as acute heart failure (oxygen, non-invasive ventilation, diuretic, vasodilator) and consideration of causes.
• Pneumonia with sepsis: antibiotics within 1 hour for septic shock.
• Evidence of pulmonary embolism with deep vein thrombosis and shock: thrombolysis or embolectomy, as the specialist team judges.
• Ventilator associated pneumothorax or post-procedure pneumothorax: chest drain.
• Hydropneumothorax or bronchopleural fistula: specialist drainage.
• Pleural effusion in a patient with cancer or tuberculosis: diagnostic aspiration.

Limitations and artefacts.

• Aerated deep lung and lesions that do not touch the pleura are not seen, so central consolidation, small deep nodules, and early interstitial disease may be missed; CT is needed when lung ultrasound is negative and suspicion persists.
• B-lines are non-specific: they occur in cardiogenic pulmonary oedema, acute respiratory distress syndrome, pneumonia, interstitial fibrosis, pulmonary contusion, and lung infarction; pattern (distribution, pleural line, clinical context) and associated findings (effusion, echocardiography) are required.
• Absent lung sliding is non-specific: it also occurs in apnoea, main stem intubation, pleurodesis, pleural adhesions, severe fibrosis, acute respiratory distress syndrome, large bullae and phrenic paralysis; the lung point confirms pneumothorax, but its absence (complete lung collapse) does not exclude it.
• A-lines occur also in normal lung, asthma, emphysema, and pulmonary embolism; do not equate A-lines with absence of disease.
• Subcutaneous emphysema, chest drains, dressings, large breasts, obesity, and rib shadows limit views.
• Consolidation may be due to pneumonia, atelectasis, infarction, contusion, or tumour; dynamic air bronchograms and clinical features help to separate them.
• Small effusion estimation is only approximate; the technique depends on position and ventilation.
• Operator dependence and the learning curve are substantial; training and quality control are needed, and images should be stored.
• Findings change rapidly (with fluid, ventilator settings, and posture), so time and conditions should be recorded.
• Pleural effusion and ascites can be confused at the diaphragm; the spine sign and the diaphragm position separate them.

Special groups.

• Children and neonates: lung ultrasound reduces radiation exposure, with high accuracy for pneumonia, bronchiolitis (subpleural consolidations, B-lines), pneumothorax and effusion; thin chest walls allow a high frequency linear probe.
• Pregnancy: safe imaging for dyspnoea and pneumonia without radiation.
• Critically ill and ventilated patients: monitoring of lung recruitment, fluid status, pneumothorax, and diaphragm function; limited by dressings and subcutaneous emphysema.
• Patients with heart failure: B-line scoring at discharge predicts readmission; used to adjust diuretic therapy.
• Patients with chronic kidney disease and dialysis: B-lines monitor volume overload.
• Patients with interstitial lung disease or connective tissue disease: B-line quantification and pleural line abnormality for screening and follow up.
• Regions: tuberculous pleural effusion and pneumonia, HIV-related lung disease (pneumocystis: bilateral B-lines and pleural thickening), heart failure from rheumatic disease and cardiomyopathy, and chest trauma are common; chest radiography may be limited by access to radiology.
• Lung ultrasound is a useful triage tool for dyspnoea, including pneumonia in children (reducing delays in treatment) and for drainage.

Follow up tests.

• Chest radiograph.
• CT of the chest (or CT pulmonary angiography).
• Echocardiography and natriuretic peptides for suspected heart failure.
• Venous duplex of the legs for suspected venous thromboembolism.
• Arterial blood gases and oximetry.
• Pleural fluid aspiration with analysis (protein, lactate dehydrogenase, glucose, pH, cell count, culture, cytology, adenosine deaminase for tuberculosis).
• Sputum microscopy and culture and tuberculosis tests.
• Blood cultures, procalcitonin and C-reactive protein.
• Serial lung ultrasound for response to treatment.
• Thoracic surgery or pulmonology referral for empyema and persistent effusion.
• Spirometry and diaphragm studies for suspected diaphragmatic paralysis.