Lung Ultrasound

Background

Respiratory complications are relatively common after surgery, especially in higher acuity and thoracic surgery patients. Lung ultrasound is very easy to perform and can be performed in a variety of patient positions. With experience, lung ultrasound is more rapid and sensitive compared to conventional chest x-ray. Lung ultrasound is used increasingly to diagnose pleural effusions, pneumothoraces (especially with eFAST in trauma) and fluid overload.

Indication

Rapid assessment of acute/subacute respiratory failure
Measure impact of therapeutic intervention (i.e. treatment of pneumonia, interval improvement post-chest tube)

Acquisition

Transducer

Curvilinear low frequency
- Better assessment of deeper images (i.e. B-lines)
- Decent visualization of pleura
- Good all-around probe
Linear high frequency
- Best for visualizing pleura

Patient Position:

Supine: anterior and lateral chest
- May be helpful to ask patient to raise arm above head and tuck hand under head to allow for easier chest wall access and to open rib spaces
- Best for assessment of Global LUS zones 1-4
Semi-lateral: lateral and posterior chest
- Best for assessment of Global LUS zones 5 and 6
Seated: posterior chest

Operator Mechanics

Operate probe with dominant hand
Place ultrasound machine on same side as operator to manipulate controls with non-dominant hand

Scanning Technique for 6-zone Supine Exam:

Zone 1: Upper anterior chest between parasternal line and anterior axillary line
Zone 2: Lower anterior chest between parasternal line and anterior axillary line
Zone 3: Upper lateral chest between anterior axillary line and posterior axillary line
Zone 4: Lower lateral chest between anterior axillary line and posterior axillary line
Zone 5: Upper posterior chest between posterior axillary line and paravertebral line
Zone 6: Lower posterior chest between posterior axillary line and paravertebral line

Begin scan with patient in supine position
Probe is placed first on superior-anterior chest (zone 1) with indicator marker directed cephalad
Examine each intercostal space in longitudinal plane followed by transverse plane once pleural space is visualized
- Position probe so that intercostal space is visualized between two rib shadows

Longitudinal

Transverse

Identify lung sliding (motion of the visceral and parietal pleura with respiration)
- If present, shimmering image of the pleural line, often described as ants marching
- Easiest to observe with the linear probe
  - Decrease depth and gain to improve visualization of lung sliding
- If lung sliding is not readily apparent, put ultrasound into M-mode with beam centred between rib spaces
  - Seashore sign = + lung sliding - three distinct layers are apparent
    - Motionless superficial skin and soft tissue = sky
      - Horizontal lines
    - Motionless muscular layer = ocean
      - Horizontal lines
    - Sliding lung tissue = beach
      - Sandy appearance
  - Barcode sign = no lung sliding
    - All three layers are motionless - gives off the appearance of a barcode

Identify A-lines
- These appear as bright horizontal lines below the pleural line at equidistant intervals
- Are reverberation artifacts and are present in normal lung
  - Can also appear in pneumothorax with reverberation artifact from the parietal pleura
- May be several A-lines

Lung Sliding

No Lung Sliding

Complete examination of zones 2-6
- May need to position patient semi-lateral to assess zones 5 and 6
Within zone 6, identify the curtain sign
- At zone 6, you should be able to observe the diaphragm and liver or spleen
- In healthy lungs, the base of the lungs will sweep down and obscure the organs transiently with inspiration, with reappearance of organs on expiration
Repeat examination on other side

Probe selection
- Can switch between linear/curvilinear probe in order to better visualize targeted structures

Pathologic features
- Absent lung sliding: discussed above
- B-lines
  - Appear as mobile hyperechoic lines or rays that descend into the thorax from the pleura
  - Occurs secondary to fluid accumulation within lung tissue
    - Can also occur at interlobar fissures and secondary to interstitial lung disease
  - ≥ 3 B-lines between two rib shadows is considered pathologic
- Lung point
  - Indicates the presence of a pneumothorax
  - Transition point between expanded and collapsed lung between two rib shadows
  - Appears as the presence of lung sliding transitioning to a sharp disappearance of lung sliding
  - As described above, can utilize M-mode if lung sliding is unclear
    - A transition from the seashore sign to the barcode sign is indicative of the presence of a lung point
- Alveolar consolidation
  - Presence of fluid filled lung tissue
  - Can appear as a spectrum of small pockets of fluid to complete hepatization of the lung (looks like a liver)
- Pleural effusions
  - Best visualized in zone 6 (fluid sinks with gravity)
  - Appears as anechoic fluid distinct from lung interstitium
  - Can sometimes visualize lung tissue vibrating within the fluid = jellyfish sign

Interpretation

Normal Lung
- Lung sliding
- Normal A-lines
- Curtain sign
- < 3 B-lines
BLUE (Bedside Lung US in Emergency) Protocol (Lichtenstein and Mezière, 2008) is a helpful algorithm to go through in order to rapidly rule in/out common causes of respiratory failure
- Protocol involves a 6 point scan, as described above
- Protocol is combined with DVT ultrasound to assess for PE

Adapted from Lichtenstein and Mezière (2008)

Medical Decision Making

Suspected pneumonia
- Treat with antibiotics
- Pleural effusion - consider drainage
Suspected pneumothorax
- Consult respirology
- Consider chest-tube placement
Pulmonary embolism suspected
- Obtain formal DVT ultrasound or CT chest
Pulmonary edema
- Reevaluate fluid management
- Consider diuresis
- Consider BiPAP
Unclear etiology respiratory failure
- Acquire other imaging

Pitfalls and Modifications

Obesity
- May have difficulty acquiring images
- Asking patient to raise arm above head will help to open rib spaces and space out subcutaneous tissue
Post-surgical
- Chest wall may be obscured by dressings or subcutaneous emphysema
Medial lesions
- Consolidations or masses located medially may be obscured by healthy aerated lung tissue

Comprehension Questions

1. Which transducer is best to visualize shallow pleural structures?

A. Curvilinear low frequency

B. Linear High Frequency

C. Phased array

2. Which transducer is best to visualize deeper lung structures?

A. Curvilinear low frequency

B. Linear High Frequency

C. Phased array

3. What do A-lines represent?

4. What do B-lines represent?

5. How many or more B-lines is considered pathologic?

A. 1

B. 3

C. 4

D. 5

E. 6

Answers

1. B

2. A

3. A-lines appear as bright horizontal lines below the pleural line at equidistant intervals. They are reverberation artifacts and are present in normal lungs.

4. B-lines appear as mobile hyperechoic lines or rays that descend into the thorax from the pleura. They occur secondary to fluid accumulation within lung tissue, but less than 3 B-lines is a normal finding.

5. B

References

Dinh, V., Deschamps, J., Ahn, J., Genobaga, S., Lang, A., Lee, V., . . . White, S., Lung Ultrasound Made Easy: Step-By-Step Guide. Retrieved from https://www.pocus101.com/lung-ultrasound-made-easy-step-by-step-guide/

Gargani, L., & Volpicelli, G. (2014). How I do it: Lung Ultrasound. Cardiovascular Ultrasound, 12(1). https://doi.org/10.1186/1476-7120-12-25

Lichtenstein, D. A., & Mezière, G. A. (2008). Relevance of lung ultrasound in the diagnosis of acute respiratory failure*: The blue protocol. Chest, 134(1), 117–125. https://doi.org/10.1378/chest.07-2800

Lichtenstein, D. A. (2016). Lung Ultrasound in the Critically Ill Neonate. Annals of Intensive Care, 4(1), 277-285. doi:10.1007/978-3-319-15371-1_32

Marini, T. J., Rubens, D. J., Zhao, Y. T., Weis, J., O’Connor, T. P., Novak, W. H., & Kaproth-Joslin, K. A. (2021). Lung Ultrasound: The essentials. Radiology: Cardiothoracic Imaging, 3(2). https://doi.org/10.1148/ryct.2021200564