Lung ultrasound

"Chest ultrasound" is a topic from Section 2.1.5 in the second edition of the CICM Syllabus for the Second Part Examination. Lung ultrasound was the subject of Question 23 from the first paper of 2025, making the lung stand out among the other organs in the chest, having been singled out for an SAQ. One must note that for the rest of the chest and mediastinum, the only ultrasonographic representative is "Echocardiography in intensive care" from the Cardiovascular section, and that the expectations in that part of the syllabus are only "Principles and practice" and "Controversies and risks", whereas for the chest ultrasound they also listed "Interpretation". What could this mean?  It is easy to overthink such discrepancies, risking a spiral of confusion and despair. The syllabus document should be regarded as the most definitive conduit between the real world and the shadowy Tartarus where stems of unwritten questions wait for their turn to bedevil the candidates. It is therefore not surprising that we might scrutinise its every detail trying to divine the future.  

Principles and practice of lung ultrasound in the ICU

That we are even doing this is counterintuitive, as even people who do not routinely use ultrasound will surely recall that it does not cope very well with air-filled cavities. The lung, being necessarily full of such cavities, poses a unique challenge, and the practice of insonating the lung therefore must rely on interpreting the different errors and artefacts of ultrasonography, rather than on the visualisation of tissue. 

Without an extensive digression on the physics of ultrasound in air, we can remind the post-primary exam reader that the extremely low acoustic impedance of air is the main reason for its lack of ultrasonographic representation. Acoustic impedance, we recall dimly, is the property of a substance (usually noted as Z) that describes its resistance to the propagation of sonic waves, defined by the ratio of the wave’s acoustic pressure (p) to its volume velocity (U),

or the product of its density and the velocity of the sound waves, 

Without overextending into First Part material, it is possible to summarise this as "the denser the material, and the faster the speed of sound in it, the higher the impedance", i.e the lower the sound volume will be after traversing the material.  Air has extremely low acoustic impedance, being particularly low in density (even though the velocity of sound in air is fairly slow). The reason for bringing this up is that differences of impedance give rise to reflection, which occurs because a wave has momentum, and air cannot sustain the momentum because it has very little mass. The ultrasound wave pushes the tissue out into the air cavity, and the air at the boundary does accelerate, but because it has so little mass it cannot accept all of the momentum, and the tissue boundary returns most of the momentum back into the tissue mass. Think of it as a large ball on a pendulum, knocking into a small ball (Dan Russel's animations are excellent for this). The small ball goes flying, but the large ball must continue to swing, as there was basically nothing to oppose its motion. In this fashion, at the boundary between tissue and air, the acoustic impedance mismatch is so great that about 99% of the sound wave is reflected. 

The result of this is that you see nothing of the normal lung on ultrasound. The only visible element is usually the pleura. Fortunately, there are several imaging artifacts we can exploit to assess the lung. In fact there is an entire alphabet of them, and beyond that many nameless signs, or those that have confusing duplicated names. Łyźniak et al (2023), after painstakingly assembling them, expressed the perfectly reasonable concern,"we are afraid that at some point the alphabet will be too short to name all findings". Considering the Polish authors are advantaged by a 32-letter alphabet with excellent diacritics, this is serious. Their Table 1 is reproduced here with minimal modification:

The Alphabet of Lung Ultrasound Lines

Line	Characteristics
Pleural line	Strongly hyperechoic line made of parietal and visceral pleura, may not be distinguished from parietal pericardium at its level
Lung line	Line representing visceral pleura in case of pleural fluid
Shred/fractal line	Shredded line on border of consolidation and normal lung
A-lines	• Long horizontal lines • Usually strongly hyperechoic, fading vertically • Reverberations at regular intervals (approximately skin-pleural line distance) • First is A1, second A2, and so forth • If lines in similar length are visible between, they are named A’, A’’, etc. • Indicate presence of air
B-lines	Defined by 7 criteria (most B-lines meet all of them): • Ring down artifacts • Arise from pleural line • Usually strongly hyperechoic • Laser-like, well defined, narrow • Do not fade (usually reach end of the screen) • Erase A-lines • Move with lung sliding
Sub-B-lines	As B lines, but arise from lung line or from shred line
C-lines	Small consolidations
E-lines	• Mainly ring down artifacts, reverberations, refractions, and posterior acoustic shadowing • Arise from horizontal stripes of subcutaneous gas
F-lines	• Small • Punctiform/round/discoid/oblique • Weakly hyperechoic • Should not be confused with air bronchograms • Do not move with lung sliding
G-lines	• They resemble A-, B-, and Z-lines (G-A, G-B, G-Z-lines) • Arise from abdominal structures
H-lines	• Horizontal lines • Mostly strongly hyperechoic reverberations • Arise from bare probe • Show directly adjacent air
I-lines	• Short (max. 2-3 cm) reverberations • Hyperechoic (less than pleural line), fading • Move with lung sliding • More often seen with high frequency probes
J-lines	• Small horizontal lines • Usually strongly hyperechoic • Components of a vertical B-line (sum to make one)
K-lines	• Electric interference artifacts • Arise anywhere on the screen
M-lines	• Horizontal artifacts • Interspersing weakly and strongly hyperechoic, fading vertically • Sometimes arising in rib’s acoustic shadow • Seem to be reverberations
N-lines	• As B-lines, but hypoechoic • Probably refraction artifacts
O-lines	• No lines (no artifacts visible, “black ultrasound lung”) • May be encountered both physiologically and in pneumothorax
P-/Pi-/π-lines	• In same place as A, A’ lines, but shorter • Usually strongly hyperechoic, fading vertically • Reverberations at regular intervals • Nearly forming a vertical artifact • If seen, usually in thin people
R-lines	As B-lines, but arise from pericardium (at lung interface)
S-lines	• Said to be generated by pacemakers • Characteristic propagation (sinuous)
T-lines	• Vertical artifacts on M-mode • Frequency of heart rate • M-mode equivalent of lung pulse
U-lines	• Shape of inverted U • Strongly hyperechoic • Arise from and represent gas within colonic haustra
W-lines	Like E-lines, but multiple and chaotic (not aligned)
X-lines	Occurrence of “B-line” and A-line on same image (“B-lines” which do not erase A-lines)
Z-lines	• Vertical lines • Usually weakly hyperechoic • III-defined reverberations • Fade • Do not erase A-lines • Do not move with lung sliding

The reader who has just buried their head in their hands in despair is reminded that nobody anywhere is expected to know all of these. Only a few accepted findings are commonly referred to, as follows:

Interpretation of lung ultrasound in the ICU

Findings associated with specific conditions:

• Normal lung can be recognised by the presence of pleural sliding and A lines. It is normal to see up to 3 B lines in an intercostal space.
• Pneumothorax is missing the pleural sliding. This is the popular M-mode "seashore sign", of which the interweb is full of good examples.
• Pulmonary oedema is characterised by multiple (>3) B-lines in multiple intercostal spaces
• Consolidation is characterised by "hepatisation", which is to say, "your lung looks like a liver". The  tissue is dense by the presence of inflammatory alveolar fluid and pulmonary vessels can be seen through the parenchyma (as well as the occasional air-filled bronchus). 
• Atelectasis can be distinguished from consolidation by the presence of static air bronchograms, which are distinct from the dynamic air bronchograms of pneumonia.
• Pleural effusion is obviously identified by the presence of hypoechoic fluid and by the "jellyfish sign", which describes the floopy movement of the collapsed lung in the pleural fluid.

Controversies and risks of lung ultrasound in the ICU

The wording of this section of the syllabus suggests that a "discuss" or "critically evaluate" quesition might easily germinate from any reasonable recent review article or new trial in this area. Many would be caught off guard by this, as the use of ultrasound is generally uncontroversial and most normal people have come to regard it as a necessary adjunct of ICU assessment. The candidate who has uncritically insonated every chest on their rounds in the last few years will be baffled by the idea that some sort of risk might arise from this, or that it might be in some way scandalous. 

To start with, in case they are not obvious, the advantages of lung ultrasound are:

• Instant bedside availability
• Low cost (beyond the initial outlay for the unit)
• No exposure to ionising radiation
• No requirements to transport the patient anywhere (although one must admit that all ultrasound does not have to be POCUS)
• For some conditions, eg. pneumothorax, it may be more sensitive: for example, theoretically even a 1mm sliver of pneumothorax can be detected by POCUS, and it can capture effusions in the posterior pleural space that is usually invisible on CXR

On the other hand:

• This is a skill that adds to the already large expected repertoire of skills for a critical care clinician, which produces an added cognitive load
• Like any other imaging modality, eg. CXR, training and experience is required for accurate interpretation
• Unlike many other imaging modalities, the expertise of the operator is directly related to the quality of the images, which affects interpretation
• Unlike CT, lung US does not represent the anatomy in perfect geometrical proportion, and can lead to incorrect measurements
• Some necessary windows may be missing due to chest wall greebling, such as various surgical dressings
• Not all of the lung volume is accessible even under the best conditions
• To persevere with a thorough lung ultrasound takes a lot longer than a single chest Xray
• Large amounts of adipose tissue interfere with ultrasnound transmission

Therefore, the answer to the question "does lung ultrasound improve the quality of care" is difficult to answer, because:

• Misunderstood findings or incorrectly acquired images may result in incorrect management; for example, an M-mode through a rib can produce a rather convincing "barcode sign" in the absence of a pneumothorax)
• For many conditions, CT or Xray radiology remains the gold standard, and by revealing clinically silent pathology lung ultrasound could actually lead to more ionising radiation.
• The finding of a small or otherwise radiologically occult pathology could instigate treatment which would otherwise have been unnecessary  (Volpicelli et al, 2023)
• The additional need to train critical care staff in lung ultrasound adds complexity into an already overfull syllabus