Contents
Background
History
Legend has it that way back when, when ultrasound was still a specialist tool of Radiologists and Sonographers, the lungs were deemed impossible to scan due to the perceived lack of meaningful echoes from air-filled structures. Then a French dude probed away* and identified patterns neatly correlating with pathology1.
* Figuratively and literally
1. Lichtenstein, D. A. (2014) “Lung ultrasound in the critically ill,” Annals of Intensive Care, 4(1), p. 1 →
Rationale
Despite early claims, Ultrasound has proven utility in identifying pathology in the lungs2. It's the ugly sibling of all other applications of ultrasound, insofar as it relies on the presence and interpretation of entirely artefactual appearances created by the scattering of sound by air-filled (and otherwise) lung, rather than the visualisation of anatomical tissue architecture. You can't see the lung fields as with an X-ray or CT, but normal and pathological pleura display specific patterns of artefacts that can inform diagnosis.
Once you're familiar with the appearances, ultrasound is more accurate than X-ray in diagnosing pneumothoraces and interstitial lung disease, in differentiating pleural effusions from consolidations, and is as accurate as CT in identifying the smallest of effusions3. All at the bedside.
Pulmonary lesions and infiltrates in COVID-19 are visible with ultrasound (at the pleural level), displaying a variety of artefacts seen in other pathologies. Early data from the initial epicentres of the pandemic reveal a strong correlation between ultrasound and CT4. Here's a table comparing the two, it's been shared far and wide and is present on near every resource on this topic:
Lung CT and equivalent Ultrasound findings5. Don't worry about the nomenclature, it's explained later
In short: CT > US > X-ray.
2. Gargani, L. and Volpicelli, G. (2014) “How I do it: Lung ultrasound,” Cardiovascular Ultrasound, 12(1) →
3. Volpicelli, G. et al. (2012) “International evidence-based recommendations for point-of-care lung ultrasound,” Intensive Care Medicine, 38(4), pp. 577–591 →
4. Lu, W. et al. (2020) “A Clinical Study of Noninvasive Assessment of Lung Lesions in Patients with Coronavirus Disease-19 (COVID-19) by Bedside Ultrasound,” Ultraschall in der Medizin - European Journal of Ultrasound →
5. Peng, Q.-Y. et al. (2020) “Findings of lung ultrasonography of novel corona virus pneumonia during the 2019–2020 epidemic,” Intensive Care Medicine, 46(5), pp. 849–850 →
How to do
You can argue either way which probe to use to scan the lungs. A high frequency linear probe is commonly used in diagnosing pneumothoraces as it visualises the pleura in great detail, however its low frequency brother has the added benefit of revealing the depth and extent of consolidations and effusions.
They're both right, somehow, but stick to a single probe to limit contact with a patient potentially overflowing with SARS-CoV-2. If you're familiar with the FAST views of the pleural cavity through the liver (here) and spleen, they're a good starting point to interrogate the lung bases.
Choose a lung preset on your machine to optimise settings. We want the device to suppress artefacts as little as possible, so frame averaging, multi-beam/compound imaging and tissue harmonic imaging be the enemy here. Less is more, fancier is worse(r).
Anterior ■, lateral ■ and posterior ■ lung zones, avoiding the scapulae
Split the lungs into zones and scan as much lung tissue as possible. Consider a cross sectional view of the thorax in a CT slice: the aim is to scan as much pleura adherent to the chest wall as possible, using that thin beam of sound emanating from the probe. A 'lawnmower' ■ technique that traverses along and between each rib space ensures adequate coverage. Be sure to hold the probe perfectly perpendicular to the contours of the chest wall to produce nice bright views of the pleura.
Start posteriorly at the bases: most changes occur here and you're placed out of the line of fire of a cough.
Normal lung
Normal inflated lung appears on ultrasound as a flat, bright pleural line with repeating horizontal reverberation artefacts ('A' lines). These lines look, smell and taste like the pleura: they're reflections of the pleural line plotted lower down the screen.
It's a strange and at first disorientating appearance created by the tremendous reflection of sound at the boundary between tissue and air, between which the speed of sound is so starkly different. Use the ribs as your anchoring landmarks.
The visceral and parietal pleura are visibly in contact, sliding over one another, shimmering between the ribs.
A view of the pleura across two ribs, scanning in the sagittal plane with the probe marker pointing towards the patient's grinning face
Normal lung: see the many, many repeated horizontal reverberations and the flat, smooth pleural line
Sliding pleura in a single rib space with a tapering comet tail sliding in from the left. This is a normal artefact in normal lung
Normal pleura in multiple rib spaces
View of the right lung base through the liver and diaphragm (the RUQ FAST view) with the lung curtain appearing on inspiration. This is normal.
Findings
Ring-down artefacts ('B' lines)
These lines aren't really there, the machine lies: they manifest when the sound beam interacts with a mixture of fluid and gas bubbles and thus aren't unique to the lungs. You'll find them in the bowel, in abscesses and in a cheekful of your favourite soft drink (I mean this literally - try this at home). They appear as vertical linear artefacts (lines) that arise from the pleura and propagate down the screen. Importantly, they don't attenuate (lose energy) and hence maintain their brightness in the entire field of view.
Scanning at greater depth nicely exaggerates their appearance, so grab your favourite low frequency probe and hammer down on the depth control.
Ring-down artefacts in multiple rib spaces. Bonus tiny effusion
Confluent (lots of them, closely packed) ring-down artefacts in two rib spaces, forming a torchlight-like appearance
Intermittent ring-down artefact appearing on expiration
Headlights from multiple rib spaces, beaming down the screen. Bonus irregular pleura, with a sliver of pleural fluid thrown in for free
A lighthouse. Choose your own metaphor, go nuts with it
A panoramic lightshow
Confluent ring-down artefacts along three rib spaces
Pleural abnormalities
Pulmonary infiltrates in COVID-19 are multifocal and typically peripheral. The latter quality allows you to visualise changes at the pleura with ultrasound, beyond which the sound beam can barely penetrate. Areas of diseased lung reveal an ill-defined and irregular pleural line that often appears thickened.
This was my original caption: Irregular pleural line with associated small hypoechoic consolidation
In simple terms, it looks rough and dirty and nasty and this is not normal
Irregular pleura from which confluent ring-down artefacts arise
Note the adjacent normal lung with (normal) reverberation artefacts within the same rib space. Two for one
An irregular, 'thickened', poorly defined pleural line
An irregular pleura scanned superficially and inline (transverse) between the ribs, providing a panoramic view
Another irregular pleural line with ring-down artefacts
And one more, for good measure
Yet another
Consolidations
Focal peripheral consolidations appear as hypoechoic collections below the pleural line. They're usually small (compared to the typically larger consolidations of bacterial pneumonias) but can be of varying size and consistency. The ultrasonic appearance is of confined regions of differing shades of grey: they look dirty. There's often associated pleural fluid, which reflects no sound so appears black.
This is a small right basal consolidation, below an irregular region of pleura and a small rim of fluid. It dances in and out of the rib spaces with respiration
A slightly larger consolidation with air bronchograms
The same lung, scanning in line with the ribs to interrogate the pleura in more detail, opening up the space
Small consolidation with focal ring-down artefacts
Slightly larger consolidation with a little surrounding pleural fluid
A comparatively massive basal consolidation; lung drowning in effusion
Superficial view of a small consolidation visible at the pleural line
A moderate consolidation with confluent ring-down artefacts emanating from within. This is dirty lung
The same, scanned at less depth
Skip zones
All of the above will appear to varying degrees in the lungs of COVID-19 patients, and rarely in isolation. Infiltrates are patchy, throughout both lungs, resulting in diseased pleura being interspersed between areas of normal lung. 'Skip zones,' 'skip lesions' or 'bits where there be normal lung too' describe this phenomenon.
Don't rely on your findings from a single point on the chest wall, that millimetre-thin slice through the pleura might be missing a huge degree of pathology elsewhere. Scan far and wide, coat with gel.
Irregular pleura and ring-down artefacts, with normal lung pattern within the same and adjacent rib space
Normal lung pattern below two rib spaces displaying ring-down artefacts
Confluent ring-down artefacts in two rib spaces below an area of normal pleura
Basal consolidation, ring-down artefacts and some normal pleura in the rib spaces above
Consolidated lung base with ring-down artefacts at the inferior portion of a single rib space (left is towards the face, right towards the feets)
Cases
Case
Male of advancing age; SOB and fever
I won't patronise: this was a patient with infective respiratory symptoms and a significant hypoxia - run of the mill probable COVID-19 (later confirmed).
Torchlights!
Poorly defined, irregular pleura and a small consolidation
More consolidated lung
The eventual CXR
Case
Syncopal sixty-something
Confirmed SARS-CoV-2 positive dude who presented following repeated loss of consciousness on minimal exertion.
Area of normal pleura (I won't say 'normal lung' as who knows what's going on at this point below the pleura)
Irregular pleura with confluent ring-down artefacts in two rib spaces at the left base
More of the same, with fleeting glances of normal pleural reverberation pattern in a single rib space. Pretty irregular looking lumpy pleura too
Confluent ring-down artefacts emanating from an irregular pleura scanned in the transverse plane of a single rib space. An area of normal pleura appears on expiration
CT slices
Case
Middle aged lady, short of breath at rest
She'd self-isolated for about a week with mild but progressive symptoms.
Irregular pleura, ring-down artefacts and a probable tiny consolidation
Widespread changes when scanning between and across rib spaces
Multiple (single and confluent) ring-down artefacts arising from a single rib space
Bit where there be normal lung too in a single rib space
Small basal consolidation, effusion and craggy pleura
Longitudinal (panoramic!) view of irregular pleura with a rim of effusion
😬
Case
Woman, short of breath
Hypoxic, pyrexial and around 10 days down the line from onset
Irregular pleura
And more, with ring-down artefacts
And even more yet
Transverse view of a region of irregular pleura, a consolidation and a small effusion
CXR
Case
Sixty-ish dude, known COVID-19
Attended with worsening symptoms over the preceding three days, now short of breath on minimal exertion. He underwent this ultrasound during his assessment: it influenced an early decision to admit despite equivocal observations.
Down-ringing (I just made that up) artefacts and lots of normal pleura
Confluent ring-down artefacts and a small effusion
Skip zones
Skip lesions (same thing)
Transverse view of irregular pleura
It looks so normal, then suddenly doesn't
Down-ringers (sorry) in a few rib spaces
CXR for comparison: much softer, far less stark changes than on ultrasound
Case
Young female, short of breath and hypoxic
She'd had close contact with a known positive case and undergone community testing, the result still pending. Attended with a week's history of increasing shortness of breath.
Essentially normal pleura throughout both lungs
This naturally led to seeking another cause of her symptoms, resulting in a bedside echo:
Parasternal long-axis view: a collapsed LV. That's a lung shadow that creeps into view with each breath: peek between each cycle, hold your position
Parasternal short-axis view: a collapsed LV with flattening of the septum and a dilated RV, dwarfing the LV. Note the tachycardia and tachypnoea. NOTE THEM
Apical four chamber view (slightly foreshortened): Dilated right heart, deviated septum and dancing apex with comparatively akinetic RV wall
A jet of tricuspid regurgitation
Subcostal view: dilated RV; rotated to visualise a dilated, fixed IVC
Not all that's hypoxic is COVID-19: she had acute right heart strain and a (sub)massive PE. Later swabbed negative for SARS-Cov-2.
CTPA