Lung ultrasound in covid-19

Coronavirus-19 (Covid19) is a respiratory virus that can trigger severe respiratory failure and acute lung injury. The typical symptoms of Covid19 are non-specific to respiratory illnesses and chest radiography lacks sensitivity and specificity. Computed tomography (CT) is much more sensitive to the detection of this virus, but also poses a number of concerns: availability, patient transport, disinfection, cost, radiation, and patient stability. Lung ultrasound (LUS) has emerged as a possible attractive alternative to CT as it is fast, portable, relatively inexpensive, and can be done at the point-of-care. While demonstrating potential findings consistent with Covid19 lung involvement (1), LUS may have prognostic value in predicting the progression of respiratory failure (2 ).

WHy the interest?

Researchers have identified a number of key sonographic findings that can be found in Covid19 including (3):
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1. Pleural line thickening and pleural line irregularities
2. A B-line pattern which may be focal, multi-focal, and confluent
3. Patterns of consolidation: multi-focal small, non-translobar, and trans-lobar with occasional mobile air bronchograms
4. Convalesence is accompanied by A-lines
5. Pleural effusions are uncommon

*N.B. Many of these findings are not unique to Covid19, and may be found it illnesses associated with acute respiratory distress syndrome (ARDS).

WHen to use lung ultrasound in covid19

While multiple large trials are pending, multiple small trials (referenced here) demonstrate that LUS appears to have strong correlation with CT imaging. Debate over when to use LUS largely ongoing, but strongest cases at this time are likely when CT is unavailable (e.g. resources, timing, etc), a patient is too unstable for CT, the potential for confounding life-threatening diagnoses (i.e. pneumothorax, pulmonary edema) and potentially (pending evidence) predicting progression.

How to: acquisition

Lung ultrasound images are acquired with the probe marker toward the head (cephalad) in the para-sagittal plane. LUS images acquired on the mid- and posterior-axillary line are uniquely in the coronal plane. The probe marker must be perpendicular to the pleural line to elicit lung artifacts. Depth for pleural based findings on the anterior thorax are approximately ~8-12 cm. Acquisition in the lower thorax must be adjusted to a depth of ~15-18 cm.

Where to use lung ultrasound

There are a number of protocols cited in the published literature (4,5,6). While more comprehensive protocols may carry additional sensitivity, they may lack feasibility in time pressured environments. On the contrary, abbreviated protocols may be appealing, but they may sacrifice sensitivity in the detection of the dorsal lung involvement that characterizes Covid-19 (1).

 

Nonetheless, whichever protocol is selected, it is likely best to capture a broad geographic territory to ensure detection of scattered parenchymal findings (Especially dorsal and ventral portions in a supine patient). Finally, while select protocols (6) emphasize paravertebral imagining, this approach is likely more technically challenging in critically ill patients when this examination would require moving the patient. This displayed protocol makes use of 6 points on each side of the thorax; 4 on the anterior chest wall and 2 on the posterior axillary line.

 

Pleural line thickening

The pleural line is thickened, irregular, and interrupted in areas affected by Covid-19. This is best seen with a high-frequency linear array probe, but can also be seen with a phased-array or curve-linear probe (with tissue harmonic imaging turned off). While evidence is wanting in this area, patients with plentiful B-lines and a smooth pleural interface are more likely to have other potential causes of respiratory failure (e.g. pulmonary edema). Other pathologies with thickened, interrupted pleura include interstitial lung disease (such as usual interstitial pneumonia or IPF and ARDS).

Subpleural opacities

Sub-pleural consolidation can also be seen as sub-pleural opacities. These are best seen with a high-frequency linear array probe. These are often seen in scattered territories across the chest wall and may be associated with B-lines as this section of the lung begins to suffer from reduced alveolar inflation. In one study of 100 patients with Covid19 (2), sub-pleural consolidations were found in 78% of patients in at least one zone.

B-line patterns

The B-line pattern may be variable. Here we can see 3-4-lines that appear with sliding in the interspace.

We can also see areas where B-lines become so dense, that they are “confluent”. This area may be pre-consolidative as the area becomes progressively more de-aerated.

We can also see areas of relatively normal lung (sliding, A-line), next to areas of affected lung (B-lines). This pattern of preserved normal next to affected B-line rich parenchyma can also be seen in ARDS.

Consolidation

Here we can see mild sub-pleural consolidation with B-lines (and not simply isolated sub-pleural anechoic areas). It is unclear whether these areas will evolve into more dense consolidation.

Here we can see dense sub-pleural consolidation (and not simply sub-pleural anechoic areas).

This clip demonstrates dense lobar consolidation with air bonchograms.

Recovery or non-involvement

Areas of lung unaffected by the virus *or are recovering from the virus may display normal sliding and horizontal A-lines.
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In this example, the lungs are affected heterogeneously: the lung is characterized by areas of ground glass opacity, preserved parenchyma, and dense dorsal consolidation on CT.
 

In this lung ultrasound, the patient has the emergence of an A-line after recovering from severe Covid19 pneumonia.

Preventing bias in lung ultrasound

As one last final point, we must emphasize that none of these findings are pathognomonic for Covid19 (7). Pre-test probability is an essential basis for making diagnoses and Covid19 biochemical testing is the gold reference standard. In select cases, further imaging (i.e. CT, echo) or serological tests will be needed. More work must be done if LUS is considered to be a test for diagnosis and prognostication. Finally, while pleural effusions are uncommon in Covid19, competing medical problems like congestive heart failure are still illnesses than can complicate patients with Covid19.

References

1.Zieleskiewicz L, Markarian T, Lopez A, et al. Comparative study of lung ultrasound and chest computed tomography scan in the assessment of severity of confirmed COVID-19 pneumonia. Intensive Care Medicine. 2020;46(9):1707-1713. doi:10.1007/s00134-020-06186-0

2. Lichter Y, Topilsky Y, Taieb P, et al. Lung ultrasound predicts clinical course and outcomes in COVID-19 patients. Intensive Care Medicine. 2020. doi:10.1007/s00134-020-06212-1

3. Peng QY, Wang XT, Zhang LN. Findings of lung ultrasonography of novel corona virus pneumonia during the 2019–2020 epidemic. Intensive Care Medicine. 2020;(87):6-7. doi:10.1007/s00134-020-05996-6

4. Smith MJ, Hayward SA, Innes SM, Miller ASC. Point-of-care lung ultrasound in patients with COVID-19 – a narrative review. Anaesthesia. 2020;75(8):1096-1104. doi:10.1111/anae.15082 

5. Soldati G, Smargiassi A, Inchingolo R, et al. Is there a role for lung ultrasound during the COVID-19 pandemic? Journal of ultrasound in medicine : official journal of the American Institute of Ultrasound in Medicine. 2020. doi:10.1002/jum.15284

6. Soldati G, Smargiassi A, Inchingolo R, et al. Proposal for international standardization of the use of lung ultrasound for COVID-19 patients; a simple, quantitative, reproducible method. Journal of Ultrasound in Medicine. 2020:1-7. doi:10.1002/jum.15285
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7. Volpicelli G, Lamorte A, Villén T. What’s new in lung ultrasound during the COVID-19 pandemic. Intensive Care Medicine. 2020;46(7):1445-1448. doi:10.1007/s00134-020-06048-9

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