Lung ultrasound, once deemed “not useful” (1) as a thoracic imaging modality, traditionally has not been utilized in departments of radiology except for imaging of pleural effusions. The modality emerged in a grass-roots style in the hands of what we now call point-of-care clinicians likely sometime in the decade of 1980. Yet publishing of articles and books and much later in peer-reviewed journals did not happen until the 1990s and 2000s (2). One landmark article was published in 2008 in Chest entitled, “Relevance of Lung Ultrasound in the Diagnosis of Acute Respiratory Failure*: The Blue Protocol” by Lichtenstein DA and Mezière GA (3). This article established a systematic method to accurately diagnosis conditions causing acute respiratory failure with an accuracy of 90% (3). Multiple articles prior (see bibliography of reference 4) and many articles since (5-12) have demonstrated the utility of bedside lung ultrasound in caring for patients with respiratory complaints and respiratory failure.
In the context of the SARS-CoV-2 pandemic, lung ultrasound has been proposed to be of major utility in rapid triage and identification of disease, classification, risk stratification and prognostication of disease with other clinical and imaging variables (13). It has been utilized to follow progression of disease and in management decisions particularly related to care in the intensive care unit (14). It has the classic advantages of point-of-care ultrasound (POCUS) over other imaging as being rapid, bedside, without ionizing radiation, accurate, repeatable, and can be correlated clinically in real-time. However, the risk of infectious transmission of the SARS-CoV-2 virus to patient and provider has to be seriously taken into consideration when choosing to perform the POCUS exam and when following a protocolized method to cleanse and disinfect the machine and transducers.
The goal of this article is to briefly review applicable principles of lung ultrasound as they relate to COVID-19, review the methodology of performing a lung ultrasound examination on a patient with suspected or confirmed COVID-19, discuss how to utilize information obtained from the lung ultrasound, and outline the principles of safely disinfecting the ultrasound system.
PRINCIPLES OF LUNG ULTRASOUND
A thorough review of lung ultrasound is not the intent of this article, but to understand lung ultrasound in COVID-19 a few principles warrant review. Firstly, the “earth-sky axis principle” (2) simply states that dependent processes of the lung (e.g. fluid/effusion) will be best found in the dependent areas of the lung (or toward the earth) and non-dependent processes of the lung (e.g. pneumothorax) will be best identified in the non-dependent areas of the lung (or toward the sky). Secondly, understanding the air-fluid ratio principle (2) is vital. As we move from 100% air as in the setting of pneumothorax to 100% fluid as in the setting of a pleural effusion our ultrasound appearance changes in a specific pattern that correlates with this air-fluid ratio.
Figure 1: Air/Fluid Ratio Principle
Lung ultrasound findings as ultrasound beam encounters 100% air (pneumothorax) yielding A-lines without lung sliding to 98% air (normal lung) yielding A-lines with lung sliding to 95% air (interstitial syndrome) yielding multiple B-lines/lung rockets to 10% air (alveolar syndrome) yielding signs of consolidation to 100% fluid (pleural effusion) yielding signs of pleural effusion.
We move from seeing A-lines without lung sliding (A’-profile) to A-lines with lung sliding (normal lung profile) to lung rockets (more than 2 B-lines per interspace) in interstitial syndrome (B-profile) to lung consolidation (C-profile) to pleural effusion. As aeration of the lung worsens in a disease, the lung ultrasound findings progress down this air-fluid ratio toward more fluid and less air. Conversely, as aeration improves the lung ultrasound findings progress toward more air and less fluid.
ULTRASOUND MACHINE SETTINGS AND PROBE SELECTION
Any ultrasound machine with the capability of B-mode imaging can be utilized for lung ultrasound. In fact, many of the modern advances in ultrasound imaging that improve image resolution and decrease artifact need to be turned off when insonating the lung. This is because most other ultrasound imaging applications rely on the sonographer viewing the anatomy, where lung ultrasound heavily relies on the imaging of artifacts created by the interface of the soft tissues of the chest wall, the pleura and the lung itself. Therefore, ideally a sonographer versed in lung ultrasound would create a preset on the ultrasound machine for the purpose of loading proper machine settings for sonography of the lung. If this resource is not at your disposal then assure that all filters, compound imaging and speckle reduction are turned off and that you are imaging in the fundamental frequency with tissue harmonic imaging disabled.
Figure 2: Types of Transducers
A. A microconvex transducer frequency range 11-3 MHz B. A phased array (sector) transducer frequency range 5-1 MHz C. A curvilinear array (convex) transducer frequency range 5-1 MHz D. A linear array transducer frequency range 12-4 MHz
A probe that images the superficial nature of the pleura as well as the depth of the lung parenchyma when pathology is present is key. Some lung experts advocate the microconvex probe is best suited for this purpose as it balances the two goals above with wide range of depth and frequency and that this probe’s ergonomics allow intercostal scanning and easier access to the posterolateral lung field in a supine patient (2). Other experts advocate the curvilinear array probe (typically utilized for abdominal applications) as its larger footprint allows scanning a wider portion of the lung at a time and that the loss of resolution of the superficial structures (i.e. pleural line) does not suffer much (15, 16). Many would add using a high-frequency probe, such as the high-frequency linear probe, when improved resolution of the superficial structures is necessary (16,17). Most would agree that the phased array probe (typically used for echocardiography) is the least desirable probe for lung ultrasound but can provide information (e.g. the presence of interstitial syndrome) if that is the only probe you have access to utilize (15). In most settings in the United States the combination of a low-frequency curvilinear probe (or a phased-array probe) along with a high-frequency linear probe is utilized in lung ultrasound. In many of the lung ultrasound in COVID-19 articles this combination of curvilinear and linear probe was utilized (16,17).
LUNG ZONES AND SCANNING TECHNIQUE
There is proposed consensus (18) but still lack of consensus in practice on numbering the anatomic lung areas into zones. It is because of this that currently an anatomically descriptive way of labelling your ultrasound images may be advisable. In the early literature (3,4), the lung ultrasound zones were divided with anterior being Zone 1 (from sternal border to anterior axillary line), lateral being Zone 2 (from anterior to posterior axillary line), and posterior being Zone 3 (from posterior axillary line to spinal vertebrae). These zones were then divided into superior and inferior halves leaving 6 areas to be evaluated per hemithorax, 12 areas in total.
Figure 3: Lung Zones
Lung Zones as described in the earlier literature with Zone 1 (Anterior) being anatomically from sternal border to anterior axillary lines, Zone 2 (Lateral) from anterior axillary lines to posterior axillary lines, and Zone 3 (Posterior) from posterior axillary lines to vertebrae. Each zone is divided into upper and lower halves resulting in 6 lung areas of investigation per hemithorax, 12 in total.
For many, this convention still is practiced. Lichtenstein in his BLUE protocol, advocates three specific points (aptly named the BLUE points - two anterior BLUE points and a PLAPS point) be evaluated in a step-wise fashion in a patient presenting in acute respiratory failure (19). A description with figures of the BLUE points can be found here https://doi.org/10.1007/s13089-011-0066-3. In the article, “International evidence-based recommendations for point-of-care ung ultrasound”, published in Intensive Care Medicine journal in 2012 an 8 zone layout was proposed (18). It is essentially removing the posterior lung zone from the 12-zone convention presented above (see figure 3). The international consensus figure can be found here https://link.springer.com/article/10.1007/s00134-012-2513-4/figures/2. A 14-zone protocol was proposed by Soldati et al to standardize lung ultrasound evaluation in COVID-19 (20). Figure 1 in this article illustrates the 14-zone proposal and can be found here https://doi.org/10.1002/jum.15285. Instead of using zones, areas or points, a “lawn-mower technique” can be utilized to actively scan the entire chest wall like mowing a lawn in overlapping rows. (See Video 1)
Video 1: The “Lawnmower” Technique
This video schematically depicts scanning the three lung zones in a “lawnmower” fashion thus insonating the entire lung in a systematic fashion. The gives the clinical sonographer a big picture view of the lung ultrasound and subsequent focus on particular areas of the lung can additionally be performed as necessary.
In the studies on COVID-19 and pneumonia in general, the dependent posteroinferior areas of the lung are integral areas to evaluate (17,21,22). Therefore, a thorough evaluation is recommended in COVID-19 using either a “lawn-mower technique” or a 12-14 zone protocol.
There is also lack of consensus on whether scanning should occur in sagittal plane (technically short axis to ribs/intercostal space (23)) alone or rotating probe to fit beam intercostally (long axis to ribs/intercostal space) to see more of the pleural line. There are reports of both techniques in the literature (2-4,15,17,19,20) with a combination of both being utilized by some as well (24). The advantage of using a short-axis orientation to the intercostal spaces is that the classic anatomical landmark of the ribs and pleural line can be consistently demonstrated (the bat sign) unless there is pathology like subcutaneous emphysema that obscures visualization. Also using the short-axis orientation to the intercostal space as convention, the quantification of artifacts, particularly B-lines, can be standardized as the intercostal distance is relatively constant (23). The advantage of a utilizing a long-axis orientation to the intercostal space is that a wider area of the lung can be seen at one time without rib shadows hindering visualization. However, this comes at a price that the ribs cannot be used as a consistent reference point in imaging nor can quantitative standardization of lung artifacts, particularly B-lines, be performed as easily. In some lung pathology, this could prove problematic.
My practice is to utilize the lawnmower technique in COVID-19 orienting the probe in short-axis to the intercostal space recording video clips in 12 zones of the lung as originally described. If I find an area difficult to image in this orientation or a finding requiring more investigation, I will change to long-axis orientation to the intercostal space to attempt to gain a more informative image of the lung at that location as necessary.
SIGNS AND PROFILES IN LUNG ULTRASOUND
If you are new to lung ultrasound you can see even in the previous sections there is a new vocabulary one must learn when discussing the findings in lung ultrasound. Daniel Lichtenstein describes 10 basic signs (2) one has to know to utilize lung ultrasound in clinical practice. They can be found in an educational video I recorded at https://youtu.be/_Q0cTG3ZlHk.
Figure 4: 10 Basic Signs of Lung Ultrasound
The 10 basic signs of lung ultrasound
1. The Bat Sign - identifies the pleural line, a crucial landmark in lung ultrasound. Obtained by being perpendicular to the long axis of the intercostal space/ribs.
2. The Seashore Sign - the M-mode depiction of normal lung sliding.
3. A-lines - horizontal, equidistant, attenuating reverberation artifact produced in aerated lung.
4. The Quad Sign - the quadrangle shape produced by adjacent rib shadows and the pleura separated by a pleural effusion, thus identifying a pleural effusion.
5. The Sinusoid Sign - the sinusoidal appearance of the M-mode depiction of respiration in a pleural effusion, thus further identifying a pleural effusion.
6. The Shred Sign - the “shredded” appearance of the lung line deep to consolidated lung resulting from the ultrasound interactions at that interface. This along with sign #7 is used to identify lung consolidation.
7. The Tissue-like Sign - the “tissue-like” appearance of consolidated lung. Others have used the term “hepatization” of the lung again comparing lung consolidation to its liver “tissue-like” appearance.
8. Lung Rockets - Lung rockets are defined as more than 2 B-lines per intercostal space. B-lines are a specific comet-tail artifact that arises from the pleural line moving with lung sliding. They are hyperechoic (bright), long (typically continue off the ultrasound screen), are well-defined, and typically erase A-lines when present. Lung rockets can be further divided into septal rockets (3-4 B-lines per intercostal space and approx. 7mm apart) and ground-glass rockets (>=5 B-lines per intercostal space and approx. 3mm or less apart). These ground-glass rockets can coalesce forming a “curtain” or “waterfall” or “light beam” appearance. Lung rockets are indicative of interstitial syndrome and further divided into septal and ground-glass processes as above. (see video 2).
9. The Stratosphere Sign - the M-mode depiction of lack of lung sliding.
10. The lung point - the B-mode or M-mode visualization of the transition point between the presence and absence of lung sliding. This is pathognomonic for pneumothorax.
Video 2: Coalescent Ground-glass Lung Rockets
This video demonstrates coalescent B-lines indicative of a ground-glass lung rocket pattern.
When performing lung ultrasound, one must evaluate each lung zone for these signs. These signs can then be used to construct lung ultrasound profiles that can be correlated with pathologic syndromes and disease entities. In Lichtenstein’s BLUE protocol paper (3), we see the accuracy of the correlation of the ultrasound signs and profiles summarized in the table in Figure 5.
Figure 5: Accuracy of Ultrasound Profiles
Table 4 from “Relevance of Lung Ultrasound in the Diagnosis of Acute Respiratory Failure*: The Blue Protocol” by Lichtenstein DA and Mezière GA https://doi.org/10.1378/chest.07-2800. This table summarizes the correlation of ultrasound signs, profiles and diseases and the accuracy of these correlations.
Because we are focusing on lung ultrasound in COVID-19, let us just look at the profiles associated with pnuemonia.
One can see that pneumonia had 4 different profiles based on that fact that the presentation of pneumonia is heterogeneous. Diffuse bilateral B-lines WITHOUT lung sliding (B’-profile) was found 11% of the time. This is due to the interstitial inflammatory/infectious nature of the disease (visualized as lung rockets on ultrasound) causing the pleura to adhere to each other and decrease or eliminate lung sliding at the affected portion of the lung. (See Video 3)
Video 3: B’-Profile
This video shows a B-profile (>2 B-lines per intercostal space) WITHOUT lung sliding (‘) thus the B’-profile. This is due to the interstitial inflammatory/infectious nature of the disease causing the pleura to adhere to each other and decrease or eliminate lung sliding at the affected portion of the lung.
A focal interstitial syndrome was seen 14% of the time as predominantly lung rockets on one side or in one area of the lungs and A-lines with lung sliding in another (A/B profile) as illustrated in Figure 6. This is due to pneumonia causing a focal interstitial process not a diffuse interstitial process as is seen more typically in cardiogenic pulmonary edema or other disease processes. Anterior alveolar consolidation (C-profile) was noted 22% of the time. This is simply due to pneumonia causing lung consolidation or alveolar syndrome, illustrated in Figure 7.
Figure 6: An A/B Profile
An example of an A/B profile, where one side or one part of the lung has an A-profile (A-lines with lung sliding) and the other side or another part of the lung has a B-profile (>2 B-lines per intercostal space).
Figure 7: C-profile
An example of lung consolidation on ultrasound (C-profile). Note the “tissue-like” appearance of the consolidated lung (“tissue-like” sign) and the “shredded” appearance of the lung line deep to the consolidated lung tissue (shred sign). This is due to the interactions of the ultrasound beam at the interface of the consolidated lung and aerated lung.
An A-profile (normal lung) seen anteriorly but PLAPS (posterolateral alveolar and/or pleural syndrome) seen posterolaterally was seen 42% of the time. (See Video 4). This is due to the dependent nature of the disease (earth-sky axis principle) causing lung consolidation and/or pleural effusion. As we will see in future sections, this profile is not typically seen in COVID-19 and is more common in bacterial pneumonia.
One may see pleural line abnormalities in pneumonia. These have varied appearances and have been described with assorted terminology from irregular, “thickened”, fragmented, “lumpy-bumpy”, and so forth (See Video 5). These have been called “C-lines” to correlate them with a C-profile as these usually represent millimetric/subcentimetric consolidations.
Video 4: PLAPS
This video shows the findings of posterolateral alveolar and/or pleural syndrome (PLAPS). Note the signs of lung consolidation (alveolar syndrome) and pleural effusion (pleural syndrome) on this video.
Video 5: Pleural Line Abnormalities
This video depicts multiple pleural line abnormalities. Note the irregularity of the pleural line and areas of millimetric consolidation. The pleural line also can appear fragmented and have comet-tail artifacts arising from the lung line.
But pleural line abnormalities have been described in other disease entities outside of pneumonia and can result from a heterogenous group of pathology that changes the interface of the pleura and underlying lung parenchyma. Lastly, one may visualize air bronchograms within the area of consolidation. These appear as hyperechoic (white) areas within the consolidated lung, seen in Figure 8 These can be further divided into static and dynamic air bronchograms (25). Therefore, if we remember the signs and profiles of pneumonia from the BLUE protocol, identifying COVID-19 pneumonia will be familiar.
Figure 8: Air bronchograms
This figure demonstrates the appearance of air bronchograms on lung ultrasound. First note the “tissue-like” pattern of the consolidated lung. Within the consolidated lung you can see a well-defined hyperechoic linear air bronchogram (white arrows) as well as other less well-defined air bronchograms (red arrows).
LUNG ULTRASOUND FINDINGS IN ARDS
A primary cause of short-term mortality in COVID-19 is acute respiratory distress syndrome (ARDS) and sepsis with multi-system organ failure. Lung ultrasound findings of ARDS are reviewed here and very similar to the lung ultrasound findings we discussed in pneumonia just more widespread. In an article by Copetti et al (26), they demonstrated the utility of chest sonography in differentiating ARDS from pulmonary edema. In that article they describe the findings of ARDS on lung ultrasound as: (1) anterior consolidation - this corresponds with the C-profile we reviewed above; (2) diminished or absent lung sliding - this corresponds with the B’-profile; (3) “spared areas” or non-homogenous distribution of B-lines - this corresponds with the A/B profile; (4) pleural line abnormalities - these corresponds with C-lines as discussed above; (5) lung pulse at times - the lung pulse is a sign we have not discussed up to this point but for the sake of this article we will correlate it with atelectasis and the C-profile. You can learn more about lung pulse from the article, “The “lung pulse”: an early ultrasound sign of complete atelectasis” at https://doi.org/10.1007/s00134-003-1930-9(27).
CT FINDINGS IN COVID-19
Now that we have reviewed the lung ultrasound findings in pneumonia and ARDS, we are prepared to move closer to discussing the lung ultrasound findings in COVID-19. Before we do so, let us briefly review the CT findings so we can notice the correlation between the CT findings and the lung ultrasound findings. CT findings of COVID-19 pneumonia were typically ground-glass opacities (GGO) with or without consolidation. One study cited 91% of COVID-19 patients had GGO and 63% had consolidation (28). Sixty-percent of the time or more, patients had bilateral disease with greater than fifty-percent having more than two lobes affected (i.e. multi-focal disease). There was a predilection for the lower lobes and peripheral lung areas (21). Frank pleural effusions were rare but small pleural effusions localized to the areas of GGO/consolidation were seen at times (14). Following the progression of the disease, CT showed in the early phase showing multiple small patchy shadows and interstitial changes near pleura or bronchi rather than lung parenchyma. With progression, the lesions increased in size, developed into GGO with infiltrative consolidation in bilateral lungs. Further progression to severe disease noted massive consolidations and “white lungs” usually without pleural effusion. With dissipation of disease, GGO and consolidations improved, were absorbed and changed into fibrosis. (29).
LUNG ULTRASOUND FINDINGS IN COVID-19
Now that we understand the lung ultrasound signs and profiles of pneumonia and ARDS as well as the CT findings of COVID-19 pneumonia, we could predict what we would expect to see on lung ultrasound. It is important to note that these findings are not specific to COVID-19 pneumonia and in fact are similar to other viral pneumonias (30-35). To review the lung ultrasound findings let us review three papers and then summarize.
A PRELIMINARY STUDY ON THE ULTRASONIC MANIFESTATIONS OF PERIPULMONARY LESIONS OF NON-CRITICAL NOVEL CORONAVIRUS PNEUMONIA (COVID-19) ( https://ssrn.com/abstract=3544750)
In this article (17), Huang et al retrospectively looked at the lung ultrasounds done on 20 COVID19 patients in Xi’an Chest hospital. All patients had an epidemiologic history, infectious symptoms, and positive blood or respiratory testing. Their lung ultrasound exam used convex and linear transducers and examined 12 lung zones.
Here are their key results with my comments below each, in italics:
1. COVID-19 foci are mainly observed in the posterior fields in both lungs, especially in the posterior lower fields.
• Therefore, scanning protocols in COVID-19 should include posterior inferior lung zones.
2. Fused B lines and waterfall signs are visible under the pleura. The B lines are in fixed position.
• This corresponds with ground-glass rockets/coalescent B-lines and B’-profile.
3. The pleural line is unsmooth, discontinuous and interrupted.
• These are pleural line abnormalities and correlates with C-lines.
4. The subpleural lesions show patchy, strip, and nodule consolidation.
• This corresponds with C-profile.
5. Air bronchogram sign or air bronchiologram sign can be seen in the consolidation.
• This corresponds with C-profile.
6. The involved interstitial tissues have localized thickening and edema, and there is localized pleural effusion around the lesions.
7. Color Doppler Flow Imaging ultrasound shows insufficient blood supply in the lesions.
8. High frequency linear array probe is suggested to be used for minor subpleural lesions, as it can provide rich information and improve diagnostic accuracy.
Of note, ultrasound missed lesions that were completely intrapulmonary and apical lesions in this study. The article is rich with multiple cases showing the CT and ultrasound comparative imaging and we will review a few cases here. I encourage readers to view the entire article.
Figure 9: COVID-19 example #1
Figures 1 & 2 from Huang et al (17) article comparing CT and lung ultrasound images of a patient with COVID-19
Figure 10: COVID-19 example #2
Figures 3 & 4 from Huang et al (17) article comparing CT and lung ultrasound images of a patient with COVID-19
Figure 11: COVID-19 example #3
Figures 11 & 12 from Huang et al (17) article comparing CT and lung ultrasound images of a patient with COVID-19.
Figure 12: COVID-19 example #4
Figures 17-19 from Huang et al (17) article comparing CT and lung ultrasound images of a patient with COVID-19.
Figure 13a: COVID-19 example #5
Figure 20-22 from Huang et al (17) article (same patient). HRCT showed large flaps of soft tissues and low-density shadows under the pleura in the posterior segment of upper lobe of the right lung, and large air bronchogram sign (red arrow). Convex array probe showed large areas of consolidation in the right posterior upper area and air bronchiologram sign (yellow arrow). The pleural line was interrupted.
Figure 13b: COVID-19 example #5
Figure 23 from Huang et al (17) article. The linear array probe showed subpleural nodular irregular echo shadow in the left subaxillary field, with fixed fusion of thick B lines.
Figure 13c: COVID-19 example #5
Image on the left is figure 24 from Huang et al (17) article. The linear array probe showed subpleural lesion with localized plueral thickening (greenarrow) and localized pleural effusion (yellow arrow) in the right posterior lower area (blue arrow). Image on the right is figure 25. Color Doppler ultrasound showed no obvious blood flow signal in the peripulmonary consolidation of the left posterior upper area, which was significantly different from that of common inflammatory bacterial pneumonia.
FINDINGS OF LUNG ULTRASOUND OF NOVEL CORONAVIRUS PNEUMONIA DURING THE 2019-2020 EPIDEMIC (https://link.springer.com/article/10.1007/s00134-020-05996-6)
In this article (14), the authors describe their experience with lung ultrasound and the general findings in 20 COVID-19 patients in China. They describe characteristic findings of COVID-19 pneumonia on lung ultrasound as:
1. Thickening of the pleural line with pleural line irregularity;
2. B lines in a variety of patterns including focal, multi-focal, and confluent;
3. Consolidations in a variety of patterns including multi-focal small, non-translobar, and translobar with occasional mobile air bronchograms;
4. Appearance of A lines during recovery phase;
5. Pleural effusions are uncommon.
The also note that early disease presented as focal interstitial syndrome (focal lung rockets). As the disease worsened the interstitial syndrome became more multi-focal with consolidation becoming present and this “alveolar-interstitial syndrome” worsened with disease progression. As above, they noted A-lines during the recovery phase. This follows our air/fluid ratio principle well and correlates with the CT data previously mentioned.
SONOGRAPHIC SIGNS AND PATTERNS OF COVID-19 PNEUMONIA (https://theultrasoundjournal.springeropen.com/articles/10.1186/s13089-020-00171-w)
This article by Volpicelli and Gargani (36) describes their experience in Italy with lung ultrasound in COVID-19. They note that the signs of COVID-19 are typical of other causes of ARDS (as we’ve already reviewed here) including “B-lines in various forms, both separate and coalescent, irregular or fragmented aspect of the pleural line, and small peripheral consolidations.” They describe a novel (in their opinion) sign they name the “light beam” that corresponds with the “waterfall sign” in the first article we reviewed in this section. The authors describe it as “a shining band-form artifact spreading down from a large portion of a regular pleural line, often appearing and disappearing with an on–off effect in the context of a normal A-lines lung pattern visible on the background.” They differentiate this from coalescent B-lines in that it arises from a regular pleural line and turns “on and off” with respiration making it likely due to the passing of diseased lung through the ultrasound beam with areas of preserved lung surrounding. One could hypothesize that this is an very early stage of ground-glass rockets corresponding with the GGO on CT that has not developed sufficiently to cause the associated pleural line abnormalities and that we can describe this with existing terminology. Nonetheless, given that two epicenters (Italy and China) have observed this finding similarly, it is important for us to note this and research it further. You can view multiple examples of this “light beam” sign at the bottom of the article page found here https://doi.org/10.1186/s13089-020-00171-w.
In summary, lung ultrasound findings in COVID-19 pneumonia are typical of viral pneumonia/ARDS and have a bilateral, multifocal distribution favoring the posteroinferior portions of the lung without large or complex pleural effusion typically. The progression of disease on lung ultrasound follows our air/fluid ration principle with initial focal lung rockets progressing to multi-focal ground-glass rockets to consolidation and worsening of consolidation. Resolution of disease displays the reverse with restoration of an A-profile as lung aeration improves.
UTILITY OF LUNG ULTRASOUND IN COVID-19
There has been significant reported utility of lung ultrasound in the COVID-19 pandemic. Authors in multiple geographic epicenters from multiple specialties and practice settings have reported their experience. Common themes of reported utility include:
1. Triage of symptomatic patients (13,16, 37). One example of a proposed protocol can be found here https://www.butterflynetwork.com/covid19/role-of-pocus (37).
2. Diagnostic suspicion/case identification (13,16). This also would include making alternate/comorbid diagnoses with lung ultrasound while evaluating the symptomatic patient for COVID-19. One example of categories of probability of the disease based on patterns of lung ultrasound findings can be found here https://link.springer.com/article/10.1007/s00134-020-06048-9/tables/1 (13).
3. Classification of disease severity and prognostic stratification (13, 14,16, 20). This could include a scoring system of severity based off of a standardized lung ultrasound examination (20) and would obviously incorporate other clinical and diagnostic variables.
4. Monitoring of disease progression (13, 14,16). This could include quantification of changes in lung aeration by lung ultrasound scoring systems (13 - see table 2).
5. Aid in treatment of intensive care unit patients with regard to ventilation and weaning (14,16).
6. Monitoring the effect of therapeutic measures (antiviral or others) (16).
7. Reducing the number of health care professionals exposed during patient assessment as well as the personal protective equipment (PPE) utilization for that patient as a single clinician could perform the history, physical exam and lung ultrasound imaging at the bedside during a single patient encounter (16).
The utility of lung ultrasound in COVID-19 is an extensive, on-going worldwide discussion. How an individual clinician applies lung ultrasound involves many factors including patient’s clinical presentation, provider’s practice environment, provider access to resources and testing, provider expertise and experience, current epidemiological trends, healthcare system capacity, infection transmission risk, perceived benefits of diagnostic information obtained by lung ultrasound, and more.
Computer vision software, artificial intelligence, computer automation and quantification, telemedicine and other methods are being researched and developed to realize the full potential of how lung ultrasound can optimize patient care both within and beyond the SARS-CoV-2 pandemic.
DISINFECTION OF THE ULTRASOUND MACHINE AND TRANSDUCERS
The major risk of lung ultrasound in suspected or confirmed COVID-19 patients is infection transmission. The ultrasound transducers and any surface of the ultrasound machine that is exposed to the SARS-CoV-2 virus can act as a fomite and thus transmit the infection to another patient or provider or any person who would come in contact with the ultrasound equipment. Therefore, it is of paramount importance that an appropriate protocol be developed and meticulously followed for the donning and doffing of PPE and disinfecting the ultrasound equipment.
Firstly, the provider should follow their institutional guidelines for PPE and consider “double-gloving” when performing an ultrasound exam removing the outer gloves after the ultrasound exam while still in the patient’s room. Organizations like the Centers for Disease Control (cdc.gov) and the World Health Organization (www.who.int) provide guidance for the proper use of PPE in COVID-19 and will not be discussed here.
Secondly, if resources allow, a dedicated ultrasound system should be reserved for use only on patient with suspected or confirmed COVID-19 disease. That ultrasound machine should be set-up with only the essential elements necessary to perform an ultrasound exam. This is to minimize any surfaces that could serve as a fomite for the coronavirus. Ideally, a portable system would be used as these can be entirely covered with a single-use disposable barrier covering for the ultrasound exam. Single use packets of ultrasound gel should be utilized not the multi-use bottles of ultrasound gel for this same reason.
Thirdly, a virucidal disinfectant that has shown effectiveness against the SARS-CoV-2 virus AND has been approved for use by your ultrasound system’s vendor should be stocked in sufficient quantity with your ultrasound system. Every surface of the ultrasound machine and probe should be disinfected before AND after each ultrasound exam. Pay close attention to proper use of the virucidal disinfectant, specifically the wet time. The United States Environmental Protection Agency (EPA) has a list of disinfectants for use against the SARS-CoV-2 virus (38) that can be found here https://www.epa.gov/pesticide-registration/list-n-disinfectants-use-against-sars-cov-2-covid-19. Contact your ultrasound vendor for a list of approved disinfectants for your particular ultrasound machine and transducers.
If the ultrasound system is going to be utilized during a procedure at risk for respiratory aerosolization, consider using a barrier covering over the entire ultrasound machine and transducers.
Lastly, there should be a reserved area for the dedicated ultrasound system to be stored when not in use and clear labeling identifying that machine’s purpose so as to minimize cross contamination. A monitoring program assuring adherence to the developed disinfection protocol is ideal to maintain a heightened awareness of the importance of following the protocol and prevent laxity in practice from developing.
A sample protocol (39) developed by the American College of Emergency Physician’s Emergency Ultrasound Section can be found here https://www.acep.org/globalassets/new-pdfs/policy-statements/guideline-on-covid-19--ultrasound-machine-and-transducer-cleaning.pdf.
As lung ultrasound has previously shown significant utility in management of symptomatically dyspneic patients, so has it shown utility in patients with COVID-19. You may utilize whatever ultrasound machine and transducers at your disposal to ultrasound the lung although most literature to date in COVID-19 suggest using curvilinear and linear probes. Ideally, a lung ultrasound preset should be programmed on your ultrasound machine for easy access of proper machine settings. The exam should be thorough scanning in 12 or more lung zones or using the “lawnmower” technique focusing especially on the posterior and inferolateral lung fields. Strict ultrasound machine and transducer disinfection measures need to be protocolized and followed meticulously to decrease the chance of infectious transmission.
Lung ultrasound in COVID-19 has the ultrasound signs and profiles of viral pneumonia and ARDS. These include: (1) lung rockets (more than 2 B-lines per intercostal space) in various patterns in a focal/multi-focal distribution representing a focal/multi-focal interstitial syndrome; (2) consolidations in various patterns but typically peripheral and posteroinferior in location; (3) pleural line abnormalities; (4) small, simple pleural effusions localized to the areas of affected lung but large or complex pleural effusions are NOT typically seen. The disease progression and regression follows the air/fluid ratio principle and lung ultrasound can be utilized to monitor real-time changes of the disease.
The extent of the utility of lung ultrasound in COVID-19 is vast and still being explored. The fullness of it’s utility will vary depending on many patient, provider and practice environment factors. In this critical time in worldwide healthcare, providers have the opportunity to learn this advantageous and pragmatic imaging modality to better care for their patients.
I presented much of this content in video format that can be found here https://youtu.be/y5QJ5O3ldE0. For additional learning, I also refer you to the references in the bibliography.
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