Lung semiotics ultrasound in COVID-19 infection

Agatella Barchitta; Mauro Pepi; Ines Paola Monte; Giuseppe Trocino; Andrea Barbieri; Quirino Ciampi; Alberto Cresti; Sofia Miceli; Licia Petrella; Frank Benedetto; Maio Daniele; Francesco Antonini-Canterin

doi:10.4103/jcecho.jcecho_53_20

REVIEW ARTICLE

Year : 2020 | Volume : 30 | Issue : 6 | Page : 1-5

Lung semiotics ultrasound in COVID-19 infection

Agatella Barchitta¹, Mauro Pepi², Ines Paola Monte³, Giuseppe Trocino⁴, Andrea Barbieri⁵, Quirino Ciampi⁶, Alberto Cresti⁷, Sofia Miceli⁸, Licia Petrella⁹, Frank Benedetto¹⁰, Maio Daniele¹¹, Francesco Antonini-Canterin¹²
¹ Emergency Medicine, S. Antonio Hospital, AO Padova, Italy
² Monzino Cardiology Center, IRCCS, Milano, Italy
³ AOU Policlinic, University of Catania, Italy
⁴ Cardiology, Hospital of Desio, Monza (MB), Italy
⁵ Division of Cardiology, Policlinico University Hospital of Modena, Italy
⁶ Cardiology, Fatebenefratelli Hospital, Benevento, Italy
⁷ Cardiology, Cardio Neuro Vascular Dep. Asl Sudest Toscana, Hospital of Grosseto, Italy
⁸ Geriatrics, AOU Mater-Domini, Catanzaro, Italy
⁹ Cardiology, “Mazzini” Hospital, Teramo, Italy
¹⁰ Cardiology, G.O.M. “Bianchi Melacrino Morelli”, Reggio Calabria, Italy
¹¹ Department of Emergency Medicine, University of Padova, Italy
¹² Rehabilitative Cardiology, Highly Specialized Rehabilitation Hospital, Motta di Livenza, Treviso

Date of Submission	21-May-2020
Date of Decision	17-Jul-2020
Date of Acceptance	06-Sep-2020
Date of Web Publication	26-Oct-2020

Correspondence Address:
Agatella Barchitta
S. Antonio Hospital, AO Padova, Street Facciolati, 71 Padova
Italy

Source of Support: None, Conflict of Interest: None

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DOI: 10.4103/jcecho.jcecho_53_20

Abstract

This paper aims to highlight the usefulness of “bedside” lung ultrasound in the context of the COVID-19 pandemic. The evaluation of lung artifacts allows to detect at the subpleural level the presence of an altered “tissue/air” ratio both in case of consolidative or not consolidative lung lesions. Furthermore, lung ultrasound allows acquiring topographical images of the lesions, establishing their extension on the lung surface as well as their evolution or regression over time, without radiation exposure. Since ultrasound semiotics is already widely known and described in other similar diseases (acute respiratory distress syndrome, interstitial flu virus, and pneumonia), thoracic ultrasound is a useful diagnostic tool in different scenarios in the COVID-19 pandemic: in the first triage of symptomatic patients, both in the prehospital setting or in the emergency department, in the prognostic stratification and monitoring of patients with pneumonia, and in the management of patients in the intensive care unit. Moreover, “bedside” lung ultrasound can reduce the number of health-care workers exposed to the virus during patient assessment and treatment.

Keywords: Artifacts, COVID-19, lung ultrasound, semiotics

How to cite this article:
Barchitta A, Pepi M, Monte IP, Trocino G, Barbieri A, Ciampi Q, Cresti A, Miceli S, Petrella L, Benedetto F, Daniele M, Antonini-Canterin F. Lung semiotics ultrasound in COVID-19 infection. J Cardiovasc Echography 2020;30, Suppl S2:1-5

How to cite this URL:
Barchitta A, Pepi M, Monte IP, Trocino G, Barbieri A, Ciampi Q, Cresti A, Miceli S, Petrella L, Benedetto F, Daniele M, Antonini-Canterin F. Lung semiotics ultrasound in COVID-19 infection. J Cardiovasc Echography [serial online] 2020 [cited 2022 Aug 13];30, Suppl S2:1-5. Available from: https://www.jcecho.org/text.asp?2020/30/6/1/299221

Introduction

The application of lung ultrasound has spread during the past 15 years in different fields and various pathologies. Lung ultrasound can be integrated with echocardiographic findings to obtain more accurate diagnoses and to analyze hemodynamic parameters. To better understand the lung lesions that can be detected with lung ultrasound in patients suffering from COVID-19, it is necessary to remember some elements of pulmonary ultrasound semiotics. Lung echography can explore the thoracic wall, the ribs and their shadow cone, the pleural line, the mediastinum, the lung parenchyma, and the diaphragm. Our purpose is to focus on lung parenchymal and pleural findings.^[1]

Procedures

Choice of the probe

All the traditional probes can be used, taking into account that:

The adult cardiac sector ultrasound transducer does not allow an accurate examination of the pleural line but provides adequate visualization of pleural effusion and lung consolidation; however, this probe permits only a generic evaluation of interstitial syndrome, but it has the important advantage of using the same probe used for echocardiography
The linear transducer is chosen for the evaluation of chest wall, pleural line, and its movement and for visualizing the homogeneity or micronodules of the lung surface
The convex transducer is versatile and has many more benefits than the others.^[2]

For COVID-19 patients, the convex and linear probes are suggested.^[3],[4]

Exploration of the chest wall

Every district (anterior lateral and posterior) of the chest wall must be explored, including the diaphragm and subdiaphragmatic areas. A brief exploration of the heart and the inferior vena cava should be performed since lung ultrasound and echocardiography are complementary. Both sides of the chest wall must be explored with a longitudinal scan from parasternal to the anterior axillary line (anterior chest wall), from the anterior to the posterior axillary line (lateral chest wall), and from posterior axillary line to paravertebral line (posterior chest wall). Both lungs can be divided into six different quadrants: two in the anterior chest wall (upper and lower), two in the lateral chest wall (upper and lower), and two in the posterior chest wall (upper and lower). The line which divides the upper quadrants from the lower is the transversal mammillary line. When an area of interest is found, it should be explored using a transversal scan.^[1],[2]

Interpretation of the Artifacts

Lung echography is the study of artifacts generated by the interaction between ultrasounds and air. These artifacts appear different according to the pulmonary air content and the homogeneity of its distribution. It is necessary to evaluate the presence of ultrasound artifacts in multiple areas bilaterally, to determine the extent of the lung surface affected by the lesions.^[5]

There are two pleural layers; the parietal layer, which is attached to the chest wall and does not move, and the visceral layer, which is adherent to the lung and follows the lung in its respiratory movements. Between the visceral and parietal pleura, there is a thin layer of pleural fluid that allows the sliding movement of the visceral pleura upon the parietal pleura (this movement is called the “pleural sliding”). After having positioned the probe in an intercostal space, it is possible to recognize the pleural line and visualize the movement of the visceral layer sliding upon the parietal layer. If lung stiffness is present (e.g., atelectasis, lung consolidation, or hyperinflation in asthma), pleural sliding is substituted by the “lung pulse,” which is a movement of the pleura synchronized with the heartbeat. However, the lung pulse is also detected in healthy patients in the pleura adjacent to the heart. If scans are acquired with M-mode, we can recognize the “seashore sign:” the motionless superficial layers generate horizontal lines (the waves), whereas the deep artifacts below the pleural line generate a sandy pattern. At the lung posterior bases, there is the “curtain sign:” with inspiration, the lung bases cover the abdominal organs (liver and spleen); in case of pleural effusion, this effect is absent, and between the lung bases and the abdominal organs, there is a dark, anechoic, or hypoechoic area. The “A-lines” are horizontal reverberation artifacts reproducing in parallel the pleural line more deeply [Figure 1]a and [Figure 1]b. The distance between every A-line is the same distance between the probe interface and the pleural line. In the healthy patient, the ultrasound pattern observed is the “Type A” (or “dry lung”): this is the classic picture of normal lung ventilation with a preserved ratio between aerated alveoli and thickness of their interlobular septa, characterized by A-line, seashore effect [Figure 1]c, and lung sliding (or lung pulse next to the heart). The “B-lines” are well-defined vertical reverberation artifacts, visualized as comet tails. The B-lines are defined with four constant criteria: (1) vertical, (2) laser-like, (3) hyperechoic reverberation, and (4) arise from the pleural line extending to the bottom of the screen without fading. The B-lines mask the A-lines and follow the movement of the pleural sliding. The finding of rare B-lines at the lung bases is normal [Figure 2].^{[1],[2],[5],[6],[7]}

Figure 1: (a) pleural line and A-line. (b) Multiple A-lines. (c) M-mode “seashore sign”

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Figure 2: Two B-lines

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Echographic Patterns

Pneumothorax

To better evaluate the COVID patient, it is fundamental to recognize ultrasound signs in some conditions such as pneumothorax (PNX), alveolar-interstitial syndrome, and pulmonary consolidations. Since these patients are often ventilated with high positive end-expiratory pressure and barotrauma can occur, recognition of the PNX is mandatory. The PNX can be demonstrated with the absence of pleural sliding, absence of B-lines, and lung pulse. These signs are highly specific. In M-mode, the seashore sign is substituted by the stratosphere sign [Figure 3]: below the pleural line, there is no sandy artifact, but instead, there are multiple transversal lines. During PNX, the air moves to the anterior chest wall, whereas the lung collapses toward the posterior chest wall. Therefore, chest exploration for PNX must be performed from the parasternal line to the lateral chest wall and then to the posterior chest wall. The point where the pleural sliding reappears indicates the margin of the PNX and is called “lung point.” Hence, if the lung point is in the lateral or posterior chest wall, the PNX will be more severe.^{[1],[2],[5],[6],[8]}

Figure 3: M-mode pneumothorax stratosphere sign

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The interstitial syndrome

The interstitial syndrome is a nonconsolidative lung parenchymal disease that includes cardiogenic edema, acute respiratory distress syndrome (ARDS), and interstitial lung diseases. The COVID-19 determines interstitial pneumonia and in the most severe patients, ARDS.^[4] The corresponding image in a computed tomography scan is the “ground-glass” lesion. The interstitial syndrome determines an ultrasound pattern characterized by the presence of multiple and bilateral B-lines. This pattern is called the “Type B” pattern. According to the European Society of Cardiology guidelines, echography must be used to search for B-lines in acute or chronic heart failure due to systolic or diastolic dysfunction. B-lines are reverberation artifacts due to an important acoustic difference between ventilated alveoli and thickened subpleural interlobular septa. In a healthy patient, the ratio between ventilated alveoli and interlobular septa determines to Type A pattern. When pulmonary aeration is reduced, such ratio is altered due to thickened interlobular septa [e.g., because of fluids, cells, or proteins, see histology in [Figure 4]a and the echography will reveal multiple B-lines [Figure 4]b. As the interlobular septa become more thickened, the number of B-lines increases until all the B-lines are completely fused, creating the image of the so-called “white lung.” The number of B-lines of each intercostal space must be counted: B-lines are defined “crowded” if more than ten B-lines are observed in an intercostal space indicating a severe interstitial syndrome. In summary, when the examined portion of the lung is completely ventilated, the pattern observed is the Type A, but as ventilation decreases and septa get thickened, the lung acquires a Type B pattern.

Figure 4: (a) Thickened interlobular septa. (b) Type B pattern in cardiogenic edema. (c) Pleural effusion

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In acute pulmonary cardiogenic edema, the B pattern is symmetrically distributed and first appears at the lung bases. B-lines are bright, have a narrow origin from the pleura, and can be distinguished one from the other. As the edema worsens, the B pattern extends gradually toward the pulmonary apexes, whereas, with treatment, the edema (and consensually the Type B pattern) regresses from the apexes to the bases. Moreover, the pleural line is thin, and bilateral pleural effusion can be observed at bases (the curtain sign disappears) [Figure 4]c.

The Type B pattern has a high sensitivity not only for cardiogenic edema but also for other noncardiogenic forms of interstitial syndromes such as acute lung injury/ARDS or interstitial lung pathologies. The element which best helps in the differential diagnosis between cardiogenic edema and ARDS is the distribution of the Type B pattern: in the ARDS, the distribution is asymmetric, areas of B pattern are alternated with spared areas, and the pleural line is irregularly thickened and interrupted by subpleural and parenchymal consolidations. Moreover, B-lines in ARDS are not bright and are alternated with focal white lung areas and spared areas [Figure 5]a and [Figure 5]b.^{[6],[7],[8],[9],[10],[11],[12],[13],[14],[15]} The acoustic window for lung ultrasound is always patent, even when transthoracic echocardiography is not feasible.

Figure 5: (a) Type B pattern in acute respiratory distress syndrome. (b) Type B pattern in ARDS with spared areas

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Alveolar syndrome (or pulmonary consolidation)

Alveolar syndrome or pulmonary consolidation appears as a hypoechoic area which interrupts the pleural line. When lung aeration drops to 10%, the lung acquires a solid parenchymal consistency, defined as a tissue-like aspect [Figure 6]a and [Figure 6]b. These findings characterize lobar pneumonia. In ARDS, there is an alveolar-interstitial syndrome since both Type B pattern and lung consolidation are present.

Figure 6: (a) Lung consolidation. (b) Lung consolidation with visible bronchograms

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Inside the tissue-like aspect, dynamic air or fluid bronchograms can be visualized as “tree-like” tubular structures with hyperechoic walls. It is important to distinguish pulmonary consolidation due to pneumonia from atelectasis.^{[1],[2],[5],[10],[11]} In pneumonia, focal crowded B-lines can be observed next to the consolidation, whereas crowded B-lines are not observed in atelectasis. Atelectasis is defined as the absence of parenchymal expansion in a lung field with the tissue-like aspect as in pneumonia. In atelectasis, the parenchyma is compressed; therefore, bronchi are not tree-like shaped but are parallel tubular structures. Atelectasis is divided into compressive (nonobstructive) and obstructive atelectasis. Compressive atelectasis is due to high pressure in the pleural cavity (e.g., pleural effusion), which leads to parenchymal collapse; in this case, the lung field involved acquires a triangular shape. The collapsed parenchyma is ventilated and expands inside the effusion during inspiration. Therefore, it is possible to see dynamic air bronchograms. Obstructive atelectasis is due to bronchial obstruction with mucus or neoplasm, and the involved area is not ventilated. In this case, the lung sliding is substituted by lung pulse, and static fluid bronchograms are visible. Lung ultrasound is a useful tool to monitor lung re-expansion during and after secretion removal in bronchoscopy. Vessels located next to the bronchi are recognized with the color Doppler.^[1],[5]

Conclusion

Bedside lung ultrasound is easily performed and allows to make the differential diagnosis of lung pathologies analyzing the artifacts and patterns mentioned before [Table 1]. Lung ultrasound had already been recognized by the cardiologist as a fundamental tool in heart failure diagnosis and monitoring, with a low-cost radiation-free approach. Lung semiotic elements have also been recognized as useful tools for COVID-19 pneumonia [Figure 7]^[4]] diagnosis and management ^[3],[4] since ultrasound semiotics is already widely known and described in other similar diseases (ARDS, interstitial flu virus, and pneumonia). Thoracic ultrasound is fundamental in different scenarios: in the first triage of symptomatic patients, both in the prehospital setting or in the emergency department for differential diagnosis of pneumonia from other noninfective lung diseases, in the prognostic stratification and monitoring of patients with pneumonia, and in the management of patients in the intensive care unit with ventilation, weaning, and monitoring of the therapeutic effect (antiviral and immunomodulating). Moreover, “bedside” lung ultrasound can reduce the number of health-care workers exposed to the virus during patient assessment and treatment.

Table 1: Description of findings in lung ultrasound

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Figure 7: Sequential interpretation of lung ultrasound findings. The blue boxes highlight ultrasound findings in the COVID-19 patient. Modified from Via et al.

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Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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