Lung ultrasonography in COVID-19

Introduction

Lung ultrasonography has been established to offer diagnostic capability similar to chest CT; it is more efficacious compared to chest radiography in the evaluation of pneumonia and the adult respiratory distress syndrome (ARDS). Besides, lung ultrasonography is easily carried out at the point of care, with the added advantage of repeated examination as required. Besides, it is cost-effective with no exposure to ionizing radiation. Lung ultrasonography has become the standard for the assessment and follow-up of critically ill patients. It may be useful in the planning of treatment strategies, including prone ventilation and the titration of positive end-expiratory pressure. Ultrasonography of the lung bears a strong correlation with the findings on high-resolution CT imaging. Lung ultrasonography has emerged as a potential tool in the diagnosis, initial assessment, and management of COVID-19 patients. 

Which probes to use?

A high-frequency linear probe is most suited to view the pleura and abnormalities that emanate just beneath the pleura, including sub-pleural ground-glassing and consolidation. A curvilinear probe (≈ 5 MHz) or a microconvex probe (≈ 8 MHz) may be used, depending on the body habitus to view deeper regions of the lung. Examination is performed in two ways; first, the probe is positioned vertically within an intercostal space with the probe marker pointing in the cranial direction. This is followed by horizontal placement within the intercostal space with the probe marker pointing laterally. 

Which areas to scan? 

A 12-zone examination (Figure 1) is generally adequate, with careful examination of the posterior zones. If the lateral position is difficult, a slight tilt to the side from the supine position is usually adequate; the microconvex probe is easy to place to examine the posterior zones of the lung from this position. It is important to scan posteriorly to the extent possible, to cover the PLAPS point, where typical features may be observed.1 The examination is carried out on the lung or abdominal preset mode. The depth is adjusted to 6–10 cm with the focal point set to the pleural line, and the gain is optimized.

Figure 1. The 12-zone ultrasonographic examination of the lung. Each hemithorax is divided into 12 zones. Yellow arrow: PLAPS point 

Typical ultrasonographic features

Pleural thickening and irregularity 

The pleural line is normally 2 mm thick. Increased thickness of the pleura with an irregular or fragmented appearance is a common finding in patients with COVID-19. This abnormality is usually patchy and may be seen in both lungs, depending on the extent of involvement. A more generalized pattern of pleural irregularity may suggest fibrosis associated with long term disease. 

Subpleural consolidations

Patchy areas of subpleural consolidation are one of the early ultrasonographic findings in COVID-19. Consolidation appears as subpleural hypoechoic areas with ragged pleural margins, referred to as the “shred” or the “fractal” sign2 (Figure 2). As the disease evolves, they become more widespread along the surface of the lung. At a later stage, consolidation extends towards the deeper areas of the lung, with increasing likelihood of respiratory failure.

Figure 2. Arrow points to a fragmented segment of the pleura with subpleural consolidation

B-lines

Sub-pleural areas of ground-glass opacities are one of the typical early features of COVID-19. This is represented on lung ultrasonography as B-lines, which arise from and move with the pleural line and extend to the bottom edge of the image, obliterating A-lines along the way. The B-lines may be seen separate from each other (Figure 3A) or coalesced to represent an ultrasonographic “white lung” (Figure 3B). A unique ultrasonographic feature in COVID-19 is a light band-like artifact that arises from a wide area of the pleural line that seems to flash on and off with respiratory excursions. This “light band” appears against a background of normal-looking A-lines.3 This artifact has also been described as the “waterfall” sign.4

Figure 3A: well-separated B-lines arising from the pleura; B: coalescent B-lines

Lobar consolidation 

Consolidation in the deeper areas of the lung with lobar involvement is seen in the late phase of COVID-19 or with bacterial superinfection. The lung assumes a “tissue-like” texture with lobar consolidation; hyperechoic lines that streak across the tissue-like lung represent ultrasonographic air bronchograms (Figure 4) The air bronchograms move back and forth with respiratory excursions, assuming a dynamic pattern. 

Figure 4. Air bronchograms in lobar consolidation. The lung assumes a tissue-like echo texture 

Pleural effusions

Unless complicated by comorbid disease, including cardiac or renal dysfunction, large pleural effusions are uncommon in COVID-19. However, small pleural effusions are often seen, associated with areas of areas of densely consolidated lung.4

Lung aeration score 

The lung aeration score is calculated by assigning a score to each zone of the lung examined.  

0: Normal. The pleural line appears continuous and regular. Horizontal A-lines are readily seen 

1: B-lines seen, but are well-separated

2: Coalescent B-lines, or the “light band” sign

3: Features of consolidation

Thus, the total score ranges from 0 to 36 (using a 12-zone approach) with higher scores indicating more severe disease. 

Ultrasonographic studies in COVID-19

Poggiali et al. evaluated the usefulness of lung ultrasonography in patients who presented with COVID-19 pneumonia to the emergency department.5 They studied 12 patients using bedside lung ultrasonography and CT scan. A diffuse B pattern with areas of sparing was observed in all patients. Posterior subpleural consolidations were less common and observed in three patients. The chest CT demonstrated bilateral lung involvement with ground-glass opacities and demonstrated a strong correlation with lung ultrasonography; a crazy-paving pattern was observed in five of 12 patients. Organizing pneumonia was detected on both ultrasonography and CT imaging in four patients.

In a retrospective study, clinical features and ultrasonographic findings were analyzed in 20 patients who were clinically diagnosed with non-critical COVID-19 infection. Ultrasonographic findings in order of frequency included B-lines, sub-pleural consolidations, localized pleural effusion and pleural thickening, and reduced blood flow in consolidated areas of the lung on color Doppler imaging.4   In another study, lung ultrasonography was carried out in 20 COVID-19 patients using a standard 12-zone protocol. Typical ultrasonographic features included thickening and fragmentation of the pleural line, multifocal, discrete and confluent B-lines, focal and lobar consolidation with dynamic air bronchograms, and reappearance of A-lines during the recovery phase of illness. Pleural effusions were uncommon.6

Lichter et al. evaluated 120 consecutive patients who underwent 12-zone lung ultrasonography within 24 hours of admission to the medical ward or intensive care unit. The examination was repeated if clinical deterioration occurred. A lung ultrasonographic score ranging from 0–36 (higher score indicating worse aeration) was calculated for each patient. Patchy pleural thickening was observed in most patients (83%); subpleural patchy consolidation in at least a single zone, was observed in 78% of patients. Clinical deterioration was associated with an increase in the lung ultrasonography score and was commonly due to involvement of anterior lung segments. The requirement for invasive ventilation and mortality were associated with a baseline lung ultrasonography score > 18. 

Summary

  • Lung ultrasonography has emerged as an important tool in the diagnosis, initial assessment, and prognostication of patients with COVID-19
  • Excellent ultrasonographic views are generally obtained using a combination of a high-frequency linear probe for superficial structures, including the pleura, and a curvilinear or microconvex probe for deeper views
  • Common ultrasonographic findings include pleural thickening, sub-pleural or lobar consolidations, discrete or coalescent B-lines, and the distinctive “light band” sign 
  • The lung aeration score is correlated to the requirement for invasive mechanical ventilation and mortality 
  • Further investigations are required to evaluate the utility of lung ultrasonography in optimizing therapeutic interventions, including PEEP titration and the use of prone ventilation among patients with COVID-19 pneumonia

References

1.         Lichtenstein DA. Lung ultrasound in the critically ill. Published online 2014:12.

2.         Biswas A, Lascano JE, Mehta HJ, Faruqi I. The Utility of the “Shred Sign” in the Diagnosis of Acute Respiratory Distress Syndrome Resulting from Multifocal Pneumonia. Am J Respir Crit Care Med. 2016;195(2):e20-e22. doi:10.1164/rccm.201608-1671IM

3.         Volpicelli G, Gargani L. Sonographic signs and patterns of COVID-19 pneumonia. Ultrasound J. 2020;12(1):22. doi:10.1186/s13089-020-00171-w

4.         Huang Y, Wang S, Liu Y, et al. A Preliminary Study on the Ultrasonic Manifestations of Peripulmonary Lesions of Non-Critical Novel Coronavirus Pneumonia (COVID-19)  In Review; 2020. doi:10.21203/rs.2.24369/v1

5.         Poggiali E, Dacrema A, Bastoni D, et al. Can Lung US Help Critical Care Clinicians in the Early Diagnosis of Novel Coronavirus (COVID-19) Pneumonia? Radiology. 2020;295(3):E6-E6. doi:10.1148/radiol.2020200847

6.         Chinese Critical Care Ultrasound Study Group (CCUSG), Peng Q-Y, Wang X-T, Zhang L-N. Findings of lung ultrasonography of novel corona virus pneumonia during the 2019–2020 epidemic. Intensive Care Med. 2020;46(5):849-850. doi:10.1007/s00134-020-05996-6

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