REVIEW ARTICLE

https://doi.org/10.5005/jp-journals-11002-0057
Newborn
Volume 2 | Issue 1 | Year 2023

Lung Ultrasound in Neonates: An Emerging Tool for Monitoring Critically Ill Infants

Arjun Verma1, Abhishek Paul2, Atnafu Mekonnen Tekleab3, Abhay Lodha4, Kei Lui5, Akhil Maheshwari6, Jan Klimek7, Pradeep Suryawanshi8

1Department of Neonatology, Mahatma Gandhi Medical College, Mahatma Gandhi University of Medical Sciences and Technology, Jaipur, Rajasthan, India

2Department of Neonatology, Sanjay Gandhi Postgraduate Institute of Medical Sciences, Lucknow, Uttar Pradesh, India

3Department of Pediatrics and Child Health, Saint Paul’s Hospital Millennium Medical College, Addis Ababa, Ethiopia

4Department of Pediatrics and Community Health Sciences, Cumming School of Medicine, University of Calgary, Calgary, Alberta, Canada

5Discipline of Pediatrics and Child Health, School of Clinical Medicine, University of New South Wales, Sydney, Australia

6Global Newborn Society, Clarksville, Maryland, United States of America

7Department of Neonatology, Westmead Hospital, Westmead, Australia

8Department of Neonatology, Bharati Vidyapeeth University Medical College, Hospital and Research Center, Pune, Maharashtra, India

Corresponding Author: Pradeep Suryawanshi, Department of Neonatology, Bharati Vidyapeeth University Medical College, Hospital, and Research Center, Pune, Maharashtra, India, Phone: +91 9923540500, e-mail: drpradeepsuryawanshi@gmail.com

How to cite this article: Verma A, Paul A, Tekleab AM, et al. Lung Ultrasound in Neonates: An Emerging Tool for Monitoring Critically Ill Infants. Newborn 2023;2(1):80–90.

Source of support: Nil

Conflict of interest: Dr. Abhay Lodha, Dr. Kei Lui and Dr. Akhil Maheshwari are associated as the Editorial Board Members of this journal and this manuscript was subjected to this journal’s standard review procedures, with this peer review handled independently of these Editorial Board Members and their research group.

Received on: 19 January 2023; Accepted on: 04 March 2023; Published on: 06 April 2023

ABSTRACT

Context: Neonatal lung ultrasound is emerging as a useful clinical tool for the assessment of lung anatomy and management of various lung pathologies. In this review, we summarize normal lung ultrasound (LUS) findings and specific features of various lung morbidities.

Evidence acquisition: A comprehensive literature search was conducted across multiple sources with relevant keywords with an additional filter of the age-group between 0 and 28 days.

Findings: Apart from the description of normal newborn lungs, clinical and radiological features of a variety of lung pathologies were evaluated and incorporated in the review. Bedside LUS has evolved to be an important point-of-care imaging modality that can help in day-to-day clinical decision-making. It can be used in differentiating respiratory distress syndrome from transient tachypnea on the newborns, in the detection of pneumothorax, and in diagnosing pneumonia, pulmonary hemorrhage, and pleural effusion. Evidence supports the use of LUS scores to decide on the need for early rescue surfactant therapy with high sensitivity and specificity. Lung ultrasound scores obtained during the first 2 weeks after birth can help predict the likelihood of chronic lung disease/bronchopulmonary dysplasia. Once validated, it could be valuable for guiding early intervention and evaluation of new treatments.

Conclusion: Neonatal lung ultrasound is emerging as a vital monitoring tool in critically ill infants with lung disease. It will be valuable in the early diagnosis, management, and prognosis of these patients.

Keywords: Bronchopulmonary dysplasia, Conventional lung ultrasound score, Modified lung ultrasound score, Pneumothorax, Point-of-care lung ultrasound, Respiratory distress syndrome, Transient tachypnea of newborn.

KEY POINTS

INTRODUCTION

Ultrasound of the lung is emerging as an important tool in the clinical care of newborn infants.1–8 This is an interesting development because sound waves are not known to penetrate air very well and consequently, the aerated lungs show many artifacts.9–11 In neonates, the relatively under-aerated lungs permit better sonic evaluation of various pathologies with higher sensitivity and specificity than in adults.12–15 Lung ultrasound can help in bedside diagnosis/evaluation of respiratory distress syndrome (RDS), pneumonia, transient tachypnea of the newborn (TTN), pleural effusion, and pneumothorax.1,5,16–22 Compared to chest radiographs, LUS is more convenient as it can be readily available at the bedside, is safer as there is no radiation exposure, and has the potential to improve care by allowing quick interpretation and repeated testing to monitor the course of disease and response to therapy.22,23–32 Ultrasound can help evaluate the response to recruitment maneuvers during high-frequency ventilation.33–36 There may be less inter-observer variability, and consequently, higher reliability.1,37,38

Frequently Used Terms and Scanning Methods

Lung ultrasound is commonly done using a high-frequency linear transducer or a hockey-stick transducer (10 Mhz or higher), in supine, lateral, and prone positions. The transducer is placed either parallel or perpendicular to the ribs. Each lung is evaluated in three regions:

  1. The anterior area between the sternum and the anterior axillary line;

  2. The lateral area between the anterior and posterior axillary lines; and

  3. The posterior area between the posterior axillary line and the spine.

Ultrasound evaluation of the lung may be performed in a 6- or 12-region approach (Fig. 1).

Figs 1A and B: (A) A hockey stick transducer (15 Mhz) used for lung ultrasound imaging; (B) Positioning of the probe parallel and perpendicular to the ribs and the blue boxes show various zones scanned in lung sonography. Left: 6-region approach showing anterior and lateral views of both the lungs. Right: 12-region approach can show upper anterior, lower anterior, upper lateral, lower lateral, upper posterior, and lower posterior views

Pleural Lines

These hyperechogenic, thin, regular, and smooth horizontal lines are typically located 0.5 cm below the rib line.1,37,39 These lines are an echo-reflection of the interface of the pleural lung surface.40–42 When viewed together, the ribs and the pleural line have been described as the ‘bat’ sign.2,40,43

  • A-lines: These reverberation artifacts are a series of linear, hyperechoic structures seen starting from just below the skin all the way to the pleural line.1,40,44 These parallel lines are separated from each other at near equal distances, and may be associated with a set of parallel rods in a ‘bamboo sign’ (Fig. 2).7,32,40,45

  • B-lines: These laser-like vertical hyperechoic artifacts arise from the pleural line and may continue up to the edge of the screen without fading (Fig. 2).46 B-lines movesynchronously with lung–sliding breathing movements.2,40,43 These lines indicate decreased aeration and accumulation of fluid in the interstitium. B-lines can be seen for 24–36 hours, even up to 48 hours in normal newborns.31,47 In many, both A and B lines are seen.40 The presence of fluid in the interstitial and alveolar space may be seen as confluent B-lines.48

Figs 2A and B: (A) Sonogram shows the normal pleural line (black arrow) and the normal pattern of A-lines (serial white arrows). Seen together, the A-lines have been viewed as a set of parallel rods and described as the ‘bamboo’ sign (serial white arrows); (B) Sonogram shows multiple simple B-lines (white arrows)

Confluent or fused B-lines can fill the entire intercostal space and erase the A-lines.49 However, the rib shadows are not obscured.32 The presence of more than two confluent B-lines in the lung field(s) has been described as alveolar-interstitial syndrome (AIS).6,48,50,51

White-out or opacified lungs

The lung fields are filled with compact B-lines (Fig. 3).12,51,52

Figs 3A to C: Evolution of B-lines as shown in the flowchart and ultrasound images. (A) Sonographic appearance of multiple simple B-lines. A- and simple B-lines can be seen in normal lungs; (B) Confluent B-lines seen covering the entire intercostal space (blue arrow) with the rib shadow in between (green arrow); (C) Compact B-lines representing the fused confluent B-lines with no rib shadows (blue arrow). The detection of compact B-lines in all the lung fields is suggestive of white-out lungs

Lung sliding

The pleural line moves with respiration, which represents the relative movement between the parietal and visceral pleura during the respiratory cycle.40 In a normal lung, the pleural line slides horizontally from one side to another during respiration and has been described as lung sliding.53

Lung consolidation

The lung tissues show air- and/or fluid bronchograms and “hepatization” with a density resembling that of the liver tissue on sonography.54 Consolidated areas larger than 1 cm, not the smaller micro-consolidated areas, are likely to be seen on radiographs.1,31,55,56

Air bronchograms

Dense, snowflake like areas seen in (severe) RDS and pneumonia.57

Lung Points

A specific sign of pneumothorax locates in the position of the gas boundary in cases of mild-to-moderate pneumothorax.40,58 Real-time ultrasound shows transition points between areas that show lung sliding and others that do not.25,59 A straight linear array high-frequency probe (5–13 MHz) may be most helpful in analyzing superficial structures such as the pleural line and providing better resolution.58,60 Double Lung Point

In infants with varying severity or differing nature of pathological changes in different areas of the lung, such as between the upper and lower lung fields, a sharp cut-off point called the double lung point can be seen.47,61

Clinical Applications

Assessment of the Severity of RDS and the Need for Surfactant

The major features of a preterm baby with RDS on LUS include pulmonary consolidation, air bronchograms, alveolar interstitial syndrome, pleural line abnormalities, and diffuse white-out lungs.1,39,62 Together, these findings raise the sensitivity and specificity of LUS to approach nearly 100%.63,64 Traditional chest radiographs for RDS generally show pan-opacified lungs, where it might be difficult to differentiate pulmonary edema from pleural effusion.55,65 Lung ultrasound can add to chest radiographs by defining various pathologies such as atelectasis, consolidation, pulmonary edema, and pleural effusion.66,67 These findings in sonography correlate with the difference in the response of each infant to surfactant and RDS management.67 To grade the severity of the RDS and the response to clinical management of RDS, LUS scoring has been proven to be a consistent and useful modality (Fig. 4).1

Figs 4A to F: Respiratory distress syndrome. A preterm infant born at 28-week gestation with a birth weight of 1100 grams had severe respiratory distress. The Silverman-Andersen respiratory severity score was 7/10. A lung ultrasound was done at birth. (A, C) Compact B-lines with a thickened pleural line; (B) Confluent B-lines; (D, E) Confluent B-lines with thick pleural lines and absent A-lines; (F) Consolidation with the shred sign. The overall score at birth was 12 which correlated with the increasing requirement for FiO2 and the need for surfactant

A 2020 systematic review and meta-analysis showed lung ultrasound (LUS) cut-off scores of > 5–6 to give pooled sensitivity and specificity rates of 88 and 80%, respectively.68 The study concluded that LUS scores can be useful for decision-making on surfactant replacement therapy and assisted ventilation.69 Similarly, the 2019 echography-guided surfactant therapy (ESTHER) trial showed that lung ultrasound performed within 3 hours after birth accurately identified infants who eventually developed severe RDS and needed surfactant.70 The use of LUS also reduced the number of ventilator days and the need for high FiO2 requirements.

Differentiation of RDS from TTN

Respiratory distress syndrome has characteristic features such as consolidation and thickened pleural lines, which are not seen in TTN.71–73 Infants with the double lung point and AIS patterns are more likely to have mild TTN,47,61 whereas those with compact B-lines and severe AIS without the double lung point and consolidation may have severe disease (Figs 4 and 5).57,72 Even though lung ultrasound might not always reliably differentiate between RDS and TTN just by detecting confluent /compact B-lines or AIS pattern and white-out,57,71 sonography would still be more useful than chest X-rays to differentiate between the two groups.74

Figs 5A to C: Transient tachypnea of the newborns. Lung ultrasound at birth in a male newborn born at 37-week gestation with respiratory distress. (A) Double lung point; (B) Confluent B-lines present in all lung fields, showing alveolar interstitial syndrome; (C) Simple B lines (white arrows) and a comet tail sign (black arrow)

Diagnosis of Pneumonia

Bacterial pneumonia is a major cause of neonatal mortality.75 It is also one of the most common hospital-acquired infections in neonatal intensive care units (NICUs).76–80 There has been a high degree of discrepancy in the interpretation of chest X-rays in diagnosing pneumonia between radiologists and neonatologists.81,82 Computed tomography (CT) is a well-accepted gold standard for diagnosing pneumonia, but it is not a good first-line investigation in neonates because of high radiation exposure, issues with feasibility, and high costs.83,84 Lung ultrasound is emerging as a good first-line, non-invasive bedside modality for diagnosis and monitoring.31,57

The major diagnostic features of pneumonia in lung ultrasound include large areas of consolidation, air bronchograms, the disappearance of lung sliding, presence of lung pulse, abnormal pleural lines, the disappearance of A-lines, and AIS patterns.25,51 Lung consolidation due to pneumonia shows includes large-sized, variably-shaped hypoechoic areas with irregular margins, which are frequently seen in the subpleural regions and are classically associated with the presence of dynamic air bronchograms within the regions of consolidation.2,25,46,85,86 The liver-like appearance of the echotexture in consolidated areas has been described as hepatization of the lung tissue.87,88

The hypoechoic consolidated lung frequently contains hyperechoic linear elements that represent air in the bronchioles, called air bronchograms.89,90 There are two types of these air bronchograms, named static and dynamic.91,92 Dynamic air bronchograms show centrifugal air movement in the bronchi in real-time lung ultrasound.85,91,93 These are of diagnostic importance in differentiating pneumonia from resorptive atelectasis, which frequently tends to be static.85,92,93

Lung pulse is another diagnostic feature seen in real-time lung ultrasounds.94 It is evocative of the apex beat and can be explained by the presence of a consolidated lung transmitting the vibrations of the beating heart.53,94 Lung pulse is also seen in atelectasis, but the pulse seen in consolidation due to pneumonia is associated with the disappearance of lung sliding.25,40 Therefore, the lung pulse can be visualized as a replacement of the normal lung sliding by pulsations that coincide with the heart rate.94 Neonatal pneumonia has been recorded to be associated with the loss of lung sliding in 75%, lung pulse in 30%, and dynamic air bronchograms in 52.5% of the cases.95

In pneumonia, several non-specific lung findings can be seen. These include abnormalities of pleural lines ranging from fine lines with a coarse appearance to prominent irregularities, disruption, to the disappearance of pleural lines.96,97 These abnormalities correlate with the severity of the inflammatory reaction.96 Some cases show associated pleural effusions, compact and confluent B-lines, and AIS patterns.96,98

Diagnosis of Pulmonary Atelectasis

Atelectasis is frequently seen in criticallyill infants.99 These lesions can prolong the duration of ventilation and hospitalization.15,100 Atelectatic areas have been classified as obstructive or non-obstructive, unilateral or bilateral, and segmental or lobar.101 On ultrasound, atelectasis can be viewed as focal or occult.102 Focal atelectasis shows as an area of consolidation that is delimited by clear margins and contains air and/or fluid bronchograms.41

In atelectatic foci, the lung pulse is regularly visible. There are air bronchograms, which have a static, hyperechoic appearance.25,103 There may be pleural line abnormalities, loss of A-lines, and findings of AIS around the atelectatic areas.1,31,103,104 Occult lung atelectasis is so-named because these lesions are difficult to see on routine radiographs.105 Lung ultrasound is useful for detecting these lesions; there are small areas of consolidation with irregular edges and punctate air bronchograms.106 Lung sliding is present, but there is no lung pulse.106 Identification of occult atelectasis is important in infants who are being weaned off assisted ventilation but still show limited lung function.24

As described above, regions affected by pneumonia show fluid bronchograms. Unlike the hyperechoic air bronchograms seen in atelectatic foci, these hypoechoic lesions can be linear or dendritic in pneumonic patches.103,104 Lung ultrasound is useful for differentiating lung consolidation due to RDS, pneumonia, and atelectasis.107 Lesions related to RDS are seen in premature infants or infants of diabetic mothers in the first few days after birth, pneumonia, and those due to atelectasis during high-severity illness or during assisted ventilation.15,57,108 The differentiating features are summarized in Table 1.

Table 1: Features of lung consolidation in RDS, pneumonia, and in atelectasis
Differentiating features RDS Pneumonia Atelectasis
Timing of the lesion Acute phase Acute phase During recovery
Static air bronchograms Punctiform arrangement Punctiform arrangement Linear parallel arrangement
Dynamic air bronchograms May be present Present (high PPV) Absent
Margins Regular/irregular margins Irregular margins only Sharp, clearlydefined margins
Pleural effusion Rare Small, parapneumonic Large effusion
Fluid bronchograms Less likely Less likely Classical for focal atelectasis
Shred sign Less likely Strong association Less likely

Detection of Pulmonary Hemorrhage

Pulmonary hemorrhage is one of the most critical emergencies in criticallyill neonates.109 It is frequently seen in extremely preterm with hemodynamically-significant patent ductus arteriosus, hypoxic-ischemic encephalopathy, and fulminant sepsis with disseminated intravascular coagulation. Chest X-ray usually reveals fluffy opacities, focal ground-glass, and sometimes, complete white-out opacities. Lung ultrasound can help in the early identification of pulmonary hemorrhage for emergency bedside management.110 Alongside supportive clinical history, supportive sonographic signs include the shred sign and pleural effusions.110

Some sonographic features overlap between infectious pneumonia and pulmonary hemorrhage.111 In pulmonary hemorrhage, large areas of consolidation can be associated with massive pleural effusions.2 In these effusions, floating fibrinous strands can be the signs of incomplete coagulation.110 The volume of the effusion may reflect the severity of bleeding and that of the overall illness.112 Lung consolidation may reflect the primary disease such as pneumonia or RDS, but some areas may be atelectatic due to airway blockage from secretions or thrombi.112

Shred sign is another classical feature of pulmonary hemorrhage (Fig. 6).113 It is characterized by thick, irregular, broken hyperechoic lines that separate the aerated and consolidated segments of the lung.114 Some non-specific features may include the absence of A-lines, abnormal pleural lines, and alveolar interstitial pattern.40 The lung ultrasound features can sometimes overlap with other differentials like pneumonia with parapneumonic effusion, RDS, and pulmonary atelectasis.25 A careful assessment of the clinical features can be helpful.

Figs 6A to D: Pneumonia and pulmonary hemorrhage. (A, B) The shred sign of consolidation. Shred sign is the transition between the normal lung and the areas with consolidation, seen as hyperechoic broken lines; (C, D) pulmonary hemorrhage with air bronchograms (green arrows). Fluid bronchograms (red arrow) in images C and D are hypoechoic linear lines seen below the hemorrhage

Evaluation for Pneumothorax

In infants with a pneumothorax, lung ultrasound can help determine the need for needle thoracostomy and chest tube drainage.14,72,115 In 2019, a systematic review and meta-analysis showed that the overall specificity of lung ultrasound in diagnosing pneumothorax in neonates was 96.7% (95% confidence interval 88.3–99.6%).14 Lung sliding is an important sign; its presence practically rules out pneumothorax. 116 Its absence has a sensitivity of 87.2% and a specificity of 99.4%. If lung sliding is absent, the comet tail sign can provide additional important information.117 Comet tails are short-path vertical reverberation artifacts that fade rapidly and appear like a comet tail. The presence of the comet tail sign also rules out pneumothorax. These findings are shown in Figure 5. To summarize these findings, the absence of lung sliding, B-lines, and comet tail sign may be suggestive of a pneumothorax.58 The lung point is the second most specific sign of pneumothorax; it is the transition point between the presence/absence of lung sliding and indicates the point of gas boundary in cases with mild-moderate pneumothorax (Fig. 7).

Figs 7A and B: Pneumothorax. Flow diagram showing a systematic approach for diagnosing pneumothorax. (A) A-lines with lung point, hypoechoic spare area, and absent lung sliding; (B) Classical barcode sign of pneumothorax on M-mode

The systematic review of 2019 showed a specificity of 100% and sensitivity of 82% in diagnosing pneumothorax.14 The lung point and presence of only A-lines with no lung sliding increase the specificity to 100% and sensitivity to 80%. Lung point denotes mild-to-moderate pneumothorax. However, the absence of lung point with absent lung sliding with A-lines and no comet tail signs or B-lines should be considered as a possible severe pneumothorax.14

The M-mode ultrasound can provide useful information in the diagnosis of pneumothorax.58 In the normal lung, a series of high echodense wavy lines above the pleural line and uniform granular dot echoes below the pleural line together giving a beach-like appearance known as a sea-shore sign.40,58 In cases with pneumothorax, the M-mode granular echo dots below the pleural line are replaced by a series of horizontal parallel lines.58 These horizontal parallel lines are diagnostic of pneumothorax and this ultrasound sign is known as the barcode or stratosphere sign.40,58,90,118 The diagnostic approach to evaluate for pneumothorax is outlined in Figure 7.

Evaluation of Chronic Lung Disease

Lung ultrasound is a promising new modality in the prediction of the risk and severity of chronic lung disease (CLD).119 Sonography has been increasingly used in NICUs worldwide in diagnosing the need for ventilation, surfactant, and the differential diagnosis for newborn respiratory distress.51 Lung ultrasound imaging can be used as a scoring tool to predict the future risk of CLD.120 A recent study used lung sonography at 1 week, 2 weeks, and 4 weeks after birth to predict CLD.121 The study used the classical lung ultrasound score which included upper anterior, lower anterior, and lateral view on both sides of the ling field (Fig. 8).37

Fig. 8: Conventional lung ultrasound scoring system. Scoring system for respiratory distress in the upper anterior, lower anterior, and lateral regions on both sides. Each area can be scored from 0 to 3, as shown in the colored boxes below the scoring sheet. Maximum scores can be as high as 18

A lung ultrasound score of ≥ 5 at 1 week after birth had a sensitivity of 71% and specificity of 80% and the area under the curve (AUC) was 0.8 on a receiver-operating curve. At 2 weeks, the sensitivity and specificity were 74% and 100% with an AUC of 0.93. The study concluded that very-low-birth-weight infants without CLD showed lower LUS scores beyond the first week after birth, whereas those at higher risk of CLD had persistently elevated LUS scores for 4 weeks and beyond.

A recent study in 2022 reported a new prediction method based on a modified lung ultrasound score.122 The investigators modified the classical lung ultrasound score by including sagittal scans of the liver and spleen using a convex probe from the lower end of the ribs.1,123–127 These datapoints were potentially useful because CLD affects the posterior and lower part of the lung more than the anterior region. The study showed that at 36 weeks post-menstrual age, this new model was a better predictor of moderate-to-severe CLD and oxygen dependence. In addition, the use of standardized lists for data collection was also useful.103,128 Overall, lung sonography seems to be a promising modality for predicting CLD, which can help in early intervention with postnatal steroids in the second week of life to improve outcomes.129,130

CONCLUSION

This review sums up the potential importance of bedside lung ultrasound in diagnosis, assessment of the severity of illness, response to therapy, and prediction of outcomes of neonatal lung disease. Lung ultrasound can help in deciding the need for surfactant, in the differentiation of TTN vs RDS, confirming the presence of pneumothorax, and the assessment of the severity of atelectasis in neonates who are difficult to extubate. It also seems to be a promising modality for predicting CLD and deciding about the need for early intervention and modalities for follow-up. There is a need for the development of expertise and standardization of lung ultrasound in neonatal ICUs.

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