To date, several studies have been published assessing the clinical usefulness of lung ultrasound in newborns with respiratory disorders [1–14]. These works focused mainly on preterm infants with RDS and transient tachypnea of the newborn (TTN); in both cases, LUS was performed primarily on the first day of life. [1–11, 13, 14].
Our research provides an additional body of evidence as it was based on post-birth lung sonograms complemented by sequential scans performed over a period of 28 days. As a result, the correlation of LUS with the severity of respiratory failure as expressed by oxygenation indices and the need for respiratory support was demonstrated beyond the first day of life.
Additionally, our study covered a diverse spectrum of patients, both before and after administration of surfactant, infants with significant posttreatment improvement and those who deteriorated in the follow-up examinations, patients with developing pneumonia and those with bronchopulmonary dysplasia (BPD). Sonograms reflected all sorts of respiratory statuses, ranging from spontaneous breathing ambient air through nasal CPAP to invasive ventilation. Thus, a close correlation between the LUS and the indices of respiratory failure was demonstrated in much more diverse clinical conditions. This allows the LUS to be treated as a more universal measure of pulmonary pathology and is not limited to the typical changes observed in neonates with RDS on the first day of life.
In studies where lung ultrasound images were quantified, four-grade scales were mainly used (a score of 0-1-2-3) [1, 3, 5, 6, 8–11]. In our work, we proposed a five-grade scale (a score of 0-1-2-3-4), defining an additional category of “white lung with fluid alveologram”. This sonographic pattern is sometimes referred to in the literature as shallow consolidations and should be distinguished from true consolidations. True consolidations are hypoechogenic triangular-shaped changes with visible air bronchogram, while fluid alveologram is a subpleural accumulation of exudative fluid, which mixed with air, fulfills the alveoli. Fluid alveologram can be observed in severe RDS [10] and, as a rule, is not seen in the first day of life but rather in the following days. Since the study covered the entire first month of life of premature babies, it seems appropriate to classify this type of change separately.
We abandoned the division of pulmonary fields into upper and lower sections. Previously, Pang et al. [10] expanded the assessment scales by including posterior pulmonary fields, dividing each of the studied lung areas into the upper and lower parts while maintaining a four-grade scale. This type of division used also by Raimondi et al. [11] is adopted from adult studies, where due to the chest size, it seems expedient.
In our modification of the lung ultrasound examination, we propose assessment of posterior and not lateral pulmonary fields. In almost all available works, ultrasound examination was limited to the anterior and lateral parts of the lungs. Very few studies have reported assessments of posterior pulmonary fields [4, 10, 11]. The “anterior-lateral” approach leads to deviation from the significant advantage of lung ultrasound over standard anteroposterior X-ray, namely, the possibility of distinguishing between changes in the anterior and posterior parts of the lungs. Additionally, it is known from daily clinical practice that most pulmonary pathologies tend to have gravitational positioning. Our study confirmed this observation, as posterior scores had significantly higher weight in the total LUS score compared to anterior scores (ls means 4.0 vs 2.2; p < 0.0001).
In the analysis, we did not focus on the prognostic value of LUS in relation to the need for surfactant treatment, which has been previously reported [5]. Because LUS is closely correlated with oxygenation indices and surfactant administration is based on the level of inspired oxygen fraction, such a relationship seems expected. However, we demonstrated significant predictive ability of the postnatal (performed within 24 hours from birth) LUS to prognose the need for mechanical ventilation on subsequent days – an aspect that has not been investigated before. Depending on the birth weight, specific LUS cut-off levels indicate infants in whom invasive ventilation is the most likely mode of respiratory support on DOL 3. The need for invasive ventilation on DOL 3 is a widely recognized endpoint in neonatal RDS and is strongly correlated with subsequent adverse outcomes. Thus, our findings may have significant practical implications. Modified LUS allows for early identification of the most vulnerable infants and may lead to earlier decisions, e.g., administration of exogenous surfactant or more aggressive respiratory support.
The analysis also provides an important observation regarding the association of oxygen requirements and the severity of lung changes. As shown in Fig. 2, the increasing severity of pulmonary changes is not accompanied by a parallel increase in oxygen demand until the LUS reaches 5–6 points, which corresponds to approximately 1/3 of the LUS maximum possible score. Simply put, this means that only when approximately one-third of the lung parenchyma is involved in the pathological process does an increased demand for oxygen appear. This observation is not surprising from a pathophysiological point of view, as for most organs (kidney, liver), the pathological process must cover a certain area of the organ to cause clinical symptoms.
However, this fact sheds new light on the applied criteria for surfactant treatment. According to the current therapeutic guidelines, the decision of surfactant administration is based on the required level of inspired oxygen [15]. With the current approach, if there is no increased oxygen requirement, surfactant is not applied. In light of our findings, this means that treatment is not administered until the severity of lung changes as measured by LUS reach at least 1/3 of its maximum possible level. The current treatment paradigm requires verification in properly designed clinical trials where surfactant would be LUS-guided, and the primary efficacy endpoint would not be the failure of CPAP, as it is at present, but the development of bronchopulmonary dysplasia.
In our study, LUS assessments had a very high interrater agreement, as reflected by Cronbach’s alpha equal to 0.99. Although all scans were assessed by two neonatologists with expert-level skills, lung ultrasound is known to be a diagnostic procedure that is easy to master and has a steep learning curve. Daily observations outside the scope of this study show that lung ultrasound examination can be easily learned during two-day courses and 1–2 months of supervised practice. It is therefore likely that the reproducibility of the LUS assessments carried out by ultrasound experts would be replicated by less experienced sonographers.
The current situation of using different scales for assessing the severity of changes in lung ultrasound in newborns is summarized in a meta-analysis published by Razak et al. [16]. In their conclusions, the authors emphasize the close correlation of ultrasound assessment to clinical parameters, which proves the usefulness of this tool.
In summary, we proposed a modified LUS, which is characterized by a high correlation with oxygenation parameters and respiratory support modes and a very high consistency of assessments between performers. We also confirmed the need to examine the lungs as a whole, including the posterior fields, taking into account the impact of gravity on the location of the lesions. The study showed that LUS is a tool that can be used for the early postnatal identification of infants at risk of invasive ventilation in the subsequent days of life. Finally, the modified LUS is suitable not only in newborns during the first days of life and remaining on noninvasive respiratory support but also in mechanically ventilated infants with various respiratory disorders during the first month of life.