Authors: Xuewei Cui, Jianhua Fu
Categories: Reviews, 2
Source: ERJ Open Research
Despite remarkable breakthroughs in diagnosis and treatment, the prevalence of bronchopulmonary dysplasia (BPD) in preterm infants and the consequent mortality have remained high over the last half-century. The pathophysiology of BPD is complicated, with several causes. In addition, infants with severe BPD are predisposed to a variety of complications that need multidisciplinary collaboration during hospitalisation and post-discharge home treatment. Consequently, early prediction, precise prevention and individualised management have become the cornerstones of therapeutic care of preterm infants with BPD, thereby improving patient survival and prognosis. BPD has an operational clinical description; however, it has various clinical phenotypes and endotypes, making accurate prediction challenging. Currently, most approaches for predicting BPD in preterm infants include invasive collection of biofluids, which is inappropriate in fragile neonates. Consequently, researchers and clinicians are becoming more interested in noninvasive monitoring for BPD prediction. Comprehensive assessments of pertinent research, however, remain scarce. In this review, we compared many noninvasive monitoring techniques that contribute to early prediction of BPD development in premature infants.
With continuous advancements in perinatal medicine and neonatal intensive care unit medical procedures over the last three decades, there has been a steady rise in the survival rate of preterm infants, particularly very-low-birthweight infants (VLBWIs) and extremely-low-birthweight infants (ELBWIs) [1]. Bronchopulmonary dysplasia (BPD) was initially identified by Northway et al. [2] in 1967. Despite impressive advancements in the diagnosis and management of BPD, there has not been a substantial drop in its incidence during the subsequent five decades [3], with up to 45% of premature infants at <29 weeks of gestational age (GA) suffering from BPD [4]. Presently, the National Institute of Child Health and Human Development (NICHD) (2001 and 2018) criteria for BPD are widely used [5, 6]. Since BPD has an operational clinical description, it is impossible to apply an objective definition of the disease to reliably predict its prognosis in terms of mortality and morbidity [7]. The “old” BPD has become less common due to the use of pulmonary surfactant and relatively sophisticated mechanical ventilation protocols, whereas the “new” BPD is characterised by alveolar hypoplasia, which is identified by a decrease in the number and an increase in the size of alveoli, simplification of structure and pulmonary microvascular dysplasia. Furthermore, the aetiology of BPD is multifactorial. Different potential prenatal and postnatal risk factors may contribute to BPD [6] (figure 1).
FIGURE 1 Lung morphogenesis and different prenatal and postnatal risk factors contributing to bronchopulmonary dysplasia (BPD). A schematic diagram depicting the development of the lungs from the embryonic stage until alveolarisation. Changes in lung anatomy as gestation advances are shown. The proportion of “old BPD” with “severe lung damage” decreases, whereas the proportion of “new BPD” with “delayed lung development” increases. Preterm infants born at a gestational age of 24–32 weeks have a significant risk of developing BPD. Different potential prenatal (genetic predisposition, infection/inflammation, IUGR, asphyxia and antenatal steroid therapy) and postnatal (mechanical ventilation damage, oxygen toxicity, nutrient deficiencies, infection and pulmonary fluid overload) risk factors may contribute to BPD. IUGR: intrauterine growth restriction; PDA: patent ductus arteriosus.
BPD remains one of the primary causes of mortality in very preterm infants (VPIs). Premature infants with BPD have a significantly higher morbidity of complications, high incidence of long-term neurodevelopmental prognosis and severe compromise on quality of life. Therefore, early prediction, accurate prevention and individual management of BPD have become major challenges in the field of perinatal and neonatal medicine [8]. Ideal BPD biomarkers would target causative pathways involved in BPD pathogenesis, such as biomarkers of immunomodulatory and inflammatory responses, angiogenic growth, epithelial damage and fibrosis, oxidative stress, vascular injury and pulmonary hypertension. Among them, early noninvasive detection of BPD using measures such as biofluid biomarkers, exhaled breath condensates, radiographical studies, lung function and future individualised biomarker “omics” have been the focus of extensive investigation in the field. Although noninvasive methods may not precisely represent the lung and blood concentrations of the respective biomarkers, the application of noninvasive methods will not damage the integrity of the skin and mucous membranes of the infants and can reduce their pain and facilitate continuous measurements. However, comprehensive reviews of pertinent research in the field remain scarce. Therefore, this review aimed to investigate recent advancements in noninvasive monitoring approaches for early prediction of BPD progression and severity within the first week of life, even at birth, with the goal of improving personalised therapy and short- and long-term prognoses (figure 2).
FIGURE 2 Noninvasive monitoring approaches for predicting bronchopulmonary dysplasia (BPD) development and severity. The use of several biomarkers (such as inflammatory markers, fibrotic and epithelial markers, oxidative stress-related markers and angiogenic growth factors) that are detectable in the umbilical cord blood, urine or tracheal aspirate have been suggested for identifying preterm infants at a high risk for BPD. Exhaled breath condensates, lung function and radiological innovations have the potential to provide new quantitative measures of pulmonary function and structure in BPD. The “omics” sciences have emerged as a promising new approach for BPD forecasting. FRC: functional residual capacity; Crs: compliance; Reff·kg^−1^: resistance per kilogram; Rrs: resistance; TV·kg^−1^: lower tidal volume per kilogram; %T-PF: ratio of time to peak tidal expiratory flow to total expiratory time; %V-PF: ratio of volume to peak tidal expiratory flow to total expiratory volume; eNO: elevated exhaled nitric oxide; ETCO: end-tidal carbon monoxide; F
ENO: exhaled nitric oxide fraction; CT: computed tomography; MRI: magnetic resonance imaging; LUS: lung ultrasound.
PubMed searches using MeSH keywords from 1967 to 2022 yielded the data for this review. We searched “Bronchopulmonary Dysplasia” OR “BPD” AND “Early Prediction” OR “Early Diagnosis” AND “Biomarkers” OR “biomarkers” AND “Noninvasive” OR “Non-invasive” AND “Monitoring” OR “Detection” AND “Preterm infants” OR “Preterm newborns” OR “Premature infants” and other similar terms to prevent missing important literature.
In recent years, there has been rapid progress in research on the pathophysiology of BPD, with the focus largely being on inflammation, oxidative stress, pulmonary interstitial fibrosis and epithelial differentiation. Furthermore, the biomarker potential of different cytokines and growth factors that influence lung development or may be associated with lung damage in preterm infants has been investigated [9]. For the most part, the noninvasive prediction approach has been studied using umbilical cord blood (UCB), urine or tracheal aspirate (TA) samples [10, 11].
In the examination of preterm infants prone to BPD, the UCB is possibly one of the simplest and earliest samples to obtain. The UCB screening provides information about fetal disturbances that cause BPD in preterm births. However, they may not precisely represent lung concentrations of the respective biomarker, and it is not feasible to determine whether pulmonary cells produced or released them (table 1).
Interleukin (IL)-6 is a cytokine associated with inflammation and immunological control that binds to receptors and acts on the signalling membrane glycoprotein (gp) 130, activating a cascade of downstream signals and regulating gene expression [12]. A study evaluating the levels of IL-6 and soluble gp130 (Sgp130) in the UCB of 134 preterm infants found that these biomarkers may be used as independent predictors of BPD. The area under the curve (AUC) was 0.849 when IL-6 was >46.125 pg·mL^−1^, demonstrating a positive predictive efficacy on BPD [13].
CD4^+^ T-cells play an essential role as immune system mediators. It is well accepted that CD4^+^ regulatory T-cells (Tregs) have an immunological suppressive function and are essential in maintaining immune homeostasis [14]. In addition, Tregs are engaged in a variety of chronic, inflammation-mediated respiratory disorders [15]. Pagel et al. [16] compared Tregs frequencies in newborns at <29 weeks of GA with and without BPD (n=57 and 56, respectively). Increased Treg frequencies were seen in preterm infants with BPD on day of life (DOL) 4–10 (p=0.0343) [16]. Misra et al. [17] detected CD4^+^ T-cells in UCB mononuclear cells in preterm infants with GA of <32 weeks. The total number of Tregs and non-Tregs in preterm infants with moderate BPD was considerably lower than the number of the aforementioned cells in those with mild or no BPD, indicating that BPD is caused by inflammatory alterations (p<0.05).
Given that pulmonary microvascular dysplasia is a potential aetiology of “new BPD”, aberrant angiogenesis may have a role in the onset of BPD [18]. During angiogenesis, the angiopoietin (ANG)/Tie-2 ligand/receptor system communicates with the vascular endothelial growth factor (VEGF) pathway to determine the development of blood vessels [19].
A prospective study of 102 preterm infants (GA <32 weeks) found that ANG-1 concentrations of UCB were considerably lower while levels of endostatin were significantly higher in infants with BPD than in those without BPD (p<0.001). The levels of ANG-1 in the UCB were inversely associated with endostatin levels in newborns diagnosed with BPD (r= −0.48; p=0.008) [20]. Endostatin, a 20-kDa angiogenesis inhibitor that is a proteolytic fragment of collagen XVIII's C-terminal non-triple helical domain, plays a role in angiogenesis. In VLBWIs with GA <32 weeks, those who developed BPD had higher circulating levels of endostatin in the UCB than those who did not develop BPD (100.7±29.7 ng·mL^−1^ versus 85.6±28.7 ng·mL^−1^; p=0.029) [21]. These findings suggest that prenatally reduced ANG-1 and high endostatin levels in the UCB are risk factors for the development of BPD.
Placental growth factor (PlGF), a VEGF family member, regulates angiogenesis via the modification of VEGF activity by competing to bind with fms-like tyrosine kinase 1 (Flt-1). Apart from its angiogenic action, the overexpression of PlGF in transgenic mice increased alveolar type II cell death, resulting in expanded airspace and pulmonary emphysema, which is pathologically comparable to BPD [22]. Tsao et al. [23] revealed that a higher level of PlGF in UCB was significantly and independently associated with a higher risk of BPD. The PlGF cut-off value for predicting BPD was 17 mg·dL^−1^, with a specificity of 95%, a sensitivity of 53% and a positive predictive value of 83%. However, extremely preterm infants with BPD-associated pulmonary hypertension (BPD-PH) at a corrected age of 36 weeks had significantly lower UCB levels of PlGF and granulocyte colony-stimulating factor (G-CSF) than the subgroups of infants with BPD only and those without BPD or pulmonary hypertension (PH). Analysis of the receiver operating characteristic (ROC) curve demonstrated that reduced levels of placental growth factor and G-CSF were good indicators of BPD-PH (AUC=0.83 and 0.76, respectively) [24].
Endoglin (ENG), a co-receptor of transforming growth factor (TGF) family members such as TGF-β1 and TGF-β3, is highly expressed in cell membranes of vascular endothelium [25]. Placental ENG is upregulated in preeclampsia, releasing soluble ENG (sENG) into the maternal circulation. sENG, which is a truncated form of the extracellular domain of ENG, has an antiangiogenic effect that may inhibit TGF-β signalling in the vasculature [26]. Circulating sENG, which has antiangiogenic properties, may promote endothelial dysfunction [27]. Elevated concentrations of sENG (>3420 pg·mL^−1^) in UCB were associated with an increased risk of severe or moderate BPD in the preeclampsia group, as determined by multivariate logistic regression analysis in a retrospective cohort study of 199 singleton newborns (GA <32 weeks, adjusted odds ratio (OR) 11.9, p=0.006) [28].
Endothelial colony-forming cells (ECFCs) are a type of circulating and resident endothelial cells that may self-renew and generate new vessels. Abnormal ECFCs function may lead to alveolar growth retardation [29]. Baker et al. [30] discovered that the ECFCs level in the UCB was significantly lower in preterm infants who later had moderate or severe BPD than in those who did not (p<0.001). All patients with BPD had lower ECFCs than those without BPD (p<0.05).
During the initial phases of lung development, fibroblast growth factor-10 (FGF-10) modulates branching morphogenesis. FGF-10 promotes the elongation and branching of developing airways [31]. A low concentration of FGF-10 in UCB is independently related to the development of BPD and its severity [32].
The generation of alveoli during lung development is dependent on the synthesis, maturity and remodelling of pulmonary epithelial cell markers as well as extracellular matrix proteins [33–35]. These two may be involved in alveolar hypoplasia, which is a crucial pathological characteristic of the “new BPD”.
Epithelial cells known as club cells border the respiratory and terminal bronchioles. Club cell secretory protein (CCSP) of 10–16 kDa (CC16 or CC10) belongs to the family of secretoglobins [36]. CC16 has been demonstrated to play a role in lung repair by significantly reducing pulmonary inflammation and improving airway regeneration in animal studies, in addition to stimulating epithelial proliferation and protecting against oxidative stress [37, 38]. Schrama et al. [39] inferred that preterm infants with low CC16 concentrations in UCB were at an increased risk of developing BPD. Low CC16 levels may indicate the onset of lung damage, an important factor in BPD progression.
Krebs yon den Lundgen-6 (KL-6) is a high-molecular-weight gp that is predominantly produced by Type II alveolar cells and induces lung fibrosis by acting as a fibroblast chemotactic factor [40]. A case–control study including 74 preterm infants of GA <32 weeks revealed that the cord plasma KL-6 concentrations in the BPD group were substantially higher than those in the non-BPD group (p<0.05). It was shown to be strongly linked with the duration of oxygen exposure (r=0.502, p=0.024). This demonstrates that KL-6 is elevated in BPD and objectively represents disease severity [41].
Matrix metalloproteinases (MMPs) have crucial roles in a variety of pathological processes, including inflammation, cardiovascular problems and respiratory system disorders [42]. Tissue inhibitors of metalloproteinases (TIMPs) regulate the action of MMPs. MMP-9, a core component of the MMP family, is a prominent element of the basement membrane of the airways [43]. Preterm infants with moderate/severe BPD have greater MMP-9/TIMP-1 ratios in the UCB than those with mild/no BPD [44].
Vitamin D, a steroid hormone, has antifibrotic, antioxidative and anti-inflammatory properties. In addition, it stimulates the production of alveolar surfactants and is involved in cell proliferation, cell differentiation, and fetal lung development and maturation [45, 46]. Moreover, vitamin D insufficiency at birth is associated with poor respiratory outcomes, particularly BPD in preterm births [47, 48]. The phenolic hydroxyl group linked to the benzene ring of vitamin E is primarily responsible for its antioxidant effects [49]. UCB levels of vitamins D and E were significantly lower in the BPD group than in the non-BPD group, and this difference was also associated with BPD severity. The vitamin D and E levels were shown to be inversely linked with the period of oxygen support required for preterm newborns with BPD [50]. A retrospective research revealed that a 25-hydroxy vitamin D level of 15.7 ng·mL^−1^ in the UCB was predictive of BPD development (AUC=0.585, 95% CI: 0.523–0.645, p=0.016) [51]. The positive predictive value was 57.1%, whereas the negative predictive value was 63.3%.
Convenience, noninvasiveness and the ability to get a large volume of specimens for repeated measurements are all benefits of urine screening tests [52]; however, the lack of information on urine protein profiles in preterm infants and high variability of protein concentrations present challenges when trying to standardise urinary biomarkers [53] (table 2).
Leukotriene E4 (LTE4) is an inflammatory marker that is associated with the severity of bronchial asthma and may be identified in the blood and urine. It is recognised as a stable and dependable by-product of the destruction of the cell membrane's phospholipid layer [54]. According to a study, urinary LTE4 concentrations on DOL 7 were higher in “atypical” BPD than in “classic” BPD (p=0.006). These findings imply that a continuous inflammatory process may be a key factor in “atypical” BPD [55].
β-2-microglobulin (B2M) is a low-molecular-weight protein that is expressed on the surface of lymphocytes and macrophages, as part of the class I major histocompatibility complex, and is amplified by systemic immunological activation [56]. Shima et al. [57] investigated the relationship between B2M urine excretion levels at birth and the presence of BPD in preterm (24**–**28 weeks) singleton newborns and discovered that those with BPD had substantially higher median B2M levels than those without BPD (p<0.0001). After adjusting for GA and other confounding variables, the determined cut-off value of urine B2M at birth (19.6×10^4^ μg·g^−1^ creatinine (Cr); AUC 0.800; p<0.0001, a positive predictive value of 69% and a negative predictive value of 85%) was associated with the development of BPD.
Beginning with acute inflammatory damage (as in respiratory distress syndrome (RDS)), oxidative stress contributes to lung injury that ultimately results in pulmonary microvascular remodelling, poor alveolarisation and, ultimately, BPD [58]. Multiple studies have demonstrated the role of oxidative stress in the development of BPD [59].
8-hydroxy-2′-deoxyguanosine (8-OHdG) is the most widely used biomarker of endogenous oxidative stress-related DNA damage [60]. We found that urine 8-OHdG levels were substantially higher in the BPD group from DOL 7**–28 than in the control group (p<0.05) in a prospective cohort study of 165 preterm babies. Furthermore, 8-OHdG (DOL 14–**28) levels were shown to be directly correlated with the development of BPD (p<0.05) [61]. Additionally, one urine lipid oxidation product, 11-dehydro-thromboxane B2 (TXB2), was higher in BPD participants (p=0.05) [62].
BPD has been associated with an increase in pulmonary neuroendocrine overexpressed bombesin-like peptide (BLP). Furthermore, in BPD, BLP regulates alveolarisation and angiogenesis [63]. The correlation between hyperoxia, airway remodelling and pulmonary neuroendocrine hyperplasia has also been shown in neonatal rat models of hyperoxia exposure [64]. During DOL 1**–**4, BPD preterm infants had a urine BLP level of >20 000 pg·mg^−1^ Cr (p≤0.001). After adjusting for all confounding variables, multivariable logistic regression analysis demonstrated that increased urine BLP levels were correlated with BPD (p≤0.001) [65].
Infants with BPD have persistent numbers of neuroendocrine cells that produce gastrin-releasing peptide (GRP) in their bronchioles [66]. GRP may modulate BPD in a mechanical way, probably as an oxidant sensor. Urinary GRP concentration in the first postnatal week was positively linked with the diagnosis of BPD at 36 weeks postmenstrual age (PMA) (90 pmol·mg^−1^ Cr versus 67 pmol·mg^−1^ Cr, p=0.005). Biomarkers of oxidative stress including allantoin and 8-OHdG were related to GRP levels over 80 pmol·mg^−1^ Cr, as were BPD, and ventilation and oxygen support durations [67].
N-terminal pro-brain natriuretic peptide (NT-proBNP) is secreted by cardiac myocytes in response to volume overload. The use of serum NT-proBNP as a screening biomarker for multiple neonatal diseases, such as diaphragmatic hernia, RDS, haemodynamically significant patent ductus arteriosus, PH and BPD, has generated increased research interest in recent years [53]. Moreover, the levels of NT-proBNP in the serum and urine are identical [68]. According to our pilot investigation, urine NT-proBNP levels between DOL 7 and 28 were significantly higher in the BPD group than in the control group (p<0.05), positively correlated with mechanical ventilation and oxygen exposure duration (r: 0.175–0.505, p<0.05), and were significantly associated with BPD development (p<0.05) [61].
Studies on BPD biomarkers have largely been conducted using TA and bronchoalveolar lavage fluid samples because they provide direct access to the pulmonary compartment [69]. Although endotracheal intubation for mechanical ventilation is intrusive, tracheal suctioning is administered as required to preserve airway patency in intubated newborns, and it is relatively noninvasive. The convenience of sample acquisition via routine endotracheal suctioning is one of the key benefits of TA examination, even though that leads to a lack of sampling of initially non-intubated infants who may subsequently develop BPD and may not necessarily represent the distal lung parenchymal milieu [70] (supplementary table S1).
Proinflammatory cytokines IL-1, IL-1β, IL-6, IL-8 and IL-16, and tumour necrosis factor-α in earlier studies [71], and interferon-γ, IL1/IL6 ratios and IL1/IL1ra ratios in more recent studies [72, 73], have been shown to be elevated in the TA and to predict adverse pulmonary outcomes, respiratory failure and, in particular, the development of BPD in preterm infants. Furthermore, investigators have hypothesised that newborns who develop BPD have a diminished capacity to produce enough quantities of IL-10 [74].
Monocyte chemoattractant protein (MCP) controls the recruitment of mononuclear cells to the lung and their subsequent activation [74]. The levels of MCP-1, -2, and -3 in the TA were higher in infants with BPD than in those without BPD [75, 76]. Nuclear factor-κB plays a critical role in regulating major proinflammatory mediators [77]. Its activation is elevated in the TA of preterm infants with BPD [69, 78].
In the first 2**–**3 weeks of life, low concentrations of TIMP-1 and TIMP-2 in the TA, high levels of MMP-8 and MMP-9, and ratio of MMP-9/TIMP-1 were attributed to the acute inflammatory process in RDS and later development of BPD in preterm infants [11, 79, 80]. The TA of premature births with BPD demonstrated an increase in TGF-β1 expression [81].
Cathepsin K (CatK) is a cysteine proteinase that has been shown to degrade matrix collagen and elastin. It has been found to be highly expressed in osteoclasts and, more recently, in lung tissue [82]. CatK expression in infants with BPD was dramatically downregulated from DOL 9**–**13 compared to that in controls in the TA (p<0.05) [83].
8-OHdG levels in the TA were comparatively higher in the BPD group than in the non-BPD group on DOL 1 and 28, and they remained elevated on DOL 28 in the BPD group (p<0.05) as reported in a birth cohort study [84]. Furthermore, predicting the development of BPD with TA 8-OHdG levels on DOL 1 (cut-off, 4.4 ng·mg^−1^) yielded an AUC of 0.77, as determined by an ROC curve analysis [84].
Lung capillary maturation is a key process regulated by VEGF, which also plays a role in normal and pathological vasculogenesis and angiogenesis. VEGF acts on cells through two different tyrosine kinase receptors, VEGF receptor (VEGFR)-1 and VEGFR-2. In VLBWIs needing oxygen and assisted mechanical ventilation, Hasan et al. [85] discovered that low VEGF levels in the TA, in combination with high soluble VEGFR (sVEGFR)-1 levels on DOL 1, were biological indicators of BPD development. In addition, preterm newborns with later BPD development had significantly increased sphingosine 1-phosphate concentrations in the TA on DOL 1 [86].
ANG-2 is an angiogenic growth factor that destabilises blood vessels [87]. Higher ANG-2 levels in TA have been correlated with the incidence of BPD or mortality in ventilated preterm newborns [88]. Endothelin-1 (ET-1) is an effective endothelial-derived vasoconstrictor and bronchoconstrictor with the highest concentration in the lungs [89]. The ET-1 concentrations in newborns with BPD were considerably higher than those who did not eventually develop BPD in the first week of life [90]. Furthermore, higher levels of FGF-2 in the TA on DOL 1 were correlated to a poorer prognosis (BPD/death) [91].
Mesenchymal stromal cells (MSCs) isolated from TA of newborns express lung-specific genes, differ from lung fibroblasts, and produce proinflammatory cytokines [92]. In a newborn mouse lung injury model, intravenous or intra-alveolar MSCs transplantation was related to decreased inflammation and fibrosis in the lung [93]. The colony/cluster ratio of MSCs in the TA was considerably lower in BPD infants, according to a cross-sectional investigation (p=0.002). In addition, the colony/cluster ratio was a significant variable linked with BPD (95% CI: 1.6–2.98, p=0.029).
Heat shock protein-70 (Hsp-70) is essential for protein folding, protein stability and cellular homeostasis. The BPD group had considerably higher TA Hsp-70 concentrations on DOL 1 and 28 than the non-BPD group. In addition, an elevated TA Hsp-70 threshold value (above 149.1 ng·mg^−1^) had a 71.9% sensitivity and 93.5% specificity for the diagnosis of BPD, demonstrating a significant predictive value [94].
Inflammation of the respiratory system is a hallmark of BPD; in addition, measuring inflammatory markers in exhaled breath condensates may be a valuable noninvasive method. Infants who are either ventilated or not may have their exhaled breath condensate analysed using gas chromatography and mass spectrometry for diagnostic purposes [95].
By monitoring end-tidal carbon monoxide (ETCO), the levels of an inflammatory biological marker (carbon monoxide) were determined [96]. May et al. [97] concluded in prospective research that the evaluation of ETCO on DOL 14 enhanced the prediction of BPD when birthweight was taken into consideration. ETCO levels on DOL 14 were consistently higher in newborns with BPD. This result re-emphasised the significance of continuing inflammation in the development of BPD.
In preterm infants with BPD, levels of fractional exhaled nitric oxide were distinctly higher than in those without BPD (p=0.006). In addition, there was a significant association of elevated exhaled nitric oxide (eNO) concentrations with BPD (95% CI: 1.397–4.824, p=0.000) [98]. In preterm newborns with existing BPD, especially those with moderate or severe BPD, there was an increase in eNO levels, which may be indicative of an inflammatory process [99].
Since 1967, radiological imaging has been an essential technique for identifying BPD. Image diagnosis and prediction are complicated in BPD. Plain chest radiography (CXR) and computed tomography (CT) scanning remain the most widely used imaging modalities in BPD [100]. Radiological innovations, especially in magnetic resonance imaging (MRI) and lung ultrasound (LUS), have the potential to provide new quantitative measures of pulmonary function and structure in BPD [101] (supplementary table S2).
CXR is the primary line of radiological investigation in neonates due to a negligible dose of ionising radiation and is thus, widely regarded as safe for serial monitoring of infants [102]. However, CXR only offers a flat image of the chest (two-dimensional, usually a coronal view) and is not sufficiently capable of detecting subtle cases of pulmonary diseases [103].
In a prospective study of 400 VLBWIs, in the first 6 h after birth, the presence of a small thymus, as measured by the ratio between the transverse diameter of the cardiothymic image at the level of the carina and that of the thorax (CT/T) <0.28, distinguished newborns with BPD [104]. Furthermore, 89.5% of newborns requiring oxygen dependency at 36 weeks PMA exhibited leaky lung syndrome with hazy-to-opaque lungs or cystic BPD with bubbly alterations on CXR at that time [105]. Kim et al. [106] reported that the pattern of interstitial pneumonia on the CXR on DOL 7 was considerably greater in the BPD group (p<0.001). It was independently related to BPD or death prediction (OR: 4.0, 95% CI: 1.1–14.4) and may be used to identify high-risk VLBWIs who may benefit from prevention strategies against BPD.
The chest radiograph thoracic area (CRTA), which can be used to predict how much the pulmonary mechanics has changed, is a sign of hyperinflation [107]. After controlling for potential confounders, the CRTA in the first 72 h of life was higher in the moderate/severe BPD group (p<0.001) and mild BPD group (p<0.05) than in the no BPD group [108]. Enhanced CRTA in BPD was shown to be substantially associated with oxygenation impairment, suggesting that CRTA may be utilised as a straightforward radiographic diagnostic index to evaluate BPD severity [109].
The CXR scoring criteria was designed by Toce et al. [110] and Greenough et al. [111]. It was discovered that the scoring system used for radiographs on DOL 7 correlated better with BPD diagnosis [112]. The relationship between the scoring system and the clinical severity of BPD was substantial (r=0.78, p<0.001) [113].
Because of its high three-dimensional anatomical resolution of the lung parenchyma and significant tissue contrast, CT is a common imaging modality for the evaluation of respiratory diseases in adults [114]. Although CT observations were more specific and objective for preterm infants with BPD than CXR, the continuous use of chest CT in infants has been limited, largely due to the consequent ionising radiation exposure in neonates who may be radiosensitive throughout development [115]. With the recent advancement of low-dose CT protocols, it is believed that newborn pulmonary imaging with CT would become more commonplace [116, 117].
Based on the quantitative investigation of pulmonary abnormalities in BPD, Shin et al. [114] developed a novel high-resolution computed tomography (HRCT) scoring system for BPD that consists of the hyperaeration and parenchymal scores. The HRCT score was more strongly correlated (r=0.646, p<0.001) with the clinical severity of BPD than the CXR score. In contrast, another study found no significant difference in the mean HRCT scores of moderate and severe BPD [118]. Moreover, CT scoring systems were directly associated with a variety of adverse physiological and clinical outcomes in neonates with BPD, such as the duration of oxygen exposure, incidence of desaturation events during sleep, severity of wheezing and rate of decline in forced expiratory volume in 1 s [119, 120]. In conclusion, CT scans have not yet been clearly characterised in their efficacy as a predictor of BPD in preterm newborns; however, they may be useful for predicting the severity and long-term respiratory consequences of BPD.
As a non-ionising modality, MRI is appropriate for paediatric imaging, especially routine and serial examination. Historically, MRI of the lung has been restricted owing to technological limitations resulting primarily from the lung parenchyma's essentially low proton density and rapid signal decay [121]. Recent improvements in MRI software acquisition methodologies have substantially eliminated these limitations, and neonatal pulmonary MRI is currently being developed as a method that yields new imaging-based biomarkers [122, 123].
Ultrashort echo-time (UTE) MRI is the most common technique for MRI imaging of pulmonary structures [124]. The use of neonatal pulmonary scoring of UTE MRI improves the independent assessment of structural abnormalities of BPD [125]. Förster et al. [126] discovered that higher T2 and lower T1 relaxation times were correlated with an elevated risk of BPD and its severity at near-term age (mean age at 36 weeks PMA) with an AUC of 0.80. MRI might play a vital role in the characterisation of certain phenotypes and could ultimately replace CT as the method for BPD imaging with high resolution [127].
LUS is accessible at the bedside, can be done immediately and serially, has a high interobserver agreement and does not involve ionising radiation. It can evaluate dynamic shifts over time and identify them even before CXR [128]. In addition, LUS has been demonstrated to be effective for pulmonary maintenance in VPIs and appears to be a potential technique for BPD prediction and diagnosis [129].
Compared to CT and MRI, perinatal LUS lacks a consistent acquisition method and cannot discriminate lung parenchyma, pulmonary vasculature and lung function in newborns with BPD [101, 130]. According to Aldecoa-Bilbao et al. [131], VPIs with BPD had substantially higher LUS score at admission, on DOL 7 and DOL 28. Patients with BPD demonstrated greater consolidations and pleural line anomalies than those without BPD (p<0.001). LUS on DOL 7 accurately predicted the NICHD 2001-BPD (AUC=0.87, p<0.001) and Jensen 2019-BPD (AUC=0.80, p<0.001). After the first week of life, the LUS score, pleural line anomalies and consolidations may aid in the diagnosis of BPD in VPI and prediction of its severity.
There is substantial evidence linking preterm delivery to reduced lung capacity, lower lung compliance, increased airway obstruction and lung resistance, impaired gas exchange, and early reduction of respiratory function [120, 132]. Owing to BPD, structural abnormalities such as alveolar simplicity, decreased number of alveoli and enlarged alveoli are the major pathophysiological basis for impaired lung function that may continue throughout childhood and contribute to COPD in adulthood [133–135]. Studies have shown that early diagnosis of pulmonary function abnormalities in preterm newborns is advantageous for early prediction of BPD development [136].
May et al. [97] prospectively assessed lung function using functional residual capacity (FRC), compliance (Crs) and resistance (Rrs) of the respiratory system on DOL 3 and 14. Results for FRC and Crs on DOL 3 and 14, and Rrs on DOL 3 were substantially different between preterm infants with and without BPD. Another study demonstrates that decreased FRC in the first 72 h after delivery is a substantial predictor of moderate to severe BPD [108]. On DOL 7, 14 and 28, those with mild and moderate BPD had higher effective airway resistance per kilogram (Reff·kg^−1^), lower tidal volume per kilogram (TV·kg^−1^), FRC, ratio of time to peak tidal expiratory flow to total expiratory time (% T-PF), and ratio of volume to peak tidal expiratory flow to total expiratory volume (% V-PF) than those without BPD (p<0.05). This demonstrated the value of plethysmography for dynamic monitoring of pulmonary function in evaluating lung development in neonates with BPD [137].
However, there is no consensus on the early lung function assessment that best predicts the development of BPD, presumably because various methodologies have been utilised in different studies [138] (supplementary table S2).
Owing to recent developments in molecular genetics and next-generation sequencing, such as whole exome sequencing and whole genome sequencing, the investigation of BPD at a molecular “omic” level without bias is now possible [139]. Genetic inheritance and epigenetic mechanisms play a role in determining BPD vulnerability. Individualised and precise BPD prediction and management is made feasible by the “omics” sciences. This review focuses on genomic, proteomic, metabolomic and microbiomic indicators in noninvasive biofluids for BPD (table 3).
Several studies have demonstrated that genetic variables have a substantial role in the development of BPD, ranging from 53% to 82% [140, 141]. Scientists have tried to determine how the complex interactions between genetic and environmental factors lead to BPD. Genomic analysis may help in the future by identifying newborns with a greater risk of this condition so that they may get personalised therapy [142]. Furthermore, epigenetic processes may regulate genetic expression by either activating or silencing the genetic material encoded by the genome [143].
Fujioka et al. [144] obtained genomic DNA from 97 VLBWIs' umbilical cords, UCB or buccal mucosa. The genotype -634C>G was significantly related to BPD. The frequency of the G allele was substantially greater in the BPD group than in the non-BPD group (p=0.02). Multivariate logistic regression revealed that VEGF-634G>C G alleles were independent BPD risk factors. Exosomal miRNA (miR) signals were discovered using a prospective cohort analysis of the TA collected from extremely premature newborns shortly after delivery. Significantly lower levels of miR-876-3p were associated with severe BPD [145].
Multiple specific proteins have been involved in the pathophysiology of BPD. Proteomic analysis may aid in the research of protein networks that provide real-time clinical status and protein function modification. However, studies using unbiased proteome analysis for BPD are not yet available [146, 147].
Ahmed et al. [148] gathered urine samples from extremely-low-gestational-age neonates within 72 h of delivery, and they used this to construct a high-throughput urine proteomics approach. A total of 94 proteins had substantial abundance differences between BPD and healthy controls, and 20 proteins were directly linked to BPD and/or other infantile lung diseases (for example, chitinase-3-like protein-1, MMP-9, and so on), as determined by analysing the urine proteome.
Dyslipidaemia (decreased major choline-containing phospholipids) and metabolic immaturity (increased oxylipins PGE1, PGE2, PGF2a, 9- and 13-HOTE, 9- and 13-HODE, and 9- and 13-KODE) were found to be positively associated with BPD severity and PH development in the analysis of UCB of 42 preterm infants using a multiplatform metabolomics strategy [149].
In recent research, Rogosch et al. [150] demonstrated that smell prints of volatile organic compounds assessed with an electronic nose varied between the TAs from preterm newborns who later had BPD and those who did not. Metabolites in fatty acid activation, and those involved in androgen and oestrogen formation and metabolism, were found to be enriched in the TA; in addition, there were tendencies towards reduction in the fatty acid β-oxidation pathway in newborns at high risk for BPD, according to a mummichog metabolomic pathway study [151]. Furthermore, neonates with BPD had increased concentrations of histidine, glutamic acid, citrulline, glycine and isoleucine in their TA during the first week of life, and higher levels of acyl carnitines C16-OH and C18:1-OH [152].
Lactate, taurine, trimethylamine N-oxide (TMAO), myoinositol (which increased) and gluconate (which decreased) were identified as significant discriminant metabolites in the urine of BPD newborns during the first 24 to 36 h of life [153]. Pintus et al. [154] collected urine samples on DOL 7 and examined them with ^1^H-nuclear magnetic resonance. Alanine and betaine (both of which were shown to be elevated in the BPD group), TMAO, lactate and glycine (decreased in the BPD group) were identified as metabolites that differentiated the BPD group from the non-BPD group.
The microbiota plays an essential role in maintaining the host's normal levels of immunity, hormones and metabolism. Although most of these bacteria cannot be cultivated, high-throughput sequencing of the bacterial 16S rRNA gene has enabled the identification of the whole microbiome by facilitating its identification, alignment, sorting and classification using publicly accessible taxonomic databases [155]. Recent research has shown that the early airway microbiome may stimulate the maturing pulmonary immune system, and dysbiosis of a normal airway microbiome may lay the foundation for BPD and future pulmonary disease.
Furthermore, numerous studies have shown that an increased microbial community turnover with age and decreased microbiome diversity may be related to the development of BPD [156, 157]. Lal et al. [158] examined the TA microbiome in 23 ELBWIs and discovered an increase in Proteobacteria in the first week of birth. Conversely, Firmicutes, such as genus Lactobacillus, which is known to have strong anti-inflammatory properties and regulate alveolar development, was less common in BPD-prone newborns than in BPD-resistant newborns (p<0.05). A longitudinal investigation demonstrated that mechanically ventilated preterm infants who were later diagnosed with severe BPD acquired fewer Staphylococcus on DOL 1 and had a relatively higher abundance of Ureaplasma [159]. Potential biomarkers for BPD include an elevated abundance of Stenotrophomonas and increased level of sn-glycerol 3-phosphoethanolamine on DOL 1. Stenotrophomonas, which may affect the content of the lower airway microbiome through its metabolite sn-glycerol 3-phosphoethanolamine, is linked to BPD development and severity [160]. This result raised the possibility that the metabolome is affected by the metabolic activity of the airway microbiome.
There is a demand for objective, precise and reliable early predictive methods to understand the development of BPD, construct predictive indicators of subsequent outcomes, and guide innovative therapeutics to avoid or reduce the severity of BPD in preterm newborns. Existing noninvasive methods for predicting the incidence and severity of BPD at a very early stage, even right after birth, were reviewed in this study. However, no established universally accepted indicator has been found. This may be because the sample sizes of the studies in the review were not large enough. Confirmative investigations with larger sample sizes and high-quality data and the evaluation of the function of noninvasive biomarkers in the identification of BPD would increase the potential for therapeutic treatments. In the future, better understanding of the pathogenesis of the disease and the potential for the development of novel biomarkers for early detection, prevention and treatment of BPD can be gained through the application of cutting-edge prediction models that integrate clinical information with risk factors, noninvasive predictors and “omic” approaches.