Bronchopulmonary dysplasia

Bronchopulmonary dysplasia (BPD), the chronic lung disease of prematurity, may be associated with long-term airflow limitation. Survivors of BPD may develop asthma-like symptoms in childhood, with a variable response to β^sub 2^-agonists. However, the pathologic pathways underlying these respiratory manifestations are still unknown. The aim of this study was to measure exhaled nitric oxide (FE^sub NO^) and lung function in a group of 31 school-age survivors of BPD. They showed variable degrees of airflow obstruction (mean FEV^sub 1^ 77.8 ± 2.3% predicted) unresponsive to β^sub 2^-agonists in 72% of the subjects. Their FE^sub NO^ values (geometric mean [95% confidence interval]: 7.7 [± 1.1] ppb) were significantly lower than in a group of healthy matched control subjects born at term (10.7 [± 1.1] ppb, p

Keywords: airway remodeling; asthma; bronchopulmonary dysplasia; exhaled nitric oxide; flow limitation

Bronchopulmonary dysplasia (BPD) was first described in premature neonates surviving respiratory distress syndrome, as a consequence of chronic lung injury induced by mechanical ventilation and exposure to high oxygen concentrations (1). Several decades later, BPD still represents a major problem in neonatal medicine despite remarkable advances in the preventive care, treatment, and monitoring of premature babies. Different stimuli acting at different postmenstrual ages may give rise to different forms of BPD, pathologically ranging from the classic pattern of pulmonary fibrosis and extensive airway remodeling to the alveolar simplification typical of "new BPD" (2-5). In most cases, however, regardless of the etiologic pathway, there is an early intense inflammatory response that persists over the first weeks (2, 3). This early event may have far-reaching consequences, extending into childhood and beyond (6). Several studies have reported significant bronchial obstruction and airway hyperresponsiveness in subjects with BPD during their childhood (6-11) and early adulthood (12). Unfortunately, the mechanisms behind these abnormalities are still not entirely understood, as the airway pathology of BPD beyond infancy has not been investigated, and no studies exist on bronchial biopsies in children with BPD. Survivors of BPD often behave like children with asthma, with recurrent wheezing, shortness of breath, and airflow limitation, and they are treated with β^sub 2^-agonists and glucocorticoids (3, 13). However, although eosinophil-mediated epithelial damage is a major mechanism of respiratory impairment in asthma (14), only scanty information is available on the nature and pathogenetic relevance of airway inflammation in survivors of BPD beyond infancy.

The recent development of noninvasive methods for assessing airway inflammation, for example, exhaled nitric oxide (FE^sub NO^) measurement and exhaled breath condensale analysis, has greatly increased our understanding of the pathogenesis of several lung diseases (15). FE^sub NO^ is the most extensively studied marker. It is thought to reflect airway inflammation in asthma (15-18) and has recently been extended to the noninvasive assessment of disease activity in several other lung diseases, such as cystic fibrosis and chronic obstructive pulmonary disease (15). The aim of this study was to evaluate FE^sub NO^ and lung function indices at school age in a cohort of survivors of BPD, comparing them with a group of matched healthy children and a group of preterm children without BPD. As control subjects with a similar degree of airflow limitation, we also included a group of children with asthma matched for airway function.

METHODS

Subjects

Children with BPD. Thirty-one school-age children with BPD were recruited to take part in the study protocol from a population of subjects born at gestalional ages under 31 weeks and with birth weights less than 2,000 g, admitted to the Neonatal Intensive Care Unit at the Department of Pediatrics in Padova between January 1990 and December 1994. To select a homogeneous group of children with BPD, we considered only patients still needing mechanical ventilation 7 days after birth because of severe neonatal respiratory failure. Patients with major congenital anomalies or suspected lung hypoplasia were excluded. BPD was defined as clinical signs of respiratory distress, chest radiograph abnormalities, and oxygen dependence at 28 days of life (1). Pertinent neonatal data and the characteristics of the subjects at the time of the school-age assessment are shown in Tables 1 and 2, respectively. At the time of inclusion in the study, all of the patients were in stable conditions and had not used steroids for at least 1 month. None of the patients was being treated with leukotriene receptor antagonists.

Preterm children without BPD. Thirty-one children born prematurely who did not develop BPD were recruited as control subjects for the BPD group (Table 2). All the children were admitted to the Neonatal Intensive Care Unit of our Department at birth because of prematurity. They were matched with children with BPD for birth weight and gestational age. They were with mild or without respiratory distress syndrome at birth, requiring no mechanical ventilation beyond the seventh day of life and oxygen supplementation beyond 2 weeks of postnatal age. Thus, none was diagnosed as having BPD according to the previously mentioned criteria, nor were they given postnatal steroids. Their neonatal data are shown in Table 1.

Healthy control children. Thirty-one healthy children born at term with no history of asthma or atopy and no respiratory symptoms in the previous 4 weeks were recruited as control subjects. They were matched for sex and age with the children with BPD (Table 2).

Children with asthma. We enrolled 31 children with asthma with anthropometric and spirometric features comparable with those of the subjects with BPD (Table 2). They were recruited among patients attending the pulmonology/allergy outpatient clinic. The diagnosis of asthma was based on clinical history, symptom frequency, physical examination, and pulmonary function parameters, according to international guidelines (19). Nineteen children had intermittent asthma, and 12 had moderate persistent asthma. None of the children had used inhaled or oral glucocorticosteroids for at least 1 month. Children with persistent asthma were not under treatment either because they spontaneously discontinued long-term controller medication before the planned visit or because they were referred for the first time to our clinic without treatment. None of the patients was being treated with leukotriene receptor antagonists. Patients were excluded from the study if they had developed a respiratory infection or had an episode of asthma exacerbation in the previous 4 weeks.

The study was approved by the local ethics committee, and all parents gave their verbal informed consent.

Study Design

A detailed clinical history was obtained, and a complete physical examination was performed before FE^sub NO^ measurement and spirometry. At least one parent was asked to attend the pulmonary function tests and allergometric studies. Spirometry was always performed after FE^sub NO^ measurement. Then the skin prick test was done. Each child was adequately instructed and trained before taking FE^sub NO^ measurements and testing pulmonary function.

Fractional Exhaled NO Measurement

FE^sub NO^ was measured with an online method using a computerized system (EBA Aerocrine, Stockholm, Sweden) following the American Thoracic Society's recommendations (18). Subjects inhaled NO-free air through the mouth to total lung capacity and exhaled through a dynamic flow restrictor with a target flow of 50 ml/second for at least 6-7 seconds (20). No nose clip was used. FE^sub NO^, expressed as ppb, was calculated as the mean of three measurements that agreed to within 10% of the mean value.

Spirometry and Reversibility to β^sub 2^-Agonists

Lung function was analyzed by flow-volume spirometry (Biomedin, Padova, Italy). FVC, FEV^sub 1^, and forced expiratory flow rate between 25% and 75% (FEF^sub 25-75%^) of FVC were measured and expressed as percentages of the standardized values predicted for normal children according to sex and height (21). The best of three maneuvers was recorded. Spirometry was also performed after administering 300 µg of inhaled salbutamol by metered dose inhaler with a spacer device (Aerochamber, Trudell, Canada). Reversibility to β^sub 2^-agonists was defined as a more than 12% increase in FEV^sub 1^ after salbutamol inhalation.

Skin Prick Tests

All subjects underwent skin prick testing with a panel of common inhalant allergens: mixed grass pollen, Parietaria, Artemisia vulgaris, Dermatophagoides pteronissynus and Dermatophagoides farinae, Alternaria, dog and cat (Lofarma, Milano, Italy). Glycerine with histamine 1:1,000 was used as a positive control and glycerine alone as a negative control. The skin tests were done on the volar side of the forearms and were considered positive if they resulted in a wheal reaction greater than 3 mm.

Statistical Analysis

Results are expressed as mean ± SEM for normally distributed data. FE^sub NO^ values were logarithmically transformed to normalize the distribution of the data and were expressed as a geometric mean with their 95% confidence intervals. One-way analysis of variance was then used to detect the overall difference between groups; the comparison between groups with a similar variance was performed using the Student-Newman-Keuls test. Correlations were evaluated by Pearson's test. Statistical significance was assumed for p values of less than 0.05. Statistical analysis was performed using SigmaStat version 3.0.

RESULTS

All survivors of BPD were intubated at birth and mechanically ventilated for respiratory distress syndrome. The mean (SEM) mechanical ventilation and oxygen dependency duration were, respectively, 26 ± 3.4 days and 115 ± 22 days (Table 1). Surfactant was routinely introduced in our unit from 1991 onward, and thus, 24 neonates received surfactant on a rescue basis, and 7 did not. Twenty of the 31 children in the study received postnatal glucocorticoid therapy. Twenty-three children had no neurologic sequelae, and eight had mild to moderate degrees of neurodevelopmental delay.

The preterm children without BPD had the same gestational age and birth weight as the children in the group with BPD (Table 1) but a markedly shorter time on mechanical ventilation and oxygen dependence (p

Survivors of BPD, preterm children without BPD, those with asthma, and healthy control subjects did not differ in age, sex, and height (Table 2), whereas weight was significantly lower in patients with BPD than in healthy control subjects or those with asthma (p = 0.004 and 0.007, respectively).

The skin prick test was positive in four children in the group with BPD, in 5 preterm children without BPD, and in 25 children with asthma.

FE^sub NO^

All 124 children enrolled in the study were able to perform FE^sub NO^ measurement. Children with BPD had significantly lower FE^sub NO^ levels than healthy control subjects or preterm children without BPD (p

There was no difference in FE^sub NO^ between children who did or did not receive surfactant (7.7 ± 1.1 and 7.6 ± 1.1 ppb, respectively; p = 0.9) or between children who were or were not given postnatal glucocorticoid therapy (7.5 ± 1.1 and 8.0 ± 1.2 ppb, respectively; p = 0.7).

Spirometry

Spirometry test results showed evidence of airflow limitation in the BPD group by comparison with the control group (Table 3), as demonstrated by significantly lower values of FEV^sub 1^, FEF^sub 25-75^, and FEV^sub 1^/FVC ratio (p

FEV^sub 1^ and FEF^sub 25-75^ were significantly higher in the preterm children without BPD than in the children with BPD and were significantly lower than in the group of healthy control subjects (Table 3).

Reversibility to salbutamol was tested in all but two patients with BPD: airflow limitation was not reversible (≤ 12%) in 21 patients (72%) (mean increase in FEV^sub 1^, 5.8 ± 0.9%). The other eight children showed a mean increase in FEV^sub 1^ of 18.8 ± 2.7%. In the group of children with BPD as a whole, the mean increase in FEV^sub 1^ after salbutamol was 9.4 ± 1.5%. No difference was found in FE^sub NO^ values between children with BPD whose airflow limitation did and did not respond to salbutamol inhalation (6.6 [± 1.1] ppb vs. 8.1 [± 1.1] ppb respectively, p = 0.3). No correlation was found between ΔFEV^sub 1^ and FE^sub NO^ levels (p = 0.7) or between FEV^sub 1^ at the baseline and after salbutamol (p = 0.1). In the preterm group without BPD, FEV^sub 1^ increased by 12% or more in 5 of the 30 cases (17%) tested.

DISCUSSION

This is the first study reporting low FE^sub NO^ levels in a group of school-age children with BPD with varying degrees of airflow obstruction by comparison with healthy control subjects, preterm subjects without BPD, and FEV^sub 1^-matched children with asthma. Although BPD and asthma share some clinical and functional features, the remarkable difference in FE^sub NO^ values suggests that the airflow limitation in these two obstructive lung diseases of childhood is related to distinct pathophysiologic pathways that ought to be properly identified.

The early stages of BPD are almost invariably characterized by an intense inflammatory response (3), followed by chronic inflammation and airway remodeling (3, 22, 23). Significant early airway changes often have far-reaching consequences: as in the previous literature (6-11), most of the children we studied at school age had some degree of airflow limitation (mean FEV^sub 1^, 77%), which failed to improve after salbutamol inhalation in 72% of cases. Only sparse information is available on the mechanisms underlying the long-term clinical and functional manifestations of BPD. Some of the clinical features and the abnormal airway physiology (airflow limitation and airway hyperresponsiveness) that BPD shares with bronchial asthma might suggest a common mechanism responsible for lung function impairment (3, 13). Unfortunately, studies on airway pathology in BPD beyond infancy are currently lacking, although clarifying this issue would have important prognostic and therapeutic implications. In fact, children with BPD are frequently treated empirically with asthma medication, although there is no evidence to support this common practice. For these reasons, we aimed to noninvasively assess the presence of airway inflammation in children with BPD by measuring their FE^sub NO^. In the lungs, NO plays a key role in the physiologic regulation of vessels and airway tone, and it can be altered in several heart-lung diseases (15). In patients with asthma, FE^sub NO^ is considered an indirect marker of eosinophilic airway inflammation (16, 17, 24, 25). Part of the NO measured in exhaled breath may be produced in the vascular endothelium, however, and FE^sub NO^ measurement has recently also been proposed as a marker of pulmonary endothelial dysfunction (26).

Our data show that flow limitation at school age in survivors of BPD was not associated with an increased NO production. This is consistent with the FE^sub NO^ results that we had previously obtained in a smaller group of children with BPD enrolled in a longitudinal pulmonary function study (11).

The comparison with children with asthma with a similar airflow limitation indicates a clear difference in the pathophysiology of flow limitation between these two obstructive lung diseases. We chose to compare children with BPD and children with asthma with similar airflow limitations to prevent any bias in FE^sub NO^ measurement related to possible differences in airway caliber. In addition, we compared children with BPD with a group of preterm children without BPD to exclude any influence of prematurity per se on FE^sub NO^. Preterm children without BPD also had a significantly worse lung function than healthy control subjects but normal FE^sub NO^ values (Table 3).

Mieskonen and colleagues recently showed significant bronchial obstruction but normal FE^sub NO^ levels in a group of school-age children born prematurely compared with healthy control subjects. Similar results were obtained by separately analyzing a subgroup of nine children with BPD (27). In our study, FE^sub NO^ values in children with BPD were slightly but significantly lower than in healthy controls, a result that may have been obscured in the Mieskonen study by the small number of children and the high expiratory flow (100-200 ml/second) used in their FE^sub NO^ measurement. As recommended by international guidelines (18), we used an expiratory flow rate of 50 ml/second, which is highly reproducible and considered the most sensitive flow for discriminating among subjects (28).

A defective NO synthesis or release in children with BPD could be related to different mechanisms. The more obvious explanation, a reduction in FE^sub NO^ related to a smaller lung size of children with BPD, is unlikely for two reasons: children with BPD have a FE^sub NO^ that is also lower than in equally premature children without BPD, and an insignificant relationship was found between vital capacity and FE^sub NO^ values. A defective NO synthesis and/or diffusion in the airway lumen could be a sequela of the epithelial damage occurring in the early phases of BPD. A complementary explanation for the lower FE^sub NO^ in patients with BPD may lie in the "vascular hypothesis" recently proposed to explain some features of BPD (29, 30), according to which the early lung injury in infants with BPD results in a dysmorphic vascular growth, with a reduction in the pulmonary vascular bed. As a result, an impaired endothelial release or reduced diffusion of NO from the endothelium into the airways would account for the lower levels of FE^sub NO^. However, at this regard, a potential limit of our study is that we did not use multiple flows analysis of FE^sub NO^ to separate conducting-airway NO output from alveolar NO production. A similar phenomenon, with reduced FE^sub NO^ values, has been described in patients with chronic obstructive pulmonary disease with cor pulmonale (31), where there is evidence of NO release being impaired in the pulmonary vasculature (32). In the light of these hypotheses, a reduced FE^sub NO^ suggests a poor or dysmorphic pulmonary development in survivors of BPD. Airway remodeling is also suggested by the lack of reversibility of airflow limitation in most of our children with BPD (33). This is consistent with the evidence of a considerable airway function tracking we previously found in a group of children with BPD whose degree of spirometric impairment at school age was found closely related to their airflow limitation at 2 years of age (11). This adds concern that moderate to severe BPD may be associated with chronic obstructive lung disease in later life (6), but longer longitudinal studies are needed to establish the clinical and functional relevance of these findings in adult life.

In conclusion, in school-age survivors of BPD, differently from what is observed in children with asthma, airflow limitation is not associated with an increase in FE^sub NO^. The low FE^sub NO^ values we found and the lack of β^sub 2^-agonist reversibility of airflow limitation in most of these subjects suggest that a distinct pathophysiologic mechanism is present in children with BPD. Further studies are necessary to determine the role of NO in the developmental biology of the lung.

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Eugenio Baraldi, Gea Bonetto, Franco Zacchello, and Marco Filippone

Department of Pediatrics, School of Medicine, University of Padova, Padova, Italy

(Received in original form March 8, 2004; accepted in final form October 4, 2004)

Correspondence and requests for reprints should be addressed to Eugenio Baraldi, Department of Pediatrics, Via Giustiniani 3, 35128 Padova, Italy. E-mail: baraldi@ pediatria.unipd.it

Am J Respir Crit Care Med Vol 171. pp 68-72, 2005

Originally Published in Press as DOI: 10.1164/rccm.200403-298OC on October 11, 2004

Internet address: www.atsjournals.org

Conflict of Interest Statement: E.B. does not have a financial relationship with a commercial entity that has an interest in the subject of this manuscript; G.B. does not have a financial relationship with a commercial entity that has an interest in the subject of this manuscript; F.Z. does not have a financial relationship with a commercial entity that has an interest in the subject of this manuscript; M.F. does not have a financial relationship with a commercial entity that has an interest in the subject of this manuscript.

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