Inhaled nitric oxide effects on lung structure and function in chronically ventilated preterm lambs

Aberrant signaling pathways of the lung mesenchyme and their contributions to the pathogenesis of bronchopulmonary dysplasia

Bronchopulmonary dysplasia (BPD) is a chronic lung disease in infants born extremely preterm, typically before 28 weeks' gestation, characterized by a prolonged need for supplemental oxygen or positive pressure ventilation beyond 36 weeks postmenstrual age. The limited number of autopsy samples available from infants with BPD in the postsurfactant era has revealed a reduced capacity for gas exchange resulting from simplification of the distal lung structure with fewer, larger alveoli because of a failure of normal lung alveolar septation and pulmonary microvascular development. The mechanisms responsible for alveolar simplification in BPD have not been fully elucidated, but mounting evidence suggests that aberrations in the cross-talk between growth factors of the lung mesenchyme and distal airspace epithelium have a key role. Animal models that recapitulate the human condition have expanded our knowledge of the pathology of BPD and have identified candidate matrix components and growth factors in the developing lung that are disrupted by conditions that predispose infants to BPD and interfere with normal vascular and alveolar morphogenesis. This review focuses on the deviations from normal lung development that define the pathophysiology of BPD and summarizes the various candidate mesenchyme-associated proteins and growth factors that have been identified as being disrupted in animal models of BPD. Finally, future areas of research to identify novel targets affected in arrested lung development and recovery are discussed.

Mechanism of reduced lung injury by high-frequency nasal ventilation in a preterm lamb model of neonatal chronic lung disease

The mechanism underlying the potentially beneficial effects of the "gentler" modes of ventilation on chronic lung disease (CLD) of the premature infant is not known. We have previously demonstrated that alveolar parathyroid hormone-related protein-peroxisome proliferator-activated receptorγ (PTHrP-PPARγ) signaling is critically important in alveolar formation, and this signaling pathway is disrupted in hyperoxia- and/or volutrauma-induced neonatal rat lung injury. Whether the same paradigm is also applicable to CLD, resulting from prolonged intermittent mandatory ventilation (IMV), and whether differential effects of the mode of ventilation on the PTHrP-PPARγ signaling pathway explain the potential benefits of the "gentler" modes of ventilation are not known. Using a well-established preterm lamb model of neonatal CLD, we tested the hypothesis that ventilatory support using high-frequency nasal ventilation (HFNV) promotes alveolar PTHrP-PPARγ signaling, whereas IMV inhibits it. Preterm lambs managed by HFNV or IMV for 21 d following preterm delivery at 132-d gestation were studied by Western hybridization and immunofluorescence labeling for key markers of alveolar homeostasis and injury/repair. In lambs managed by IMV, the abundance of key homeostatic alveolar epithelial-mesenchymal markers was reduced, whereas it was significantly increased in the HFNV group, providing a potential molecular mechanism by which "gentler" modes of ventilation reduce neonatal CLD.

Inhaled nitric oxide in preterm infants: an individual-patient data meta-analysis of randomized trials

BACKGROUND

Inhaled nitric oxide (iNO) is an effective therapy for pulmonary hypertension and hypoxic respiratory failure in term infants. Fourteen randomized controlled trials (n = 3430 infants) have been conducted on preterm infants at risk for chronic lung disease (CLD). The study results seem contradictory.

DESIGN/METHODS

Individual-patient data meta-analysis included randomized controlled trials of preterm infants (<37 weeks' gestation). Outcomes were adjusted for trial differences and correlation between siblings.

RESULTS

Data from 3298 infants in 12 trials (96%) were analyzed. There was no statistically significant effect of iNO on death or CLD (59% vs 61%: relative risk [RR]: 0.96 [95% confidence interval (CI): 0.92-1.01]; P = .11) or severe neurologic events on imaging (25% vs 23%: RR: 1.12 [95% CI: 0.98-1.28]; P = .09). There were no statistically significant differences in iNO effect according to any of the patient-level characteristics tested. In trials that used a starting iNO dose of >5 vs ≤ 5 ppm there was evidence of improved outcome (interaction P = .02); however, these differences were not observed at other levels of exposure to iNO. This result was driven primarily by 1 trial, which also differed according to overall dose, duration, timing, and indication for treatment; a significant reduction in death or CLD (RR: 0.85 [95% CI: 0.74-0.98]) was found.

CONCLUSIONS

Routine use of iNO for treatment of respiratory failure in preterm infants cannot be recommended. The use of a higher starting dose might be associated with improved outcome, but because there were differences in the designs of these trials, it requires further examination.

Pathogenesis and treatment of bronchopulmonary dysplasia

PURPOSE OF REVIEW

Bronchopulmonary dysplasia (BPD) is a chronic lung disease of infancy affecting mostly premature infants with significant morbidity and mortality. Improved survival of very immature infants has led to increased numbers of infants with this disorder. Acute and chronic lung injury and impaired postnatal lung growth are thought to be responsible for the development of BPD. Whereas changes in clinical practice have improved the clinical course and outcomes for infants with BPD, over the past decade, the overall incidence of BPD has not changed. This review will describe the prenatal and postnatal factors that contribute to the pathogenesis of BPD as well as current and experimental therapies for treatment of BPD.

RECENT FINDINGS

The factors that contribute to the pathogenesis of BPD are well described; however, recent studies have better defined how these factors modulate lung growth. Inflammation, proinflammatory cytokines and altered angiogenic gene signaling contribute to lung injury and impair prenatal and postnatal lung growth resulting in BPD; however, to date no therapy has been identified that potently and consistently prevents or reverses their effects on lung growth. We will discuss the cell signaling pathways affected in BPD and current therapies available for modulating these pathways.

SUMMARY

Despite current advances in neonatal care, BPD remains a heavy burden on healthcare resources. New treatments directed at either reducing lung injury or improving lung growth are under study.

Inhaled nitric oxide for respiratory failure in preterm infants

BACKGROUND

Inhaled nitric oxide (iNO) is effective in term infants with hypoxic respiratory failure. The pathophysiology of respiratory failure and the potential risks of iNO differ substantially in preterm infants, necessitating study in this population.

OBJECTIVES

To determine the effect of treatment with iNO on death, bronchopulmonary dysplasia (BPD), intraventricular hemorrhage (IVH), and neurodevelopmental disability in preterm newborn infants with respiratory disease.

SEARCH STRATEGY

Standard methods of the Cochrane Neonatal Review Group were used. MEDLINE, EMBASE, Healthstar and the Cochrane Central Register of Controlled Trials (The Cochrane Library) were searched covering the years from 1985 to 2010. In addition, the abstracts of the Pediatric Academic Societies were also searched.

SELECTION CRITERIA

Randomized and quasi-randomized studies in preterm infants with respiratory disease that compared the effects of iNO gas to control, with or without placebo were eligible.

DATA COLLECTION AND ANALYSIS

Standard methods of the Cochrane Neonatal Review Group were used.

MAIN RESULTS

Fourteen randomized controlled trials of inhaled nitric oxide therapy in preterm infants were found. The trials have been grouped post hoc into three categories depending on entry criteria; entry in the first three days of life based on oxygenation criteria, routine use in preterm babies with pulmonary disease, and later enrolment based on an increased risk of BPD. No overall analyses were performed.Nine trials of early rescue treatment of infants based on oxygenation criteria demonstrated no significant effect of iNO on mortality or BPD. Three studies with routine use of iNO in infants with pulmonary disease also demonstrated no significant reduction in death or BPD [typical RR 0.93 (95% CI 0.86 to 1.01)] although this small effect approached significance. Later treatment with iNO based on the risk of BPD (two trials) demonstrated no significant benefit for this outcome in analyses which are possible using summary data.There is no clear effect of iNO on the frequency of all grades of IVH or of severe IVH. Early rescue treatment was associated with a non-significant 20% increase in severe IVH.No effect on the incidence of neurodevelopmental impairment was found.

AUTHORS' CONCLUSIONS

iNO as rescue therapy for the very ill preterm infant does not appear to be effective. Early routine use of iNO in preterm infants with respiratory disease does not affect serious brain injury or improve survival without BPD. Later use of iNO to prevent BPD might be effective, but requires further study.

L-citrulline attenuates arrested alveolar growth and pulmonary hypertension in oxygen-induced lung injury in newborn rats

Bronchopulmonary dysplasia (BPD) is characterized by arrested alveolar development and complicated by pulmonary hypertension (PH). NO promotes alveolar growth. Inhaled NO (iNO) ameliorates the BPD phenotype in experimental models and in some premature infants. Arginosuccinate synthetase (ASS) and arginosuccinate lyase (ASL) convert L-citrulline to L-arginine; L-citrulline is regenerated during NO synthesis from L-arginine. Plasma levels of these NO precursors are low in PH. We hypothesized that L-citrulline prevents experimental O2-induced BPD in newborn rats. Rat pups were assigned from birth through postnatal day (P) 14 to room air (RA), RA + L-citrulline, 95% hyperoxia (BPD model), and 95%O2 + L-citrulline. Rat pups exposed to hyperoxia had fewer and enlarged air spaces and decreased capillary density, mimicking human BPD. This was associated with decreased plasma L-arginine and L-citrulline concentrations on P7. L-citrulline treatment significantly increased plasma L-arginine and L-citrulline concentrations and increased ASL protein expression in hyperoxia. L-citrulline preserved alveolar and vascular growth in O2-exposed pups and decreased pulmonary arterial medial wall thickness (MWT) and right ventricular hypertrophy (RVH). Increased lung arginase (ARG) activity in O2-exposed pups was reversed by L-citrulline treatment. L-citrulline supplementation prevents hyperoxia-induced lung injury and PH in newborn rats. L-citrulline may represent a novel therapeutic alternative to iNO for prevention of BPD.

Opposing regulation of human alveolar type II cell differentiation by nitric oxide and hyperoxia

Clinical trials demonstrated decreasing rates of bronchopulmonary dysplasia in preterm infants with hypoxic respiratory failure treated with inhaled nitric oxide (iNO). However, the molecular and biochemical effects of iNO on developing human fetal lungs remain vastly unknown. By using a well-characterized model of human fetal alveolar type II cells, we assessed the effects of iNO and hyperoxia, independently and concurrently, on NO-cGMP signaling pathway and differentiation. Exposure to iNO increased cGMP levels by 40-fold after 3 d and by 8-fold after 5 d despite constant expression of phosphodiesterase-5 (PDE5). The levels of cGMP declined significantly on exposure to iNO and hyperoxia at 3 and 5 d, although expression of soluble guanylyl cyclase (sGC) was sustained. Surfactant proteins B and C (SP-B, SP-C) and thyroid transcription factor (TTF)-1 mRNA levels increased in cells exposed to iNO in normoxia but not on exposure to iNO plus hyperoxia. Collectively, these data indicate an increase in type II cell markers when undifferentiated lung epithelial cells are exposed to iNO in room air. However, hyperoxia overrides these potentially beneficial effects of iNO despite sustained expression of sGC.

Congenital diaphragmatic hernia: endothelin-1, pulmonary hypertension, and disease severity

RATIONALE

Endothelin-1 (ET1) is dysregulated in pulmonary hypertension (PH). It may be important in the pathobiology of congenital diaphragmatic hernia (CDH).

OBJECTIVES

We hypothesized that ET1 levels in the first month would be higher in infants with CDH who subsequently expired or were discharged on oxygen (poor outcome). We further hypothesized that ET1 levels would be associated with concurrent severity of PH.

METHODS

We sampled plasma at 24 to 48 hours, and 1, 2, and 4 weeks of age in 40 prospectively enrolled newborns with CDH. We performed echocardiograms to estimate pulmonary artery pressure at less than 48 hours of age and weekly to 4 weeks. PH was classified in relationship to systemic blood pressure (SBP): less than 2/3 SBP, 2/3 SBP-systemic is related to pressure, or systemic-to-suprasystemic pressure.

MEASUREMENTS AND MAIN RESULTS

ET1 levels at 1 and 2 weeks were higher in infants with poor outcome compared with infants discharged on room air (median and interquartile range: 27.2 [22.6, 33.7] vs. 19.1 [16.1, 29.5] pg/ml, P = 0.03; and 24.9 [17.6, 39.5] vs. 17.4 [13.7, 21.8] pg/ml, P = 0.01 at 1 and 2 weeks, respectively). Severity of PH was significantly associated with increasing ET1 levels at 2 weeks (16.1 [13.7, 21.8], 21.0 [17.4, 31.1], and 23.6 [21.9, 39.5] pg/ml for increasing PH class, P = 0.03). Increasing severity of PH was also associated with poor outcome at that time (P = 0.001).

CONCLUSIONS

Infants with CDH and poor outcome have higher plasma ET1 levels and severity of PH than infants discharged on room air. Severity of PH is associated with ET1 levels.

Chronic lung disease in preterm lambs: effect of daily vitamin A treatment on alveolarization

Neonatal chronic lung disease is characterized by failed formation of alveoli and capillaries, and excessive deposition of matrix elastin, which are linked to lengthy mechanical ventilation (MV) with O(2)-rich gas. Vitamin A supplementation has improved respiratory outcome of premature infants, but there is little information about the structural and molecular manifestations in the lung that occur with vitamin A treatment. We hypothesized that vitamin A supplementation during prolonged MV, without confounding by antenatal steroid treatment, would improve alveolar secondary septation, decrease thickness of the mesenchymal tissue cores between distal air space walls, and increase alveolar capillary growth. We further hypothesized that these structural advancements would be associated with modulated expression of tropoelastin and deposition of matrix elastin, phosphorylated Smad2 (pSmad2), cleaved caspase 3, proliferating cell nuclear antigen (PCNA), VEGF, VEGF-R2, and midkine in the parenchyma of the immature lung. Eight preterm lambs (125 days' gestation, term approximately 150 days) were managed by MV for 3 wk: four were treated with daily intramuscular Aquasol A (vitamin A), 5,000 IU/kg, starting at birth; four received vehicle alone. Postmortem lung assays included quantitative RT-PCR and in situ hybridization, immunoblot and immunohistochemistry, and morphometry and stereology. Daily vitamin A supplementation increased alveolar secondary septation, decreased thickness of the mesenchymal tissue cores between the distal air space walls, and increased alveolar capillary growth. Associated molecular changes were less tropoelastin mRNA expression, matrix elastin deposition, pSmad2, and PCNA protein localization in the mesenchymal tissue core of the distal air space walls. On the other hand, mRNA expression and protein abundance of VEGF, VEGF-R2, midkine, and cleaved caspase 3 were increased. We conclude that vitamin A treatment partially improves lung development in chronically ventilated preterm neonates by modulating expression of tropoelastin, deposition of elastin, and expression of vascular growth factors.

Inhaled nitric oxide in preterm infants: a systematic review and individual patient data meta-analysis

Background

Preterm infants requiring assisted ventilation are at significant risk of both pulmonary and cerebral injury. Inhaled Nitric Oxide, an effective therapy for pulmonary hypertension and hypoxic respiratory failure in the full term infant, has also been studied in preterm infants. The most recent Cochrane review of preterm infants includes 11 studies and 3,370 participants. The results show a statistically significant reduction in the combined outcome of death or chronic lung disease (CLD) in two studies with routine use of iNO in intubated preterm infants. However, uncertainty remains as a larger study (Kinsella 2006) showed no significant benefit for iNO for this combined outcome. Also, trials that included very ill infants do not demonstrate significant benefit. One trial of iNO treatment at a later postnatal age reported a decrease in the incidence of CLD. The aim of this individual patient meta-analysis is to confirm or refute these potentially conflicting results and to determine the extent to which patient or treatment characteristics may explain the results and/or may predict benefit from inhaled Nitric Oxide in preterm infants.

Methods/Design

The Meta-Analysis of Preterm Patients on inhaled Nitric Oxide (MAPPiNO) Collaboration will perform an individual patient data meta-analysis to answer these important clinical questions. Studies will be included if preterm infants receiving assisted ventilation are randomized to receive inhaled Nitric Oxide or to a control group. The individual patient data provided by the Collaborators will be analyzed on an intention-to-treat basis where possible. Binary outcomes will be analyzed using log-binomial regression models and continuous outcomes will be analyzed using linear fixed effects models. Adjustments for trial differences will be made by including the trial variable in the model specification.

Discussion

Thirteen (13) trials, with a total of 3567 infants are eligible for inclusion in the MAPPiNO systematic review. To date 11 trials (n = 3298, 92% of available patients) have agreed to participate. Funding was successfully granted from Ikaria Inc as an unrestricted grant. A collaborative group was formed in 2006 with data collection commencing in 2007. It is anticipated that data analysis will commence in late 2009 with results being publicly available in 2010.

Transforming growth factor-beta modulates the expression of nitric oxide signaling enzymes in the injured developing lung and in vascular smooth muscle cells

Nitric oxide signaling has an important role in regulating pulmonary development and function. Expression of soluble guanylate cyclase (sGC) and cGMP-dependent protein kinase I (PKGI), both critical mediators of nitric oxide (NO) signaling, is diminished in the injured newborn lung through unknown mechanisms. Recent studies suggest that excessive transforming growth factor-beta (TGF-beta) activity inhibits injured newborn lung development. To explore mechanisms that regulate pulmonary NO signaling, we tested whether TGF-beta decreases sGC and PKGI expression in the injured developing lung and pulmonary vascular smooth muscle cells (SMC). We found that chronic oxygen-induced lung injury decreased pulmonary sGCalpha(1) and PKGI immunoreactivity in mouse pups and that exposure to a TGF-beta-neutralizing antibody prevented this reduction of sGC and PKGI protein expression. In addition, TGF-beta(1) decreased expression of NO signaling enzymes in freshly isolated pulmonary microvascular SMC/myofibroblasts, suggesting that TGF-beta has a direct role in modulating NO signaling in the pup lung. Moreover, TGF-beta(1) decreased sGC and PKGI expression in pulmonary artery and aortic SMC from adult rats and mice, suggesting a general role for TGF-beta in modulating NO signaling in vascular SMC. Although other cytokines decrease sGC mRNA stability, TGF-beta did not modulate sGCalpha(1) or PKGIbeta mRNA turnover in vascular SMC. These studies indicate for the first time that TGF-beta decreases NO signaling enzyme expression in the injured developing lung and pulmonary vascular SMC. Moreover, they suggest that TGF-beta-neutralizing molecules might counteract the effects of injury on NO signaling in the newborn lung.

Inhaled nitric oxide improves lung structure and pulmonary hypertension in a model of bleomycin-induced bronchopulmonary dysplasia in neonatal rats

Whether inhaled nitric oxide (iNO) prevents the development of bronchopulmonary dysplasia (BPD) in premature infants is controversial. In adult rats, bleomycin (Bleo) induces lung fibrosis and pulmonary hypertension, but the effects of Bleo on the developing lung and iNO treatment on Bleo-induced neonatal lung injury are uncertain. Therefore, we sought to determine whether early and prolonged iNO therapy attenuates changes of pulmonary vascular and alveolar structure in a model of BPD induced by Bleo treatment of neonatal rats. Sprague-Dawley rat pups were treated with Bleo (1 mg/kg ip daily) or vehicle (controls) from day 2 to 10, followed by recovery from day 11 to 19. Treatment groups received early ( days 2–10), late ( days 11–19), or prolonged iNO therapy (10 ppm; days 2–19). We found that compared with controls, Bleo increased right ventricular hypertrophy (RVH), and pulmonary arterial wall thickness, and reduced vessel density alveolarization. In each iNO treatment group, iNO decreased RVH ( P < 0.01) and wall thickness ( P < 0.01) and restored vessel density after Bleo ( P < 0.05). iNO therapy improved alveolarization for each treatment group after Bleo; however, the values remained abnormal compared with controls. Prolonged iNO treatment had greater effects on lung structure after bleomycin than late treatment alone. We conclude that Bleo induces lung structural changes that mimic BPD in neonatal rats, and that early and prolonged iNO therapy prevents right ventricle hypertrophy and pulmonary vascular remodeling and partially improves lung structure.

Epidemiology of bronchopulmonary dysplasia

First described more than 40 years ago, bronchopulmonary dysplasia (BPD) remains one of the most serious and vexing challenges in the care of very preterm infants. Affecting approximately one-quarter of infants born <1500g birth weight, BPD is associated with prolonged neonatal intensive care unit hospitalization, greater risk of neonatal and post-neonatal mortality and a host of associated medical and neurodevelopmental sequelae. This seminar focuses on the epidemiology and definition of BPD as well as the current evidence pertaining to a number of potential preventive treatments for BPD: non-invasive respiratory support technologies, inhaled nitric oxide, vitamin A, and caffeine.

Clinical and economic effects of iNO in premature newborns with respiratory failure at 1 year

BACKGROUND

The long-term consequences of inhaled nitric oxide (iNO) use in premature newborns with respiratory failure are unknown. We therefore studied the clinical and economic outcomes to 1 year of corrected age after a randomized controlled trial of prophylactic iNO.

METHODS

Premature newborns (gestational age <or=34 w, birth weight 500-1250 g) with respiratory failure randomly received 5 ppm iNO or placebo within 48 h of birth until 21 d or extubation. We assessed clinical outcomes via in-person neurodevelopmental evaluation at 1 y corrected age and telephone interviews every 3 m. We estimated costs from detailed hospital bills and interviews, converting all costs to 2008 US$. Of 793 trial subjects, 631 (79.6%) contributed economic data, and 455 (77.1% of survivors) underwent neurodevelopmental evaluation.

RESULTS

At 1 y corrected age, survival was not different by treatment arm (79.2% iNO vs. 74.5% placebo, P = .12), nor were other post-discharge outcomes. For subjects weighing 750-999 g, those receiving iNO had greater survival free from neurodevelopmental impairment (67.9% vs. 55.6%, P = .04). However, in subjects weighing 500-749 g, iNO led to greater oxygen dependency (11.7% vs. 4.0%, P = .04). Median total costs were similar ($235,800 iNO vs. $198,300 placebo, P = .19). Quality-adjusted survival was marginally better with iNO (by 0.011 quality-adjusted life-years/subject). The incremental cost-effectiveness ratio was $2.25 million/quality-adjusted life-year.

CONCLUSIONS

Subjects in both arms commonly experienced neurodevelopmental and pulmonary morbidity, consuming substantial health care resources. Prophylactic iNO beginning in the first days of life did not lower costs and had a poor cost-effectiveness profile.

Adrenomedullin promotes lung angiogenesis, alveolar development, and repair

Bronchopulmonary dysplasia (BPD) and emphysema are significant global health problems at the extreme stages of life. Both are characterized by alveolar simplification and abnormal distal airspace enlargement due to arrested development or loss of alveoli, respectively. Both lack effective treatments. Mechanisms that inhibit distal lung growth are poorly understood. Adrenomedullin (AM), a recently discovered potent vasodilator, promotes angiogenesis and has protective effects on the cardiovascular and respiratory system. Its role in the developing lung is unknown. We hypothesized that AM promotes lung angiogenesis and alveolar development. Accordingly, we report that lung mRNA expression of AM increases during normal alveolar development. In vivo, intranasal administration of the AM antagonist, AM22-52 decreases lung capillary density (12.4 +/- 1.5 versus 18 +/- 1.5 in control animals; P < 0.05) and impairs alveolar development (mean linear intercept, 52.3 +/- 1.5 versus 43.8 +/- 1.8 [P < 0.05] and septal counts 62.0 +/- 2.7 versus 90.4 +/- 3.5 [P < 0.05]) in neonatal rats, resulting in larger and fewer alveoli, reminiscent of BPD. This was associated with decreased lung endothelial nitric oxide synthase and vascular endothelial growth factor-A mRNA expression. In experimental oxygen-induced BPD, a model of arrested lung vascular and alveolar growth, AM attenuates arrested lung angiogenesis (vessel density, 6.9 +/- 1.1 versus 16.2 +/- 1.3, P < 0.05) and alveolar development (mean linear intercept, 51.9 +/- 3.2 versus 44.4 +/- 0.7, septal counts 47.6 +/- 3.4 versus 67.7 +/- 4.0, P < 0.05), an effect in part mediated by inhibition of apoptosis. AM also prevents pulmonary hypertension in this model, as assessed by decreased right ventricular hypertrophy and pulmonary artery medial wall thickness. Our findings suggest a role for AM during normal alveolar development. AM may have therapeutic potential in diseases associated with alveolar injury.

Inhaled nitric oxide for preterm premature rupture of membranes, oligohydramnios, and pulmonary hypoplasia

We sought to determine if inhaled nitric oxide (iNO) administered to preterm infants with premature rupture of membranes (PPROM), oligohydramnios, and pulmonary hypoplasia improved oxygenation, survival, or other clinical outcomes. Data were analyzed from infants with suspected pulmonary hypoplasia, oligohydramnios, and PPROM enrolled in the National Institute of Child Health and Development Neonatal Research Network Preemie Inhaled Nitric Oxide (PiNO) trial, where patients were randomized to receive placebo (oxygen) or iNO at 5 to 10 ppm. Outcome variables assessed were PaO (2) response, mortality, bronchopulmonary dysplasia (BPD), and severe intraventricular hemorrhage (IVH) or periventricular leukomalacia (PVL). Twelve of 449 infants in the PiNO trial met criteria. Six infants received iNO and six received placebo. The iNO group had a mean increase in PaO (2) of 39 +/- 50 mm Hg versus a mean decrease of 11 +/- 15 mm Hg in the control group. Mortality was 33% versus 67%, BPD (2/5) 40% versus (2/2) 100%, and severe IVH or PVL (1/5) 20% versus (1/2) 50% in the iNO and control groups, respectively. None of these changes were statistically significant. Review of a limited number of cases from a large multicenter trial suggests that iNO use in the setting of PPROM, oligohydramnios, and suspected pulmonary hypoplasia improves oxygenation and may decrease the rate of BPD and death without increasing severe IVH or PVL. However, the small sample size precludes definitive conclusions. Further studies are required to determine if iNO is of benefit in this specific patient population.

Inhaled nitric oxide for preterm neonates

The evidence for the benefits of inhaled nitric oxide (iNO) on gas exchange, cytokine-induced lung inflammation, and vascular dysfunction has been demonstrated by several animal and human studies. The use of iNO in extremely low birth weight neonates for the prevention of adverse outcomes like chronic lung disease and neurologic injury has been investigated, but the findings remain inconclusive. This review briefly outlines the biologic rationale for the use of iNO in preterm neonates and the results on the outcome measures of bronchopulmonary dysplasia and brain injury from the recent clinical trials. This article focuses on the potential toxicities, persistent controversies, and unanswered questions regarding the use of this treatment modality in this patient population at high risk for adverse outcomes.

Respiratory function during infancy in survivors of the INNOVO trial

RATIONALE

Despite encouraging reports suggesting that inhaled nitric oxide (iNO) appear to improve outcome in hypoxemic term and near term infants by improving oxygenation and reducing need for ECMO, the long-term benefits of iNO remain unclear. This study aimed to compare lung function at approximately 1 year in infants who were and were not randomly allocated to iNO as part of their neonatal management for severe respiratory failure at birth. Furthermore, results were compared to lung function of healthy infants.

METHODS

Maximal expiratory flow at functional residual capacity (V'maxFRC) was measured at approximately 1 year of age (corrected for any prematurity) in survivors of the INNOVO trial. Results were expressed as Z-scores, adjusted for sex and body size, based on data from healthy controls using identical techniques.

RESULTS

Technically satisfactory results were obtained in 30 infants (53% < 34 weeks gestation), 19 of whom were randomized to receive iNO V'maxFRC. Z-score was significantly reduced in infants with prior respiratory failure, whether or not they had been allocated to iNO (mean (SD) Z-score: -2.0 (1.2) and -2.6 (1.1), respectively, 95% CI difference; iNO vs. no iNO: -0.3; 1.6, P = 0.2). There was significant respiratory morbidity in both groups during the first year of life.

CONCLUSIONS

These results suggest that airway function remains reduced at 1 year of age following severe respiratory failure at birth, whether or not iNO is administered.

Inhaled nitric oxide in the term newborn

Inhaled nitric oxide is a selective pulmonary vasodilator that serves integral diagnostic and therapeutic roles in the clinical management of near-term and term newborns with hypoxemic respiratory failure and pulmonary hypertension. Its proper use and potential limitations in current clinical practice are reviewed.

Inhaled nitric oxide in the treatment of preterm infants

Inhaled nitric oxide (iNO) has been used successfully in select term and near-term infants with respiratory failure. The use of iNO in the premature infant population, however, remains controversial. This article will review some of the current literature regarding the use of iNO in premature infants and discuss current recommendations and future research directions.

One-year respiratory outcomes of preterm infants enrolled in the Nitric Oxide (to prevent) Chronic Lung Disease trial

OBJECTIVE

To identify whether inhaled nitric oxide treatment decreased indicators of long-term pulmonary morbidities after discharge from the neonatal intensive care unit.

STUDY DESIGN

The Nitric Oxide (to Prevent) Chronic Lung Disease trial enrolled preterm infants (<1250 g) between 7 to 21 days of age who were ventilated and at high risk for bronchopulmonary dysplasia. Follow-up occurred at 12 +/- 3 months of age adjusted for prematurity; long-term pulmonary morbidity and other outcomes were reported by parents during structured blinded interviews.

RESULTS

A total of 456 infants (85%) were seen at 1 year. Compared with control infants, infants randomized to inhaled nitric oxide received significantly less bronchodilators (odds ratio [OR] 0.53 [95% confidence interval 0.36-0.78]), inhaled steroids (OR 0.50 [0.32-0.77]), systemic steroids (OR 0.56 [0.32-0.97]), diuretics (OR 0.54 [0.34-0.85]), and supplemental oxygen (OR 0.65 [0.44-0.95]) after discharge from the neonatal intensive care unit. There were no significant differences between parental report of rehospitalizations (OR 0.83 [0.57-1.21]) or wheezing or whistling in the chest (OR 0.70 [0.48-1.03]).

CONCLUSIONS

Infants treated with inhaled nitric oxide received fewer outpatient respiratory medications than the control group. However, any decision to institute routine use of this dosing regimen should also take into account the results of the 24-month neurodevelopmental assessment.

Effects of positive end-expiratory pressure, inhaled nitric oxide and surfactant on expression of proinflammatory cytokines and growth factors in preterm piglet lungs

We hypothesized that imbalance of proinflammatory cytokines and growth factors (GFs) in immature lungs of early postnatal life may be affected by protective ventilation strategy, and evaluated correlations of these aspects. Preterm neonate piglets were mechanically ventilated with low tidal volume and 5-6 or 10-12 cm H2O positive end-expiratory pressure (PEEP) with or without surfactant and inhaled nitric oxide (iNO) for 6 h, followed by biochemical, biophysical, and histopathological assessment of lung injury severity. Compared with surfactant and the control, iNO combined with lower PEEP exerted better oxygenation, lower activity of myeloperoxidase, lower expression of mRNA of interleukin (IL)-1beta, IL-6, IL-8, and platelet derived growth factor-B (PDGF-B), but higher expression of insulin-like growth factor-I (IGF-I), whereas that of tumor necrosis factor-alpha, keratinocyte GF, hepatocyte GF, vascular endothelial growth factor, and TGF-beta1 had no or modest changes. IL-1beta, IL-6 mRNA were closely correlated to PDGF-B mRNA and myeloperoxidase, but inversely to IGF-I mRNA, Pao2/FiO2 and dynamic lung compliance at 6 h. These results indicate that the association of lower PEEP and iNO may be more protective than surfactant on preventing lung injury and facilitating reparation by affecting the expression of proinflammatory cytokines and GFs.

Nasal ventilation alters mesenchymal cell turnover and improves alveolarization in preterm lambs

RATIONALE

Bronchopulmonary dysplasia (BPD) is a frequent cause of morbidity in preterm infants that is characterized by prolonged need for ventilatory support in an intensive care environment. BPD is characterized histopathologically by persistently thick, cellular distal airspace walls. In normally developing lungs, by comparison, remodeling of the immature parenchymal architecture is characterized by thinning of the future alveolar walls, a process predicated on cell loss through apoptosis.

OBJECTIVES

We hypothesized that minimizing lung injury, using high-frequency nasal ventilation to provide positive distending pressure with minimal assisted tidal volume displacement, would increase apoptosis and decrease proliferation among mesenchymal cells in the distal airspace walls compared with a conventional mode of support (intermittent mandatory ventilation).

METHODS

Accordingly, we compared two groups of preterm lambs: one group managed by high-frequency nasal ventilation and a second group managed by intermittent mandatory ventilation. Each group was maintained for 3 days.

MEASUREMENTS AND MAIN RESULTS

Oxygenation and ventilation targets were sustained with lower airway pressures and less supplemental oxygen in the high-frequency nasal ventilation group, in which alveolarization progressed. Thinning of the distal airspace walls was accompanied by more apoptosis, and less proliferation, among mesenchymal cells of the high-frequency nasal ventilation group, based on morphometric, protein abundance, and mRNA expression indices of apoptosis and proliferation.

CONCLUSIONS

Our study shows that high-frequency nasal ventilation preserves the balance between mesenchymal cell apoptosis and proliferation in the distal airspace walls, such that alveolarization progresses.

Blunted hypoxic pulmonary vasoconstriction in experimental neonatal chronic lung disease

RATIONALE

Neonatal chronic lung disease (CLD), caused by prolonged mechanical ventilation (MV) with O(2)-rich gas, is the most common cause of long-term hospitalization and recurrent respiratory illness in extremely premature infants. Recurrent episodes of hypoxemia and associated ventilator adjustments often lead to worsening CLD. The mechanism that causes these hypoxemic episodes is unknown. Hypoxic pulmonary vasoconstriction (HPV), which is partially controlled by O(2)-sensitive voltage-gated potassium (K(v)) channels, is an important adaptive response to local hypoxia that helps to match perfusion and ventilation in the lung.

OBJECTIVES

To test the hypothesis that chronic lung injury (CLI) impairs HPV.

METHODS

We studied preterm lambs that had MV with O(2)-rich gas for 3 weeks and newborn rats that breathed 95%-O(2) for 2 weeks, both of which resulted in airspace enlargement and pulmonary vascular changes consistent with CLD.

MEASUREMENTS AND MAIN RESULTS

HPV was attenuated in preterm lambs with CLI after 2 weeks of MV and in newborn rats with CLI after 2 weeks of hyperoxia. HPV and constriction to the K(v)1.x-specific inhibitor, correolide, were preferentially blunted in excised distal pulmonary arteries (dPAs) from hyperoxic rats, whose dPAs exhibited decreased K(v)1.5 and K(v)2.1 mRNA and K(+) current. Intrapulmonary gene transfer of K(v)1.5, encoding the ion channel that is thought to trigger HPV, increased O(2)-sensitive K(+) current in cultured smooth muscle cells from rat dPAs, and restored HPV in hyperoxic rats.

CONCLUSIONS

Reduced expression/activity of O(2)-sensitive K(v) channels in dPAs contributes to blunted HPV observed in neonatal CLD.

Airway function in infants treated with inhaled nitric oxide for persistent pulmonary hypertension

RATIONALE

Inhaled nitric oxide (iNO), used for treatment of persistent pulmonary hypertension of newborn (PPHN), is an oxygen free radical with potential for lung injury. Deferring ECMO with iNO in these neonates could potentially have long-term detrimental effects on lung function. We studied respiratory morbidity (defined as occurrence of respiratory infections requiring treatment, episodes of wheezing, and/or need for ongoing medications following discharge) and airway function at 1 year postnatal age in term neonates treated with iNO but not ECMO for PPHN, and compared data from similar infants recruited to the UK ECMO Trial randomized to receive ECMO or conventional management (CM).

METHODS

Maximal expiratory flow at FRC (V(') (maxFRC)) was measured in infants treated with iNO for PPHN (oxygenation index >or=25) at birth.

RESULTS

V(') (maxFRC) was measured in 23 infants and expressed as z-scores, to adjust for sex and body size and compared to data from 71 (46 ECMO, 25 CM) infants studied at a similar age in the ECMO Trial. Respiratory morbidity was low in iNO group. V(') (maxFRC) z-score was lower than predicted in all groups (P < 0.001), with no significant difference between those treated with iNO [mean (SD) z-score: -1.65 (1.2)] and those treated with ECMO [-1.59 (1.2)] or CM [-2.1(1.0)]. Within iNO, ECMO and CM groups; 26%, 37% and 56%, respectively, had V(') (maxFRC) z-scores below normal.

CONCLUSIONS

Respiratory outcome at 1 year in iNO treated neonates with moderately severe PPHN is encouraging, with no apparent increase in respiratory morbidity when compared to the general population. Sub-clinical reductions in airway function are evident at 1 year, suggesting that continuing efforts to minimize lung injury in the neonatal period are warranted to maximize lung health in later life.

Plasma biomarkers of oxidative stress: relationship to lung disease and inhaled nitric oxide therapy in premature infants

OBJECTIVES

Inhaled nitric oxide treatment for ventilated premature infants improves survival without bronchopulmonary dysplasia. However, there has been no information regarding possible effects of this therapy on oxidative stress. We hypothesized that inhaled nitric oxide therapy would not influence concentrations of plasma biomarkers of oxidative stress.

PATIENTS AND METHODS

As part of the Nitric Oxide Chronic Lung Disease Trial, we collected blood samples at specified intervals from a subpopulation of 100 infants of <1250 g birth weight who received inhaled nitric oxide (20 ppm, weaned to 2 ppm) or placebo gas for 24 days. Plasma was assayed for total protein and for 3-nitrotyrosine and carbonylation by using immunoassays.

RESULTS

The demographic characteristics and primary outcome for the infants were representative of the entire group of infants who were in the Nitric Oxide Chronic Lung Disease Trial. For all infants at baseline, before receiving study gas, the concentration of total protein was inversely correlated with the respiratory severity score, and plasma carbonyl was positively correlated with severity score, supporting an association between oxidative stress and severity of lung disease. Infants who survived without bronchopulmonary dysplasia had 30% lower protein carbonylation concentrations at study entry than those who had an adverse outcome. At each of 3 time points (1-10 days) during exposure to study gas, there were no significant differences between control and treated infants for concentrations of plasma protein, 3-nitrotyrosine, and carbonylation.

CONCLUSIONS

Inhaled nitric oxide treatment for premature infants who are at risk for bronchopulmonary dysplasia does not alter plasma biomarkers of oxidative stress, which supports the safety of this therapy.

Mechanical ventilation uncouples synthesis and assembly of elastin and increases apoptosis in lungs of newborn mice

Prolonged mechanical ventilation (MV) with O2-rich gas inhibits lung growth and causes excess, disordered accumulation of lung elastin in preterm infants, often resulting in chronic lung disease (CLD). Using newborn mice, in which alveolarization occurs postnatally, we designed studies to determine how MV with either 40% O2or air might lead to dysregulated elastin production and impaired lung septation. MV of newborn mice for 8 h with either 40% O2or air increased lung mRNA for tropoelastin and lysyl oxidase, relative to unventilated controls, without increasing lung expression of genes that regulate elastic fiber assembly (lysyl oxidase-like-1, fibrillin-1, fibrillin-2, fibulin-5, emilin-1). Serine elastase activity in lung increased fourfold after MV with 40% O2, but not with air. We then extended MV with 40% O2to 24 h and found that lung content of tropoelastin protein doubled, whereas lung content of elastin assembly proteins did not change (lysyl oxidases, fibrillins) or decreased (fibulin-5, emilin-1). Quantitative image analysis of lung sections showed that elastic fiber density increased by 50% after MV for 24 h, with elastin distributed throughout the walls of air spaces, rather than at septal tips, as in control lungs. Dysregulation of elastin was associated with a threefold increase in lung cell apoptosis (TUNEL and caspase-3 assays), which might account for the increased air space size previously reported in this model. Our findings of increased elastin synthesis, coupled with increased elastase activity and reduced lung abundance of proteins that regulate elastic fiber assembly, could explain altered lung elastin deposition, increased apoptosis, and defective septation, as observed in CLD.

The effect of inhaled nitric oxide on pulmonary function in preterm infants

OBJECTIVE

Bronchopulmonary dysplasia (BPD) in preterm infants is associated with impaired alveolar growth, inflammation and airway hyperreactivity. In animal models of BPD, inhaled nitric oxide (NO) improves alveolar growth and inhibits airway smooth muscle proliferation. This study was designed to assess the effect of inhaled NO on resistance and compliance in ventilated preterm infants with evolving BPD.

STUDY DESIGN

Expiratory resistance and compliance of the respiratory system were measured in 71 ventilated preterm infants, < or = 32 weeks gestation, randomized to NO (n=34) versus placebo (n=37) for > or = 24 days at 7 to 21 days of life.

RESULT

At baseline expiratory resistance (231+/-71 versus 215+/-76 cm H(2)O l(-1) s(-1)) and compliance (0.49+/-0.14 versus 0.53+/-0.13 ml cm H(2)O(-1) kg(-1)) were comparable between placebo and NO groups, respectively. There was no effect of NO on expiratory resistance or compliance at 1 h, 1 week or 2 weeks of study gas administration.

CONCLUSION

NO had no short- or medium-term effect on expiratory resistance or compliance in ventilated preterm infants.

Early inhaled nitric oxide treatment decreases apoptosis of endothelial cells in neonatal rat lungs after vascular endothelial growth factor inhibition

Vascular endothelial growth factor (VEGF) receptor blockade impairs lung growth and decreases nitric oxide (NO) production in neonatal rat lungs. Inhaled NO (iNO) treatment after VEGF inhibition preserves lung growth in infant rats by unknown mechanisms. We hypothesized that neonatal VEGF inhibition disrupts lung growth by causing apoptosis in endothelial cells, which is attenuated by early iNO treatment. Three-day-old rats received SU-5416, an inhibitor of VEGF receptor, or its vehicle and were raised in room air with or without iNO (10 ppm). SU-5416 reduced alveolar counts and lung vessel density by 28% ( P < 0.005) and 21% ( P < 0.05), respectively, as early as at 7 days of age. SU-5416 increased lung active caspase-3 protein by 60% at 5 days of age ( P < 0.05), which subsided by 7 days of age, suggesting a transient increase in lung apoptosis after VEGF blockade. Apoptosis primarily colocalized to lung vascular endothelial cells, and SU-5416 increased endothelial cell apoptotic index by eightfold at 5 days of age ( P <0.0001). iNO treatment after SU-5416 prevented the increases in lung active caspase-3 and in endothelial cell apoptotic index. There was no difference in alveolar type 2 cell number between control and SU-5416-treated rats. We conclude that neonatal VEGF receptor inhibition causes transient apoptosis in pulmonary endothelium, which is followed by persistently impaired lung growth. Early iNO treatment after VEGF inhibition reduces endothelial cell apoptosis in neonatal lungs. We speculate that enhancing endothelial cell survival after lung injury may preserve neonatal lung growth in bronchopulmonary dysplasia.

cGMP-dependent protein kinase I interacts with TRIM39R, a novel Rpp21 domain-containing TRIM protein

Nitric oxide modulates vascular smooth muscle cell (SMC) cytoskeletal kinetics and phenotype, in part, by stimulating cGMP-dependent protein kinase I (PKGI). To identify molecular targets of PKGI, an interaction trap screen in yeast was performed using a cDNA encoding the catalytic region of PKGI and a human lung cDNA library. We identified a cDNA that encodes a putative PKGI-interactor that is a novel variant of TRIM39, a member of the really interesting new gene (RING) finger family of proteins. Although this TRIM39 variant encodes the NH(2)-terminal RING finger (RF), B-box, and coiled-coil (RBBC) domains of TRIM39, instead of a complete COOH-terminal B30.2 domain, this TRIM39 isoform contains the COOH-terminal portion of Rpp21, a component of RNase P. RT-PCR demonstrated that the TRIM39 variant, which we refer to as TRIM39R, is transcribed in the human fetal lung and in rat pulmonary artery SMC. Indirect immunofluorescence using an antibody generated against the conserved domains of TRIM39 and TRIM39R revealed the proteins in speckled intranuclear structures in human acute monocytic leukemia (THP-1) and human epidermal carcinoma line (HEp-2) cells. PKGI phosphorylated a typical PKGI/PKA phosphorylation domain in a conserved region of TRIM39 and TRIM39R. Additional studies demonstrated that PKGI interacts with both isoforms of TRIM39 in yeast cells and phosphorylates both isoforms of TRIM39 in human cell lines. Although PKGI has been observed to interact with proteins that regulate cytoskeletal function and gene expression, this investigation shows for the first time that PKGI interacts with tripartite motif (TRIM) proteins, which, through diverse molecular pathways, are often observed to regulate important aspects of cellular homeostasis.

Surfactant function and composition in premature infants treated with inhaled nitric oxide

OBJECTIVES

We hypothesized that inhaled nitric oxide treatment of premature infants at risk for bronchopulmonary dysplasia would not adversely affect endogenous surfactant function or composition.

METHODS

As part of the Nitric Oxide Chronic Lung Disease Trial of inhaled nitric oxide, we examined surfactant in a subpopulation of enrolled infants. Tracheal aspirate fluid was collected at specified intervals from 99 infants with birth weights <1250 g who received inhaled nitric oxide (20 ppm, weaned to 2 ppm) or placebo gas for 24 days. Large-aggregate surfactant was analyzed for surface activity with a pulsating bubble surfactometer and for surfactant protein contents with an immunoassay.

RESULTS

At baseline, before administration of study gas, surfactant function and composition were comparable in the 2 groups, and there was a positive correlation between minimum surface tension and severity of lung disease for all infants. Over the first 4 days of treatment, minimum surface tension increased in placebo-treated infants and decreased in inhaled nitric oxide-treated infants. There were no significant differences between groups in recovery of large-aggregate surfactant or contents of surfactant protein A, surfactant protein B, surfactant protein C, or total protein, normalized to phospholipid.

CONCLUSIONS

We conclude that inhaled nitric oxide treatment for premature infants at risk of bronchopulmonary dysplasia does not alter surfactant recovery or protein composition and may improve surfactant function transiently.

Inhaled nitric oxide for the prevention of bronchopulmonary dysplasia

This review provides the readers with background information on the state of the art and science of inhaled nitric oxide (iNO) as therapy for prevention or amelioration of bronchopulmonary dysplasia (BPD) in preterm infants. The goal is to review and critique relevant published information. A total of six clinical trials, all placebo-controlled, four out of six blinded, four out of six multi-centered with a predetermined outcome of reduction in death or BPD, have been reported in full text. These definitive studies have included a total of > 2100 preterm, mostly very preterm, infants. Their designs were informed by results of earlier non-definitive studies which cumulatively enrolled > 350 preterm patients. This very substantial experience provides a firm framework for asserting that iNO will be useful in this population of patients. The use of iNO can reduce the occurrence of BPD and possibly the severity of the disorder. Optimal time of initiation, dosing (both initial dose, duration of treatment and possibly the route of administration) and most importantly, optimal patient subset selection, are not determined. Any clear adverse neurological finding in iNO-treated infants will of course limit or halt the use of this promising therapy.

Disruption of NO-cGMP signaling by neonatal hyperoxia impairs relaxation of lung parenchyma

Exposure of immature lungs to hyperoxia for prolonged periods contributes to neonatal lung injury and airway hyperreactivity. We studied the role of disrupted nitric oxide-guanosine 3',5'-cyclic monophosphate (NO-cGMP) signaling in impairing the relaxant responses of lung tissue from hyperoxia-exposed rat pups. Pups were exposed to >/=95% O(2) or room air for 7 days starting from days 1, 5, or 14. The animals were killed, lungs were removed, and 1-mm-thick lung parenchymal strips were prepared. Lung parenchymal strips of room air or hyperoxic pups were preconstricted using bethanechol and then graded electrical field stimulation (EFS) was applied to induce relaxation. EFS-induced relaxation of lung parenchymal strips was greater at 7 and 12 days than at 21 days in room air-exposed rat pups. Hyperoxic exposure significantly reduced relaxation at 7 and 12 days but not 21 days compared with room air exposure. NO synthase blockade with N(omega)-nitro-l-arginine methyl ester diminished relaxant responses in room air but not in hyperoxic pups at 12 days. After incubation with supplemental l-arginine, the relaxation response of hyperoxic strips was restored. cGMP, a key mediator of the NO signaling pathway, also decreased in strips from hyperoxic vs. room air pups and cGMP levels were restored after incubation with supplemental l-arginine. In addition, arginase activity was significantly increased in hyperoxic lung parenchymal strips compared with room air lung parenchymal strips. These data demonstrate disruption of NO-cGMP signaling in neonatal rat pups exposed to hyperoxia and show that bioavailability of the substrate l-arginine is implicated in the predisposition of this model to airway hyperreactivity.

Inhaled NO as a therapeutic agent

In 1991, Frostell and colleagues reported that breathing low concentrations of nitric oxide (NO) decreased pulmonary artery pressure (PAP) in awake lambs with experimental pulmonary hypertension (PH) [Frostell C, Fratacci MD, Wain JC, Jones R, Zapol WM. Inhaled nitric oxide. A selective pulmonary vasodilator reversing hypoxic pulmonary vasoconstriction. Circulation 1991;83:2038-47]. Subsequently, efforts of multiple research groups studying animals and patients led to approval of inhaled NO by the US Food and Drug Administration in 1999 and the European Medicine Evaluation Agency and European Commission in 2001. Inhaled NO is currently indicated for the treatment of term and near-term neonates with hypoxemia and PH. Since regulatory approval, several studies have suggested that NO inhalation can prevent chronic lung disease in premature infants. In addition, unanticipated systemic effects of inhaled NO may lead to treatments for a variety of disorders including ischemia-reperfusion injury. This review summarizes the pharmacology and physiological effects of breathing NO. The application of inhaled NO to hypoxemic neonates with PH is discussed including recent studies exploring the use of inhaled NO to prevent bronchopulmonary dysplasia in premature infants. This review also highlights the application of inhaled NO to treat adults with cardiopulmonary disease, strategies to augment the efficacy of inhaled NO, and potential applications of the systemic effects of the gas.

Neurodevelopmental outcomes of premature infants with severe respiratory failure enrolled in a randomized controlled trial of inhaled nitric oxide

OBJECTIVES

We hypothesized that inhaled nitric oxide (iNO) would not decrease death or neurodevelopmental impairment (NDI) in infants enrolled in the National Institute of Child Health and Human Development Preemie iNO Trial (PiNO) trial, nor improve neurodevelopmental outcomes in the follow-up group.

STUDY DESIGN

Infants <34 weeks of age, weighing <1500 g, with severe respiratory failure were enrolled in the multicenter, randomized, controlled trial. NDI at 18 to 22 months corrected age was defined as: moderate to severe cerebral palsy (CP; Mental Developmental Index or Psychomotor score Developmental Index <70), blindness, or deafness.

RESULTS

Of 420 patients enrolled, 109 who received iNO (52%) and 98 who received placebo (47%) died. The follow-up rate in survivors was 90%. iNO did not reduce death or NDI (78% versus 73%; relative risk [RR], 1.07; 95% CI, 0.95-1.19), or NDI or Mental Developmental Index <70 in the follow-up group. Moderate-severe CP was slightly higher with iNO (RR, 2.41; 95% CI, 1.01-5.75), as was death or CP in infants weighing <1000 g (RR, 1.22; 95% CI, 1.05-1.43).

CONCLUSIONS

In this extremely ill cohort, iNO did not reduce death or NDI or improve neurodevelopmental outcomes. Routine iNO use in premature infants should be limited to research settings until further data are available.

Inhaled nitric oxide in infants >1500 g and <34 weeks gestation with severe respiratory failure

OBJECTIVE

Inhaled nitric oxide (iNO) use in infants >1500 g, but <34 weeks gestation with severe respiratory failure will reduce the incidence of death and/or bronchopulmonary dysplasia (BPD).

STUDY DESIGN

Infants born at <34 weeks gestation with a birth weight >1500 g with respiratory failure were randomly assigned to receive placebo or iNO.

RESULTS

Twenty-nine infants were randomized. There were no differences in baseline characteristics, but the status at randomization showed a statistically significant difference in the use of high-frequency ventilation (P=0.03). After adjustment for oxygenation index entry strata, there was no difference in death and/or BPD (adjusted relative risk (RR) 0.80, 95% confidence interval (CI) 0.43 to 1.48; P=0.50), death (adjusted RR 1.26, 95% CI 0.47 to 3.41; P=0.65) or BPD (adjusted RR 0.40, 95% CI 0.47 to 3.41; P=0.21).

CONCLUSIONS

Although sample size limits our ability to make definitive conclusions, this small pilot trial of iNO use in premature infants >1500 g and <34 weeks with severe respiratory failure suggests that iNO does not affect the rate of BPD and/or death.

Dysregulation of pulmonary elastin synthesis and assembly in preterm lambs with chronic lung disease

Failed alveolar formation and excess, disordered elastin are key features of neonatal chronic lung disease (CLD). We previously found fewer alveoli and more elastin in lungs of preterm compared with term lambs that had mechanical ventilation (MV) with O(2)-rich gas for 3 wk (MV-3 wk). We hypothesized that, in preterm more than in term lambs, MV-3 wk would reduce lung expression of growth factors that regulate alveolarization (VEGF, PDGF-A) and increase lung expression of growth factors [transforming growth factor (TGF)-alpha, TGF-beta(1)] and matrix molecules (tropoelastin, fibrillin-1, fibulin-5, lysyl oxidases) that regulate elastin synthesis and assembly. We measured lung expression of these genes in preterm and term lambs after MV for 1 day, 3 days, or 3 wk, and in fetal controls. Lung mRNA for VEGF, PDGF-A, and their receptors (VEGF-R2, PDGF-Ralpha) decreased in preterm and term lambs after MV-3 wk, with reduced lung content of the relevant proteins in preterm lambs with CLD. TGF-alpha and TGF-beta(1) expression increased only in lungs of preterm lambs. Tropoelastin mRNA increased more with MV of preterm than term lambs, and expression levels remained high in lambs with CLD. In contrast, fibrillin-1 and lysyl oxidase-like-1 mRNA increased transiently, and lung abundance of other elastin-assembly genes/proteins was unchanged (fibulin-5) or reduced (lysyl oxidase) in preterm lambs with CLD. Thus MV-3 wk reduces lung expression of growth factors that regulate alveolarization and differentially alters expression of growth factors and matrix proteins that regulate elastin assembly. These changes, coupled with increased lung elastase activity measured in preterm lambs after MV for 1-3 days, likely contribute to CLD.

Pulmonary nitric oxide synthases and nitrotyrosine: findings during lung development and in chronic lung disease of prematurity

BACKGROUND

Nitric oxide mediates and modulates pulmonary transition from fetal to postnatal life. NO is synthesized by 3 nitric oxide synthase isoforms. One key pathway of nitric oxide metabolism results in nitrotyrosine, a stable, measurable marker of nitric oxide production.

OBJECTIVE

The purpose of this study was to assess, by semiquantitative immunohistochemistry, nitric oxide synthase isoforms and nitrotyrosine at different airway and vascular tree levels in the lungs of neonates at different gestational ages and to compare results in control groups to those in infants with chronic lung disease.

DESIGN/METHODS

Formalin-fixed, paraffin-embedded, postmortem lung blocks were prepared for immunohistochemistry using antibodies to each nitric oxide synthase isoform and to nitrotyrosine. Blinded observers evaluated the airway and vascular trees for staining intensity (0-3 scale) at 5 levels and 3 levels, respectively. The control population consisted of infants from 22 to 42 weeks' gestation who died in < 48 hours. Results were compared with gestation-matched infants with varying severity of chronic lung disease.

RESULTS

In control and chronic lung disease groups, 22 to 42 weeks' gestation, staining for all 3 of the nitric oxide synthase isoforms was found in the airway epithelium from the bronchus to the alveolus or distal-most airspace. The abundance or distribution of nitric oxide synthase-3 staining in the airways did not show significant correlation with gestational age or severity of chronic lung disease. In the vascular tree, intense nitric oxide synthase-3 and moderate nitric oxide synthase-2 staining was found; nitric oxide synthase-1 was not consistently stained. Nitrotyrosine did stain in the pulmonary tree. Compared with controls where nitrotyrosine staining was minimal, regardless of gestation, in infants with chronic lung disease there was more than fourfold increase between severe chronic lung disease (n = 12) and either mild chronic lung disease or control infants (n = 16).

CONCLUSIONS

All 3 of the nitric oxide synthase isoforms and nitrotyrosine are detectable by immunohistochemistry early in lung development. Nitric oxide synthase ontogeny shows no significant changes in abundance or distribution with advancing gestational age nor with chronic lung disease. Nitrotyrosine is significantly increased in severe chronic lung disease.

Endotoxin-induced nitric oxide production rescues airway growth and maturation in atrophic fetal rat lung explants

Inflammation induces premature maturation of the fetal lung but the signals causing this effect remain unclear. We determined if nitric oxide (NO) synthesis, evoked by Escherichia coli lipopolysaccharide (LPS, 2 microg ml-1), participated in this process. Fetal rat lung airway surface complexity rose 2.5-fold over 96h in response to LPS and was associated with increased iNOS protein expression and activity. iNOS inhibition by N6-(1-iminoethyl)-L-lysine-2HCl (L-NIL) abolished this and induced airway atrophy similar to untreated explants. Surfactant protein-C (SP-C) expression was also induced by LPS and abolished by L-NIL. As TGFbeta suppresses iNOS activity, we determined if feedback regulation modulated NO-dependent maturation. LPS induced TGFbeta1 release and SMAD4 nuclear translocation 96 h after treatment. Treatment of explants with a blocking antibody against TGFbeta1 sustained NO production and airway morphogenesis whereas recombinant TGFbeta1 antagonized these effects. Feedback regulation of NO synthesis by TGFbeta may, thus, modulate airway branching and maturation of the fetal lung.

Pathology of bronchopulmonary dysplasia

Over the past three decades, advances in prenatal and neonatal intensive care have contributed to marked improvements in survival rates for extremely immature infants born during the canalicular phase of lung development at 24 to 26 weeks, a time when alveolar and distal vascular development is rapidly occurring. The histopathological lesions of severe airway injury and alternating sites of overinflation and fibrosis in "old" BPD have been replaced in "new" BPD with the pathologic changes of large, simplified alveolar structures, a dysmorphic capillary configuration, and variable interstitial cellularity and/or fibroproliferation. Airway and vascular lesions, when present, tend to be present in infants, who over time develop more severe disease. The concept that "new" BPD results in an arrest in alveolization should be modified to that of an impairment in alveolization as evidence shows that short ventilatory times and/or the use of nCPAP allow continued alveolar formation.

Inhaled nitric oxide in preterm infants undergoing mechanical ventilation

BACKGROUND

Bronchopulmonary dysplasia in premature infants is associated with prolonged hospitalization, as well as abnormal pulmonary and neurodevelopmental outcome. In animal models, inhaled nitric oxide improves both gas exchange and lung structural development, but the use of this therapy in infants at risk for bronchopulmonary dysplasia is controversial.

METHODS

We conducted a randomized, stratified, double-blind, placebo-controlled trial of inhaled nitric oxide at 21 centers involving infants with a birth weight of 1250 g or less who required ventilatory support between 7 and 21 days of age. Treated infants received decreasing concentrations of nitric oxide, beginning at 20 ppm, for a minimum of 24 days. The primary outcome was survival without bronchopulmonary dysplasia at 36 weeks of postmenstrual age.

RESULTS

Among 294 infants receiving nitric oxide and 288 receiving placebo birth weight (766 g and 759 g, respectively), gestational age (26 weeks in both groups), and other characteristics were similar. The rate of survival without bronchopulmonary dysplasia at 36 weeks of postmenstrual age was 43.9 percent in the group receiving nitric oxide and 36.8 percent in the placebo group (P=0.042). The infants who received inhaled nitric oxide were discharged sooner (P=0.04) and received supplemental oxygen therapy for a shorter time (P=0.006). There were no short-term safety concerns.

CONCLUSIONS

Inhaled nitric oxide therapy improves the pulmonary outcome for premature infants who are at risk for bronchopulmonary dysplasia when it is started between 7 and 21 days of age and has no apparent short-term adverse effects. (ClinicalTrials.gov number, NCT00000548 [ClinicalTrials.gov] .).

Early inhaled nitric oxide therapy in premature newborns with respiratory failure

BACKGROUND

The safety and efficacy of early, low-dose, prolonged therapy with inhaled nitric oxide in premature newborns with respiratory failure are uncertain.

METHODS

We performed a multicenter, randomized trial involving 793 newborns who were 34 weeks of gestational age or less and had respiratory failure requiring mechanical ventilation. Newborns were randomly assigned to receive either inhaled nitric oxide (5 ppm) or placebo gas for 21 days or until extubation, with stratification according to birth weight (500 to 749 g, 750 to 999 g, or 1000 to 1250 g). The primary efficacy outcome was a composite of death or bronchopulmonary dysplasia at 36 weeks of postmenstrual age. Secondary safety outcomes included severe intracranial hemorrhage, periventricular leukomalacia, and ventriculomegaly.

RESULTS

Overall, there was no significant difference in the incidence of death or bronchopulmonary dysplasia between patients receiving inhaled nitric oxide and those receiving placebo (71.6 percent vs. 75.3 percent, P=0.24). However, for infants with a birth weight between 1000 and 1250 g, as compared with placebo, inhaled nitric oxide therapy reduced the incidence of bronchopulmonary dysplasia (29.8 percent vs. 59.6 percent); for the cohort overall, such treatment reduced the combined end point of intracranial hemorrhage, periventricular leukomalacia, or ventriculomegaly (17.5 percent vs. 23.9 percent, P=0.03) and of periventricular leukomalacia alone (5.2 percent vs. 9.0 percent, P=0.048). Inhaled nitric oxide therapy did not increase the incidence of pulmonary hemorrhage or other adverse events.

CONCLUSIONS

Among premature newborns with respiratory failure, low-dose inhaled nitric oxide did not reduce the overall incidence of bronchopulmonary dysplasia, except among infants with a birth weight of at least 1000 g, but it did reduce the overall risk of brain injury. (ClinicalTrials.gov number, NCT00006401 [ClinicalTrials.gov].).

Neonatal lung and airway injury: a role for neurotrophins

Maintenance of patency in distal airways is essential for gas exchange in neonatal life, and its disruption may have long-lasting effects on respiratory function. However, neural mechanisms that regulate caliber of intrapulmonary airways during early postnatal life, and their disruption by hyperoxic exposure, have not been well characterized. We have previously shown that cholinergically mediated airway contractile responses in rat pups are upregulated after hyperoxic exposure, and that increased expression of neuropeptides, such as substance P, may be contributory. More recently, we have documented impairment of neurally mediated airway relaxation in response to hyperoxic stress associated with loss of nitric oxide and prostaglandin-induced airway relaxation as well as inhibition of long chain myosin phosphatase. Our most recent data demonstrate significantly enhanced expression of the neurotrophin, brain-derived neurotrophic factor (BDNF) and its high affinity specific tyrosine kinase B (TrkB) receptor in hyperoxia-exposed airway smooth muscle. The existence of a BDNF-TrkB receptor autocrine and paracrine loops in the airways provides a basis for understanding local regulatory mechanisms of airway homeostasis. A mechanistic role for BDNF-TrkB signaling in hyperoxia-induced airway hyperreactivity in early postnatal life could serve to modulate both afferent and efferent neural pathways that result in enhanced contractile responses of immature airways exposed to hyperoxic stress. Greater insight into these neural pathways may lead to future preventive strategies for preterm infants surviving neonatal intensive care and developing chronic lung disease.