Treatment with exogenous surfactant stimulates endogenous surfactant synthesis in premature infants with respiratory distress syndrome

Gastric Aspirate Phosphatidylcholine Species in Preterm Neonates Receiving Aerosolized Surfactant

OBJECTIVE

To characterize phosphatidylcholine (PC) molecular species in serial gastric aspirates as biomarkers for lung maturity, delivery of aerosolized surfactant (AS), and need for intubation.

METHODS

In a phase II clinical trial of aerosolized surfactant in preterm neonates with respiratory distress syndrome receiving noninvasive ventilation, infants received a maximum of 2 doses of nebulized beractant. Gastric aspirates were collected before and after each dose and were analyzed for PCs using liquid chromatography mass spectrometry.

RESULTS

Of 149 infants enrolled, gastric aspirates were obtained before (n = 91) and after (n = 94) dose 1, and before (n = 56) and after (n = 57) dose 2 of nebulized beractant. The mean ± SD values of birthweight, gestational age, and age at collection of baseline gastric aspirate were 1.7 ± 0.6 kg, 31.7 ± 2.8 weeks, and 5.5 ± 1.7 hours, respectively. The most abundant PC in beractant and gastric aspirates was PC(16:0/16:0). Advancing gestational age and number of antenatal corticosteroid doses predicted increased gastric aspirate PC(16:0/16:0), whereas maternal diabetes predicted a decrease. Several PCs increased significantly (P < .05) after nebulized beractant, consistent with effective aerosol delivery. Infants who received intubation within 72 hours of birth were more likely to have lower PC(16:0/16:0) levels in baseline gastric aspirates compared with those who did not (P = .024).

CONCLUSIONS

PC molecular species in gastric aspirates of preterm neonates are potentially novel and precise biomarkers to assess lung maturity, aerosol delivery, and need for endotracheal intubation.

Porcine versus bovine surfactant therapy for RDS in preterm neonates: pragmatic meta-analysis and review of physiopathological plausibility of the effects on extra-pulmonary outcomes

BACKGROUND

While porcine seems to be superior to bovine surfactants in terms of respiratory outcomes, it is unclear if a surfactant can improve extra-pulmonary outcomes in preterm neonates with respiratory distress syndrome and if there is any physiopathological/biological mechanism linking surfactant therapy to these outcomes. We aim to fill these knowledge gaps.

METHODS

Systematic and pragmatic review coupled with meta-analysis of randomized controlled trials of bovine or porcine surfactants administered to treat RDS in preterm neonates; common extra-pulmonary neonatal intensive care outcomes were considered. As additional analysis, animal or human translational studies about mechanisms linking surfactant replacement to extra-pulmonary neonatal outcomes were also systematically reviewed.

RESULTS

Porcine surfactant is associated with lower incidence of patent ductus arteriosus (OR:0.655; 95%CI:0.460-0.931); p = 0.018; 12 trials; 1472 patients); prenatal steroids (coeff.:-0.009, 95%CI:-0.03-0.009, p = 0.323) and gestational age (coeff.:0.079, 95%CI:-0.18-0.34, p = 0.554) did not influence this effect size. No significant differences were found between porcine and bovine surfactants on neonatal intensive care unit length of stay (mean difference (days):-2.977; 95%CI:-6.659-0.705; p = 0.113; 8 trials; 855 patients), intra-ventricular hemorrhage of any grade (OR:0.860; 95%CI:0.648-1.139); p = 0.293; 15 trials; 1703 patients), severe intra-ventricular hemorrhage (OR:0.852; 95%CI:0.624-1.163); p = 0.313; 15 trials; 1672 patients), necrotizing entero-colitis (OR:1.190; 95%CI:0.785-1.803); p = 0.412; 9 trials; 1097 patients) and retinopathy of prematurity (OR:0.801; 95%CI:0.480-1.337); p = 0.396; 10 trials; 962 patients).

CONCLUSIONS

Physiopathological mechanisms explaining the effect of surfactant have been found for patent ductus arteriosus only, while they are lacking for all other endpoints. Porcine surfactant is associated with lower incidence of PDA than bovine surfactants. As there are no differences in terms of other extra-pulmonary outcomes and no physiopathological plausibility, these endpoints should not be used in future trials.

REGISTRATION

PROSPERO n.CRD42018100906.

Effect of two different early rescue surfactant treatments on mortality in preterm infants with respiratory distress syndrome

INTRODUCTION

Although current evidence suggests that initial dose of 200 mg/kg poractant alfa reduces mortality in the treatment of respiratory distress syndrome (RDS), these data were obtained in a highly heterogeneous group of patients and neither of them addressed mortality as primary outcome.

OBJECTIVE

The aim of this study was to investigate the effects of poractant alfa and beractant on mortality when administered as early rescue surfactant therapy in very preterm infants.

METHODS

We retrospectively evaluated preterm infants followed in our unit between May 2017 and November 2018 whose gestational age (GA) was ≤28 weeks and received surfactant within the first 2 hours of life. Morbidities and mortality rates were compared between infants who received initial doses of 200 mg/kg poractant alfa and 100 mg/kg beractant.

RESULTS

Data from 200 infants who met the inclusion criteria were analyzed. There were 112 patients in the poractant alfa group and 88 patients in beractant group. Mean gestational age in these groups was 26 ± 2 and 25.8 ± 1.8 weeks (P = 0.45) and mean birth weight was 812 ± 243 and 840 ± 208 g (P = 0.39), respectively. The poractant alfa and beractant groups had similar rates of overall mortality (53.5% vs 56.8%), mortality in first 7 days (30.5% vs 25.8%), and beyond day 7 (16.4% vs 13.3%) (P > 0.05). There were no differences in the incidence of preterm morbidities among the two groups.

CONCLUSION

We were unable to demonstrate the superiority of poractant in terms of mortality in very preterm infants with RDS. These findings need to be supported by multicenter, randomized controlled trials.

Tracing exogenous surfactant in vivo in rabbits by the natural variation of 13C

BACKGROUND

Respiratory Distress Syndrome (RDS) is a prematurity-related breathing disorder caused by a quantitative deficiency of pulmonary surfactant. Surfactant replacement therapy is effective for RDS newborns, although treatment failure has been reported. The aim of this study is to trace exogenous surfactant by ¹³C variation and estimate the amount reaching the lungs at different doses of the drug.

METHODS

Forty-four surfactant-depleted rabbits were obtained by serial bronchoalveolar lavages (BALs), that were merged into a pool (BAL pool) for each animal. Rabbits were in nasal continuous positive airway pressure and treated with 0, 25, 50, 100 or 200 mg/kg of poractant alfa by InSurE. After 90 min, rabbits were depleted again and a new pool (BAL end experiment) was collected. Disaturated-phosphatidylcholine (DSPC) was measured by gas chromatography. DSPC-Palmitic acid (PA) ¹³C/¹²C was analyzed by isotope ratio mass spectrometry. One-way non-parametric ANOVA and post-hoc Dunn's multiple comparison were used to assess differences among experimental groups.

RESULTS

Based on DSPC-PA ¹³C/¹²C in BAL pool and BAL end experiment, the estimated amount of exogenous surfactant ranged from 61 to 87% in dose-dependent way (p < 0.0001) in animals treated with 25 up to 200 mg/kg. Surfactant administration stimulated endogenous surfactant secretion. The percentage of drug recovered from lungs did not depend on the administered dose and accounted for 31% [24-40] of dose.

CONCLUSIONS

We reported a risk-free method to trace exogenous surfactant in vivo. It could be a valuable tool for assessing, alongside the physiological response, the delivery efficiency of surfactant administration techniques.

Porcine vs bovine surfactant therapy for preterm neonates with RDS: systematic review with biological plausibility and pragmatic meta-analysis of respiratory outcomes

Background

Bovine surfactants are known to be clinically equivalent but it is unclear if porcine or bovine surfactants at their licensed dose should be preferred to treat respiratory distress syndrome in preterm neonates.

Methods

We performed a comprehensive review of biochemical and pharmacological features of surfactants to understand the biological plausibility of any clinical effect. We then performed a pragmatic meta-analysis comparing internationally marketed porcine and bovine surfactants for mortality and respiratory outcomes. Search for randomised controlled trials with no language/year restrictions and excluding “grey” literature, unpublished or non-peer reviewed reports was conducted, following Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines and the most recent methodological recommendations.

Results

Sixteen articles were included in the review and 14 in the meta-analysis (1491 neonates). 200 mg/kg poractant-α (a porcine surfactant) was associated with lower BPD/mortality (OR 0.632[95%CI:0.494, 0.809];p < 0.001),BPD (OR 0.688[95%CI:0.512, 0.925];p = 0.013), retreatment (OR 0.313[95%CI:0.187, 0.522];p < 0.0001), airleaks (OR 0.505[95%CI:0.308, 0.827];p = 0.006) and lung haemorrhage (OR 0.624[95%CI:0.388, 1];p = 0.051). Gestational age is associated with effect size for BPD (coefficient: 0.308 [95%CI:0.063, 0.554];p = 0.014) and surfactant retreatment (coefficient: -0.311 [95%CI:-0.595, − 0.028];p = 0.031).

Conclusion

200 mg/kg poractant-α is associated with better respiratory outcomes compared to bovine surfactants at their licensed dose. The effect of poractant-α on BPD and surfactant retreatment is greater at lowest and highest gestational ages, respectively.

Trial registration

PROSPERO n.42017075251.

Electronic supplementary material

The online version of this article (10.1186/s12931-019-0979-0) contains supplementary material, which is available to authorized users.

Metabolism of a synthetic compared with a natural therapeutic pulmonary surfactant in adult mice

Secreted pulmonary surfactant phosphatidylcholine (PC) has a complex intra-alveolar metabolism that involves uptake and recycling by alveolar type II epithelial cells, catabolism by alveolar macrophages, and loss up the bronchial tree. We compared the in vivo metabolism of animal-derived poractant alfa (Curosurf) and a synthetic surfactant (CHF5633) in adult male C57BL/6 mice. The mice were dosed intranasally with either surfactant (80 mg/kg body weight) containing universally ¹³C-labeled dipalmitoyl PC (DPPC) as a tracer. The loss of [U¹³C]DPPC from bronchoalveolar lavage and lung parenchyma, together with the incorporation of ¹³C-hydrolysis fragments into new PC molecular species, was monitored by electrospray ionization tandem mass spectrometry. The catabolism of CHF5633 was considerably delayed compared with poractant alfa, the hydrolysis products of which were cleared more rapidly. There was no selective resynthesis of DPPC and, strikingly, acyl remodeling resulted in preferential synthesis of polyunsaturated PC species. In conclusion, both surfactants were metabolized by similar pathways, but the slower catabolism of CHF5633 resulted in longer residence time in the airways and enhanced recycling of its hydrolysis products into new PC species.

Estimating the contribution of surfactant replacement therapy to the alveolar pool: An in vivo study based on 13 C natural abundance in rabbits

Variation of the isotopic abundance of selected nutrients and molecules has been used for pharmacological and kinetics studies under the premise that the administered molecule has a different isotopic enrichment from the isotopic background of the recipient subject. The aim of this study is to test the feasibility of assessing the contribution of exogenous surfactant phospholipids to the endogenous alveolar pool in vivo after exogenous surfactant replacement therapy in rabbits. The study consisted in measuring the consistency of ¹³ C/¹² C ratio of disaturated-phosphatidylcholine palmitate (DSPC-PA) in 7 lots of poractant alfa, produced over a year, and among bronchoalveolar lavages of 20 rabbits fed with a standard chow. A pilot study was performed in a rabbit model of lavage-induced surfactant deficiency: 7 control rabbits and 4 treated with exogenous surfactant. The contribution of exogenous surfactant to the alveolar pool was assessed after intra-tracheal administration of 200 mg/kg of poractant alfa. The ¹³ C content of DSPC-PA was measured by isotope ratio mass spectrometry. The mean DSPC-PA ¹³ C/¹² C ratio of the 7 lots of poractant alfa was -18.8‰ with a SD of 0.1‰ (range: -18.9‰; -18.6‰). The mean ¹³ C/¹² C ratio of surfactant DSPC recovered from the lung lavage of 20 rabbits was -28.8 ± 1.2‰ (range: -31.7‰; -25.7‰). The contribution of exogenous surfactant to the total alveolar surfactant could be calculated in the treated rabbits, and it ranged from 83.9% to 89.6%. This pilot study describes a novel method to measure the contribution of the exogenous surfactant to the alveolar pool. This method is based on the natural variation of ¹³ C, and therefore it does not require the use of chemically synthetized tracers. This method could be useful in human research and especially in surfactant replacement studies in preterm infants.

Non-invasive ventilation and surfactant treatment as the primary mode of respiratory support in surfactant-deficient newborn piglets

BackgroundNasal continuous positive airway pressure (NCPAP) and nasal intermittent positive pressure ventilation (NIPPV), forms of non-invasive ventilation (NIV) for respiratory support, are increasingly being chosen as the initial treatment for neonates with surfactant (SF) deficiency. Our objective was to compare NCPAP with NIPPV with or without SF administration as a primary mode of ventilation.MethodsTwenty-four newborn piglets with SF-deficient lung injury produced by repetitive bronchoalveolar lavages were randomly assigned to NCPAP or NIPPV, with or without SF administration (InSurE method). We evaluated pulmonary, systemic (hemodynamic and oxygen metabolism), and cerebral effects.ResultsSF-deficient piglets developed respiratory distress (F_iO2:1, pH<7.2, P_aCO2>70 mm Hg, P_aO2<70 mm Hg, and C_dyn<0.5 ml/cmH₂O/kg). Gradual improvements in pulmonary status were observed in both NIV groups, with NIPPV achieving lower lung inflammation markers and injury scores. Both SF-treated groups obtained significantly better respiratory outcomes than groups not treated with SF before NIV. All NIV-treated groups showed low brain injury scores.ConclusionIn spontaneously breathing SF-deficient newborn piglets, NIPPV is a suitable NIV strategy. SF administration in combination with NCPAP or NIPPV improves pulmonary status providing extra protection against pulmonary injury. No injury to the developing brain was observed to be associated with these NIV strategies, with or without SF therapy.

Neonatal Respiratory Diseases in the Newborn Infant: Novel Insights from Stable Isotope Tracer Studies

Respiratory distress syndrome is a common problem in preterm infants and the etiology is multifactorial. Lung underdevelopment, lung hypoplasia, abnormal lung water metabolism, inflammation, and pulmonary surfactant deficiency or disfunction play a variable role in the pathogenesis of respiratory distress syndrome. High-quality exogenous surfactant replacement studies and studies on surfactant metabolism are available; however, the contribution of surfactant deficiency, alteration or dysfunction in selected neonatal lung conditions is not fully understood. In this article, we describe a series of studies made by applying stable isotope tracers to the study of surfactant metabolism and lung water. In a first set of studies, which we call 'endogenous studies', using stable isotope-labelled intravenous surfactant precursors, we showed the feasibility of measuring surfactant synthesis and kinetics in infants using several metabolic precursors including plasma glucose, plasma fatty acids and body water. In a second set of studies, named 'exogenous studies', using stable isotope-labelled phosphatidylcholine tracer given endotracheally, we could estimate surfactant disaturated phosphatidylcholine pool size and half-life. Very recent studies are focusing on lung water and on the endogenous biosynthesis of the surfactant-specific proteins. Information obtained from these studies in infants will help to better tailor exogenous surfactant treatment in neonatal lung diseases.

Acute and sustained effects of aerosolized vs. bolus surfactant therapy in premature lambs with respiratory distress syndrome

BACKGROUND

Surfactant (SF) instillation may produce acute deleterious effects on gas exchange and both systemic and cerebral hemodynamics. Our aim was to compare the effects of aerosolized SF (SF-aero) with those of bolus SF (SF-bolus) administration on gas exchange, lung mechanics, and cardiovascular function in premature lambs with respiratory distress syndrome (RDS).

METHODS

Fourteen preterm lambs (85% gestation) were randomly assigned to receive SF-aero or SF-bolus. Oxygenation index (OI), PaCO2, cardiovascular parameters, carotid blood flow (CBF), lung compliance (mean dynamic compliance), and tidal volume (VT) were measured every 30 min for 6 h. Biochemical and histological analyses were performed.

RESULTS

After delivery, lambs developed severe RDS (inspiratory fraction of oxygen: 1; pH < 7.15; PaCO2 > 80 mm Hg; PaO2 < 30 mm Hg, mean dynamic compliance < 0.08 ml/cm H2O/kg). By 60 min after treatment, both groups showed an improvement in OI, PaCO2, mean dynamic compliance, and VT that was maintained until the end of the experiment. PaCO2 and CBF increased significantly in the SF-bolus group during the first 15-30 min, without concomitant changes in cardiovascular parameters, whereas in the SF-aero group, PaCO2 and CBF decreased gradually. SF-aero induced less alveolar hemorrhage and inflammation.

CONCLUSION

SF-aero produced improvements in gas exchange and lung mechanics similar to those produced by bolus administration but with less lung injury and fewer cerebral hemodynamic changes.

Applications of stable isotopes in clinical pharmacology

This review aims to present an overview of the application of stable isotope technology in clinical pharmacology. Three main categories of stable isotope technology can be distinguished in clinical pharmacology. Firstly, it is applied in the assessment of drug pharmacology to determine the pharmacokinetic profile or mode of action of a drug substance. Secondly, stable isotopes may be used for the assessment of drug products or drug delivery systems by determination of parameters such as the bioavailability or the release profile. Thirdly, patients may be assessed in relation to patient-specific drug treatment; this concept is often called personalized medicine. In this article, the application of stable isotope technology in the aforementioned three areas is reviewed, with emphasis on developments over the past 25 years. The applications are illustrated with examples from clinical studies in humans.

Effect of exogenous surfactant therapy on levels of pulmonary surfactant proteins A and D in preterm infants with respiratory distress syndrome

OBJECTIVES

To examine whether exogenous pulmonary surfactants (exPS) substitute for or merely supplement endogenous pulmonary surfactants (enPS) by looking at sequential changes in the surfactant proteins (SP) SP-A and SP-D in alveolar pools.

METHODS

Fourteen preterm infants with RDS treated with an artificial surfactant were compared to five normal-term infants without RDS who were treated with artificial ventilation at birth.

RESULTS

Immediately after birth, SP-A and SP-D were essentially absent in the alveolar pools of the RDS group, but were present at normal levels in the controls. Treatment with exPS apparently stimulated enPS production.

CONCLUSIONS

In infants who responded well to exPS therapy, the SP concentration reached essentially normal levels within 48-72 h after birth.

Dosing of porcine surfactant: effect on kinetics and gas exchange in respiratory distress syndrome

OBJECTIVE

The goal was to study exogenous surfactant disaturated phosphatidylcholine (DSPC) kinetics in preterm infants with respiratory distress syndrome (RDS) who were treated with 100 or 200 mg/kg porcine surfactant.

METHODS

Sixty-one preterm infants with RDS undergoing mechanical ventilation received, within 24 hours after birth, 100 mg/kg (N = 40) or 200 mg/kg (N = 21) porcine surfactant mixed with [U-(13)C]dipalmitoylphosphatidylcholine. Clinical and respiratory parameters were recorded, and DSPC half-life and pool size and endogenous DSPC synthesis rate were calculated.

RESULTS

Clinical characteristics and short-term outcomes did not differ between groups. In the 100 mg/kg group, 28 infants (70%) received a second dose after 25 +/- 11 hours and 9 (22.5%) a third dose after 41 +/- 11 hours; in the 200 mg/kg group, 6 infants (28.6%) received a second dose after 33 +/- 8 hours and 1 a third dose. The DSPC half-life was longer in the 200 mg/kg group (first dose: 32 +/- 19 vs 15 +/- 15 hours [P = .002]; second dose: 43 +/- 32 vs 21 +/- 13 hours [P = .025]). DSPC synthesis rates and pool sizes before the first and second doses did not differ between the groups. The 200 mg/kg group exhibited a greater reduction in the oxygenation index than did the 100 mg/kg group after the first (P = .009) and second (P = .018) doses.

CONCLUSIONS

Porcine surfactant given to preterm infants with RDS at a dose of 200 mg/kg resulted in a longer DSPC half-life, fewer retreatments, and better oxygenation index values.

Decreased surfactant phosphatidylcholine synthesis in neonates with congenital diaphragmatic hernia during extracorporeal membrane oxygenation

Purpose

Congenital diaphragmatic hernia (CDH) may result in severe respiratory insufficiency with a high morbidity. The role of a disturbed surfactant metabolism in the pathogenesis of CDH is unclear. We therefore studied endogenous surfactant metabolism in the most severe CDH patients who required extracorporeal membrane oxygenation (ECMO).

Methods

Eleven neonates with CDH who required ECMO and ten ventilated neonates without significant lung disease received a 24-h infusion of the stable isotope [U-¹³C] glucose. The ¹³C-incorporation into palmitic acid in surfactant phosphatidylcholine (PC) isolated from serial tracheal aspirates was measured. Mean PC concentration in epithelial lining fluid (ELF) was measured during the first 4 days of the study.

Results

Fractional surfactant PC synthesis was decreased in CDH-ECMO patients compared to controls (2.4 ± 0.33 vs. 8.0 ± 2.4%/day, p = 0.04). The control group had a higher maximal enrichment (0.18 ± 0.03 vs. 0.09 ± 0.02 APE, p = 0.04) and reached this maximal enrichment earlier (46.7 ± 3.0 vs. 69.4 ± 6.6 h, p = 0.004) compared to the CDH-ECMO group, which reflects higher and faster precursor incorporation in the control group. Surfactant PC concentration in ELF was similar in both groups.

Conclusion

These results show that CDH patients who require ECMO have a decreased surfactant PC synthesis, which may be part of the pathogenesis of severe pulmonary insufficiency and has a negative impact on weaning from ECMO.

Randomized clinical trial comparing two natural surfactant preparations to treat respiratory distress syndrome

OBJECTIVES

Natural surfactant preparations have been shown to reduce the severity and mortality of respiratory distress syndrome (RDS) in preterm infants. The objective of this study was to compare the efficacy of two natural surfactants, namely SF-RI 1 (Alveofact) and barectant (Survanta), on the incidence of chronic lung disease (CLD) and other associated complications of RDS in preterm infants.

METHODS

Preterm infants with RDS requiring artificial ventilation were randomly selected to receive an initial dose of either Alveofact or Survanta. The two treatment groups were tested for variation in gas exchange, ventilatory settings and neonatal complications such as CLD and mortality.

RESULTS

After 5 days the Survanta-treated infants had a lower fraction of inspired oxygen (FiO2) compared with the Alveofact-treated infants. There were no differences in the ventilatory settings. More infants in the Survanta group were extubated at 3 days and fewer required the use of postnatal steroids. Less CLD and duration of oxygenation were experienced by the Survanta-treated group.

CONCLUSIONS

Improved oxygenation and reduced ventilatory requirements were greater with Survanta compared to Alveofact, which in turn was associated with a trend towards a lower incidence of serious pulmonary complications.

Metabolic precursors of surfactant disaturated-phosphatidylcholine in preterms with respiratory distress

Our objective was to study the metabolic precursors of surfactant disaturated-phosphatidylcholine (DSPC) in preterm infants with respiratory distress syndrome (RDS) on mechanical ventilation. We performed 46 DSPC kinetic studies in 23 preterms on fat-free parenteral nutrition and mechanical ventilation (birth weight = 1167 +/- 451 g, gestational age = 28.5 +/- 2.0 weeks). Eight infants received a simultaneous intravenous infusion of U(13)C-glucose and [16,16,16](2)H-palmitate, eight infants received U(13)C-glucose and (2)H(2)O, and seven received U(13)C-palmitate and (2)H(2)O. Surfactant DSPC kinetics were calculated from the isotopic enrichments of DSPC-palmitate from sequential tracheal aspirates and its metabolic precursors in plasma or urine. DSPC fractional synthesis rate (FSR) was 17 +/- 11, 21 +/- 16, and 15 +/- 6%/day from glucose, palmitate, and body water, respectively (P = 0.36). DSPC-FSR from U(13)C-glucose and (2)H(2)O were significantly correlated and yielded similar estimates (difference of -0.1 +/- 3%) (P = 0.91). The difference in the 15 infants receiving palmitate versus (2)H(2)O or palmitate versus glucose was +6.0 +/- 12%/day (P = 0.21). There was a significant correlation between DSPC-FSRs from plasma glucose and plasma FFA. The contribution of glucose versus palmitate to DSPC-FSR was 49 +/- 20% versus 51 +/- 20%, respectively. Plasma glucose and FFA showed similar contributions to DSPC-FSR in infants with RDS and fat-free parenteral nutrition. FSRs from (2)H(2)O or glucose were highly correlated.

Pulmonary surfactant kinetics of the newborn infant: novel insights from studies with stable isotopes

Deficiency or dysfunction of the pulmonary surfactant plays a critical role in the pathogenesis of respiratory diseases of the newborn. After a short review of the pulmonary surfactant, including its role in selected neonatal respiratory conditions, we describe a series of studies conducted by applying two recently developed methods to measure surfactant kinetics. In the first set of studies, namely 'endogenous studies', which used stable isotope-labeled intravenous surfactant precursors, we have shown the feasibility of measuring surfactant synthesis and kinetics in infants using several metabolic precursors, including plasma glucose, plasma fatty acids and body water. In the second set of studies, namely 'exogenous studies', which used a stable isotope-labeled phosphatidylcholine (PC) tracer given endotracheally, we estimated the surfactant disaturated phosphatidylcholine (DSPC) pool size and half-life. The major findings of our studies are presented here and can be summarized as follows: (a) the de novo synthesis and turnover rates of the surfactant (DSPC) in preterm infants with respiratory distress syndrome (RDS) are very low with either precursor; (b) in preterm infants with RDS, pool size is very small and half-life much longer than what has been reported in animal studies; (c) patients recovering from RDS who required higher continuous positive airway pressure pressure after extubation or reintubation have a lower level of intrapulmonary surfactant than those who did well after extubation; (d) term newborn infants with pneumonia have greatly accelerated surfactant catabolism; and (e) infants with uncomplicated congenital diaphragmatic hernia (CDH) and on conventional mechanical ventilation have normal surfactant synthesis, but those requiring extracorporeal membrane oxygenated (ECMO) do not. Information obtained from these studies in infants will help to better tailor exogenous surfactant treatment in neonatal lung diseases.

Dexamethasone therapy in preterm infants developing bronchopulmonary dysplasia: effect on pulmonary surfactant disaturated-phosphatidylcholine kinetics

The role of corticosteroid in severe bronchopulmonary dyplasia (BPD) is still debated. Scanty data are available on the corticosteroids effect on surfactant metabolism. Our objective was to compare surfactant kinetics in preterm infants with developing BPD, before and after dexamethasone (DEXA) treatment. Twenty-eight studies were performed in 14 preterm infants (birth weight 786 +/- 192 g, gestational age 26 +/- 1 wk) on high ventilatory setting, before (age 22 +/- 11 d) and after (age 33 +/- 11 d) DEXA. C-labeled dipalmitoyl-phosphatidylcholine (DPPC) was administered endotrachelly to trace pulmonary surfactant. Surfactant disaturated-phosphatidylcholine (DSPC) kinetics and pools were calculated from DSPC C-enrichment curves of serial tracheal aspirates and bi-compartmental analysis. Total protein and myeloperoxidase (MPO) activity in tracheal aspirates were also measured and expressed per ml of Epithelial Lining Fluid (ELF). After DEXA, DSPC alveolar pool increased significantly from 8.2 +/- 7.6 to 10.6 +/- 11.3 mg/kg (p = 0.039), total proteins and MPO were reduced from 8.8 +/- 8.6 to 3.1 +/- 2.1 mg/ml ELF (p = 0.046) and from 1822 +/- 1224 to 1261 +/- 987 mU/mlELF (p = 0.028) respectively. In conclusion, DEXA treatment in mechanically ventilated preterm infants with severe respiratory failure and at high risk of developing BPD, significantly reduced inflammatory markers and increased alveolar surfactant DSPC pool.

What's new in surfactant? A clinical view on recent developments in neonatology and paediatrics

Surfactant therapy has significantly changed clinical practice in neonatology over the last 25 years. Recent trials in infants with respiratory distress syndrome (RDS) have not shown superiority of any natural surfactant over another. Advancements in the development of synthetic surfactants are promising, yet to date none has been shown to be superior to natural preparations. Ideally, surfactant would be administered without requiring mechanical ventilation. An increasing number of studies investigate the roles of alternative modes of administration and the use of nasal continuous positive airway pressure to minimise the need for mechanical ventilation. Whether children with other lung diseases benefit from surfactant therapy is less clear. Evidence suggests that infants with meconium aspiration syndrome and children with acute lung injury/acute respiratory distress syndrome may benefit, while no positive effect of surfactant is seen in infants with congenital diaphragmatic hernia. However, more research is needed to establish potential beneficial effects of surfactant administration in children with lung diseases other than RDS. Furthermore, genetic disorders of surfactant metabolism have recently been linked to respiratory diseases of formerly unknown origin. It is important to consider these disorders in the differential diagnosis of unexplained respiratory distress although no established treatment is yet available besides lung transplantation for the most severe cases.

CONCLUSION

Research around surfactant is evolving and recent developments include further evolution of synthetic surfactants, evaluation of surfactant as a therapeutic option in lung diseases other than RDS and the discovery of genetic disorders of surfactant metabolism. Ongoing research is essential to continue to improve therapeutic prospects for children with serious respiratory disease involving disturbances in surfactant.

Surfactant phosphatidylcholine metabolism in neonates with meconium aspiration syndrome

OBJECTIVE

Because meconium directly inhibits surfactant function, we sought to determine the effect of meconium on endogenous surfactant synthesis and clearance.

STUDY DESIGN

We studied surfactant phosphatidylcholine kinetics with the use of stable isotopes in 11 newborn infants with meconium aspiration syndrome (MAS) who required extracorporeal membrane oxygenation (ECMO). For comparison we studied 6 neonates with persistent pulmonary hypertension (PPHN) on ECMO and 10 term neonates ventilated for non-pulmonary indications and not on ECMO. All patients received a 24-hour [U- 13C]glucose infusion as precursor for the palmitic acid in surfactant phosphatidylcholine.

RESULTS

In the meconium group, the maximal 13C-incorporation in phosphatidylcholine (PC) was half of that in controls (0.09 +/- 0.01 vs 0.18 +/- 0.03 atom percent excess [APE], P = .027). There was a trend toward lower surfactant synthesis in the MAS group (3.3 +/- 0.7%/day) and PPHN group (2.6 +/- 0.3%/day) compared with controls 8.0 +/- 2.4%/day, P = .058). Significantly lower PC concentrations in tracheal aspirates were found in the MAS group (4.4 +/- 2.6 mg/mL) and PPHN group (3.6 +/- 2.0 mg/mL) compared with controls (12.8 +/- 2.6 mg/mL, P = .01). Endogenously synthesized surfactant had a similar half-life in all groups, ranging from 63 to 98 hours.

CONCLUSION

We conclude that surfactant synthesis is disturbed and that surfactant PC concentrations are low in infants with MAS on ECMO.

Surfactant metabolism in the neonate

With the use of stable isotope-labeled intravenous precursors for surfactant phosphatidylcholine (PC) synthesis, it has been shown that the de novo synthesis rates in preterm infants with respiratory distress syndrome (RDS) are very low as are turnover rates. This is consistent with animal data. Surfactant therapy does not inhibit endogenous surfactant synthesis, and prenatal corticosteroids stimulate it. With the use of stable isotope-labeled PC given endotracheally, surfactant pool size was estimated. It turned out to be low in RDS, as expected. Similar studies were performed in term neonates with severe lung diseases. In general, patients with lung injury show a lower surfactant synthesis. The controversy around surfactant in congenital diaphragmatic hernia (CDH) persists: studies on CDH with and without extracorporeal membrane oxygenation yielded different results. In severe meconium aspiration syndrome surfactant synthesis was found to be decreased but surfactant pool size was maintained. It is possible and safe to study surfactant metabolism in human neonates with the use of stable isotopes. This can help in answering clinical questions and has the potential to bring new in vitro and animal findings about surfactant metabolism to the patient.

Metabolic kinetics of pulmonary surfactant in newborn infants using endogenous stable isotope techniques

We compared kinetic indices of pulmonary surfactant metabolism in premature infants (n = 41) with respect to i) tracer ([1-(13)C1]acetate, [U-(13)C6]glucose, and [1,2,3,4-(13)C4] palmitate), ii) phospholipid (PL) pool (total PLs or disaturated PLs), or iii) instrumentation [gas chromatography/mass spectrometry (GC/MS) or GC-combustion-isotope ratio mass spectometry (GC-C-IRMS)]. Tracer incorporation was measured in PLs extracted from serial tracheal aspirates after a 24 h tracer infusion. The fractional catabolic rate (FCR), representing the total fractional turnover from all sources of surfactant production, was independent of tracer. The fractional synthesis rate of surfactant PL from plasma palmitate was significantly higher than that from palmitate synthesized de novo from acetate, and these two sources of palmitate together accounted for only half of the total surfactant production in preterm infants. [U-(13)C6]glucose showed significant recycling of the (13)C label in intermediary metabolism, distinguishable by GC-MS but not by GC-C-IRMS, resulting in a slower apparent FCR when GC-C-IRMS was used. The extracted PL pool did not affect the surfactant metabolic indices. We suggest that FCR should be used as a primary measure of surfactant turnover kinetics and that tracers labeling both de novo synthesis (acetate and glucose) and preformed pathways (plasma palmitate) can be used to partition the fractional contribution of each pathway to total production.

Septicemia in the first week of life in a Norwegian national cohort of extremely premature infants

OBJECTIVES

To investigate the incidence, causes, predictors, and outcomes of septicemia in the first week of life in a national cohort of extremely premature infants.

METHODS

A prospective study of survival of all infants with gestational age of <28 weeks or birth weight of <1000 g who were born in Norway in 1999-2000 was performed. Data on the maternal prenatal history, delivery, and neonatal course, including detailed information on episodes of microbiologically verified septicemia, were collected on predefined forms. Septicemia was reported in 2 groups, ie, episodes diagnosed on the day of delivery (ie, very early-onset septicemia [VEOS]) and episodes diagnosed from day 2 to day 7 of life (ie, early-onset septicemia [EOS]). Logistic regression models were used for the selection of variables for predictor analysis in each group.

RESULTS

Of 462 included infants, VEOS occurred for 15 (32.5 per 1000 population) and EOS for 15 (35.5 per 1000 population). The most prevalent bacteria were Escherichia coli in VEOS (n = 9) and staphylococci (coagulase-negative staphylococci and Staphylococcus aureus) (n = 15) in EOS. Case fatality rates were 40% and 13%, respectively. Independent predictive factors for VEOS were clinical chorioamnionitis (odds ratio [OR]: 10.5; 95% confidence interval [CI]: 3.3-33.4) and high maternal age (OR: 1.2; 95% CI: 1.0-1.3), whereas not receiving systemic antibiotic therapy within 2 days of age (OR: 13.6; 95% CI: 3.7-50.2) and receiving nasal continuous positive airway pressure (n-CPAP) support at 24 hours of age (OR: 9.8; 95% CI: 2.5-38.4) independently predicted septicemia after the first day of life.

CONCLUSIONS

Whereas vertically transmitted septicemia was dominated by Gram-negative bacteria, with predictors being exclusively of maternal origin, EOS was dominated by typically nosocomial flora, with n-CPAP treatment at 24 hours of age being a powerful predictor. Early n-CPAP treatment, as opposed to mechanical ventilation, as a powerful predictor of septicemia in the early neonatal period, even with adjustment for early systemic antibiotic treatment, is a new observation among extremely premature infants that warrants additional study.

Endogenous surfactant metabolism in newborn infants with and without respiratory failure

Studies using stable isotopically labeled glucose and palmitate as precursors of pulmonary surfactant synthesis have demonstrated slow surfactant turnover in premature infants with respiratory distress syndrome (RDS). However, only limited data about surfactant turnover are available for term infants. Because acetate is a direct precursor of de novo synthesized surfactant fatty acid, we measured [1-13C1]acetate incorporation into surfactant of term infants without respiratory dysfunction (control group), preterm infants with RDS, and term infants with primary respiratory failure to determine whether stable isotopically labeled acetate would yield similar results to previous studies of preterm infants with RDS and, furthermore, would distinguish normal from abnormal surfactant turnover. Despite similar amounts of phospholipids and acetate precursor enrichment, the control group had higher fractional synthetic rate and shorter half-life of clearance than preterm infants with RDS, (fractional synthetic rate, 15.4 +/- 2.4 versus 2.2 +/- 0.4%/d, p < 0.001; half-life of clearance, 27 +/- 3 versus 105 +/- 11 h, p < 0.001). Term infants with severe respiratory failure had a lower fractional synthetic rate than those with mild disease (2.9 +/- 0.6 versus 13.8 +/- 3.5%/d, p = 0.014) and a reduced amount of phospholipids recovered from tracheal aspirates (54 +/- 17 versus 300 +/- 28 nmol, severe versus mild disease, respectively, p < 0.001). The amount of phospholipids in tracheal aspirates correlated inversely with disease severity, (r = -0.75, p = 0.01). We conclude that normal surfactant turnover in term infants is faster than in preterm infants with RDS. Surfactant turnover in term infants with severe respiratory failure is similar to that of preterm infants with RDS, suggesting either delayed maturity of the surfactant system or disruption from the underlying disease process.

Influence of dietary long-chain PUFA on premature baboon lung FA and dipalmitoyl PC composition

One of the major survival challenges of premature birth is production of lung surfactant. The lipid component of surfactant, dipalmitoyl PC (DPPC), increases in concentration in the period before normal term birth via a net shift in FA composition away from unsaturates. We investigated the influence of dietary DHA and arachidonic acid (AA) on lung FA composition and DPPC concentration in term and preterm baboons. Pregnant animals/neonates were randomized to one of four groups: breast-fed (B), term formula-fed (T-, preterm formula-fed (P-, and preterm fed formula supplemented with DHA-AA (P+). Breast milk contained 0.68%wt DHA and the P+ group formula contained 0.61%wt DHA. In the preterm groups (P- and P+), pregnant females received a course of antenatal corticosteroids. At the adjusted age of 4 wk, neonate lung tissue was harvested, and FA composition and DPPC were analyzed. Palmitate was approximately 28%wt of lung total FA and no significant differences were found among the four treatment groups. In contrast, DPPC in the B group lung tissue was significantly greater than DPPC in the unsupplemented groups, but not compared with the P+ group. The B and P+ groups were not significantly different in DHA and AA, but were different compared with the unsupplemented (T, P-) groups. These results indicate that LCP supplementation increases lung DHA and AA, without compromising overall lung 16:0 or DPPC. The shift in FA composition toward greater unsaturation in the groups consuming LCP supported improved surfactant lipid concentration in preterm neonate lungs.

Surfactant phosphatidylcholine half-life and pool size measurements in premature baboons developing bronchopulmonary dysplasia

Because minimal information is available about surfactant metabolism in bronchopulmonary dysplasia, we measured half-lives and pool sizes of surfactant phosphatidylcholine in very preterm baboons recovering from respiratory distress syndrome and developing bronchopulmonary dysplasia, using stable isotopes, radioactive isotopes, and direct pool size measurements. Eight ventilated premature baboons received (2)H-DPPC (dipalmitoyl phosphatidylcholine) on d 5 of life, and radioactive (14)C-DPPC with a treatment dose of surfactant on d 8. After 14 d, lung pool sizes of saturated phosphatidylcholine were measured. Half-life of (2)H-DPPC (d 5) in tracheal aspirates was 28 +/- 4 h (mean +/- SEM). Half-life of radioactive DPPC (d 8) was 35 +/- 4 h. Saturated phosphatidylcholine pool size measured with stable isotopes on d 5 was 129 +/- 14 micro mol/kg, and 123 +/- 11 micro mol/kg on d 14 at autopsy. Half-lives were comparable to those obtained at d 0 and d 6 in our previous baboon studies. We conclude that surfactant metabolism does not change during the early development of bronchopulmonary dysplasia, more specifically, the metabolism of exogenous surfactant on d 8 is similar to that on the day of birth. Surfactant pool size is low at birth, increases after surfactant therapy, and is kept constant during the first 2 wk of life by endogenous surfactant synthesis. Measurements with stable isotopes are comparable to measurements with radioactive tracers and measurements at autopsy.

Endogenous pulmonary surfactant metabolism is not affected by mode of ventilation in premature infants with respiratory distress syndrome

OBJECTIVE

To determine if high frequency oscillatory ventilation (HFOV) decreases surfactant production in premature infants with respiratory distress syndrome (RDS).

STUDY DESIGN

We randomized 19 infants <28 weeks of gestation to either HFOV (n = 8) or conventional ventilation (CV, n = 11) at 24 hours of life. After a 24-hour continuous infusion of uniformly labeled carbon 13 glucose (U-(13)C(6)) glucose, we measured (13)C enrichment in surfactant phosphatidylcholine (PC) in tracheal aspirate samples using gas chromatography/mass spectrometry. We calculated the fractional synthetic rate (FSR) of surfactant PC from labeled glucose and its half-life of clearance (T(1/2)).

RESULTS

FSR did not differ between groups (4.7% +/- 2.7%/day CV vs 4.2% +/- 3.1%/day HFOV, P =.7). T(1/2) was 79 +/- 18 hours in the CV group and 76 +/- 23 hours in the HFOV group (P =.7). Neither degree of ventilatory support nor supplemental oxygen exposure correlated with surfactant metabolic indices. Three of 4 infants who died from RDS within the first month of life had a shorter T(1/2) than 14 of 15 infants who survived.

CONCLUSION

Surfactant metabolism is similar in preterm infants ventilated with HFOV and CV. Shortened surfactant half-life may characterize a subset of preterm infants with lethal RDS.

Biology of surfactant

Surfactant is a metabolically active assembly of phospholipids and surfactant-specific proteins that is essential for normal lung mechanics. The surfactant proteins SP-A and SP-D also have important innate host defense functions. Surfactant metabolism in the developing lung differs from the adult lung by having slower kinetics of secretion with a longer half-life and more efficient recycling. Ventilation styles that injure the lung also result in altered surfactant function.