Endogenous surfactant turnover in preterm infants with respiratory distress syndrome studied with stable isotope lipids

Surfactant status assessment and personalized therapy for surfactant deficiency or dysfunction

Surfactant is a pivotal neonatal drug used both for respiratory distress syndrome due to surfactant deficiency and for more complex surfactant dysfunctions (such as in case of neonatal acute respiratory distress syndrome). Despite its importance, indications for surfactant therapy are often based on oversimplified criteria. Lung biology and modern monitoring provide several diagnostic tools to assess the patient surfactant status and they can be used for a personalized surfactant therapy. This is desirable to improve the efficacy of surfactant treatment and reduce associated costs and side effects. In this review we will discuss these diagnostic tools from a pathophysiological and multi-disciplinary perspective, focusing on the quantitative or qualitative surfactant assays, lung mechanics or aeration measurements, and gas exchange metrics. Their biological and technical characteristics are described with practical information for clinicians. Finally, available evidence-based data are reviewed, and the diagnostic accuracy of the different tools is compared. Lung ultrasound seems the most suitable tool for assessing the surfactant status, while some other promising tests require further research and/or development.

Pulmonary Surfactant in Adult ARDS: Current Perspectives and Future Directions

Acute respiratory distress syndrome (ARDS) is a major cause of hypoxemic respiratory failure in adults, leading to the requirement for mechanical ventilation and poorer outcomes. Dysregulated surfactant metabolism and function are characteristic of ARDS. A combination of alveolar epithelial damage leading to altered surfactant synthesis, secretion, and breakdown with increased functional inhibition from overt alveolar inflammation contributes to the clinical features of poor alveolar compliance and alveolar collapse. Quantitative and qualitative alterations in the bronchoalveolar lavage and tracheal aspirate surfactant composition contribute to ARDS pathogenesis. Compared to neonatal respiratory distress syndrome (nRDS), replacement studies of exogenous surfactants in adult ARDS suggest no survival benefit. However, these studies are limited by disease heterogeneity, variations in surfactant preparations, doses, and delivery methods. More importantly, the lack of mechanistic understanding of the exact reasons for dysregulated surfactant remains a significant issue. Moreover, studies suggest an extremely short half-life of replaced surfactant, implying increased catabolism. Refining surfactant preparations and delivery methods with additional co-interventions to counteract surfactant inhibition and degradation has the potential to enhance the biophysical characteristics of surfactant in vivo.

Evolution of Ultrasound-Assessed Lung Aeration and Gas Exchange in Respiratory Distress Syndrome and Transient Tachypnea of the Neonate

OBJECTIVE

To use clinical, lung ultrasound, and gas exchange data to clarify the evolution of lung aeration and function in neonates with respiratory distress syndrome (RDS) and transient tachypnea of the neonate (TTN), the most common types of neonatal respiratory failure.

STUDY DESIGN

In this prospective observational cohort study, lung aeration and function were measured with a semiquantitative lung ultrasound score (LUS) and transcutaneous blood gas measurement performed at 1 hour (time point 0), 6 hours (time point 1), 12 hours (time point 2), 24 hours (time point 3) and 72 hours (time point 4) of life. Endogenous surfactant was estimated using lamellar body count (LBC). LUS, oxygenation index (OI), oxygen saturation index (OSI), and transcutaneous pressure of carbon dioxide (PtcCO₂) were the primary outcomes. All results were adjusted for gestational age.

RESULTS

Sixty-nine neonates were enrolled in the RDS cohort, and 58 neonates were enrolled in the TTN cohort. LUS improved over time (within-subjects, P < .001) but was worse for the RDS cohort than for the TTN cohort at all time points (between-subjects, P < .001). Oxygenation improved over time (within-subjects, P = .011 for OI, P < .001 for OSI) but was worse for the RDS cohort than for the TTN cohort at all time points (between-subjects, P < .001 for OI and OSI). PtcCO₂ improved over time (within-subjects, P < .001) and was similar in the RDS and TTN cohorts at all time points. Results were unchanged after adjustment for gestational age. LBC was associated with RDS (β = -0.2 [95% CI, -0.004 to -0.0001]; P = .037) and LUS (β = -3 [95% CI, -5.5 to -0.5]; P = .019).

CONCLUSIONS

For the first 72 hours of life, the RDS cohort had worse lung aeration and oxygenation compared with the TTN cohort at all time points. CO₂ clearance did not differ between the cohorts, whereas both lung aeration and function improved in the first 72 hours of life.

Techniques to evaluate surfactant activity for a personalized therapy of RDS neonates

According to both European and American Guidelines, preterm neonates have to be treated by nasal continuous air pressure (CPAP) early in the delivery room. The administration of surfactant should be reserved only for babies with respiratory distress syndrome (RDS) with increased oxygen requirement, according to different thresholds of FiO₂. However, these oxygenation thresholds do not fully take into consideration the lung physiopathology and mechanics or the lung surfactant biology of RDS neonates. Since surfactant replacement therapy (SRT) seems to be more effective if it is initiated within the first 3 hours after birth, the use of a reliable bench-to-bedside biological test able to predict as soon as possible the necessity of SRT will help optimise individualised therapies and personalise the actual collective strategy used to treat RDS neonates. With this in mind, in the present review several quantitative and qualitative biological tests to assess the surfactant status in RDS neonates are introduced as potential candidates for the early prediction of SRT requirement, summarising the state-of-the-art in the evaluation of surfactant activity.

Pulmonary surfactant: a unique biomaterial with life-saving therapeutic applications

Pulmonary surfactant is a complex lipoprotein mixture secreted into the alveolar lumen by type 2 pneumocytes, which is composed by tens of different lipids (approximately 90% of its entire mass) and surfactant proteins (approximately 10% of the mass). It is crucially involved in maintaining lung homeostasis by reducing the values of alveolar liquid surface tension close to zero at end-expiration, thereby avoiding the alveolar collapse, and assembling a chemical and physical barrier against inhaled pathogens. A deficient amount of surfactant or its functional inactivation is directly linked to a wide range of lung pathologies, including the neonatal respiratory distress syndrome. This paper reviews the main biophysical concepts of surfactant activity and its inactivation mechanisms, and describes the past, present and future roles of surfactant replacement therapy, focusing on the exogenous surfactant preparations marketed worldwide and new formulations under development. The closing section describes the pulmonary surfactant in the context of drug delivery. Thanks to its peculiar composition, biocompatibility, and alveolar spreading capability, the surfactant may work not only as a shuttle to the branched anatomy of the lung for other drugs but also as a modulator for their release, opening to innovative therapeutic avenues for the treatment of several respiratory diseases.

Lung Ultrasound to Monitor Extremely Preterm Infants and Predict Bronchopulmonary Dysplasia. A Multicenter Longitudinal Cohort Study

Rationale: Lung ultrasound is useful in critically ill patients with acute respiratory failure. Given its characteristics, it could also be useful in extremely preterm infants with evolving chronic respiratory failure, as we lack accurate imaging tools to monitor them. Objectives: To verify if lung ultrasound can monitor lung aeration and function and has good reliability to predict bronchopulmonary dysplasia in extremely preterm neonates. Methods: A multicenter, international, prospective, longitudinal, cohort, diagnostic accuracy study consecutively enrolling inborn neonates with gestational age 30⁺⁶ weeks or younger. Lung ultrasound was performed on the 1, 7, 14, and 28 days of life, and lung ultrasound scores were calculated and correlated with simultaneous blood gases and work of breathing score. Gestational age-adjusted lung ultrasound scores were created, verified in multivariate models, and subjected to receiver operator characteristics (ROC) analyses to predict bronchopulmonary dysplasia at 36 weeks postmenstrual age. Measurements and Main Results: Mean lung ultrasound scores are different between infants developing (n = 72) or not developing (n = 75) bronchopulmonary dysplasia (P < 0.001 at any time point). Lung ultrasound scores significantly correlate with oxygenation metrics and work of breathing at any time point (P always < 0.0001). Gestational age-adjusted lung ultrasound scores significantly predict bronchopulmonary dysplasia at 7 (area under ROC curve, 0.826-0.833; P < 0.0001) and 14 (area under ROC curve, 0.834-0.858; P < 0.0001) days of life. Bronchopulmonary dysplasia severity and gestational age-adjusted lung ultrasound scores are significantly correlated at 7 and 14 days (P always < 0.0001). Conclusions: Lung ultrasound scores allow monitoring of lung aeration and function in extremely preterm infants. Gestational age-adjusted scores significantly predict the occurrence of bronchopulmonary dysplasia, starting from the seventh day of life.

Respiratory distress syndrome in preterm neonates in the era of precision medicine: A modern critical care-based approach

Respiratory distress syndrome (RDS) was recognized to be caused by primary surfactant deficiency almost 70 years ago and continuous positive airway pressure was introduced approximately 50 years ago. Since then, there have been many developments in neonatology; we know many things but others are still controversial. The more we know, the more questions arise. However, this review aims to indicate what is more needed to understand and how should be the modern approach to RDS in the era of precision medicine. The review is divided between new concepts and new tools. We will explain the interaction between steroids, CPAP and surfactant, as well as the surfactant catabolism and the diagnosis of NARDS; lung ultrasound and new tools to optimize CPAP will also be covered. How these concepts are integrated in the author's personal experience is also illustrated.

Personalized Medicine for the Management of RDS in Preterm Neonates

Continuous positive airway pressure and surfactant represent the first- and second-line treatment for respiratory distress syndrome in preterm neonates, as European and American guidelines, since 2013 and 2014, respectively, started to recommend surfactant replacement only when continuous positive airway pressure fails. These recommendations, however, are not personalized to the individual physiopathology. Simple clinical algorithms may have improved the diffusion of neonatal care, but complex medical issues can hardly be addressed with simple solutions. The treatment of respiratory distress syndrome is a complex matter and can be only optimized with personalization. We performed a review of tools to individualize the management of respiratory distress syndrome based on physiopathology and actual patients' need, according to precision medicine principles. Advanced oxygenation metrics, lung ultrasound, electrical impedance tomography, and both quantitative and qualitative surfactant assays were examined. When these techniques were investigated with diagnostic accuracy studies, reliability measures have been meta-analysed. Amongst all these tools, quantitative lung ultrasound seems the more developed for the widespread use and has a higher diagnostic accuracy (meta-analytical AUC = 0.952 [95% CI: 0.951-0.953]). Surfactant adsorption (AUC = 0.840 [95% CI: 0.824-0.856]) and stable microbubble test (AUC = 0.800 [95% CI: 0.788-0.812]) also have good reliability, but need further industrial development. We advocate for a more accurate characterization and a personalized approach of respiratory distress syndrome. With the above-described currently available tools, it should be possible to personalize the treatment of respiratory distress syndrome according to physiopathol-ogy.

In Vitro Functional and Structural Characterization of A Synthetic Clinical Pulmonary Surfactant with Enhanced Resistance to Inhibition

CHF5633 is a novel synthetic clinical pulmonary surfactant preparation composed by two phospholipid species, dipalmitoyl phosphatidylcholine (DPPC) and palmitoyloleoyl phosphatidylglycerol (POPG), and synthetic analogues of the hydrophobic surfactant proteins SP-B and SP-C. In this study, the interfacial properties of CHF5633 in the absence and in the presence of inhibitory serum proteins have been assessed in comparison with a native surfactant purified from porcine lungs and with poractant alpha, a widely used clinical surfactant preparation. The study of the spreading properties of CHF5633 in a Wilhelmy balance, its ability to adsorb and accumulate at air-liquid interfaces as revealed by a multiwell fluorescence assay, and its dynamic behavior under breathing-like compression-expansion cycling in a Captive Bubble Surfactometer (CBS), all revealed that CHF5633 exhibits a good behavior to reduce and sustain surface tensions to values below 5 mN/m. CHF5633 shows somehow slower initial interfacial adsorption than native surfactant or poractant alpha, but a better resistance to inhibition by serum proteins than the animal-derived clinical surfactant, comparable to that of the full native surfactant complex. Interfacial CHF5633 films formed in a Langmuir-Blodgett balance coupled with epifluorescence microscopy revealed similar propensity to segregate condensed lipid domains under compression than films made by native porcine surfactant or poractant alpha. This ability of CHF5633 to segregate condensed lipid phases can be related with a marked thermotropic transition from ordered to disordered membrane phases as exhibited by differential scanning calorimetry (DSC) of CHF5633 suspensions, occurring at similar temperatures but with higher associated enthalpy than that shown by poractant alpha. The good interfacial behavior of CHF5633 tested under physiologically meaningful conditions in vitro and its higher resistance to inactivation by serum proteins, together with its standardized and well-defined composition, makes it a particularly useful therapeutic preparation to be applied in situations associated with lung inflammation and edema, alone or in combined strategies to exploit surfactant-facilitated drug delivery.

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.

Estimation of early life endogenous surfactant pool and CPAP failure in preterm neonates with RDS

Background

It is not known if the endogenous surfactant pool available early in life is associated with the RDS clinical course in preterm neonates treated with CPAP. We aim to clarify the clinical factors affecting surfactant pool in preterm neonates and study its association with CPAP failure.

Methods

Prospective, pragmatic, blind, cohort study. Gastric aspirates were obtained (within the first 6 h of life and before the first feeding) from 125 preterm neonates with RDS. Surfactant pool was measured by postnatal automated lamellar body count based on impedancemetry, without any pre-analytical treatment. A formal respiratory care protocol based on European guidelines was applied. Clinical data and perinatal risk factors influencing RDS severity or lamellar body count were real-time recorded. Investigators performing lamellar body count were blind to the clinical data and LBC was not used in clinical practice.

Results

Multivariate analysis showed gestational age to be the only factor significantly associated with lamellar body count (standardized β:0.233;p = 0.023). Lamellar body count was significantly higher in neonates with CPAP success (43.500 [23.750–93.750]bodies/μL), than in those failing CPAP (20.500 [12.250–49.750] bodies/μL;p = 0.0003).LBC had a moderate reliability to detect CPAP failure (AUC: 0.703 (0.615–0.781);p < 0.0001; best cut-off: ≤30,000 bodies/μL). Upon adjustment for possible confounders, neither lamellar body count, nor its interaction factor with gestational age resulted associated with CPAP failure.

Conclusions

Early postnatal lamellar body count on gastric aspirates in CPAP-treated preterm neonates with RDS is significantly influenced only by gestational age. Lamellar bodies are not associated with CPAP failure. Thus, the endogenous surfactant pool available early in life only has a moderate reliability to predict CPAP failure.

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.

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.

Phospholipid composition and kinetics in different endobronchial fractions from healthy volunteers

Background

Alterations in surfactant phospholipid compositions are a recognized feature of many acute and chronic lung diseases. Investigation of underlying mechanisms requires assessment of surfactant phospholipid molecular composition and kinetics of synthesis and turnover. Such studies have recently become possible in humans due to the development of stable isotope labelling combined with advances in analytical methods in lipidomics. The objectives of this study are to compare phospholipid molecular species composition and phosphatidylcholine synthesis and turnover in surfactant isolated from various endobronchial compartments in healthy adults.

Methods

Healthy adults (N = 10) were infused with methyl-D₉-choline chloride and samples of induced sputum, tracheal wash and small volume bronchoalveolar lavage fluid were obtained subsequently at intervals up to 96 hours. Surfactant phospholipid composition and incorporation of stable isotope into surfactant phosphatidylcholine were determined by electrospray ionisation mass spectrometry.

Results

While molecular species compositions of phospholipids were similar for all three sample types, dipalmitoylphosphatidylcholine content was highest in lavage, intermediate in tracheal wash and lowest in sputum. Methyl-D₉-choline incorporation into surfactant phosphatidylcholine was lower for sputum at 24 hours but reached equilibrium with other sample types by 48 hours. Fractional methyl-D₉-dipalmitoylphosphatidylcholine incorporation for all sample types was about 0.5% of the endogenous composition. Lysophosphatidylcholine enrichment was twice than that of phosphatidylcholine.

Conclusions

Tracheal secretions may be of value as a surrogate to assess bronchoalveolar lavage fluid surfactant molecular composition and metabolism in healthy people. Despite minor differences, the phospholipid molecular composition of induced sputum also showed similarities to that of bronchoalveolar lavage fluid. Detailed analysis of newly synthesized individual phosphatidylcholine species provided novel insights into mechanisms of surfactant synthesis and acyl remodelling. Lysophosphatidylcholine methyl-D₉ incorporation patterns suggest that these species are secreted together with other surfactant phospholipids and are not generated in the air spaces by hydrolysis of secreted surfactant phosphatidylcholine. Application into patient populations may elucidate potential underlying pathophysiological mechanisms that lead to surfactant alterations in disease states.

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.

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.

Surfactant status in preterm neonates recovering from respiratory distress syndrome

OBJECTIVE

The goal was to establish whether reduced amounts of pulmonary surfactant contribute to postextubation respiratory failure in preterm infants recovering from respiratory distress syndrome.

METHODS

We prospectively recruited preterm infants who needed mechanical ventilation and exogenous surfactant for treatment of moderate/severe respiratory distress syndrome and could not be extubated before day 3 of life. (13)C-labeled dipalmitoyl-phosphatidylcholine was administered endotracheally as tracer before extubation, for estimation of surfactant disaturated phosphatidylcholine pool size and half-life. Patients were retrospectively divided into 3 groups, that is, extubation failure if, after extubation, they needed reintubation or continuous positive airway pressure treatment of >or=6 cmH(2)O and fraction of inspired oxygen of >0.4, extubation success if they did not meet the failure criteria, and not extubated if they needed ongoing ventilation. Clinical and respiratory parameters were recorded hourly.

RESULTS

Reliable kinetic data could be obtained for 63 of the 88 enrolled neonates. Sixteen, 23, and 24 neonates were categorized in the extubation failure, extubation success, and not extubated groups, respectively. Clinical and demographic characteristics did not differ between the extubation failure and extubation success groups. Disaturated phosphatidylcholine pool size was smaller in the extubation failure group than in the extubation success group (25 +/- 12 vs 43 +/- 24 mg/kg) and was 37 +/- 32 mg/kg in the not extubated group. Disaturated phosphatidylcholine half-life was 19 +/- 7, 24 +/- 12, and 28 +/- 18 hours in the extubation failure, extubation success, and not extubated groups, respectively.

CONCLUSIONS

In a selected population of preterm infants with moderate/severe respiratory distress syndrome who could not be extubated in the first 3 days of life, infants who were reintubated or needed high continuous positive airway pressure settings after extubation had a smaller disaturated phosphatidylcholine pool size than did those who were successfully extubated or needed low continuous positive airway pressure settings.

Probing phospholipid dynamics by electrospray ionisation mass spectrometry

Recent advances in electrospray ionisation mass spectrometry (ESI-MS) have greatly facilitated the analysis of phospholipid molecular species in a growing diversity of biological and clinical settings. The combination of ESI-MS and metabolic labelling employing substrates labelled with stable isotopes is especially exciting, permitting studies of phospholipid synthesis and turnover in vivo. This review will first describe the methodology involved and will then detail dynamic lipidomic studies that have applied the stable isotope incorporation approach. Finally, it will summarise the increasing number of studies that have used ESI-MS to characterise structural and signalling phospholipid molecular species in development and disease.

Measurement of pulmonary surfactant disaturated-phosphatidylcholine synthesis in human infants using deuterium incorporation from body water

The aim of the study was to determine surfactant palmitate disaturated-phosphatidylcholine (DSPC-PA) synthesis in vivo in humans by the incorporation of deuterium from total body water into DSPC-PA under steady state condition. We studied three newborns and one infant (body weight (BW) 4.6 +/- 2.9 kg, gestational age 37.5 +/- 2 weeks, age 9 +/- 9 days) and four preterm newborns (BW 1.3 +/- 0.6 kg, gestational age 30.3 +/- 2.5 weeks, postnatal age 8.8 +/- 9.2 h). All infants were mechanically ventilated during the study and the four preterm infants received exogenous surfactant at the start of the study. We administered 0.44 g (2)H(2)O/kg BW as a bolus intravenously, followed by 0.0125 g (2)H(2)O/kg BW every 6 h to maintain deuterium enrichment at plateau over 2 days. Urine samples and tracheal aspirates (TA) were obtained prior to dosing and every 6 h thereafter. Isotopic enrichment curves of DSPC-PA from sequential TA and urine deuterium enrichments were analyzed by Gas Chromatography-Isotope Ratio-Mass Spectrometry (GC-IRMS) and normalized for Vienna Standard Mean Ocean Water. Enrichment data were used to measure DSPC-PA fractional synthesis rate (FSR) from the linear portion of the DSPC-PA enrichment rise over time, relative to plateau enrichment of urine deuterium. Secretion time (ST) was defined as the time lag between the start of the study and the appearance of DSPC-PA deuterium enrichment in TA. Data were given as mean +/- SD. All study infants reached deuterium-steady state in urine. DSPC-PA FSR was 6.5 +/- 2.8%/day (range 2.6-10.2). FSR for infants who did not receive exogenous surfactant was 5.7 +/- 3.5%/day (range 2.6-9.9%/day) and 7.3 +/- 2.1%/day (range 5.1-10.2%/day) in the preterms, whereas DSPC-PA ST was 10 +/- 10 h and 31 +/- 10 h respectively. Surfactant DSPC-PA synthesis can be measured in humans by the incorporation of deuterium from body water. This study is a simpler and less invasive method compared to previously published methods on surfactant kinetics by means of stable isotopes.

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.

A dual stable isotope tracer method for the measurement of surfactant disaturated-phosphatidylcholine net synthesis in infants with congenital diaphragmatic hernia

The aim of the study was to measure for the first time in humans surfactant disaturated-phosphatidylcholine (DSPC) net synthesis and kinetics by using a novel, dual stable isotope tracer approach. Ten infants with congenital diaphragmatic hernia [CDH; birth weight, 3.4 +/- 0.2; gestational age, 39.8 +/- 0.4 wk] and 6 age-matched control subjects with no lung disease (birth weight, 3.2 +/- 0.3 kg; gestational age, 39.1 +/- 1.1 wk), all of whom were admitted to the neonatal intensive care unit (Padua, Italy), were studied. All infants received simultaneously an intratracheal (carbon-13 di-palmitoyl-phosphatidylcholine) and an i.v. (deuterated palmitic acid) stable isotope tracer. Isotopic enrichment curves of DSPC from sequential tracheal aspirates were analyzed by mass spectrometry. DSPC kinetic data were expressed as mean +/- SEM and compared by the Mann-Whitney test. DSPC net synthesis from plasma palmitate was nearly identical in infants with CDH and control subjects (8.6 +/- 2.2 and 8.1 +/- 1.5 mg. kg(-1). d(-1); P = 0.7). DSPC apparent pool size was 36.7 +/- 7.5 and 58.5 +/- 9.1 mg/kg (P = 0.07) and half-life was 26.7 +/- 4.5 and 50.3 +/- 9.7 h (P = 0.03) in infants with CDH and control subjects, respectively. Both DSPC turnover and percentage of catabolism/recycling significantly correlated with duration of mechanical ventilation. In conclusion, the measurements of net DSPC synthesis and catabolism/recycling were reported for the first time in humans. Mean net DSPC synthesis was approximately 8 mg. kg(-1). d(-1). No significant differences were found between control subjects and infants with CDH. DSPC turnover was faster in infants with CDH, presumably reflecting an increased DSPC catabolism/recycling. Whether this may ultimately lead to a secondary surfactant deficiency in infants with CDH is still to be ascertained.

Mass Spectrometric Analysis of Surfactant Metabolism in Human Volunteers Using Deuteriated Choline

Surfactant reduces surface tension at pulmonary air-liquid interfaces. Although its major component is dipalmitoyl-phosphatidylcholine (PC16:0/16:0), other PC species, principally palmitoylmyristoyl-PC, palmitoylpalmitoleoyl-PC, and palmitoyloleoyl-PC, are integral components of surfactant. The composition and metabolism of PC species depend on pulmonary development, respiratory rate, and pathologic alterations, which have largely been investigated in animals using radiolabeled precursors. Recent advances in mass spectrometry and availability of precursors carrying stable isotopes make metabolic experiments in human subjects ethically feasible. We introduce a technique to quantify surfactant PC synthesis in vivo using deuteriated choline coupled with electrospray ionization tandem mass spectrometry. Endogenous PC from induced sputa of healthy volunteers comprised 54.0 +/- 1.5% PC16:0/16:0, 9.7 +/- 0.7% palmitoylmyristoyl-PC, 10.0 +/- 1.0% palmitoylpalmitoleoyl-PC, and 13.1 +/- 0.3% palmitoyloleoyl-PC. Infusion of deuteriated choline chloride (3.6 mg/kg body weight) over 3 hours resulted in linear incorporation into PC over 30 hours. After a plateau of 0.61 +/- 0.04% labeled PC between 30 and 48 hours, incorporation decreased to 0.30 +/- 0.02% within 7 days. Compared with native PC, fractional label was initially lower for PC16:0/16:0 (31.9 +/- 8.3%) but was higher for palmitoyloleoyl-PC (21.0 +/- 1.2%), and equilibrium was achieved after only 48 hours. We conclude that infusion of deuteriated choline and electrospray ionization tandem mass spectrometry is useful to investigate surfactant metabolism in humans in vivo.

Pulmonary surfactant disaturated-phosphatidylcholine (DSPC) turnover and pool size in newborn infants with congenital diaphragmatic hernia (CDH)

In animal CDH models, surfactant deficiency contributes to the pathophysiology of the condition but information on human disease is very limited. The aim of our study was to investigate surfactant kinetics in CDH newborns. We studied surfactant disaturated-phosphatidylcholine (DSPC) half-life, turnover and apparent pool size by stable isotope methodology in CDH newborns with no ExtraCorporeal Membrane Oxygenation (ECMO) support (n = 13, birth weight (BW) 3.2 +/- 2.2 kg, gestational age (GA) 39 +/- 0.4 wks, postnatal age 43 +/- 11 h) and in 8 term infants with no lung disease (CONTROLS, BW 2.7 +/- 0 kg, GA 38 +/- 0.8 wks, postnatal age 96 +/- 26 h). We administered a trace dose of 13C-palmitic acid dipalmitoyl-phosphatidylcholine (DPPC) through the endotracheal (ET) tube and we measured DSPC kinetics by gas chromatography-mass spectrometry from DSPC13C-enrichment decay curves obtained from sequential tracheal aspirates. DSPC amount from tracheal aspirates (TA-DSPC) was measured by gas chromatography. In CDH infants DSPC half-life was shorter (24 +/- 4 and 53 +/- 11 h, p = 0.01), turnover faster (0.6 +/- 0.1 and 1.5 +/- 0.3 d-1 p = 0.01), apparent pool size smaller (34 +/- 6 and 57 +/- 7 mg/kg body weight, p = 0.02) and tracheal aspirates DSPC amount lower (2.4 +/- 0.4 and 4.6 +/- 0.5 mg/mL Epithelial Lining Fluid (ELF), p = 0.007) than in CONTROLS. In conclusion surfactant kinetics is grossly abnormal in mechanically ventilated CDH. Whether alterations of DSPC kinetics in CDH infants are caused by a primary surfactant deficiency or are secondary to oxygen therapy and ventilator support has still to be determined.

Surfactant protein-A and phosphatidylglycerol suppress type IIA phospholipase A2 synthesis via nuclear factor-kappaB

We previously showed that surfactant inhibits the synthesis of type IIA secretory phospholipase A2 (sPLA2-IIA) by alveolar macrophages. These cells have been identified as the main source of this enzyme in an animal model of acute lung injury. The aim of the present study was to identify the surfactant components involved in the inhibition of sPLA2-IIA expression in alveolar macrophages and the signaling pathways that mediate this inhibition. Our results show that various surfactant preparations can inhibit sPLA2-IIA expression in endotoxin-stimulated alveolar macrophages. Both the surfactant protein (SP)-A and the surfactant phospholipid fraction inhibit this expression. The surfactant phospholipid dioleylphosphatidylglycerol (DOPG) abolishes sPLA2-IIA expression, whereas dipalmitoylphosphatidylcholine does not. Chromatographic analysis and confocal microscopy revealed that phosphatidylglycerol was rapidly incorporated and metabolized by alveolar macrophages and that its metabolites accumulate in the cytosol. Nuclear factor-kappaB (NF-kappaB) modulates sPLA2-IIA expression in endotoxin-activated alveolar macrophages, and surfactant preparations, surfactant phospholipid fraction, SP-A, and DOPG indeed suppressed NF-kappaB activation. In summary, our results show that SP-A and DOPG play a role in the surfactant-mediated inhibition of sPLA2-IIA expression in alveolar macrophages and that this inhibition occurs via a downregulation of NF-kappaB activation.

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.

Surfactant kinetics in preterm infants on mechanical ventilation who did and did not develop bronchopulmonary dysplasia

OBJECTIVE

To characterize surfactant kinetics in vivo in two groups of premature infants on different levels of mechanical ventilation and at different risk of developing bronchopulmonary dysplasia.

DESIGN

Controlled observational study in two independent groups of infants.

SETTING

Neonatal intensive care unit.

PATIENTS

Thirteen preterm infants (26 +/- 0.5 wks, birth weight 801 +/- 64 g) on high ventilatory setting and who finally all developed bronchopulmonary dysplasia (MechVentBPD), and eight (26 +/- 0.5 wks, birth weight 887 +/- 103 g) who had minimal or no lung disease and of whom none developed bronchopulmonary dysplasia (MechVentNoBPD).

MEASUREMENTS AND MAIN RESULTS

Endotracheal 13C-labeled dipalmitoyl-phosphatidylcholine was administered and subsequent measurements of the 13C enrichment of surfactant-disaturated phosphatidylcholine (DSPC) from serial tracheal aspirates were made by gas chromatography-mass spectrometry. We calculated disaturated phosphatidylcholine pharmacokinetic variables in terms of half-life and apparent pool size from the enrichment decay curves over time. DSPC concentration from tracheal aspirates was expressed as milligrams/milliliter epithelial lining fluid (ELF-DSPC). Data are presented as mean +/- se. In MechVentBPD infants vs. MechVentNoBPD, ELF-DSPC was much reduced, 2.9 +/- 0.6 vs. 9.4 +/- 3.0 mg/mL ELF (p =.03), half-life was shorter, 19.4 +/- 2.8 vs. 42.5 +/- 6.3 hrs (p =.002), and apparent pool size larger, 136 +/- 21 vs. 65.8 +/- 16.0 mg/kg (p =.057). In MechVentBPD, apparent DSPC pool size positively correlated with mean airway pressure x Fio(2) and inversely correlated with ELF-DSPC. ELF-DSPC was inversely correlated with mean airway pressure x Fio(2). No significant correlations were found in the MechVentNoBPD group.

CONCLUSIONS

MechVentBPD infants showed profound alteration of surfactant kinetics compared with preterm infants with minimal lung disease, and these alterations were correlated with severity of ventilatory support.

In vitro sensitization of human bronchus by beta2-adrenergic agonists

Incubation of human distal bronchi from 48 patients for 15 h with 10(-7) M fenoterol induced sensitization characterized by an increase in maximal contraction to endothelin-1 (ET-1) and acetylcholine (ACh). Incubation of human bronchi with 10(-6), 3 x 10(-6), and 10(-5) M forskolin (an adenyl cyclase activator) reproduced sensitization to ET-1 and ACh. The sensitizing effect of fenoterol was inhibited by coincubation with gliotoxine (a nuclear factor-kappaB inhibitor), dexamethasone, indomethacin (a cyclooxygenase inhibitor), GR-32191 (a TP prostanoid receptor antagonist), MK-476 (a cysteinyl leukotriene type 1 receptor antagonist), SR-140333 + SR-48968 + SR-142801 (neurokinin types 1, 2, and 3 tachykinin receptor antagonists) with or without HOE-140 (a bradykinin B(2) receptor antagonist), SB-203580 (an inhibitor of the 38-kDa mitogen-activated protein kinase, p38(MAPK)), or calphostin C (a protein kinase C blocker). Our results suggest that chronic exposure to fenoterol induces proinflammatory effects mediated by nuclear factor-kappaB and pathways involving leukotrienes, prostanoids, bradykinin, tachykinins, protein kinase C, and p38(MAPK), leading to the regulation of smooth muscle contraction to ET-1 and ACh.

Surfactant synthesis and kinetics in infants with congenital diaphragmatic hernia

In animal models of congenital diaphragmatic hernia (CDH), surfactant deficiency contributes to the pathophysiology of the disease; however, information on CDH in humans is limited. We compared surfactant disaturated phosphatidylcholine (DSPC) synthesis and metabolism, by stable isotope technology, in newborn infants with CDH and in control subjects. DSPC amount, total proteins, and surfactant protein-A (SP-A) from tracheal aspirates were also measured. DSPC and SP-A were significantly lower in 14 infants with CDH than in the eight control subjects. Mean DSPC was 2.3 +/- 1.3 mg/ml of epithelial lining fluid (ELF) in infants with CDH and 4.6 +/- 1.5 mg/ml of ELF in control subjects (p = 0.001). Mean SP-A in infants with CDH and in control subjects was 16.2 +/- 9.3 and 61.2 +/- 30.6 microg/ml of ELF, respectively (p = 0.03). DSPC kinetics was measured in 12 of 14 infants with CDH and in 5 of 8 control subjects. Secretion time was 8.3 +/- 5.5 and 8.5 +/- 2.5 hours and peak time 51.9 +/- 15.2 and 51 +/- 13 hours in infants with CDH and in control subjects, respectively. Fractional synthesis rate was not different for infants with CDH and control subjects (p = 0.4). In conclusion, surfactant DSPC synthesis and kinetics were not significantly deranged in infants with CDH compared with control subjects. Other factors, such as lower surface area or increased DSPC catabolism, may contribute to surfactant pool alteration in CDH.

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.