Pulmonary hemodynamics in neonatal lambs resuscitated with 21%, 50%, and 100% oxygen

Sex-dependent changes in the pulmonary vasoconstriction potential of newborn rats following short-term oxygen exposure

BACKGROUND

Chronic exposure to supplemental oxygen (O(2)) induces lung damage and mortality in a sex-dependent manner. The effect of short-term hyperoxia on the newborn pulmonary vasculature is unknown but is, however, of clinical significance in the neonatal resuscitation context. We hypothesize that short-term hyperoxia has a sex-dependent effect on the pulmonary vasculature.

METHODS

Following 1-h 100% O(2) exposure, the pulmonary arteries and lung tissues of newborn rats were evaluated.

RESULTS

Superoxide dismutase 3 (SOD3) expression in female pups' lungs was increased as compared with that in the lungs of male pups. As compared with air-treated pups, the response of male pups to thromboxane was increased by O(2), whereas the opposite effect was documented in the vessels of female pups. The enhanced force of hyperoxia-exposed arteries of the male pups was suppressed with superoxide or peroxynitrite scavengers, and increased lung SOD activity and hydrogen peroxide content were seen in female, but not in male, rats. Hyperoxia induced an increase in lung tissue oxidative products and Rho-kinase (ROCK) activity in male, but not in female, pups.

CONCLUSION

A lower lung SOD content and failure to upregulate SOD activity facilitates peroxynitrite generation and ROCK activation in hyperoxia-exposed males, predisposing them to pulmonary vasoconstriction. These observations, if relevant to humans, may explain the increased mortality and higher incidence of pulmonary hypertension in male neonates.

Decreases in manganese superoxide dismutase expression and activity contribute to oxidative stress in persistent pulmonary hypertension of the newborn

A rapid increase in the synthesis and release of nitric oxide (NO) facilitates the pulmonary vasodilation that occurs during birth-related transition. Alteration of this transition in persistent pulmonary hypertension of the newborn (PPHN) is associated with impaired function of endothelial nitric oxide synthase (eNOS) and an increase in oxidative stress. We investigated the hypothesis that a decrease in expression and activity of mitochondrial localized manganese superoxide dismutase (MnSOD) in pulmonary artery endothelial cells (PAEC) increases oxidative stress and impairs eNOS function in PPHN. We isolated PAEC and pulmonary arteries from fetal lambs with PPHN induced by prenatal ductus arteriosus ligation or sham ligation (control). We investigated MnSOD expression and activity, tyrosine nitration of MnSOD, and mitochondrial O(2)(-) levels in PAEC from control and PPHN lambs. We introduced exogenous MnSOD via an adenoviral vector (ad-MnSOD) transduction into PAEC and pulmonary arteries of PPHN lambs. The effect of ad-MnSOD was investigated on: mitochondrial O(2)(-) levels, MnSOD and eNOS expression and activity, intracellular hydrogen peroxide (H(2)O(2)) levels, and catalase expression in PAEC. MnSOD mRNA and protein levels and activity were decreased and MnSOD tyrosine nitration was increased in PPHN-PAEC. ad-MnSOD transduction of PPHN-PAEC increased its activity two- to threefold, decreased mitochondrial O(2)(-) levels, and increased H(2)O(2) levels and catalase expression. ad-MnSOD transduction improved eNOS expression and function and the relaxation response of PPHN pulmonary arteries. Our observations suggest that decreased MnSOD expression and activity contribute to the endothelial dysfunction observed in PPHN.

The pulmonary circulation in neonatal respiratory failure

The pulmonary circulation rapidly adapts at birth to establish lungs as the site of gas exchange. Abnormal transition at birth and/or parenchymal lung disease can result in neonatal hypoxemic respiratory failure. This article reviews the functional changes in pulmonary hemodynamics and structural changes in pulmonary vasculature secondary to (1) normal and abnormal transition at birth, and (2) diseases associated with neonatal hypoxemic respiratory failure. Various management strategies to correct respiratory failure are also discussed.

Brief hyperoxia increases mitochondrial oxidation and increases phosphodiesterase 5 activity in fetal pulmonary artery smooth muscle cells

AIMS

Oxygen is a pulmonary vasodilator, but data suggest high O(2) concentrations impede that response. We previously reported 24 h of 100% O(2) increased phosphodiesterase 5 (PDE5) activity in fetal pulmonary artery smooth muscle cells (FPASMC) and in ventilated neonatal lambs. PDE5 degrades cyclic GMP (cGMP) and inhibits nitric oxide (NO)-mediated cGMP-dependent vasorelaxation. We sought to determine the mechanism by which hyperoxia initiates reactive oxygen species (ROS) production and regulates PDE5.

RESULTS

Thirty minutes of hyperoxia increased mitochondrial ROS versus normoxia (30.3±1.7% vs. 21.1±2.8%), but had no effect on cytosolic ROS, measured by roGFP, a ratiometric protein thiol redox sensor. Hyperoxia increased PDE5 activity (220±39%) and decreased cGMP responsiveness to NO (37±17%). Mitochondrial catalase overexpression attenuated hyperoxia-induced mitochondrial roGFP oxidation, compared to FPASMC infected with empty adenoviral vector (50±3% of control) or mitochondrial superoxide dismutase. MitoTEMPO, mitochondrial catalase, and DT-3, a cGMP-dependent protein kinase I alpha inhibitor, decreased PDE5 activity (32±13%, 26±21%, and 63±10% of control, respectively), and restored cGMP responsiveness to NO (147±16%,172±29%, and 189±43% of control, respectively). C57Bl6 mice exposed to 90%-100% O(2) for 45 min±mechanical ventilation had increased PA PDE5 activity (206±39% and 235±75%, respectively).

INNOVATION

This is the first description that hyperoxia induces ROS in the mitochondrial matrix prior to the cytosol. Our results indicate that short hyperoxia exposures can produce significant changes in critical cellular signaling pathways.

CONCLUSIONS

These results indicate that mitochondrial matrix oxidant signals generated during hyperoxia, specifically H(2)O(2), activate PDE5 in a cGMP-dependent protein kinase-dependent manner in pulmonary vascular smooth muscle cells.

Oxidative stress and persistent pulmonary hypertension of the newborn treated with inhaled nitric oxide and different oxygen concentrations

OBJECTIVES

The aim of this study was to determine the effects of inhaled NO with different oxygen concentrations on the inflammatory cascade in newborns with hypoxic respiratory failure secondary to persistent pulmonary hypertension.

METHODS

60 newborns received iNO and 30 of them received an initial oxygen concentration of 45% (group 1), while the other 30 newborns received an initial oxygen concentration of 80% (group 2). The levels of inflammatory cytokines (IL-6, IL-8, TNF-α) were measured. The clinical outcome was also recorded.

RESULTS

The findings show that interleukin concentrations (IL-6, IL-8, TNF-α) were significantly decreased between 0 and 72 hours (p < 0.01) in the newborns exposed to initial oxygen concentration of 45% and significantly increased in the other group.

CONCLUSIONS

When inhaled, NO was co-administered with concentration of O(2) <45%, anti-inflammatory responses occurred, in accord with evidence in the published literature. The benefits of iNO on the clinical outcome in the current study demonstrate that inhaled NO in both groups was associated with improved short-term oxygenation.

Pulmonary vasodilator therapy in the NICU: inhaled nitric oxide, sildenafil, and other pulmonary vasodilating agents

The perinatal transition from fetal to extrauterine life requires a dramatic change in the circulatory pattern as the organ of gas exchange switches from the placenta to the lungs. Pulmonary hypertension can occur during early newborn life, and present as early respiratory failure or as a complication of more chronic diseases, such as bronchopulmonary dysplasia. The most effective pharmacotherapeutic strategies for infants with persistent pulmonary hypertension of the newborn are directed at selective reduction of pulmonary vascular resistance. This article discusses currently available therapies for pulmonary hypertension, their biologic rationales, and evidence for their clinical effectiveness.

Hydrogen peroxide regulates extracellular superoxide dismutase activity and expression in neonatal pulmonary hypertension

We previously demonstrated that superoxide and H(2)O(2) promote pulmonary arterial vasoconstriction in a lamb model of persistent pulmonary hypertension of the newborn (PPHN). Because extracellular superoxide dismutase (ecSOD) augments vasodilation, we hypothesized that H(2)O(2)-mediated ecSOD inactivation contributes to pulmonary arterial vasoconstriction in PPHN lambs. ecSOD activity was decreased in pulmonary arterial smooth muscle cells (PASMCs) isolated from PPHN lambs relative to controls. Exposure to 95% O(2) to mimic hyperoxic ventilation reduced ecSOD activity in control PASMCs. In both cases, these events were associated with increased protein thiol oxidation, as detected by the redox sensor roGFP. Accordingly, exogenous H(2)O(2) decreased ecSOD activity in control PASMCs, and PEG-catalase restored ecSOD activity in PPHN PASMCs. In intact animal studies, ecSOD activity was decreased in fetal PPHN lambs, and in PPHN lambs ventilated with 100% O(2) relative to controls. In ventilated PPHN lambs, administration of a single dose of intratracheal PEG-catalase enhanced ecSOD activity, reduced superoxide levels, and improved oxygenation. We propose that H(2)O(2) generated by PPHN and hyperoxia inactivates ecSOD, and intratracheal catalase enhances enzyme function. The associated decrease in extracellular superoxide augments vasodilation, suggesting that H(2)O(2) scavengers may represent an effective therapy in the clinical management of PPHN.

Therapeutic approaches using nitric oxide in infants and children

Pulmonary hypertension contributes significantly to the morbidity and mortality associated with many pediatric pulmonary and cardiac diseases. Nitric oxide, a gas molecule, is a unique pharmaceutical agent that can be inhaled and thus delivered directly to the lung. Inhaled nitric oxide was approved by the FDA in 1999 as a therapy for infants with persistent pulmonary hypertension. Since then, the use of inhaled nitric oxide has expanded to other neonatal and pediatric conditions, and our knowledge of its properties and mechanisms of action has increased tremendously. This review discusses the physiology of nitric oxide signaling, the most common indications for its clinical use, and promising new investigations that may enhance endogenous production of nitric oxide and/or improve vascular response to it.

Pulmonary hemodynamics and vascular reactivity in asphyxiated term lambs resuscitated with 21 and 100% oxygen

An increase in oxygen tension is an important factor in decreasing pulmonary vascular resistance (PVR) at birth. Birth asphyxia results in acidosis and increased PVR. We determined the effect of resuscitation with 21 vs. 100% O(2) on pulmonary hemodynamics, pulmonary arterial (PA) reactivity, and oxidant stress in a lamb model of in utero asphyxia. Term fetal lambs were acutely asphyxiated by intrauterine umbilical cord occlusion for 10 min resulting in acidosis (pH 6.96 ± 0.05 and Pco(2) 103 ± 5 Torr), bradycardia, systemic hypotension, and increased PVR. Lambs were treated with 30 min of resuscitation with 21% or 100% O(2) (n = 6 each). Pa(O(2)) was significantly elevated with 100% O(2) resuscitation compared with 21% O(2) (430 ± 38 vs. 64 ± 8 Torr), but changes in pH and Pa(CO(2)) were similar. The 100% O(2) induced greater increase in pulmonary blood flow and decrease in PVR at 1 min of life, but subsequent values were similar to 21% O(2) group between 2 and 30 min of life. Oxygen uptake from the lung and systemic oxygen extraction was similar between the two groups. Pulmonary arteries showed increased staining for superoxide anions and increased contractility to norepinephrine following resuscitation with 100% O(2). The increased PA contractility induced by 100% O(2) was reversed by scavenging superoxide anions with superoxide dismutase and catalase. We conclude that resuscitation of asphyxiated lambs with 100% O(2) increases Pa(O(2)) but does not improve lung oxygen uptake, decrease PVR at 30 min, or increase systemic oxygen extraction ratios. Furthermore, 100% O(2) also induces oxidative stress and increases PA contractility. These findings support the new neonatal resuscitation guidelines recommending 21% O(2) for initial resuscitation of asphyxiated neonates.

Oxygen supplementation in the delivery room: updated information

Resuscitation is the most common procedure performed in neonatology. However, new contributions based on scientific evidence challenge the traditional procedures. Of these new contributions, the use of room-air instead of 100% oxygen and titration of the oxygen inspiratory fraction according to oximetry and heart rate represent a new approach in the resuscitation of both term and preterm newborns.

Pulmonary hypertension and the asphyxiated newborn

Persistent pulmonary hypertension of the newborn may occur with perinatal asphyxia, either because of direct effects of hypoxia/ischemia on pulmonary arterial function or indirectly because both are associated with meconium aspiration syndrome or perinatal sepsis/pneumonia. Therapies for persistent pulmonary hypertension of the newborn have the potential to affect cerebral function and cerebral perfusion in infants with hypoxic ischemic encephalopathy. Our literature review concludes that hyperventilation should be avoided, bicarbonate therapy is unproven, and hypoxia and hyperoxia should both be avoided. Nitric oxide improves pulmonary artery pressure and systemic perfusion. The effects of inotropic agents on cerebral perfusion or outcomes are uncertain.

Shock: a common consequence of neonatal asphyxia

Neonatal asphyxia is associated with multi-organ hypoxia-ischemia and subsequent dysfunction. The cardiovascular system is frequently affected, causing signs of shock and complicating the neonatal circulatory transition. Hypothermia therapy can improve outcome from neonatal asphyxia without adversely affecting cardiovascular stability. Therapy directed at the cardiovascular system can improve short-term measures of systemic blood flow, but to date has not been demonstrated to improve long-term outcome.

Phosphodiesterases: emerging therapeutic targets for neonatal pulmonary hypertension

Pulmonary hypertension in the neonate is associated with multiple underlying problems such as respiratory distress syndrome, meconium aspiration syndrome, congenital diaphragmatic hernia, bronchopulmonary dysplasia, sepsis, or congenital heart disease. Because of the heterogeneous group of disorders, the therapeutic approach and response often depends on the underlying disease. In many of these conditions, there is evidence that cyclic nucleotide signaling and specifically phosphodiesterases (PDEs) are disrupted. PDE inhibitors represent an emerging class of pulmonary vasodilators in adults. Studies are now under way to evaluate the utility, efficacy, and safety of such therapies in infants with pulmonary hypertension.

Current and future therapeutic options for persistent pulmonary hypertension in the newborn

Persistent pulmonary hypertension of the newborn (PPHN) is a potentially life-threatening condition that is characterized by supra-systemic pulmonary vascular resistance causing right-to-left shunting through the ductus arteriosus and/or foramen ovale, leading to a vicious cycle of hypoxemia, acidosis and further pulmonary vasoconstriction. Advances in neonatology including surfactant instillation, high-frequency ventilation, extracorporeal membrane oxygenation and, most importantly, inhaled nitric oxide (INO), have revolutionized the management of PPHN. However, given that INO does not improve oxygenation in a significant proportion (30-40%) of cases, there is an urgent need to consider other therapeutic options for PPHN. The issue is more important for developing nations with a higher PPHN-related health burden and limited resources. This article discusses the evidence about INO in term and preterm neonates in brief, and focuses mainly on the potential alternative drugs in the management of PPHN.

Why are we still using oxygen to resuscitate term infants?

This article summarizes the historical background for the use of oxygen during newborn resuscitation and describes some of the research and the process of changing the previous practice from a high- to a low-oxygen approach. Findings of a recent Cochrane review suggest that more than 100,000 newborn lives might be saved globally each year by changing from 100 to 21% oxygen for newborn resuscitation. This estimate represents one of the largest yields for a simple therapeutic approach to decrease neonatal mortality in the history of pediatric research. Available data also suggest that, for the very low birth weight infant, use of the low-oxygen approach should be considered with the understanding that some of the smallest and sickest preterm neonates will need some level of oxygen supplementation during the first minutes of postnatal life. As more data are needed for the very preterm population, creation of strict guidelines for these infants would be premature at present. However, it can be stated that term and late preterm infants in need of resuscitation should, in general, be started on 21% oxygen, and if resuscitation is not started with 21% oxygen, a blender should be available, enabling the administration of the lowest FiO(2) possible to keep heart rate and SaO(2) within the target range. For extremely low birth weight infants, initial FiO(2) could be between 0.21 and 0.30 and adjusted according to the response in SaO(2) and heart rate.

SOD and inhaled nitric oxide normalize phosphodiesterase 5 expression and activity in neonatal lambs with persistent pulmonary hypertension

Phosphodiesterase 5 (PDE5) and soluble guanylate cyclase (sGC) are key regulators of cGMP and pulmonary vascular tone. We sought to determine the impact of mechanical ventilation with O(2) with or without inhaled nitric oxide (iNO) or recombinant human Cu/Zn SOD (rhSOD) on sGC, PDE5, and cGMP in the ovine ductal ligation model of persistent pulmonary hypertension of the newborn (PPHN). PPHN lambs were ventilated with 100% O(2) for 24 h alone or combined with either inhalation of 20 parts per million (ppm) iNO continuously or a single intratracheal dose of rhSOD (5 mg/kg). Ventilated PPHN lambs were compared with PPHN fetuses, control fetuses, and 1-day-old spontaneously breathing lambs (1DSB). In the small pulmonary arteries of 1DSB lambs, sGC expression increased, PDE5 expression decreased, and cGMP concentrations increased relative to fetal levels. In PPHN lambs ventilated with 100% O(2), sGC activity increased to levels comparable with 1DSB levels. However, PDE5 expression and activity increased, and cGMP levels remained at fetal levels. Addition of either iNO or rhSOD decreased PDE5 expression and activity in PPHN lambs and increased cGMP levels to levels comparable with 1DSB lambs. These data suggest that ventilation of PPHN lambs with 100% O(2) impairs cGMP-mediated vasodilation in part due to increased PDE5 expression and activity. The addition of either iNO or rhSOD normalized PDE5 and cGMP levels. Thus therapies designed to decrease PDE5 and increase cGMP, such as iNO and rhSOD, may prove useful in the treatment of PPHN in newborn infants.

Neonatal pulmonary hypertension

When the normal cardiopulmonary transition fails to occur, the result is persistent pulmonary hypertension of the newborn. Severe persistent pulmonary hypertension of the newborn is estimated to occur in 2 per 1000 live-born term infants, and some degree of pulmonary hypertension complicates the course of >10% of all neonates with respiratory failure. This review article discusses the vascular abnormalities that are associated with neonatal pulmonary hypertension, including recognition of its role in severe bronchopulmonary dysplasia in preterm infants. A systematic review of the evidence for common therapies including inhaled nitric oxide, high-frequency ventilation, surfactant, and extracorporeal life support is included. Finally, this field is rapidly evolving, and the rationale for promising new treatment approaches is reviewed, including inhibition of phosphodiesterases and scavengers of reactive oxygen species.

Exposure to supplemental oxygen and its effects on oxidative stress and antioxidant enzyme activity in term newborn lambs

The optimal oxygen concentration for the resuscitation of term infants remains controversial. We studied the effects of 21 versus 100% oxygen immediately after birth, and also exposure for 24 h to 100% oxygen, on oxidant lung injury and lung antioxidant enzyme (AOE) activities in term newborn lambs. Lambs at 139 d gestation were delivered and ventilated with 21% (RAR) or 100% (OXR) for 30 min. A third group of newborn lambs were ventilated with 100% O2 for 24 h (OX24). Oxidized glutathione levels in whole blood were significantly different among the groups with lower values in the RAR group, and these values correlated highly with partial pressure of arterial oxygen (Pao2). The reduced to oxidized glutathione ratio was significantly different among the groups, the ratio decreasing with increasing oxygen exposure. Lipid hydroperoxide (LPO) activity was significantly higher in the OXR and OX24 groups. AOE activity was higher in the whole lung and in red cell lysate in the OX24 group. Increased myeloperoxidase (MPO) activity, percent neutrophils, and proteins in lung lavage suggested inflammation in the OX24 group after maximal oxygen exposure. We conclude that even relatively brief exposure of the lung to 100% oxygen increases systemic oxidative stress and lung oxidant injury in ventilated term newborn lambs.

Regulation of phosphodiesterase 3 in the pulmonary arteries during the perinatal period in sheep

The role of cAMP in the pulmonary vasculature during the transition from intrauterine to extrauterine life is poorly understood. We hypothesized that cAMP levels are regulated by alterations in phosphodiesterase 3 (PDE3), which hydrolyzes cAMP. PDE3 protein expression and hydrolytic activity were increased in the resistance pulmonary arteries (PA) from spontaneously breathing 1-d-old (1dSB) lambs relative to equivalent-gestation fetuses. This was accompanied by a decrease in steady-state cAMP. Ventilation with 21% O2 and 100% O2 for 24 h disrupted the normal transition, whereas ventilation with 100% O2 + inhaled NO (iNO) for 24 h restored both PDE3 activity and cAMP to 1dSB levels. Consistent with these findings, relaxation to milrinone, a PDE3 inhibitor, was greater in the PA isolated from 1dSB and 100% O2 + iNO lambs, relative to fetal, 21% O2, and 100% O2 lambs. In conclusion, PDE3 expression and activity in the PA dramatically increase after birth, with a concomitant decrease in steady-state cAMP. Ventilation with either 21% O2 or 100% O2 blunts this PDE3 increase, whereas iNO restores PDE3 activity to levels equivalent to 1dSB lambs. The vasodilatory effects of milrinone were most pronounced in vessels from lambs with the highest PDE3 activity, i.e., 1dSB and 100% O2 + iNO lambs. Thus, milrinone may be most beneficial when used in conjunction with iNO.

Oxygen concentration and pulmonary hemodynamics in newborn lambs with pulmonary hypertension

The effect of oxygen concentration on lowering pulmonary vascular resistance (PVR) during resuscitation in a model of persistent pulmonary hypertension of the newborn (PPHN) is not known. PPHN was induced in fetal lambs by ductal ligation 9 d before delivery. After delivery by cesarean section, resuscitation of PPHN lambs with 21%, 50%, or 100% O2 (n = 6 each) for 30 min produced similar decreases in PVR. Lambs were then ventilated with 50% O2 for 60 min and exposed to inhaled nitric oxide (iNO, 20 ppm). Initial resuscitation with 100% O2 significantly impaired the subsequent response to iNO compared with 21% O2 (42 +/- 9% vs 22 +/- 4% decrease from baseline PVR). Finally, each lamb was randomly and sequentially ventilated with 10%, 21%, 50%, or 100% O2. PVR decreased with increased concentrations of inhaled O2 up to 50%, there being no additional decrease in PVR with 100% O2. When PVR was correlated with Pao2, the maximal change in PVR was achieved at Pao2 values <60 mm Hg. We conclude that resuscitation with 100% O2 does not enhance pulmonary vasodilation compared with 21% and 50% O2, but impairs the subsequent response to iNO in PPHN lambs. Hypoxia increases PVR but hyperoxia does not confer significant additional pulmonary vasodilation in lambs with PPHN.

Initiating delivery room stabilization/resuscitation in very low birth weight (VLBW) infants with an FiO(2) less than 100% is feasible

BACKGROUND

Oxygen exposure during delivery room (DR) resuscitation, even when brief, is potentially toxic. A practice plan (PP) was introduced for very low birth weight (VLBW) infants < or = 1500 g as follows: initial FiO(2) from 0.21 to 1.0 using blenders, oxygen guided by oximetry to maintain saturation between 85% to 95% from birth.

OBJECTIVE

To determine whether the initiating FiO(2) could be safely lowered, and by doing so whether the number of infants with a PaO(2) >80 mm Hg could be minimized on admission, as well as lowering oxygen requirement at 24 h.

METHODS

In all, 53 infants admitted between June 2006 and June 2007 were evaluated and compared with 47 infants from 2004 managed with 100 % oxygen (historical comparison group (HC)).

RESULT

Stabilization/Resuscitation included intubation (n=28) and continuous positive airway pressure (CPAP) (n=25); no cardiopulmonary resuscitation (CPR). The heart rate increased rapidly in all cases. The initiating FiO(2) decreased from 0.42 to 0.28 over 12 months (P=0.00005); 14 (26%) were resuscitated with room air. Correspondingly, the pH increased from 7.24 to 7.30 (P=0.002) and PCO(2) decreased from 53 to 41 (P=0.001). A comparison of infants during the PP with the HC revealed that 36/53 versus 21/47 had an initial PaO(2) <80 mm Hg (P=0.02); the median PaO(2), that is, 64 versus 86 and saturation, that is, 95% versus 99% on admission were significantly lower. The median FiO(2) at 24 h was 0.25 versus 0.40.

CONCLUSION

DR resuscitation of VLBW infants can be initiated with less oxygen even with room air without concomitant overt morbidity. This change was associated with more infants with an initial PaO(2) <80 mm Hg and lower saturation values on admission as well as a lower FiO(2) requirement at 24 h.

Advances in the diagnosis and management of persistent pulmonary hypertension of the newborn

Rapid evaluation of a neonate who is cyanotic and in respiratory distress is essential for achieving a good outcome. Persistent pulmonary hypertension of the newborn (PPHN) can be a primary cause or a contributing factor to respiratory failure, particularly in neonates born at 34 weeks or more of gestation. PPHN represents a failure of normal postnatal adaptation that occurs at birth in the pulmonary circulation. Rapid advances in therapy in recent years have led to a remarkable decrease in mortality for the affected infants. Infants who survive PPHN are at significant risk for long-term hearing and neurodevelopmental impairments, however. This review focuses on the diagnosis, recent advances in management, and recommendations for the long-term follow-up of infants who have PPHN.

Nitric oxide and beyond: new insights and therapies for pulmonary hypertension

Persistent pulmonary hypertension of the newborn (PPHN) contributes significantly to the morbidity and mortality associated with meconium aspiration syndrome. This review article discusses new insights into the vascular abnormalities that are associated with PPHN, including the recent recognition of the importance of oxidant stress in its pathogenesis. Recent data are presented showing that treatment with high oxygen concentrations may increase production of oxygen free radicals. The rationale for the use of inhaled nitric oxide, and strategies for enhancing nitric oxide signaling are discussed. Finally, the rationale for new treatment approaches is reviewed, including inhibition of cyclic guanosine monophosphate-specific phosphodiesterases and scavengers of reactive oxygen species.

Refining the role of oxygen administration during delivery room resuscitation: what are the future goals?

Oxygen was discovered more than 200 years ago and was thought to be both essential and beneficial for all animal life. Although it is now over 100 years since oxygen was first shown to damage biological tissues exposed to high concentrations, and more than 50 years since it was implicated in the aetiology of retinopathy of prematurity, the use of 100% oxygen was still recommended for the resuscitation of all babies at birth as recently as 2000. However, the 2005 International Liaison Committee on Resuscitation (ILCOR) recommendations allow for the initiation of resuscitation with concentrations of oxygen between 21 and 100%. There are strong arguments in favour of a radical curtailment of the use of oxygen in resuscitation at birth, and for devoting resources to defining the margins of safety for its use in the neonatal period in general.

Superoxide dismutase restores eNOS expression and function in resistance pulmonary arteries from neonatal lambs with persistent pulmonary hypertension

Endothelial nitric oxide (NO) synthase (eNOS) expression and activity are decreased in fetal lambs with persistent pulmonary hypertension (PPHN). We sought to determine the impact of mechanical ventilation with O(2) with or without inhaled NO (iNO) or recombinant human SOD (rhSOD) on eNOS in the ductal ligation model of PPHN. PPHN lambs and age-matched controls were ventilated with 100% O(2) for 24 h alone or combined with 20 ppm iNO continuously or a single dose of rhSOD (5 mg/kg) given intratracheally at delivery. In 1-day spontaneously breathing lambs, eNOS expression in resistance pulmonary arteries increased relative to fetal levels. eNOS expression increased in control lambs ventilated with 100% O(2), but not in PPHN lambs. Addition of iNO or rhSOD increased eNOS expression and decreased generation of reactive oxygen species (ROS) in PPHN lambs relative to those ventilated with 100% O(2) alone. However, only rhSOD restored eNOS function, increased tetrahydrobiopterin (BH(4)), a critical cofactor for eNOS function, and restored GTP cyclohydrolase I expression in isolated vessels and lungs from PPHN lambs. These data suggest that ventilation of PPHN lambs with 100% O(2) increases ROS production, blunts postnatal increases in eNOS expression, and decreases available BH(4) in PPHN lambs. Although the addition of iNO or rhSOD diminished ROS production and increased eNOS expression, only rhSOD improved eNOS function and levels of available BH(4). Thus therapies designed to decrease oxidative stress and restore eNOS coupling, such as rhSOD, may prove useful in the treatment of PPHN in newborn infants.

Evaluation and management of the cyanotic neonate

The infant presenting to the emergency department with cyanosis requires rapid assessment, diagnosis, and initiation of therapy. In this article, the potential challenges in recognizing cyanosis are discussed, including the presence of higher concentrations of fetal hemoglobin, and its oxygen binding characteristics. A systematic approach to the diagnosis of cyanosis is presented, based on an understanding of the normal transitional physiology, and how diseases of the airway, lung, and circulatory system may disrupt these processes. Strategies for initial emergency department management of lung and cardiac disease are presented.

Blunted hypoxic pulmonary vasoconstriction in experimental neonatal chronic lung disease

RATIONALE

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

OBJECTIVES

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

METHODS

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

MEASUREMENTS AND MAIN RESULTS

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

CONCLUSIONS

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

Achievement of targeted saturation values in extremely low gestational age neonates resuscitated with low or high oxygen concentrations: a prospective, randomized trial

OBJECTIVE

Extremely low gestational age neonates have very low oxygen saturation in utero and an immature antioxidant defense system. Abrupt increases in oxygen saturation after birth may cause oxidative stress. We compared achievement of a targeted oxygen saturation of 85% at 10 minutes of life when resuscitation was initiated with low or high fractions of inspired oxygen and levels were adjusted according to preductal pulse oxygen saturation values.

METHODS

A prospective, randomized, clinical trial was performed in 2 level III neonatal referral units. Patients of < or = 28 weeks of gestation who required active resuscitation were randomly assigned to the low-oxygen group (fraction of inspired oxygen: 30%) or the high-oxygen group (fraction of inspired oxygen: 90%). Every 60 to 90 seconds, the fraction of inspired oxygen was increased in 10% steps if bradycardia occurred (< 100 beats per minute) or was decreased in similar steps if pulse oxygen saturation reached values of > 85%. Preductal pulse oxygen saturation was continuously monitored.

RESULTS

The fraction of inspired oxygen in the low-oxygen group was increased stepwise to 45% and that in the high-oxygen group was reduced to 45% to reach a stable pulse oxygen saturation of approximately 85% at 5 to 7 minutes in both groups. No differences in oxygen saturation in minute-to-minute registers were found independent of the initial fraction of inspired oxygen used 4 minutes after cord clamping. No differences in mortality rates in the early neonatal period were detected.

CONCLUSIONS

Resuscitation can be safely initiated for extremely low gestational age neonates with a low fraction of inspired oxygen (approximately 30%), which then should be adjusted to the infant's needs, reducing the oxygen load to the neonate.