Considerations in the management of hypoxemic respiratory failure and persistent pulmonary hypertension in term and late preterm neonates.

Recent advances in our understanding of neonatal pulmonary circulation and the underlying pathophysiology of hypoxemic respiratory failure (HRF)/persistent pulmonary hypertension of the newborn (PPHN) have resulted in more effective management strategies. Results from animal studies demonstrate that low alveolar oxygen tension (PAO2) causes hypoxic pulmonary vasoconstriction, whereas an increase in oxygen tension to normoxic levels (preductal arterial partial pressure of oxygen (PaO2) between 60 and 80 mm Hg and/or preductal peripheral capillary oxygen saturation between 90% and 97%) results in effective pulmonary vasodilation. Hyperoxia (preductal PaO2 >80 mm Hg) does not cause further pulmonary vasodilation, and oxygen toxicity may occur when high concentrations of inspired oxygen are used. It is therefore important to avoid both hypoxemia and hyperoxemia in the management of PPHN. In addition to oxygen supplementation, therapeutic strategies used to manage HRF/PPHN in term and late preterm neonates may include lung recruitment with optimal mean airway pressure and surfactant, inhaled and intravenous vasodilators and 'inodilators'. Clinical evidence suggests that administration of surfactant or inhaled nitric oxide (iNO) therapy at a lower acuity of illness can decrease the risk of extracorporeal membrane oxygenation/death, progression of HRF and duration of hospital stay. Milrinone may be beneficial as an inodilator and may have specific benefits following prolonged exposure to iNO plus oxygen owing to inhibition of phosphodiesterase (PDE)-3A. Additionally, sildenafil, and, in selected cases, hydrocortisone may be appropriate options after hyperoxia and oxidative stress owing to their effects on PDE-5 activity and expression. Continued investigation into these and other interventions is needed to optimize treatment and improve outcomes.

Challenges, priorities and novel therapies for hypoxemic respiratory failure and pulmonary hypertension in the neonate.

Future priorities for the management of hypoxemic respiratory failure (HRF) and pulmonary hypertension include primary prevention of neonatal lung diseases, 'precision medicine' and translating promising clinical and preclinical research into novel therapies. Promising areas of investigation include noninvasive ventilation strategies, emerging pulmonary vasodilators (for example, cinaciguat, intravenous bosentan, rho-kinase inhibitors, peroxisome proliferator-activated receptor-γ agonists) and hemodynamic support (arginine vasopressin). Research challenges include the optimal timing for primary prevention interventions and development of validated biomarkers that predict later disease or serve as surrogates for long-term respiratory outcomes. Differentiating respiratory disease endotypes using biomarkers and experimental therapies tailored to the underlying pathobiology are central to the concept of 'precision medicine' (that is, prevention and treatment strategies that take individual variability into account). The ideal biomarker should be expressed early in the neonatal course to offer an opportunity for effective and targeted interventions to modify outcomes. The feasibility of this approach will depend on the identification and validation of accurate, rapid and affordable point-of-care biomarker tests. Trials targeting patient-specific pathobiology may involve less risk than traditional randomized controlled trials that enroll all at-risk neonates. Such approaches would reduce trial costs, potentially with fewer negative trials and improved health outcomes. Initiatives such as the Prematurity and Respiratory Outcomes Program, supported by the National Heart, Lung, and Blood Institute, provide a framework to develop refined outcome measures and early biomarkers that will enhance our understanding of novel, mechanistic therapeutic targets that can be tested in clinical trials in neonates with HRF.

Inhaled nitric oxide use in preterm infants in California neonatal intensive care units.

OBJECTIVE: To describe inhaled nitric oxide (iNO) exposure in preterm infants and variation in neonatal intensive care unit (NICU) use. STUDY DESIGN: This was a retrospective cohort study of infants, 22 to 33+6/7 weeks of gestational age (GA), during 2005 to 2013. Analyses were stratified by GA and included population characteristics, iNO use over time and hospital variation. RESULTS: Of the 65 824 infants, 1718 (2.61%) received iNO. Infants, 22 to 24+6/7 weeks of GA, had the highest incidence of iNO exposure (6.54%). Community NICUs (n=77, median hospital use rate 0.7%) used less iNO than regional NICUs (n=23, median hospital use rate 5.8%). In 22 to 24+6/7 weeks of GA infants, the median rate in regional centers was 10.6% (hospital interquartile range 3.8% to 22.6%). CONCLUSION: iNO exposure varied with GA and hospital level, with the most use in extremely premature infants and regional centers. Variation reflects a lack of consensus regarding the appropriate use of iNO for preterm infants.

Pilot trial of late booster doses of surfactant for ventilated premature infants.

OBJECTIVE: Many premature infants at risk for bronchopulmonary dysplasia experience episodes of surfactant dysfunction with reduced surfactant protein B (SP-B). In this study, we investigated the safety and responses to booster doses of surfactant. STUDY DESIGN: A total of 87 infants, 500 to 1250 g birth weight, who were ventilated at 7 to 10 days received 2 or 3 doses of Infasurf (Calfactant, Forest Pharmaceuticals, St Louis, MO, USA) within a 1-week period. RESULT: For 184 doses, occurrence rates of transient bradycardia (13) and plugged endotracheal tube (5) were low, and no other adverse effects were noted. Treatment transiently improved the respiratory severity score (FiO(2) × mean airway pressure), SP-B content (+75%) and surface properties of isolated surfactant. Levels of eight proinflammatory cytokines in tracheal aspirate were interrelated and unchanged from baseline after surfactant treatment. CONCLUSION: Booster doses of surfactant for premature infants with lung disease are safe and transiently improve respiratory status as well as composition and function of endogenous surfactant.

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.

sGC and PDE5 are elevated in lambs with increased pulmonary blood flow and pulmonary hypertension.

Utilizing aortopulmonary vascular graft placement, we established a lamb model of pulmonary hypertension that mimics congenital heart disease with increased pulmonary blood flow. We previously demonstrated that endothelial nitric oxide synthase (eNOS) is increased in lambs at age 4 wk. However, these lambs display a selective impairment of endothelium-dependent pulmonary vasodilation that is suggestive of a derangement downstream of NO release. Thus our objective was to characterize potential alterations in the expression and activity of soluble guanylate cyclase (sGC) and phosphodiesterase type 5 (PDE5) induced by increased pulmonary blood flow and pulmonary hypertension. Late-gestational fetal lambs (n = 10) underwent in utero placement of an aortopulmonary vascular graft (shunt). Western blotting analysis on lung tissue from 4-wk-old shunted lambs and age-matched controls showed that protein for both subunits of sGC was increased in shunted lamb lungs compared with age-matched controls. Similarly, cGMP levels were increased in shunted lamb lungs compared with age-matched controls. However, PDE5 expression and activity were also increased in shunted lambs. Thus although cGMP generation was increased, concomitant upregulation of PDE5 expression and activity may have (at least partially) limited and accounted for the impairment of endothelium-dependent pulmonary vasodilation in shunted lambs.

Recombinant human superoxide dismutase enhances the effect of inhaled nitric oxide in persistent pulmonary hypertension.

We investigated the pulmonary vascular effects of superoxide dismutase (SOD) alone and in combination with inhaled nitric oxide (iNO) in newborn lambs with persistent pulmonary hypertension (PPHN) following prenatal ligation of the ductus arteriosus. In in vitro experiments, pretreatment with SOD significantly enhanced vascular relaxation in response to the NO donor S-nitrosyl-acetylpenicillamine (SNAP) in fifth-generation pulmonary arteries isolated from lambs with PPHN. In vivo treatment of fully instrumented newborn lambs with a single intratracheal dose of recombinant human CuZn SOD (rhSOD; 5 mg/kg) produced selective dilation of the pulmonary circulation. Further studies, of the combination of rhSOD and iNO, showed enhancement of the pulmonary vascular effects of iNO after brief periods of inhalation of 5 ppm and 80 ppm NO. We conclude that rhSOD reduces pulmonary vascular resistance and facilitates the action of iNO in a lamb model of PPHN. This suggests that rhSOD may prove to be an effective adjunctive treatment for newborns with PPHN.

C-type natriuretic peptide system in fetal ovine pulmonary vasculature.

C-type natriuretic peptide (CNP) is a recently described endothelium-derived relaxing factor. CNP relaxes vascular smooth muscle and inhibits smooth muscle proliferation by binding to natriuretic peptide receptor (NPR) type B (NPR-B) and producing cGMP. Lung parenchyma and fifth-generation pulmonary arteries (PA) and veins (PV) were isolated from late-gestation fetal lambs. All three types of NPR mRNA were detected in PA and PV by RT-PCR. CNP and NPR-B immunostaining was positive in pulmonary vascular endothelium and medial smooth muscle. CNP concentration-response curves of PA and PV were compared with those of atrial natriuretic peptide (ANP) by use of standard tissue bath techniques. CNP relaxed PV significantly better than PA. ANP relaxed PA and PV equally, but ANP relaxed PA significantly better than CNP. Pretreating PA and PV with natriuretic peptide receptor blocker (HS-142-1) or cGMP-dependent protein kinase inhibitor Rp-beta-phenyl-1- N2-etheno-8-bromoguanosine 3',5'-cyclic monophosphorothionate significantly inhibited the CNP relaxation response, indicating that the response was mediated through the NPR-cGMP pathway. We conclude that CNP is important in mediating pulmonary venous tone in the fetus.

Pulmonary hypertension alters soluble guanylate cyclase activity and expression in pulmonary arteries isolated from fetal lambs.

The nitric oxide (NO)-guanosine 3',5'-cyclic monophosphate (cGMP) signaling pathway plays an important role in the pulmonary vascular transition at birth. We studied pulmonary arteries and veins isolated from normal late-gestation fetal lambs and from fetal lambs with persistent pulmonary hypertension (PPHN) following prenatal ligation of the ductus arteriosus. We additionally used double immunolabeling and immunoblot analysis to determine relative vascular contents of endothelial nitric oxide synthase (NOS-III) and soluble guanylate cyclase (sGC). Cyclic GMP content and sGC activity were significantly lower in arteries from hypertensive lambs than controls. A rank order for contents of both soluble guanylate cyclase and NOS-III was observed by both immunolabeling and immunoblotting: Control vein = Hypertensive vein > Control artery > Hypertensive artery. Our data demonstrate that the relative expression of sGC correlates well with the relative expression of NOS-III, and indicate the potential importance of soluble guanylate cyclase in the regulation of the perinatal pulmonary circulation. These data may help us understand vascular mechanisms producing PPHN, as well as patterns of response to exogenous NO.

Altered endothelium-dependent relaxations in lambs with high pulmonary blood flow and pulmonary hypertension.

Congenital heart disease associated with increased pulmonary blood flow produces pulmonary hypertension. To characterize vascular alterations in the nitric oxide (NO)-cGMP cascade induced by increased pulmonary blood flow and pulmonary hypertension, 10 fetal lambs underwent in utero placement of an aortopulmonary vascular graft (shunt). When the lambs were 4-6 wk of age, we assessed responses of pulmonary arteries (PAs) and pulmonary veins (PVs) isolated from lungs of control and shunted lambs. PVs from control and shunted lambs relaxed similarly to exogenous NO (S-nitrosyl-acetyl-penicillamine), to NO produced endogenously (zaprinast and A-23187), and to cGMP (atrial natriuretic peptide). In contrast, relaxations to A-23187 and zaprinast were blunted in PAs isolated from shunted lambs relative to controls. Inhibitors of NO synthase (NOS) and soluble guanylate cyclase constricted control but not shunt PAs, indicating reduced basal NOS activity in shunt PAs. Pretreatment of shunt PAs with the substrates L-arginine and sepiapterin, a precursor for tetrahydrobiopterin synthesis, did not augment A-23187 relaxations. However, pretreatment with superoxide dismutase and catalase significantly enhanced A-23187 relaxations in shunt PAs. We conclude that increased pulmonary blood flow induces an impairment of endothelium-dependent relaxation that is selective to PAs. The impaired relaxation may be mediated in part by excess superoxide production.

Site-specific effect of guanosine 3',5'-cyclic monophosphate phosphodiesterase inhibition in isolated lamb lungs.

OBJECTIVE: To determine the effect of combining inhaled nitric oxide (NO) with an inhibitor of guanosine 3',5'-cyclic monophosphate-specific phosphodiesterase on total and segmental lung resistances. STUDY DESIGN: A controlled laboratory study in isolated blood-perfused lungs prepared from lambs. SETTING: Animal research facility affiliated with a university teaching hospital. SUBJECTS: Five newborn lambs at <48 hrs of life. INTERVENTIONS: Isolated blood-perfused lungs were prepared and treated with indomethacin (40 microg/mL) to inhibit prostaglandin synthesis. After a baseline period of normoxia (28% oxygen), pulmonary hypertension was induced with the thromboxane mimetic U46619 (0.1-0.4 microg/kg/min). During pulmonary hypertension, lungs were studied with inhaled NO only, with infusion of zaprinast only (0.25 mg/kg bolus and 0.05 mg/kg/min infusion), and with a combination of the two. For each study condition, the total pressure decrease across the lung was measured, and the inflow-outflow occlusion technique was used to partition the total pressure gradient measured at constant flow (100 mL/kg/min) into gradients across relatively noncompliant large arteries and veins and more compliant small arteries and veins. MEASUREMENTS AND MAIN RESULTS: U46619 infusion produced significant pulmonary vasoconstriction. The combination of inhaled NO and zaprinast decreased the total pressure decrease across the lung significantly more than NO alone. This effect was primarily attributable to a significantly greater decrease in gradient across the small artery segment after inhaled NO and zaprinast compared with NO alone. CONCLUSIONS: Guanosine 3',5'-cyclic monophosphate phosphodiesterase inhibition with zaprinast enhances the effect of inhaled NO, particularly in conditions in which small arteries represent the site of resistance. Phosphodiesterase inhibition may be a promising adjunct to inhaled NO for the treatment of persistent pulmonary hypertension.

Heterogeneous distribution of type I nitric oxide synthase in pulmonary vasculature of ovine fetus.

The nitric oxide/guanosine 3',5'-cyclic monophosphate pathway plays an essential role in mediating pulmonary vasodilation at birth. Small resistance arteries in the fetal lung are vessels of major significance in the regulation of pulmonary vascular tone. The present study is to determine that type I nitric oxide synthase (NOS-I) is present in ovine fetal pulmonary vasculature and that NOS-I is distributed heterogeneously in ovine fetal pulmonary circulation. We used reduced nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d) histochemistry and NOS-I immunohistochemistry to localize NOS-I in fetal sheep lungs and showed a colocalization for NADPH-d activity with NOS-I immunoreactivity. Strong NOS-I immunoreactivity was observed exclusively in the endothelium of the terminal bronchiole and respiratory bronchiole-associated arteries. As a comparison, adult sheep lung did not show positive immunoreactivity in the pulmonary endothelium. NOS-I was absent in the umbilical or systemic arteries from the ovine fetus, whereas abundant NOS-III immunoreactivity was present in these arteries. We conclude that NOS-I is present uniquely in the ovine fetal pulmonary circulation as opposed to the adult pulmonary or the fetal systemic circulation. NOS-I is distributed heterogeneously in the ovine pulmonary vasculature. We speculate that NOS-I plays an active role in the regulation of perinatal pulmonary circulation.

Successful lysis of an obstructive aortic and renal artery thrombus in a neonate on extracorporeal membrane oxygenation.

We describe a neonate on venoarterial extracorporeal membrane oxygenation (ECMO) with acute renal failure associated with extensive aortic and bilateral renal artery thrombosis. Concurrent anticoagulation and continuous systemic thrombolytic therapy with tissue plasminogen activator (t-PA) resulted in complete thrombolysis as evaluated by Doppler flow. The relative risk and benefits of thrombolytic therapy in heparinized patients undergoing ECMO needs to be further studied.

The pathophysiology of pulmonary hypertension in congenital heart disease.

Congenital heart disease with increased pulmonary blood flow commonly leads to the development of pulmonary hypertension and increased vascular reactivity. These serious sequelae are associated with the following two major categories of congenital heart defects: those resulting in increased pulmonary blood flow and increased pulmonary arterial pressure and those resulting in increased pulmonary venous pressure. Recent evidence that the pulmonary vascular endothelium is an important determinant of vascular tone has led to the hypothesis that endothelial injury, secondary to congenital heart disease with increased pulmonary blood flow, disrupts these regulatory mechanisms and thereby plays a role in the development of pulmonary hypertension and its associated increased vascular reactivity. In many animal models, endothelial dysfunction is a precursor for smooth muscle dysfunction, and there is an apparent progression from endothelial dysfunction to smooth muscle dysfunction as vascular changes progress. We established a chronic model of pulmonary hypertension with increased pulmonary blood flow in young lambs by placing a systemic-to-pulmonary shunt in utero. In this model, we found significant physiologic and molecular alternations of both the nitric oxide (NO) and endothelin signaling pathways, two important mechanisms by which the endothelium regulates pulmonary vascular tone. These alterations occur extremely early and precede severe anatomic changes. Early endothelial damage may contribute to the development of pulmonary hypertension and its associated enhanced pulmonary vascular reactivity.

Pulmonary vascular biology during neonatal transition.

Although the normal pulmonary vascular transition at birth takes place quickly in the delivery room, it has its basis in the complex structural and biochemical development of the lung. It is not surprising that this process can be easily disrupted by factors such as prematurity, intrauterine hypoxia, and parenchymal lung disease. The stimuli that initiate and maintain the transition are only beginning to be understood. Understanding the normal structure and function of the neonatal lung provides the foundation that will enable clinicians to enhance resuscitation to accomplish a normal transition in the delivery room.

The cGMP-specific phosphodiesterase inhibitor E4021 dilates the pulmonary circulation.

We investigated the pulmonary vascular effects of E4021, a potent inhibitor of cGMP-specific phosphodiesterase, in control late-gestation fetal lambs, and in newborn lambs with persistent pulmonary hypertension (PPHN) after prenatal ligation of the ductus arteriosus. E4021 alone significantly relaxed fifth-generation pulmonary arteries isolated from control fetal lambs, an effect completely blocked after inhibition of nitric oxide synthase (NOS). In contrast, E4021 did not relax pulmonary arteries isolated from hypertensive lambs. Pretreatment with E4021 (10(-7) M) significantly enhanced relaxations to the NO donor S-nitrosyl-acetyl-penicilamine (SNAP) in arteries from both control and hypertensive lambs. In control, fully instrumented fetal lambs, infusions of E4021 (31 microgram/min) selectively dilated the pulmonary circulation, an effect again blocked after inhibition of NO synthase. Further studies were performed in newborn lambs with PPHN to study the vascular effects of E4021 alone, and in combination with inhaled NO. E4021 alone (1 to 100 microgram/kg/min) decreased pulmonary artery pressure (Ppa) in a dose-dependent fashion, and had minimal effect on systemic pressure. At the highest dose (100 microgram/kg/min), the dilation was selective for the pulmonary circulation. In subsequent protocols, E4021 (10 microgram/kg/min) significantly decreased Ppa and pulmonary vascular resistance (PVR), but these pulmonary vascular effects were not enhanced after NO inhalation at 0.5 or 5 ppm. We speculate that the lack of enhancement was due to the dramatic effects of E4021 alone. Potent, specific phosphodiesterase inhibitors such as E4021 may prove to be useful in the treatment of PPHN.

Alveolar capillary dysplasia: diagnostic potential for cardiac catheterization.

OBJECTIVE: Alveolar capillary dysplasia is a rare cause of persistent pulmonary hypertension of the newborn. Infants with this condition die despite maximal medical intervention including inhaled nitric oxide therapy and extracorporeal membrane oxygenation. To date, diagnosis of this lethal condition was made by open lung biopsy or during postmortem examination. We examined the possibility that distinct cardiac catheterization findings could be used in the diagnosis of this lethal disorder. STUDY DESIGN: We present three infants with fatal persistent pulmonary hypertension of the newborn refractory to extracorporeal membrane oxygenation and inhaled nitric oxide therapy, two with postmortem autopsy confirmation of alveolar capillary dysplasia. Each infant underwent cardiac catheterization to complete the diagnostic evaluations. RESULTS: Significant right ventricular hypertension and normal pulmonary venous return were demonstrated, but a markedly diminished or absent capillary blush phase was noted in each infant. This finding is distinct from the normal capillary blush seen in infants with persistent pulmonary hypertension of the newborn of other etiologies. CONCLUSION: Cardiac catheterization may provide a useful alternative to tissue examination in the diagnosis of alveolar capillary dysplasia.

Pulmonary and systemic effects of the phosphodiesterase inhibitor dipyridamole in newborn lambs with persistent pulmonary hypertension.

Nitric oxide (NO) relaxes vascular smooth muscle by increasing the intracellular concentration of cGMP. In the pulmonary circulation, cGMP is inactivated by specific phosphodiesterases (PDE5). Dipyridamole, a clinically approved drug, has inhibitory activity against PDE5 and has been reported to augment the response to inhaled NO in persistent pulmonary hypertension of the newborn (PPHN). We wished to determine whether dipyridamole alone, or in combination with NO, can be used to treat a newborn lamb model of PPHN. In newborn lambs with PPHN, dipyridamole infused at 0.02 mg/kg/min for 45 min alone, or in combination with 5 ppm of inhaled NO for the final 15 min, significantly decreased pulmonary and systemic blood pressure, decreased pulmonary vascular resistance, and increased pulmonary blood flow. There was no significant difference between the pulmonary vascular effects of 5 ppm NO alone compared with the effects of NO combined with dipyridamole. In control lambs, the 45-min infusion of dipyridamole did not change pulmonary pressure whereas systemic pressure decreased by 28 +/- 3%. These systemic effects in control lambs persisted 90 min after discontinuing the dipyridamole infusion. Systemic arteries isolated from both control and PPHN lambs were significantly more sensitive to dipyridamole than pulmonary arteries. We conclude that dipyridamole has significant hemodynamic effects in both the pulmonary and systemic circulations of newborn lambs with pulmonary hypertension as well as in the systemic circulation of newborn control lambs. The pronounced effects of dipyridamole on the systemic circulation limits its utility as an adjunct to inhaled NO in the treatment of PPHN.