Involvement of thromboxane A2 and prostacyclin in the early pulmonary hypertension after porcine meconium aspiration

Approach to the Connection between Meconium Consistency and Adverse Neonatal Outcomes: A Retrospective Clinical Review and Prospective In Vitro Study

Whether meconium-stained amniotic fluid (MSAF) serves as an indicator of fetal distress is under debate; however, the presence of MSAF concerns both obstetricians and pediatricians because meconium aspiration is a major contributor to neonatal morbidity and mortality, even with appropriate treatment. The present study suggested that thick meconium in infants might be associated with poor outcomes compared with thin meconium based on chart reviews. In addition, cell survival assays following the incubation of various meconium concentrations with monolayers of human epithelial and embryonic lung fibroblast cell lines were consistent with the results obtained from chart reviews. Exposure to meconium resulted in the significant release of nitrite from A549 and HEL299 cells. Medicinal agents, including dexamethasone, L-Nω-nitro-arginine methylester (L-NAME), and NS-398 significantly reduced the meconium-induced release of nitrite. These results support the hypothesis that thick meconium is a risk factor for neonates who require resuscitation, and inflammation appears to serve as the primary mechanism for meconium-associated lung injury. A better understanding of the relationship between nitrite and inflammation could result in the development of promising treatments for meconium aspiration syndrome (MAS).

Thromboxane promotes smooth muscle phenotype commitment but not remodeling of hypoxic neonatal pulmonary artery

Background

Persistent pulmonary hypertension of the newborn (PPHN) is characterized by vasoconstriction and pulmonary vascular remodeling. Remodeling is believed to be a response to physical or chemical stimuli including pro-mitotic inflammatory mediators such as thromboxane. Our objective was to examine the effects of hypoxia and thromboxane signaling ex vivo and in vitro on phenotype commitment, cell cycle entry, and proliferation of PPHN and control neonatal pulmonary artery (PA) myocytes in tissue culture.

Methods

To examine concurrent effects of hypoxia and thromboxane on myocyte growth, serum-fed first-passage newborn porcine PA myocytes were randomized into normoxic (21 % O₂) or hypoxic (10 % O₂) culture for 3 days, with daily addition of thromboxane mimetic U46619 (10⁻⁹ to 10⁻⁵ M) or diluent. Cell survival was detected by MTT assay. To determine the effect of chronic thromboxane exposure (versus whole serum) on activation of arterial remodeling, PPHN was induced in newborn piglets by a 3-day hypoxic exposure (FiO₂ 0.10); controls were 3 day-old normoxic and day 0 piglets. Third-generation PA were segmented and cultured for 3 days in physiologic buffer, Ham’s F-12 media (in the presence or absence of 10 % fetal calf serum), or media with 10⁻⁶ M U46619. DNA synthesis was measured by ³H-thymidine uptake, protein synthesis by ³H-leucine uptake, and proliferation by immunostaining for Ki67. Cell cycle entry was studied by laser scanning cytometry of nuclei in arterial tunica media after propidium iodide staining. Phenotype commitment was determined by immunostaining tunica media for myosin heavy chain and desmin, quantified by laser scanning cytometry.

Results

Contractile and synthetic myocyte subpopulations had differing responses to thromboxane challenge. U46619 decreased proliferation of synthetic and contractile myocytes. PPHN arteries exhibited decreased protein synthesis under all culture conditions. Serum-supplemented PA treated with U46619 had decreased G1/G0 phase myocytes and an increase in S and G2/M. When serum-deprived, PPHN PA incubated with U46619 showed arrested cell cycle entry (increased G0/G1, decreased S and G2/M) and increased abundance of contractile phenotype markers.

Conclusions

We conclude that thromboxane does not initiate phenotypic dedifferentiation and proliferative activation in PPHN PA. Exposure to thromboxane triggers cell cycle exit and myocyte commitment to contractile phenotype.

A "cure" for preeclampsia: Improving neonatal outcomes by overcoming excess fetal placental vascular resistance

From a broad perspective there are only three arterial systems that respond to relative hypoxia with vasoconstriction. They are the placental, the pulmonic and the renal vascular beds. The renal system's adaptation to hypoxia is markedly different from the other two circulatory beds and will not be further considered here. Regional vasoconstriction is adaptive in the placenta and lung because it redirects red blood cells from areas of relative hypoxia to more oxygenated areas thereby maximizing oxygen uptake for a given cardiac output. The fetal placental and pulmonary vascular systems are unique because their smooth muscle cells have a unique and possibly identical potassium channel that responds to hypoxia by closing, thereby depolarizing the cell membrane allowing calcium ion influx and muscle contraction. It may be that a variety of initial causes of temporary or local placental hypoxia initiate a cascade of first fetal placental then maternal vasoconstriction and endothelial activation leading to the clinical syndrome we call preeclampsia. The response cascades seen in preeclampsia, which for purposes of this article I will abbreviate as (PECL), after development of widespread vasoconstriction, will also be seen to be identical or at least parallel in pulmonary hypertension (PAH). This means that some or all of the pharmacotherapies presently used, tested or considered in early PAH may also have a therapeutic effect in PECL by reducing fetal placental arterial resistance thereby increasing fetal placental flow. This would allow increased oxygen and other nutrient uptake and possibly increased fetal cardiac output in the face of reduced fetal cardiac work. This may allow a delay in delivery in which fetuses grow and are better oxygenated in preterm PECL, improving neonatal outcomes.

Right-to-left shunting in the ductus arteriosus is induced readily by intense crying and rapid postural change in neonates with meconium-stained amniotic fluid

OBJECTIVE

To investigate postnatal changes in the direction of blood flow through the ductus arteriosus in neonates with meconium-stained amniotic fluid, we measured preductal and postductal oxygen saturation in normal neonates, neonates with meconium-stained amniotic fluid, and a neonate with persistent pulmonary hypertension of the newborn.

DESIGN

Prospective, observational case series report.

SETTING

A single, tertiary neonatal intensive care unit.

PATIENTS

Twelve normal neonates, seven neonates with meconium-stained amniotic fluid, and a neonate with persistent pulmonary hypertension of the newborn.

INTERVENTIONS

SpO2 is simultaneously monitored in the right upper and lower limbs after birth.

MEASUREMENTS AND MAIN RESULTS

Compared with normal neonates, three neonates with meconium-stained amniotic fluid required longer than +2 SD of the mean time for the postductal SpO2 to reach 90% and/or 95%. In a neonate with meconium-stained amniotic fluid, intense crying triggered frequent decreases to <70% in the postductal SpO2 from 25 mins after birth, while the preductal SpO2 remained at 95% or above. When the other newborn with meconium-stained amniotic fluid was held in the father's arms after 98 mins, the postductal SpO2 decreased rapidly to <80%, while the preductal SpO2 remained at 95%. Thus, 5% or greater difference between the preductal and postductal SpO2 was observed from 25 mins after birth until 120 mins in all neonates with meconium-stained amniotic fluid, whereas the difference disappeared after 25 mins in 12 normal neonates. In a neonate with persistent pulmonary hypertension of the newborn who required vigorous resuscitation, 5% or greater difference between the preductal and postductal SpO2 levels was observed until 6 hrs after birth.

CONCLUSIONS

Right-to-left shunting in the ductus arteriosus may be induced readily by intense crying and rapid postural change in infants with meconium-stained amniotic fluid. It is important to monitor SpO2 at both pre- and postductal regions until 120 mins after birth in neonates with meconium-stained amniotic fluid and to subject these infants to minimal manipulations.

Sildenafil prevents the increase of extravascular lung water and pulmonary hypertension after meconium aspiration in newborn piglets

Meconium aspiration syndrome causes respiratory failure after birth and in vivo monitoring of pulmonary edema is difficult. The objective of the present study was to assess hemodynamic changes and edema measured by transcardiopulmonary thermodilution in low weight newborn piglets. Additionally, the effect of early administration of sildenafil (2 mg/kg vo, 30 min after meconium aspiration) on this critical parameter was determined in the meconium aspiration syndrome model. Thirty-eight mechanically ventilated anesthetized male piglets (Sus scrofa domestica) aged 12 to 72 h (1660 ± 192 g) received diluted fresh human meconium in the airway in order to evoke pulmonary hypertension (PHT). Extravascular lung water was measured in vivo with a PiCCO monitor and ex vivo by the gravimetric method, resulting in an overestimate of 3.5 ± 2.3 mL compared to the first measurement. A significant PHT of 15 Torr above basal pressure was observed, similar to that of severely affected humans, leading to an increase in ventilatory support. The vascular permeability index increased 57%, suggesting altered alveolocapillary membrane permeability. Histology revealed tissue vessel congestion and nonspecific chemical pneumonitis. A group of animals received sildenafil, which prevented the development of PHT and lung edema, as evaluated by in vivo monitoring. In summary, the transcardiopulmonary thermodilution method is a reliable tool for monitoring critical newborn changes, offering the opportunity to experimentally explore putative therapeutics in vivo. Sildenafil could be employed to prevent PHT and edema if used in the first stages of development of the disease.

Meconium aspiration delays normal decline of pulmonary vascular resistance shortly after birth through lung parenchymal injury

BACKGROUND

Persistent pulmonary hypertension of the newborn is often associated with meconium aspiration syndrome (MAS) or perinatal asphyxia.

OBJECTIVE

To determine the effect of meconium or asphyxia on pulmonary arterial pressure and circulating levels of vasoactive substances, we conducted a prospective study of 54 term infants, including infants with meconium-stained amniotic fluid with normal (MSAF) or abnormal (MAS) chest X-ray findings, infants with perinatal asphyxia, and controls. The purpose of this study was to determine the group most likely to have elevated pulmonary arterial pressure and a disturbed balance between vasoactive substances.

METHODS

To estimate the pulmonary arterial pressure by echocardiography, we used the ratio of the right to left systolic ventricular pressure (RVP/LVP ratio). We measured the plasma concentrations of endothelin-1 (ET-1), cyclic guanosine monophosphate (cGMP) as an indicator of nitric oxide (NO) production, and 6-keto-prostaglandin F(1)α (6-keto-PGF(1)α) for the estimation of prostacyclin concentration. We also measured KL-6 as a marker of lung injury.

RESULTS

The RVP/LVP ratio was significantly higher in the MAS group than the other groups on day 0. Although ET-1 and 6-keto-PGF(1)α levels were comparable among all groups, the cGMP level on days 3-5 and the KL-6 level throughout the first postnatal week were significantly higher in the MAS group.

CONCLUSIONS

It is possible that meconium aspiration delays normal decline of pulmonary vascular resistance shortly after birth through lung parenchymal injury. The subsequent increase of cGMP in MAS may be an adaptive response to prevent further elevation of pulmonary arterial pressure by inducing NO.

Glucocorticoids in the treatment of neonatal meconium aspiration syndrome

Meconium aspiration syndrome is a serious neonatal disease with complex pathophysiology. With respect to the contribution of meconium-induced lung edema, inflammation and vasoconstriction on the course of the disease, glucocorticoids are increasingly used in the treatment of MAS despite the fact that principal questions on the choice of GCs derivative, mode of delivery and dosing have not been answered yet. To bring a complex insight into the topic, this article reviews the pathomechanisms of MAS, mechanisms of action of GCs, as well as the advantages and disadvantages of GCs administration in experimental models and newborns with MAS.

Studies of meconium-induced lung injury: inflammatory cytokine expression and apoptosis

To review current literature related to cellular mechanisms of meconium-induced lung injury (MILI). Review of published experimental in vitro and in vivo MAS studies using human and animal lung cells. We found that meconium induces expression of cytokines and angiotensin II (ANG II)-induced apoptotic process in the lung cells. We postulate that inflammatory cytokines induce ANG II expression, which causes apoptotic cell death after binding to its AT1 receptors. We also demonstrated expression of serpins associated with meconium instillation into the lungs. Serpins are proteins that inhibit cellular proteases and elastases. Expression of serpins may be an attempt to recover lung from these injurious effects. In summary our studies show that whereas meconium induces inflammatory cytokines and subsequent cell apoptosis, the lung cells also try to protect themselves by inducing serpins. The balance of these interactions will determine the residual damage. We believe these new findings are very important in understanding of MILI.

Inflammatory Response and Apoptosis in Newborn Lungs after Meconium Aspiration

An important feature of meconium-instilled newborn lungs is an inflammatory response and apoptotic cell death. It was recently demonstrated by our group and supported by several other investigators in a relatively short period of time. Apoptosis exists also in healthy lungs, but in meconium-instilled lungs its level is usually dramatically higher. Apoptosis is characterized by loss of cell function, decrease in cell size, and its morphology. Apoptosis plays an important role in normal cell life, but increased levels of apoptosis induce great damage for any tissues. Apoptosis in the lungs has been greatly overlooked for the past decade, and meconium-induced apoptosis is a relatively new event and not effectively studied at the present time. This Review summarized current knowledge regarding meconium-induced inflammation and apoptosis in newborn lungs.

PGE2/TXB2 imbalance in neonatal hypoxemic respiratory failure

BACKGROUND

An imbalance of vaso-constrictor and -dilator mediators has been implicated in the pathogenesis of the pulmonary hypertension accompanying neonatal hypoxemic respiratory failure (NHRF).

AIM

To characterize plasma PGE2, TXB2 and their ratio in normal newborns and in those with NHRF.

METHODS

Twenty newborns with NHRF received inhaled PGE1 (IPGE1) by jet nebulizer in doses of 25, 50, 150 and 300 ng/kg/min followed by weaning. Blood for PGE2 and TXB2 assay using EIA was available in 8 neonates with NHRF prior to IPGE1. Umbilical cord arterial samples were also obtained at delivery from 10 normal newborns to serve as controls.

RESULTS

Compared to normal newborns, those with NHRF had significantly lower PGE2/TXB2 ratios after controlling for preterm gestation (< 37 weeks) and postnatal age (p < 0.05). Notably, all subjects except one in the NHRF group had a value of < 1.0 (range 0.1-1.2) compared to a value of > 1.0 in all subjects in the Control group (range 1.1-5.2).

CONCLUSIONS

Lower PGE2/TXB2 ratio in subjects with NHRF compared with controls reflects a predominance of vaso-constrictor activity in these patients as the basis of pulmonary hypertension. Plasma PGE2/TXB2 ratio may have important implications for the diagnosis and treatment of NHRF.

Thromboxane hypersensitivity in hypoxic pulmonary artery myocytes: altered TP receptor localization and kinetics

Hypoxia-induced neonatal persistent pulmonary hypertension (PPHN) is characterized by sustained vasospasm and increased thromboxane (TxA2)-to-prostacyclin ratio. We previously demonstrated that moderate hypoxia induces myocyte TxA2 hypersensitivity. Here, we examined TxA2 prostanoid receptor (TP-R) localization and kinetics following hypoxia to determine the mechanism of hypoxia-induced TxA2 hypersensitivity. Primary cultured neonatal pulmonary artery myocytes were exposed to 10% O2 (hypoxic myocytes; HM) or 21% O2 (normoxic myocytes; NM) for 3 days. PPHN was induced in neonatal piglets by in vivo exposure to 10% FiO2 for 3 days. TP-R was studied in whole lung sections from pigs with hypoxic PPHN- and age-matched controls; intracellular localization was studied by immunocytochemistry. TP-R affinity was studied in cultured myocytes by saturation binding kinetics using 3H-SQ-29548 and competitive binding kinetics by coincubation with U-46619. Phosphorylation and coupling were examined in immunoprecipitated TP-R. We report distal propagation of TP-R expression in PPHN, extending to pulmonary arteries <50 microm. In HM, intracellular TP-R moves towards the perinuclear region, mirroring a change in endoplasmic reticulum (ER) morphology. TP-R kinetics also alter in HM membranes, with decreased Kd and Bmax (maximal binding sites). Additionally, in hypoxia, 3H-SQ-29548 is displaced at lower concentration of U-46619 than in normoxia, suggesting increased agonist affinity. Phosphorylation of serine residues on HM TP-R was significantly decreased compared with NM; this difference correlated with increased Galphaq coupling in hypoxia and was ablated by incubation with PKA. We conclude that the TP-R is normally desensitized in the neonatal pulmonary circuit by PKA-mediated regulatory phosphorylation, decreasing ligand affinity and coupling to Galphaq; this protection is lost following hypoxic exposure. Also, the appearance of TP-R in resistance arteries after development of hypoxic PPHN may contribute to increased pulmonary arterial pressure.

Pancreatic phospholipase A2 contributes to lung injury in experimental meconium aspiration

To investigate the role of pancreatic (group I) secretory PLA2 (sPLA2-I) in the pathogenesis of meconium aspiration syndrome, human particulate meconium or its supernatant either before or after extraction of PLA2-I was insufflated into rat lungs. In addition, the pulmonary effects of intra-tracheal human and bovine PLA2-I were studied. Lungs with saline instillation served as controls. Intrapulmonary particulate meconium (both before and after PLA2-I extraction), unlike meconium supernatant, resulted in markedly elevated lung tissue PLA2 catalytic activity and human PLA2-I concentrations when compared with controls. On the other hand, tissue concentrations of the group II PLA2 remained unchanged in all meconium lungs. Pulmonary PLA2-I concentrations further correlated positively with lung injury scores. Instillation of meconium-derived human PLA2-I, at a concentration of one-third of that in particulate meconium, did not raise PLA2 activity or concentrations of PLA2-I or PLA2-II in the lung tissue from the control level, but still resulted in significantly elevated lung wet/dry ratio and injury score. In contrast, insufflation of bovine pancreatic PLA2 increased the lung tissue enzyme activity and wet/dry ratio from the control level, but had no effect on the type II PLA2 concentration or lung injury score. Our data thus indicate that human pancreatic PLA2, introduced in high amounts within aspirated meconium especially in particulate form, is a potent inducer of lung tissue inflammatory injury.

Dexamethasone alleviates meconium-induced airway hyperresponsiveness and lung inflammation in rabbits

The effects of dexamethasone on in vitro airway reactivity associated with lung inflammation were investigated in rabbits with meconium aspiration. Oxygen-ventilated adult rabbits received an intratracheal bolus of 4 ml/kg body weight of saline (Sal, n = 4) or human meconium (25 mg/ml). Thirty minutes later, meconium-instilled animals intravenously received 0.5 mg/kg of dexamethasone (Dexa, n = 6), or were left without treatment (Meco, n = 5). The animals were ventilated for a further 5 hr and then sacrificed. The left lungs were lavaged with saline, and the white blood cell (WBC) count was estimated. Tracheal and right-lung tissue strips were placed into organ chambers with Krebs-Henseleit solution. Cumulative doses of histamine (10(-8)-10(-3) mol/l) and acetylcholine (10(-8)-10(-3) mol/l) were added to the chambers, and recordings of contractions were made after a 30-min loading phase with a tension of 4 grams, and another 30-min adaptation phase with a tension of 2 g. Tracheal smooth muscle in vitro reactivity to histamine was higher in the Meco than in the Sal group, and dexamethasone decreased the reactivity compared to the Meco group (P < 0.05). Lung tissue in vitro reactivity to histamine was slightly higher in the Meco than in the Sal group (P > 0.05), and dexamethasone decreased the reactivity compared to both the Meco and Sal groups (P < 0.05). No between-group differences were observed in tracheal or lung in vitro reactivity to acetylcholine (P > 0.05). In the Meco group, blood WBC (P > 0.05) and neutrophil (P < 0.05) counts were lower than in the Sal and Dexa groups. Lung neutrophils and eosinophils were higher in both the Meco and Dexa groups than in the Sal group (P < 0.01). Dexamethasone decreased neutrophils (P < 0.05) compared to the Meco group. Meconium-induced airway hyperreactivity to histamine and lung inflammation were alleviated by dexamethasone.

Persistent pulmonary hypertension of the newborn: not a honeymoon anymore

Persistent pulmonary hypertension of the newborn is a disorder of transition to extrauterine life, in which the newly born baby cannot decrease the high pulmonary vascular resistance and low pulmonary blood flow, characteristic of the fetus, to that of a low pulmonary vascular resistance and high pulmonary blood flow necessary for postnatal survival. The syndrome primarily affects the neonate 34 weeks postmenstrual age and greater. The article will summarize the latest understanding of the pathophysiology and review innovations in management strategies that have greatly decreased mortality and morbidity since the advent of neonatal intensive care units.

Hypoxia induces hypersensitivity and hyperreactivity to thromboxane receptor agonist in neonatal pulmonary arterial myocytes

PPHN, caused by perinatal hypoxia or inflammation, is characterized by an increased thromboxane-prostacyclin ratio and pulmonary vasoconstriction. We examined effects of hypoxia on myocyte thromboxane responsiveness. Myocytes from 3rd-6th generation pulmonary arteries of newborn piglets were grown to confluence and synchronized in contractile phenotype by serum deprivation. On the final 3 days of culture, myocytes were exposed to 10% O2 for 3 days; control myocytes from normoxic piglets were cultured in 21% O2. PPHN was induced in newborn piglets by 3-day hypoxic exposure (Fi(O2) 0.10); pulmonary arterial myocytes from these animals were maintained in normoxia. Ca2+ mobilization to thromboxane mimetic U-46619 and ATP was quantified using fura-2 AM. Three-day hypoxic exposure in vitro results in increased basal [Ca2+]i, faster and heightened peak Ca2+ response, and decreased U-46619 EC50. These functional changes persist in myocytes exposed to hypoxia in vivo but cultured in 21% O2. Blockade of Ca2+ entry and store refilling do not alter peak U-46619 Ca2+ responses in hypoxic or normoxic myocytes. Blockade of ryanodine-sensitive or IP3-gated intracellular Ca2+ channels inhibits hypoxic augmentation of peak U-46619 response. Ca2+ response to ryanodine alone is undetectable; ATP-induced Ca2+ mobilization is unaltered by hypoxia, suggesting no independent increase in ryanodine-sensitive or IP3-linked intracellular Ca2+ pool mobilization. We conclude hypoxia has a priming effect on neonatal pulmonary arterial myocytes, resulting in increased resting Ca2+, thromboxane hypersensitivity, and hyperreactivity. We postulate that hypoxia increases agonist-induced TP-R-linked IP3 pathway activation. Myocyte thromboxane hyperresponsiveness persists in culture after removal from the initiating hypoxic stimulus, suggesting altered gene expression.

Meconium aspiration induces neuronal injury in piglets

AIM

Meconium aspiration-induced hypertensive lung injury, especially when connected with perinatal asphyxia, has been associated with brain damage. We aimed to determine the neuronal injury induced by pulmonary meconium contamination alone and with concurrent asphyxia.

METHODS

36 anaesthetized and ventilated newborn piglets were haemodynamically monitored for 6 h. Seven piglets without concurrent asphyxia and seven piglets with asphyxia were instilled with a bolus of human meconium intratracheally. Seven piglets had only asphyxia and 15 piglets served as controls. The brains were studied histologically.

RESULTS

Meconium aspiration did not change systemic haemodynamics acutely, while its combination with asphyxia diminished the abrupt postasphyxic systemic hypertensive peak and resulted in a transient increase in carotid artery flow, not seen after isolated asphyxia. Systemic pressure declined after 4 h in all insulted groups, but only isolated asphyxia was associated with a sustained decrease in carotid artery flow. Arterial oxygenation remained normal, except during the acute insults. Brain examination after meconium instillation indicated neuronal injury, especially in the CA3 region of the hippocampus. Asphyxia resulted in neuronal injury in the cortical, cerebellar and hippocampal hilus regions.

CONCLUSION

Severe meconium aspiration itself may result in hippocampal neuronal injury.

Meconium inhibits phagocytosis and stimulates respiratory burst in alveolar macrophages

The meconium aspiration syndrome is an important cause of respiratory distress in newborn infants. Alveolar macrophages (AMs) provide a first line of defense in the lower respiratory tract against inhaled pathogens and particles such as meconium. In this study, we examined the effect of meconium on two primary macrophage functions: phagocytosis and respiratory burst. Short-term exposure of rat NR8383 AMs to sterile meconium from human or equine neonates (1.2-24 mg/mL) produced a dose-dependent decrease in phagocytosis of fluorescent latex beads. This effect was not due to decreased cell viability or to an elevation of intracellular cAMP. The effect of short-term exposure to meconium on the respiratory burst response in AMs was quantified using flow cytometry to measure oxidation of dichlorofluorescin diacetate. A robust respiratory burst was triggered when AMs were exposed to 12 or 24 mg/mL meconium. This effect was attenuated but not eliminated by filtration of the meconium. However, subsequent to meconium exposure, AMs had a reduced respiratory burst in response to stimulation with phorbol myristate acetate. In addition, AMs that were exposed to meconium for an extended period (24 h) showed DNA fragmentation indicative of apoptosis. Meconium therefore may interfere with AM function by inducing oxidative stress and apoptosis. Tissue injury from release of reactive oxygen species by AMs may be important in the pathophysiology of the meconium aspiration syndrome.

Pathophysiologic mechanisms of persistent pulmonary hypertension of the newborn

Persistent pulmonary hypertension of the newborn (PPHN), among the most rapidly progressive and potentially fatal of vasculopathies, is a disorder of vascular transition from fetal to neonatal circulation, manifesting as hypoxemic respiratory failure. PPHN represents a common pathway of vascular injury activated by numerous perinatal stresses: hypoxia, hypoglycemia, cold stress, sepsis, and direct lung injury. As with other multifactorial diseases, a single inciting event may be augmented by multiple concurrent/subsequent phenomena that result in differing courses of disease progression. I review the various mechanisms of vascular injury involved in neonatal pulmonary hypertension: endothelial dysfunction, inflammation, hypoxia, and mechanical strain, in the context of downstream effects on pulmonary vascular endothelial-myocyte interactions and myocyte phenotypic plasticity.

Meconium aspiration induces oxidative injury in the hippocampus of newborn piglets

BACKGROUND

Meconium aspiration-induced hypertensive lung injury has been associated with neuronal damage in the newborn, but the mechanisms of the injury are poorly known.

AIMS

The aim of the study was to determine the contribution of oxidative stress to the brain damage after pulmonary meconium contamination.

STUDY DESIGN

Sixteen anesthetized and ventilated newborn piglets were studied for 6 h. Eight piglets were instilled with a bolus of human meconium intratracheally and eight piglets with saline instillation served as controls. Brain tissue lipid peroxidation products (TBARS), reduced glutathione (GSH), myeloperoxidase activity and oxidized DNA were analyzed as indicators of oxidative stress.

RESULTS

Meconium aspiration did not change the systemic or carotid hemodynamics, but caused a well-established pulmonary hypertensive response. Sustained increase in additional oxygen demand was also observed after meconium insult, but no actual hypoxemia or hypercarbia was evident during the whole study period. Myeloperoxidase activity was elevated in the cerebellum after pulmonary meconium instillation, whereas concentrations of peroxidation products and glutathione were similar in the cortical, cerebellar and hippocampal regions of the two groups. Still, the amount of oxidized DNA was increased in the hippocampus of the meconium-aspirated piglets when compared to controls.

CONCLUSIONS

Our data thus suggest that oxidative injury associated with pulmonary, but not systemic, hemodynamic disturbances may contribute to hippocampal damage after meconium aspiration in newborns.

Meconium passage in utero: mechanisms, consequences, and management

Meconium passage in newborn infants is a developmentally programmed event normally occurring within the first 24 to 48 hours after birth. Intrauterine meconium passage in near-term or term fetuses has been associated with fetomaternal stress factors and/or infection, whereas meconium passage in postterm pregnancies has been attributed to gastrointestinal maturation. Despite these clinical impressions, little information is available on the mechanism(s) underlying the normal meconium passage that occurs immediately after birth or during the intrauterine period of fetal development. Birth itself is a stressful process and it is possible that fetal stress-mediated biochemical events may regulate the meconium passage occurring either during labor or after birth. Aspiration of meconium during intrauterine life may result in or contribute to meconium aspiration syndrome (MAS), representing a continued leading cause of perinatal death. This article reviews aspects of meconium passage in utero, its consequences, and management.

Complement C5a is a key mediator of meconium-induced neutrophil activation

Meconium aspiration syndrome is a serious condition of the newborn characterized by pulmonary inflammation with substantial neutrophil infiltration. We recently showed that meconium is a potent activator of complement. The aim of the present study was to investigate a possible role for complement in meconium-induced neutrophil activation. Meconium was incubated in human whole blood anticoagulated with lepirudin, a specific thrombin inhibitor that does not affect complement activation. Complement activation was detected by measuring the terminal complement complex. Neutrophil oxidative burst and changes in CD11b and L-selectin expression were measured by flow cytometry. Complement was inhibited using the MAb 166-32 and 137-26, which block factor D and neutralize C5a, respectively. Meconium markedly activated the neutrophils, as revealed by up-regulation of CD11b, accentuation of L-selectin shedding, and induction of oxidative burst. Complement inhibition using the anti-factor D antibody completely (95-100%) blocked meconium-induced changes in CD11b and L-selectin expression, whereas oxidative burst was reduced by 60-70%. The anti-C5a antibody inhibited the neutrophil activation to the same extent as anti-factor D. The data suggest that complement activation is largely responsible for the neutrophil inflammatory responses induced by meconium in vitro and that C5a is a key mediator of this response.

The diseases treated with ECMO: focus on PPHN

Neonatal extracorporeal support is most often required for neonatal hypoxemic respiratory failure, usually accompanied by persistent pulmonary hypertension of the newborn (PPHN). PPHN is a clinical syndrome that results from the failure of pulmonary vascular transition to extrauterine life. Infants typically present shortly after birth with respiratory distress and cyanosis, but a structurally normal heart. The incidence of PPHN is estimated at 0.2% of live-born term infants. Respiratory failure and hypoxemia in the term newborn result from a heterogeneous group of disorders, and the therapeutic approach and response often depend on the underlying disease. PPHN can largely be thought of as one of three types: (1) the abnormally constricted pulmonary vasculature which is the most common type and includes diagnoses such as meconium aspiration syndrome, respiratory distress syndrome, and sepsis; (2) the structurally abnormal vasculature, which is often termed idiopathic PPHN; or (3) the hypoplastic vasculature such as is seen in congenital diaphragmatic hernia, or alveolar capillary dysplasia, a rare malformation of lung development. The pathophysiology of each type is dependent on the point in gestation when the normal transition to extrauterine life fails. This article will discuss the known pathophysiology in PPHN and new treatment modalities.

Intravenous immunoglobulin g attenuates pulmonary hypertension but induces local neutrophil influx in meconium aspiration in piglets

BACKGROUND

Pulmonary hypertension and inflammation are well-identified pathogenetic features in meconium aspiration syndrome of newborns, but current approaches to their treatment or prevention are still often unsatisfactory.

OBJECTIVES

To investigate the possible protective effects of human intravenous immunoglobulin G (IVIG) on the hypertensive and inflammatory lung injury in severe neonatal meconium aspiration.

METHODS

Eleven newborn (10-12 days old) ventilated and catheterized piglets that received an intratracheal bolus (3 ml/kg) of a 65-mg/ml mixture of human meconium were studied for 6 h. IVIG was infused in 5 piglets 30 min before meconium administration, and 6 piglets served as controls and received the vehicle only.

RESULTS

Meconium instillation induced a biphasic pulmonary hypertensive response, which was significantly diminished by IVIG pretreatment. Similarly, IVIG improved the oxygenation of the piglets, but the intrapulmonary shunt fraction or systemic hemodynamic parameters did not differ between the study groups, except of a minor decrease in the mean arterial blood pressure caused by IVIG. The blood leukocyte count was comparable in the 2 groups. The lung tissue ultrastructural and histological changes, number of apoptotic cells and phospholipase A2 activity were similar in the 2 groups. The amount of neutrophil accumulation, assessed by myeloperoxidase activity, was however significantly increased in macroscopically damaged lung tissue after IVIG administration.

CONCLUSIONS

Our results thus indicate that IVIG treatment of newborns with severe meconium aspiration significantly diminishes the pulmonary hypertensive response and improves oxygenation, but the effects do not extend to protection of lung cellular injury.

Meconium induces only localized inflammatory lung injury in piglets

Neonatal meconium aspiration often produces severe respiratory distress due to an inflammatory pulmonary injury, but the extension of this damaging reaction to the noncontaminated lung regions is still uncertain. To investigate the presence of generalized pulmonary inflammatory response, 31 anesthetized and ventilated neonatal piglets (1-3 d) were studied. Meconium (n = 16) or saline (n = 15) was instilled unilaterally into the right lung, and analysis of the lung tissue or bronchoalveolar lavage (BAL) fluid from both lungs was performed after 12 h. Meconium increased the wet/dry weight ratio, histologic tissue injury score and tissue myeloperoxidase activity as well as BAL fluid total cell count in the contaminated lung. Tumor necrosis factor-alfa concentrations in BAL fluid did not however differ significantly. Furthermore, in the meconium-instilled lungs the tissue and lavage fluid catalytic activity of phospholipase A2 (PLA2) and tissue PLA2 group-I and group-II concentrations were significantly elevated. Although BAL fluid catalytic activity of PLA2 was moderately increased also in the meconium noninstilled lung, significant inflammatory injury in this lung was absent. The results thus indicate that meconium aspiration induces severe local inflammation and lung injury, but significant generalized pulmonary inflammatory damage in the pathogenesis of meconium aspiration syndrome is unlikely.

Meconium aspiration stimulates cyclooxygenase-2 and nitric oxide synthase-2 expression in rat lungs

To study the impact of meconium aspiration on the biosynthesis of prostaglandins and nitric oxide, we investigated the effects of intratracheal meconium instillation on the expression of cyclooxygenase-1 (COX-1) and -2 (COX-2) and endothelial (NOS-3) and inducible (NOS-2) nitric oxide synthase in rat lungs. Anesthetized, tracheotomized, and ventilated rats received 3 mL/kg human meconium suspension intratracheally (n = 19), and 14 control rats received an equal volume of saline. Ten rats were pretreated with indomethacin, and 13 rats were pretreated with dexamethasone. The lungs were ventilated with 70% oxygen for 3 h after the insult, and the level of COX-1, COX-2, NOS-2, and NOS-3 mRNA in lung tissue was analyzed by Northern blot hybridization. Furthermore, the expression and localization of the enzyme proteins was analyzed by immunohistochemistry. COX-1 and NOS-3 were clearly expressed in the lungs of control rats, whereas the level of COX-2 and NOS-2 expression was minimal. Meconium administration did not affect the expression of COX-1, but COX-2 expression was up-regulated in the respiratory epithelium and alveolar macrophages. Meconium also induced up-regulation of NOS-2 in the pulmonary epithelium, vascular endothelium, and macrophages. Indomethacin pretreatment did not affect the enzyme expressions, whereas dexamethasone administration significantly inhibited the meconium-induced COX-2 and NOS-2 up-regulation. Our data thus indicate that intrapulmonary meconium up-regulates lung COX-2 and NOS-2 gene expression, suggesting an important role for prostaglandins and nitric oxide in the meconium aspiration-induced pulmonary inflammation and hemodynamic changes.

Enhanced release of thromboxane A(2) after exposure of human airway epithelial cells to meconium

Meconium aspiration syndrome (MAS) is a cause of significant morbidity and mortality in the perinatal period. Despite the clinical relevance of MAS, its pathogenesis is poorly understood. Epithelial cell-derived prostanoids are involved in the regulation of several cellular functions within the lung, including the control of tone and reactivity of airway and vascular smooth muscle. In this study, we evaluated whether exposure to meconium affects the metabolic function of human airway epithelial cells. Monolayers of A549 cells, a transformed human epithelial cell line, were incubated with various concentrations of meconium. Control cells were incubated with serum-free medium in a similar manner. The supernatant fluid was removed at various time points and assayed for thromboxane A(2) (TXA(2)) production. The latter was accomplished by measuring its immediate and stable metabolite thromboxane B(2), using an enzyme-linked immunosorbent assay (ELISA). In selected experiments, the modulatory effects of indomethacin (10(-6) M), dexamethasone (10(-6) M), and L-nitroarginine methyl ester (L-NAME, 10(-6) M) on TXA(2) production were evaluated. Results were expressed in terms of pg/mg protein (mean +/- SE). We found that exposure to meconium produced a significant release of TXA(2) from A549 cells. Indomethacin, dexamethasone, and in part, L-NAME inhibited meconium-induced release of TXA(2). Our findings demonstrate that meconium enhances the production of thromboxanes from A549 cells, suggesting that airway epithelial cells and their metabolic products may play an important role in the pathogenesis of MAS.

Advances in the treatment of the meconium aspiration syndrome

Meconium aspiration syndrome (MAS) is a common cause of respiratory distress in neonates. In many affected children, the complex nature of meconium aspiration syndrome contributes to the apparent lack of response to standard therapies. Over the past decade, several new therapies have been suggested to be more effective than "conventional" management in treating meconium aspiration syndrome. These include: anti-inflammatory drugs, medications to counter the effect of prostaglandin-related compounds, high-frequency ventilation, exogenous surfactant, inhaled nitric oxide and liquid ventilation. There are, however, scant evidence-based data to justify routine use of any of those therapies. Additional prospective, well-controlled, randomized trials of diverse therapies are needed to elucidate the optimum management of MAS.

Combination therapy with inhaled nitric oxide and intravenous dobutamine during pulmonary hypertension in the rabbit

Combination therapy with an intravenous inovasodilator and inhaled nitric oxide (NO) may be appropriate in patients with pulmonary hypertension and associated right ventricular failure. We examined whether dobutamine and inhaled NO would have additive pulmonary vasodilator effects in experimental pulmonary hypertension. Pulmonary hypertension was produced in anesthetized, mechanically ventilated rabbits by infusion of U46619, a thromboxane analogue. Dobutamine was administered in increasing doses (2.5-20 microg/kg/min) with and without inhaled NO (40 ppm). Dobutamine produced dose-dependent decreases in pulmonary vascular resistance (PVR) and mean arterial pressure (MAP) and increases in cardiac output (CO). Inhaled NO alone decreased pulmonary artery pressure (PAP) and PVR with no effect on MAP or CO. The effects of dobutamine and inhaled NO were additive, so that at each dose of dobutamine, inhaled NO decreased PAP and PVR with no effect on systemic hemodynamics. This study suggests that the combination of dobutamine and inhaled NO should produce additive pulmonary vasodilation in patients with pulmonary hypertension and associated right ventricular dysfunction.