Oxygen metabolism and oxygenation of the newborn

The premature infant is to some extent protected from hypoxia, however defense against hyperoxia is poorly developed. The optimal assessment of oxygenation is to measure oxygen delivery and extraction. At the bedside PaO₂ and SpO₂ are approximations of oxygenation at the tissue level. After birth asphyxia it is crucial to know whether or not to give oxygen supplementation, when, how much, and for how long. Oxygen saturation targets in the delivery room have been studied, but the optimal targets might still be unknown because factors like gender and delayed cord clamping influence saturation levels. However, SpO₂ > 80% at 5 min of age is associated with favorable long term outcome in preterm babies. Immature infants most often need oxygen supplementation beyond the delivery room. Predefined saturation levels, and narrow alarm limits together with the total oxygen exposure may impact on development of oxygen related diseases like ROP and BPD. Hyperoxia is a strong trigger for genetic and epigenetic changes, contributing to the development of these conditions and perhaps lifelong changes.

Attenuation of lipopolysaccharide-induced oxidative stress and apoptosis in fetal pulmonary artery endothelial cells by hypoxia

Pulmonary vascular endothelial injury resulting from lipopolysaccharide (LPS) and oxygen toxicity contributes to vascular simplification seen in the lungs of premature infants with bronchopulmonary dysplasia. Whether the severity of endotoxin-induced endothelial injury is modulated by ambient oxygen tension (hypoxic intrauterine environment vs. hyperoxic postnatal environment) remains unknown. We posited that ovine fetal pulmonary artery endothelial cells (FPAEC) will be more resistant to LPS toxicity under hypoxic conditions (20-25 Torr) mimicking the fetal milieu. LPS (10 microg/ml) inhibited FPAEC proliferation and induced apoptosis under normoxic conditions (21% O(2)) in vitro. LPS-induced FPAEC apoptosis was attenuated in hypoxia (5% O(2)) and exacerbated by hyperoxia (55% O(2)). LPS increased intracellular superoxide formation, as measured by 2-hydroxyethidium (2-HE) formation, in FPAEC in normoxia and hypoxia. 2-HE formation in LPS-treated FPAEC increased in parallel with the severity of LPS-induced apoptosis in FPAEC, increasing from hypoxia to normoxia to hyperoxia. Differences in LPS-induced apoptosis between hypoxia and normoxia were abolished when LPS-treated FPAEC incubated in hypoxia were pretreated with menadione to increase superoxide production. Apocynin decreased 2-HE formation, and attenuated LPS-induced FPAEC apoptosis under normoxic conditions. We conclude that ambient oxygen concentration modulates the severity of LPS-mediated injury in FPAEC by regulating superoxide levels produced in response to LPS.

Sensitivity and Specificity of Prenatal Features of Physiological Shunts to Predict Neonatal Clinical Status in Transposition of the Great Arteries

BACKGROUND

Although prenatal diagnosis of transposition of the great arteries (TGA) reduces neonatal mortality, the preoperative course can be complicated in infants with a restrictive foramen ovale (FO) or a ductus arteriosus (DA) constriction. We sought to determine the specificity and sensitivity of prenatal features of physiological shunts in predicting postnatal clinical status in prenatally diagnosed TGA in babies delivered in a tertiary care center providing all facilities for neonatal urgent care.

METHODS AND RESULTS

The outcomes of 130 fetuses with TGA were reviewed over a period of 5.5 years. Restriction of the FO and/or constriction of the DA could be analyzed in 119/130 fetuses at 36+/-2.7 weeks of gestation. Twenty-four out of 119 had at least 1 abnormal shunt (23 FO, 5 DA, and 4 both). Thirteen of 130 neonates had profound hypoxemia (PaO2<25 mm Hg) and metabolic acidosis (pH <7.15) in the first 30 minutes and required immediate balloon atrioseptostomy. Two who had abnormal FO and DA died despite aggressive resuscitation. The specificity and sensitivity of the fetal echo in predicting neonatal emergency were 84% and 54%, respectively. The specificity and sensitivity of a combination of restrictive FO and DA constriction were 100% and 31%, respectively.

CONCLUSIONS

Restriction of the FO and/or of the DA has a high specificity to predict the need for emergency neonatal care in fetuses with TGA, but the sensitivity is too low to detect all high-risk fetuses. Exceptional procedures should be considered for fetuses that have a combination of restrictive FO and DA constriction.

New concepts in abnormalities of respiratory control in children

PURPOSE OF REVIEW

Respiratory control disorders such as apnea of prematurity, apparent life-threatening events, sudden infant death syndrome, and central hypoventilation are relatively frequent conditions in the pediatric age range and are associated with substantial morbidity and mortality. The explosion of technological breakthroughs in biology and medicine has facilitated our understanding of the fundamental mechanisms that govern the development of brain regions underlying respiratory control functions.

RECENT FINDINGS

Recent critically important discoveries encompass the identification of neurons that constitute the central respiratory rhythm generator in the brainstem, the conceptual framework allowing for many neurons located in multiple strategic regions within the brain to coordinate central chemosensitivity, the discovery of long-term and short-term plasticity in hypoxic ventilatory regulation, and the recent uncovering of specific gene mutations in children affected with congenital central hypoventilation syndrome.

SUMMARY

While the developmental aspects of control breathing are only now being actively explored in the context of our current understanding, it is likely that such efforts will yield important novel approaches to the clinical and pharmacologic management of these disorders in the near future.

Congenital disorder of oxygen sensing: association of the homozygous Chuvash polycythemia VHL mutation with thrombosis and vascular abnormalities but not tumors

Adaptation to hypoxia is critical for survival and regulates multiple processes, including erythropoiesis and vasculogenesis. Chuvash polycythemia is a hypoxia-sensing disorder characterized by homozygous mutation (598C>T) of von Hippel-Lindau gene (VHL), a negative regulator of hypoxia sensing. Although endemic to the Chuvash population of Russia, this mutation occurs worldwide and originates from a single ancient event. That VHL 598C>T homozygosity causes elevated normoxic levels of the transcription factor hypoxia inducible factor-1alpha (HIF-1alpha), serum erythropoietin and hemoglobin is known, but the disease phenotype has not been documented in a controlled manner. In this matched cohort study, VHL 598C>T homozygosity was associated with vertebral hemangiomas, varicose veins, lower blood pressures, and elevated serum vascular endothelial growth factor (VEGF) concentrations (P <.0005), as well as premature mortality related to cerebral vascular events and peripheral thrombosis. Spinocerebellar hemangioblastomas, renal carcinomas, and pheochromocytomas typical of classical VHL syndrome were not found, suggesting that overexpression of HIF-1alpha and VEGF is not sufficient for tumorigenesis. Although hemoglobin-adjusted serum erythropoietin concentrations were approximately 10-fold higher in VHL 598C>T homozygotes than in controls, erythropoietin response to hypoxia was identical. Thus, Chuvash polycythemia is a distinct VHL syndrome manifested by thrombosis, vascular abnormalities, and intact hypoxic regulation despite increased basal expression of hypoxia-regulated genes.

Phospholipid composition of myocardium in children with normoxemic and hypoxemic congenital heart diseases

Samples of myocardial tissue were obtained during cardiac surgery from children operated for different types of normoxemic and hypoxemic congenital heart diseases. The phospholipid composition was analyzed by thin layer chromatography. The concentration of total phospholipids (PL), phosphatidylcholine and phosphatidylethanolamine (PE) was found lower in atrial tissue of both normoxemic and hypoxemic groups in comparison with the ventricles. When comparing the difference between hypoxemic and normoxemic defects, hypoxemia was found to increase the concentration of total PL, PE and phosphatidylserine in ventricles and total PL and PE in the atria. The increased level of particular phospholipid species may represent adaptive mechanisms to hypoxemia in children with congenital heart diseases.

Neonatal congenital diaphragmatic hernia and extracorporeal membrane oxygenation

OBJECTIVE

To describe the outcome of infants with congenital diaphragmatic hernia (CDH) presenting early who were referred for possible extracorporeal membrane oxygenation (ECMO).

DESIGN

Retrospective descriptive study.

SETTING

Neonatal Intensive Care Unit of the Royal Alexandra Hospital, Edmonton.

PATIENTS

Fifteen infants referred to our program since its introduction, in February 1989; 13 received ECMO. The criterion for ECMO was the presence of an oxygen index of more than 40 on three occasions within 2 hours.

INTERVENTION

ECMO was performed by means of cannulation of the right carotid artery and jugular vein for 111.0 hours on average.

RESULTS

In 5 of the 13 infants who underwent ECMO the procedure was performed after surgical repair; all were successfully weaned off ECMO, and the cannula was removed without incident. In the remaining eight ECMO was started before surgical repair; of the six who received it during repair four ultimately survived. None of the previously described predictors of outcome for CDH, including diagnosis before 25 weeks' gestation (in six cases), were useful in determining the survival of the patients. Bleeding was the most common complication and cause of death.

CONCLUSION

ECMO is associated with survival in infants with CDH who fail to respond to conventional therapy and who have a poor prognosis according to previously established criteria.

Direct embryotoxicity of cocaine in rats: effects on mitochondrial activity, cardiac function, and growth and development in vitro

Day 10 rat embryos were exposed to cocaine HCl (10-100 microM) in vitro in 20% (designated normoxic) and 10-12% (designated moderately hypoxic) oxygen and examined the following day. In normoxia, it caused prompt and significant decreases in heart rates and significant reductions in measures of growth and development and diameters of the vitelline arteries. In moderate hypoxia, cocaine exposure resulted in axially asymmetric defects reported previously only in embryos exposed to extreme hypoxia or to hypoxia generated by redox cyclers. Day 10 or 11 embryos or isolated hearts from the latter stage were incubated with cocaine under normoxic conditions. Acute and significant concentration-dependent decreases in heart rates occurred on day 10. The rates in day 11 embryos and in isolated hearts from day 11 embryos were less sensitive than those on day 10. Cocaine also significantly inhibited the activity of the terminal electron transport system of the mitochondria of embryos. Maternal cocaine exposure has been associated with uterine vasoconstriction and decreases in fetal oxygenation. The latter has been shown to stimulate glucose uptake. We hypothesize that placental vasoconstriction limits the ability of embryos to meet the increased glucose demands induced by hypoxia. The developmental toxicity of nutrient and oxygen deprivation is further enhanced by significant decreases of mitochondrial activity. We propose therefore that compromised energy supplies form the basis of the developmental toxicity of cocaine.

Hypobaric hypoxia-induced intrauterine growth retardation

2-week-pregnant New Zealand White rabbits were placed into a hypobaric chamber and maintained at 3,657 m (12,000 ft). Upon delivery (31 +/- 1 days) or at cesarean section (on day 30), all newborns were sacrificed and organ weights were compared to those of control pups maintained at 128 m (420 ft). Significant findings in the hypobaric animals included a diminished birth weight, placental weight, and liver weight per body weight. Placental weight per body weight was similar in both groups. The brain weight per body weight, brain to liver ratio, and right ventricle to left ventricle plus intraventricular septal weight ratio were significantly increased in the hypobaric pups.

Tolazoline and dopamine therapy in neonatal hypoxia and pulmonary vasospasm

Severe hypoxia unresponsive to maximum ventilatory support occurs both in idiopathic respiratory distress syndrome and meconium aspiration. We recently encountered a 980 g female infant with respiratory distress syndrome and 3 300 g female infant with meconium aspiration and persistant fetal circulation whose clinical course necessitated the use of tolazoline and dopamine to reduce pulmonary and to stabilize systemic pressures. The infant with respiratory distress syndrome responded with a PaO2 increase of 2.7 kPa while the infant with persistant fetal circulation and meconium aspiration showed a 51.6 kPa rise. Combined pharmacologic therapy may have a role in improving oxygenation status in severely hypoxemic infants receiving maximum support.

Special problems of the newborn

Rapid diagnosis on the basis of simple clinical signs and immediate, aggressive therapy are required in handling such newborn problems as persistence of fetal circulatory patterns, respiratory distress syndrome, and acute necrotizing enterocolitis.

Persistent fetal circulation: an evolving clinical and radiographic concept of pulmonary hypertension of the newborn

The roentgenographic presentations of 11 newborn infants with hypoxemia secondary to pulmonary vasospasm and subsequent right-to-left shunting of blood through the foramen ovale and/or ductus arteriosus (persistent fetal circulation) are described (P. F. C. Syndrome). One infant had radiographically normal lungs, while ten had pulmonary parenchymal abnormalities including hyaline membrane disease [4], meconium aspiration syndrome [4], or an ill defined pattern of retained lung fluid [2]. The roentgenographic appearance of the lungs, however, was discordant with the severe hypoxemia observed in most. Heart size was variable but some degree of cardiomegaly was commonly present. Tolazoline, a potent vasodilator, was useful diagnostically and may have resulted in increased survival. An expanded clinical and roentgeonographic concept of the PFC syndrome is suggested.