Hypoxemia and reoxygenation with 21% or 100% oxygen in newborn pigs: changes in blood pressure, base deficit, and hypoxanthine and brain morphology

Oxygenation of the newborn. The impact of one molecule on newborn lives

Hypoxanthine is a purine metabolite which increases during hypoxia and therefore is an indicator of this condition. Further, when hypoxanthine is oxidized to uric acid in the presence of xanthine oxidase, oxygen radicals are generated. This was the theoretical basis for suggesting and studying, beginning in the 1990s, resuscitation of newborn infants with air instead of the traditional 100% O2. These studies demonstrated a 30% reduction in mortality when resuscitation of term and near term infants was carried out with air compared to pure oxygen. The mechanism for this is not fully understood, however the hypoxanthine -xanthine oxidase system increases oxidative stress and plays a role in regulation of the perinatal circulation. Further, hyperoxic resuscitation inhibits mitochondrial function, and one reason may be that genes involved in ATP production are down-regulated. Thus, the study of one single molecule, hypoxanthine, has contributed to the global prevention of an estimated 2-500,000 annual infant deaths.

Supplemental Oxygen in the Newborn: Historical Perspective and Current Trends

Oxygen is the final electron acceptor in aerobic respiration, and a lack of oxygen can result in bioenergetic failure and cell death. Thus, administration of supplemental concentrations of oxygen to overcome barriers to tissue oxygen delivery (e.g., heart failure, lung disease, ischemia), can rescue dying cells where cellular oxygen content is low. However, the balance of oxygen delivery and oxygen consumption relies on tightly controlled oxygen gradients and compartmentalized redox potential. While therapeutic oxygen delivery can be life-saving, it can disrupt growth and development, impair bioenergetic function, and induce inflammation. Newborns, and premature newborns especially, have features that confer particular susceptibility to hyperoxic injury due to oxidative stress. In this review, we will describe the unique features of newborn redox physiology and antioxidant defenses, the history of therapeutic oxygen use in this population and its role in disease, and clinical trends in the use of therapeutic oxygen and mitigation of neonatal oxidative injury.

Oxygen for respiratory support of moderate and late preterm and term infants at birth: Is air best?

Oxygen has been used for newborn infant resuscitation for more than two centuries. In the last two decades, concerns about oxidative stress and injury have changed this practice. Air (FiO₂ 0.21) is now preferred as the starting point for respiratory support of infants 34 weeks gestation and above. These recommendations are derived from studies that were conducted on asphyxiated, term infants, recruited more than 10 years ago using strategies that are not commonly used today. The applicability of these recommendations to current practice, is uncertain. In addition, whether initiating respiratory support with air for infants with pulmonary disorders provides sufficient oxygenation is also unclear. This review will address these concerns and provide suggestions for future steps to address knowledge and practice gaps.

Newborn Resuscitation in Settings Without Access to Supplemental Oxygen

Low- and middle-income countries and resource-limited regions are major contributors to perinatal and infant mortality. Oxygen is widely used for resuscitation in high- and middle-income settings. However, oxygen supplementation is not available in resource-limited regions. Oxygen supplementation for resuscitation at birth has adverse effects in human/animal model studies. There has been a change with resultant recommendations for restrictive oxygen use in neonatal resuscitation. Neonatal resuscitation without supplemental oxygen decreases mortality and morbidities. Oxygen in resource-limited settings for neonatal resuscitation is ideal as a backup for selected resuscitations but should not be a limiting factor for implementing basic life-saving efforts.

Assessment of cerebral perfusion with contrast-enhanced ultrasound during constriction of the neck mimicking malposition of the BD Odon Device™: a study in newborn piglets

OBJECTIVE

The BD Odon Device™ is a new instrument for operative vaginal birth with potential for preventing maternal, fetal and newborn morbidity/mortality during a complicated second stage of labour. The device is a plastic sleeve with an air chamber inflated around the baby's head which is gently pulled through the birth canal. The aim was to monitor changes in cerebral circulation during constriction of the neck to evaluate a risk of potential malposition of the device.

DESIGN

Randomised prospective study.

POPULATION OR SAMPLE

Twelve newborn piglets.

METHODS

The anaesthetised piglets were exposed to hypoxia until base excess was -20 mmol/l and/or mean arterial blood pressure had decreased to 20 mmHg. At reoxygenation, an air chamber was inflated around the neck to 300 mmHg and the piglets randomised into three groups: 10 (n = 5), 5 (n = 5) or 2 (n = 2) minutes' occlusion. Cerebral perfusion was evaluated with transcranial contrast-enhanced ultrasound at four time-points, and analysed in the carotid arteries, basal ganglia, cortex and whole brain. Statistical analysis used ANOVA, linear mixed model, Kruskal-Wallis H-test.

MAIN OUTCOME MEASURES

Perfusion parameters; peak intensity, time to peak intensity, upslope, mean transit time, area under the curve.

RESULTS

The haemodynamic response was comparable between groups. Perfusion parameters showed a slight increase at end hypoxia followed by a decrease during occlusion, especially in the cortex (P = 0.00-0.2). After deflation, perfusion returned towards baseline values.

CONCLUSIONS

Simulation of malposition of the Odon Device was performed using a newborn hypoxic piglet model. Considerable compression of the neck vessels was applied, with only a moderate decrease in perfusion and with restoration of haemodynamics/cerebral perfusion after decompression.

TWEETABLE ABSTRACT

Malposition of Odon Device™ in a piglet model revealed a reversible decrease in cerebral perfusion during neck constriction.

Oxygen therapy of the newborn from molecular understanding to clinical practice

Oxygen is one of the most critical components of life. Nature has taken billions of years to develop optimal atmospheric oxygen concentrations for human life, evolving from very low, peaking at 30% before reaching 20.95%. There is now increased understanding of the potential toxicity of both too much and too little oxygen, especially for preterm and asphyxiated infants and of the potential and lifelong impact of oxygen exposure, even for a few minutes after birth. In this review, we discuss the contribution of knowledge gleaned from basic science studies and their implication in the care and outcomes of the human infant within the first few minutes of life and afterwards. We emphasize current knowledge gaps and research that is needed to answer a problem that has taken Nature a considerably longer time to resolve.

Targeted Oxygen in the Resuscitation of Preterm Infants, a Randomized Clinical Trial

BACKGROUND AND OBJECTIVES

Lower concentrations of oxygen (O₂) (≤30%) are recommended for preterm resuscitation to avoid oxidative injury and cerebral ischemia. Effects on long-term outcomes are uncertain. We aimed to determine the effects of using room air (RA) or 100% O₂ on the combined risk of death and disability at 2 years in infants <32 weeks' gestation.

METHODS

A randomized, unmasked study designed to determine major disability and death at 2 years in infants <32 weeks' gestation after delivery room resuscitation was initiated with either RA or 100% O₂ and which were adjusted to target pulse oximetry of 65% to 95% at 5 minutes and 85% to 95% until NICU admission.

RESULTS

Of 6291 eligible patients, 292 were recruited and 287 (mean gestation: 28.9 weeks) were included in the analysis (RA: n = 144; 100% O₂: n = 143). Recruitment ceased in June 2014, per the recommendations of the Data and Safety Monitoring Committee owing to loss of equipoise for the use of 100% O₂. In non-prespecified analyses, infants <28 weeks who received RA resuscitation had higher hospital mortality (RA: 10 of 46 [22%]; than those given 100% O₂: 3 of 54 [6%]; risk ratio: 3.9 [95% confidence interval: 1.1-13.4]; P = .01). Respiratory failure was the most common cause of death (n = 13).

CONCLUSIONS

Using RA to initiate resuscitation was associated with an increased risk of death in infants <28 weeks' gestation. This study was not a prespecified analysis, and it was underpowered to address this post hoc hypothesis reliably. Additional data are needed.

Genotype-specific alterations in vascular smooth muscle cell function in cystic fibrosis piglets

BACKGROUND

The most common CF-causing mutations interfere with CFTR trafficking from the endoplasmic reticulum (CFTR-F508del) or prematurely terminate transcription (CFTR-null). We suspected that genotype-specific patterns of CFTR expression would have differential effects on smooth muscle cell calcium signaling and hence vascular tone. We hypothesized that compared to wild-type or CFTR-null aorta, aorta from CFTR-F508del (dF) piglets will have reduced endoplasmic reticulum calcium mobilization and decreased vasoconstriction.

METHODS

Aortic reactivity was assessed by myography, and ratiometric calcium imaging was performed in isolated vascular smooth muscle cells.

RESULTS

Aorta from dF piglets had reduced myogenic tone (P<0.001) and decreased constriction to KCl (P<0.05). Combined inhibition of ryanodine and IP3 receptors decreased wild-type and CFTR-null responses to levels seen in dF aorta. Compared to wild-type cells, dF-expressing smooth muscle cells had reduced calcium transients, while CFTR-null cells had decreased baseline intracellular calcium concentrations.

CONCLUSIONS

Expression of CFTR-F508del interferes with smooth muscle cell calcium handling and decreases aortic responsiveness.

Hyperoxia and hypoxia in children resuscitated from cardiac arrest

BACKGROUND

Ischemia depletes antioxidant reserves and impairs mitochondrial electron transport. Oxygen within blood reperfusing ischemic tissue can form free radicals, worsen oxidative stress, and exacerbate tissue injury (reperfusion injury). One strategy for limiting reperfusion injury is to limit delivery of "luxuriant" oxygen during or after reperfusion. Resuscitation guidelines for children with cardiac arrest recommend early weaning of supplemental oxygen as tolerated. There are currently no studies demonstrating the frequency and outcomes of hyperoxia and hypoxia after pediatric cardiac arrest.

OBJECTIVE

To determine the frequency and outcomes of hyperoxia and hypoxia in patients following resuscitation from pediatric cardiac arrest admitted to a tertiary care center.

DESIGN AND METHODS

This is a retrospective observational cohort study. Charts of children resuscitated from cardiac arrest and admitted to our hospital from 2004 to 2008 were reviewed. Partial pressures of oxygen (PaO2) obtained within the first 24 hours following return of spontaneous circulation and mortality at 6 months was recorded. Children who did not survive the initial 48 hours, patients having undergone extracorporeal oxygenation or had congenital heart disease, and those in whom arterial blood gases were not obtained were excluded.

RESULTS

Seventy-four patients met inclusion criteria. Of these, 38 (51%) had at least one arterial blood gases with a PaO2 > 300 mm Hg and 10 (14%) had a PaO2 < 60 mm Hg in the first 24 hours. Neither hyperoxia nor hypoxia on initial arterial blood gases (p = 0.912 and p = 0.384) nor any arterial blood gases within the first 24 hours after cardiac arrest (p = 0.325 and p = 0.553) was associated with 6-month mortality.

CONCLUSIONS

Hyperoxia occurs commonly within the first 24 hours of management in children resuscitated from cardiac arrest.

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.

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.

Oxygen saturation monitoring for the preterm infant: the evidence basis for current practice

Many of the morbid conditions associated with extreme immaturity are potentiated by an excess of free radicals occurring in infants who developmentally have decreased levels of antioxidants. The optimal oxygen saturation values for the resuscitation, stabilization, and ongoing care of the very low birth weight infant remain largely undefined. We have reviewed the currently available evidence for clinical oxygen use in the newborn period. Until the results of further studies are available, a reasonable approach to resuscitation would include initial resuscitation with 30-40% oxygen for very preterm infants using targeted SpO2 values and blended oxygen during the first 10 min. For ongoing management of preterm infants, SpO2 targets of 85-93% seem to be most appropriate, with alarm limits set within 1 to 2% of these targets with intermittent audits to ensure compliance. There is no strong evidence to support the use of altered limits for the infant who develops early evidence of retinopathy of prematurity. Further prospective studies are required to evaluate the effects of varied oxygen targets on long-term outcome.

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.

Perivascular nitric oxide and superoxide in neonatal cerebral hypoxia-ischemia

Decreased cerebral blood flow (CBF) has been observed following the resuscitation from neonatal hypoxic-ischemic injury, but its mechanism is not known. We address the hypothesis that reduced CBF is due to a change in nitric oxide (NO) and superoxide anion O(2)(-) balance secondary to endothelial NO synthase (eNOS) uncoupling with vascular injury. Wistar rats (7 day old) were subjected to cerebral hypoxia-ischemia by unilateral carotid occlusion under isoflurane anesthesia followed by hypoxia with hyperoxic or normoxic resuscitation. Expired CO(2) was determined during the period of hyperoxic or normoxic resuscitation. Laser-Doppler flowmetry was used with isoflurane anesthesia to monitor CBF, and cerebral perivascular NO and O(2)(-) were determined using fluorescent dyes with fluorescence microscopy. The effect of tetrahydrobiopterin supplementation on each of these measurements and the effect of apocynin and N(omega)-nitro-L-arginine methyl ester (L-NAME) administration on NO and O(2)(-) were determined. As a result, CBF in the ischemic cortex declined following the onset of resuscitation with 100% O(2) (hyperoxic resuscitation) but not room air (normoxic resuscitation). Expired CO(2) was decreased at the onset of resuscitation, but recovery was the same in normoxic and hyperoxic resuscitated groups. Perivascular NO-induced fluorescence intensity declined, and O(2)(-)-induced fluorescence increased in the ischemic cortex after hyperoxic resuscitation up to 24 h postischemia. L-NAME treatment reduced O(2)(-) relative to the nonischemic cortex. Apocynin treatment increased NO and reduced O(2)(-) relative to the nonischemic cortex. The administration of tetrahydrobiopterin following the injury increased perivascular NO, reduced perivascular O(2)(-), and increased CBF during hyperoxic resuscitation. These results demonstrate that reduced CBF follows hyperoxic resuscitation but not normoxic resuscitation after neonatal hypoxic-ischemic injury, accompanied by a reduction in perivascular production of NO and an increase in O(2)(-). The finding that tetrahydrobiopterin, apocynin, and L-NAME normalized radical production suggests that the uncoupling of perivascular NOS, probably eNOS, due to acquired relative tetrahydrobiopterin deficiency occurs after neonatal hypoxic-ischemic brain injury. It appears that both NOS uncoupling and the activation of NADPH oxidase participate in the changes of reactive oxygen concentrations seen in cerebral hypoxic-ischemic injury.

Resuscitation of preterm neonates by using room air or 100% oxygen

OBJECTIVE

In this study of preterm neonates of <32 weeks, we prospectively compared the use of room air versus 100% oxygen as the initial resuscitation gas.

METHODS

A 2-center, prospective, randomized, controlled trial of neonates with gestational ages of 23 to 32 weeks who required resuscitation was performed. The oxygen group was initially resuscitated with 100% oxygen, with decreases in the fraction of inspired oxygen after 5 minutes of life if pulse oxygen saturation was >95%. The room air group was initially resuscitated with 21% oxygen, which was increased to 100% oxygen if compressions were performed or if the heart rate was <100 beats per minute at 2 minutes of life. Oxygen was increased in 25% increments if pulse oxygen saturation was <70% at 3 minutes of life or <80% at 5 minutes of life.

RESULTS

Twenty-three infants in the oxygen group (mean gestational age: 27.6 weeks; range: 24-31 weeks; mean birth weight: 1013 g; range: 495-2309 g) and 18 in the room air group (mean gestational age: 28 weeks; range: 25-31 weeks; mean birth weight: 1091 g; range: 555-1840 g) were evaluated. Every resuscitated patient in the room air group met rescue criteria and received an increase in the fraction of inspired oxygen by 3 minutes of life, 6 patients directly to 100% and 12 with incremental increases. Pulse oxygen saturation was significantly lower in the room air group from 2 to 10 minutes (pulse oxygen saturation at 3 minutes: 55% in the room air group vs 87% in the oxygen group). Heart rates did not differ between groups in the first 10 minutes of life, and there were no differences in secondary outcomes.

CONCLUSIONS

Resuscitation with room air failed to achieve our target oxygen saturation by 3 minutes of life, and we recommend that it not be used for preterm neonates.

Epinephrine versus dopamine to treat shock in hypoxic newborn pigs resuscitated with 100% oxygen

Shock and tissue hypoperfusion are common after asphyxia. We compared systemic and regional hemodynamic effects of epinephrine and dopamine in the treatment of shock and hypotension in asphyxiated newborn piglets resuscitated with 100% oxygen. Twenty-four piglets (1-3 days old; weight, 1.4-2.6 kg) were acutely instrumented to measure cardiac index (CI), carotid, mesenteric and renal arterial blood flows, and mean systemic (SAPs) and pulmonary arterial pressures (PAPs). Piglets had normocapnic alveolar hypoxia (F(IO2)=0.08-0.10) for 50 min and reoxygenated with F(IO2)=1.0 for 1 h then F(IO2)=0.21 for 3.5 h. After 2 h reoxygenation, either dopamine (2 microg kg(-1) min(-1)) or epinephrine (0.2 microg kg(-1) min(-1)) was given for 30 min in a blinded randomized manner, which was then increased to maintain SAP (within 10% of baseline, pressure-driven dose) for 2 h. Hypoxia caused hypotension (SAP, 44%+/-3% of baseline), cardiogenic shock (CI, 41%+/-4%), and metabolic acidosis (mean pH, 7.04-7.09). Upon reoxygenation, hemodynamic parameters immediately recovered but gradually deteriorated during 2 h with SAP at 45+/-1 mmHg, CI at 74+/-9% of baseline, and pH 7.32+/-0.03. Low doses of either drug had no significant systemic and renal hemodynamic response. Epinephrine (0.3-1.5 microg kg(-1) min(-1)) for 2 h increased SAP and CI (with higher stroke volume) and decreased pulmonary vascular resistance (with reduced PAP-SAP ratio), whereas the responses with dopamine (10-25 microg kg(-1) min(-1)) were modest. Low-dose epinephrine improved mesenteric and carotid arterial flows, whereas the pressure-driven doses of epinephrine and dopamine increased carotid and mesenteric arterial flows, respectively. To treat shock in asphyxiated newborn piglets resuscitated with 100% oxygen, epinephrine exhibits an inotropic action compared with dopamine, whereas both catecholamines can increase carotid and mesenteric perfusion.

Resuscitation with 21 or 100% oxygen in hypoxic nicotine-pretreated newborn piglets: possible neuroprotective effects of nicotine

BACKGROUND

Perinatal asphyxia is a major concern in perinatal medicine. Resuscitation and ways to prevent and minimize adverse outcomes after perinatal asphyxia are subject to extensive research.

OBJECTIVES

In this study we hypothesized that, prior to hypoxia, intravenously administered nicotine might have an effect on how newborn piglets tolerate hypoxia, with regard to the time and degree of damage inflicted, due to its suggested neuroprotective abilities, and further that resuscitation with 21 compared with 100% oxygen in nicotine-pretreated animals would cause less cerebral damage.

METHODS

Thirty anesthetized newborn piglets were randomized to either hypoxia or control groups, and pretreatment with either saline or nicotine. In addition, the nicotine/hypoxia group was randomized to resuscitation with either 21 or 100% oxygen for 15 min following hypoxia.

RESULTS

We found significantly more necrosis in the striatum and cortex combined (p = 0.036), and in the striatum alone (p = 0.026), in the animals pretreated with nicotine and resuscitated with 100% when compared to 21% oxygen. There was no significant difference in the cerebellum. We also found significantly increased tolerance to hypoxia as measured by the time interval that the animals endured hypoxia: 103.8 +/- 28.2 min in the nicotine-pretreated animals vs. 66.5 +/- 19.5 min in the saline-pretreated animals (p = 0.035).

CONCLUSION

Nicotine enhances newborn piglets' ability to endure hypoxia, and resuscitation with 21% oxygen inflicts less necrosis than 100% oxygen. The potential neuroprotective effects of nicotine in the newborn brain should be further investigated.

Long-term histological outcome after post-hypoxic treatment with 100% or 40% oxygen in a model of perinatal hypoxic-ischemic brain injury

Hypoxic newborns have traditionally been given supplemental oxygen, and until recently, guidelines for neonatal resuscitation recommended that 100% oxygen be used. Exposure to 100% oxygen after hypoxic injury, however, may exacerbate oxidative stress. The current study evaluated the effect of exposure to 100, 40 or 21% oxygen after neonatal hypoxic-ischemic injury on the severity of brain injury after long-term survival. The severity of histological brain injury was not different in animals exposed to 100% oxygen versus room air. Male animals treated with 40% oxygen post-hypoxia had the lowest mean total histology scores, but this was not statistically significant due to the large variation in injury within each treatment group. These results support the growing number of studies in human infants and experimental animals that show no benefit of 100% oxygen over room air for neonatal resuscitation. Our results suggest that post-hypoxia treatment with 40% oxygen may be beneficial, particularly in males. Further studies of the effects of different concentrations of oxygen on brain injury are warranted and should have sufficient power to examine sex differences.

RESUSCITATION WITH 21% OR 100% OXYGEN IS EQUALLY EFFECTIVE IN RESTORING PERFUSION AND OXYGEN METABOLISM IN THE LIVER OF HYPOXIC NEWBORN PIGLETS

The differential effects of the use of high or low oxygen levels during resuscitation on the neonatal liver are unknown. We compared the hepatic hemodynamics and oxygen metabolism in hypoxic newborn piglets resuscitated with 21% or 100% oxygen. Twenty-seven piglets (age, 1-3 days; weight, 1.5-2.0 kg) were acutely instrumented to measure cardiac output, hepatic artery, and portal venous blood flows (hepatic artery flow index [HAFI] and portal venous flow index [PVFI], respectively). The animals underwent 2 h of hypoxia (fraction of inspired oxygen, 0.10-0.15), then reoxygenation with 21% (n = 9) or 100% (n = 9) oxygen for 1 h, then 1 h with 21% oxygen. The controls (n = 9) were sham-operated without hypoxia-reoxygenation. Oxygen transport and plasma lactate concentrations were studied. Hypoxic animals had hypotension and decreased cardiac index with metabolic acidosis (mean pH, 7.00-7.02; P < 0.05 vs. controls). The PVFI and the total hepatic blood flow (THFI = PVFI + HAFI), despite the absence of significant change in HAFI, decreased to 16 +/- 2 mL/min/kg and 19 +/- 3 mL/min/kg, respectively (versus 24 +/- 2 mL/min/kg and 28 +/- 2 mL/min/kg of controls; P < 0.05). Fifteen minutes after reoxygenation, the cardiac index improved, PVFI recovered, HAFI was maintained, and THFI was not different between the groups. The hepatic oxygen consumption decreased (59%; P < 0.05) and the extraction increased (89%; P < 0.001) during hypoxia. Similarly, on reoxygenation, the hepatic oxygen consumption improved; however, extraction decreased versus controls on 100% but not on 21% oxygen (P < 0.05). The plasma lactate concentrations increased in both groups with hypoxia and were not different during reoxygenation between the group administered with 21% oxygen and the group administered with 100% oxygen. The hypoxic neonatal liver has reduced hepatic blood flow but has relatively preserved HAFI, and oxygen consumption recovered similarly on reoxygenation with 21% and 100% oxygen. The increased oxygen extraction during hypoxia normalized in 21% but reduced in 100% reoxygenation, with no differences in plasma lactate concentrations.

Effect of resuscitation with 21% oxygen and 100% oxygen on NMDA receptor binding characteristics following asphyxia in newborn piglets

The present study investigated the effect of reventilation with 21% and 100% oxygen following asphyxia in newborn piglets on NMDA receptor binding characteristics, Na(+), K(+)-ATPase activity, and lipid peroxidation. After achieving a heart rate less than 60 beats per minute, asphyxiated piglets were reventilated with 21% oxygen or 100% oxygen. (3)[H]MK-801 binding showed the Bmax in the 21% and 100% groups to be 1.53 +/- 0.43 and 1.42 +/- 0.35 pmol/mg protein (p = ns). Values for Kd were 4.56 +/- 1.29 and 4.17 +/- 1.05 nM (p = ns). Na(+), K(+)-ATPase activity in the 21% and 100% groups were 23.5 +/- 0.9 and 24.4 +/- 3.9 micromol Pi/mg protein/h (p = ns). Conjugated dienes (0.05 +/- 0.02 vs. 0.07 +/- 0.03 micromol/g brain) and fluorescent compounds (0.54 +/- 0.05 vs. 0.78 +/- 0.19 microg quinine sulfate/g brain), were similar in both groups (p = ns). Though lipid peroxidation products trended higher in the 100% group, these data show that NMDA receptor binding and Na(+), K(+)-ATPase activity were similar following reventilation with 21% or 100% oxygen after a single episode of mild asphyxia.

Room air resuscitation of the depressed newborn: A systematic review and meta-analysis

Understanding of the potential dangers of hyperoxia in the newborn is growing. Several studies have examined the use of room air for the resuscitation of newborns.

OBJECTIVE

To assess the effects of room air resuscitation versus 100% oxygen resuscitation on mortality at 1 week and 1 month in asphyxiated newborn infants.

STUDY DESIGN

Systematic review and meta-analysis of seven randomized and quasi-randomised controlled trials comparing room air and 100% oxygen resuscitation of newborn infants.

RESULTS

Compared to the 100% oxygen resuscitation group, neonates in the room air resuscitation group had a lower mortality both in the first week of life (odds ratio 0.70, 95% CI 0.50, 0.98) and at 1 month and beyond (odds ratio 0.63, 95% CI 0.42, 0.94). The incidence of severe hypoxic ischemic encephalopathy (Stage II and Stage III) was similar between the two groups.

CONCLUSION

This meta-analysis supports the hypothesis that room air is superior to 100% oxygen as the initial choice for resuscitating clinically depressed newborns as it may result in a lower mortality rate. However, adequately powered studies of long-term neurodevelopmental outcomes are not yet available.

Earlier Apgar score increase in severely depressed term infants cared for in Swedish level III units with 40% oxygen versus 100% oxygen resuscitation strategies: a population-based register study

OBJECTIVES

The aim of this study was to evaluate whether a resuscitation strategy based on administration of 40% oxygen influences mortality rates and rates of improvement in 5-minute Apgar scores, compared with a strategy based on 100% oxygen administration.

METHODS

A population-based study evaluated data from 4 Swedish perinatal level III centers during the period of 1998 to 2003. During this period, the centers used either of 2 resuscitation strategies (initial oxygen administration of 40% or 100%). Live-born, singleton, term infants with 1-minute Apgar scores of <4, with a birth weight appropriate for gestational age, and without major malformations were included in the study (n = 1223).

RESULTS

Infants born in hospitals using a 40% oxygen strategy had a more rapid Apgar score increase than did infants born in hospitals using a 100% oxygen strategy; however, no difference remained at 10 minutes. The mean Apgar score increased from 2.01 at 1 minute to 6.74 at 5 minutes in the 2 hospitals initiating resuscitation with 40% oxygen, compared with 2.01 to 6.38 in the 2 hospitals using 100% oxygen, with a mean difference in Apgar score increases of 0.36. At 5 minutes, 44.3% of infants born in the hospitals using 100% oxygen had an Apgar score of <7, compared with 34.0% of infants at the hospitals using 40% oxygen. At 10 minutes, the mean Apgar scores were 8.16 at the hospitals using 40% oxygen and 8.07 at the hospitals using 100% oxygen. There were no significant differences in rates of neonatal death, hypoxic ischemic encephalopathy, or seizures in relation to the 2 oxygen strategies.

CONCLUSION

Severely depressed term infants born in hospitals initiating resuscitation with 40% oxygen had earlier Apgar score recovery than did infants born in hospitals using a 100% oxygen strategy.

Air or 100% oxygen in neonatal resuscitation?

For more than 100 years, three principles have guided the treatment of neonatal asphyxia: maintain body heat, free air passages of obstructions, and stimulate respiration by supplying air to the lungs for oxygenation of the blood. This article addresses the question of which gas, air or 100% oxygen, is best supplied to the lungs to stimulate respiration. Evidence-based studies are presented and discussed.

Room air resuscitation versus oxygen resuscitation in the delivery room

The use of 100% oxygen for delivery room resuscitation is currently the recommended standard of the American Academy of Pediatrics and the Neonatal Resuscitation Program. However, there is mounting evidence from animal and human studies suggesting that resuscitation with room air (RA, 21% oxygen), including positive pressure ventilation with bag and face mask, may be as effective as that with 100% oxygen, and that the use of 100% oxygen may pose a risk of adverse physiologic sequelae. Resuscitation with RA has been demonstrated to result in faster recovery and improved neonatal mortality in comparison to 100% oxygen resuscitation. In addition, studies of normal oxygen saturation immediately after birth suggest delivery room personnel may be rushing to high saturation unnecessarily. The question for perinatal medical and nursing personnel involved in newborn resuscitation in the delivery room is whether the use of RA reduces the possible adverse effects of 100% oxygen, including delay in short-term stabilization, death, neurological disability, and possible secondary oxygen free radical injury. A systematic synopsis of both animal studies and human studies involving the advantages, disadvantages, possible risks, and short- and long-term effects of these 2 methods of resuscitation is presented.

Hyperoxia with 100% Oxygen following Hypoxia-Ischemia Increases Brain Damage in Newborn Rats

OBJECTIVE

To describe the effect of reoxygenation with 100% O2 as compared to the effect of room air in newborn rat brains after asphyxia.

METHODS

Experimental asphyxia (carotid artery ligation followed by hypoxic exposure with 8% O2 for 2 h) was performed on 7-day-old rats. After hypoxia-ischemia the rats were reoxygenated with either 100% O2 (hyperoxia group) or 21% O2 (room air group) for 24 h and then returned to the dam. The rats were killed 1 week after the experiment to study the cerebral cortex and hippocampus.

RESULTS

Rats reoxygenated with 100% O2 post-asphyxia showed more frequency of cortical damage (10 of 24 rats) than those reoxy genated with room air (3 of 24 rats) (chi2 test, p = 0.02).

CONCLUSION

We consider that hyperoxia with 100% oxygen after hypoxia-ischemia can cause more damage in the cerebral cortex than room air in newborn rats.

Cardiac function, myocardial glutathione, and matrix metalloproteinase-2 levels in hypoxic newborn pigs reoxygenated by 21%, 50%, or 100% oxygen

After asphyxia, it is standard to resuscitate the newborn with 100% oxygen, which may create a hypoxia-reoxygenation process that may contribute to subsequent myocardial dysfunction. We examined the effects of graded reoxygenation on cardiac function, myocardial glutathione levels, and matrix metalloproteinase (MMP)-2 activity during recovery. Thirty-two piglets (1-3 days old, weighing 1.5-2.1 kg) were anesthetized and instrumented for continuous monitoring of cardiac index, and systemic and pulmonary arterial pressures. After 2 h of hypoxia, piglets were randomized to receive reoxygenation for 1 h with 21%, 50%, or 100% oxygen (n = 8 each), followed by 3 h at 21% oxygen. At 2 h of hypoxemia (PaO2 32-34 mmHg), the animals had hypotension, decreased cardiac index, and elevated pulmonary arterial pressure (P < 0.001 vs. controls). Upon reoxygenation, cardiac function recovered in all groups with higher cardiac index and lower systemic vascular resistance in the 21% group at 30 min of reoxygenation (P < 0.05 vs. controls). Pulmonary artery pressure normalized in an oxygen-dependent fashion (100% = 50% > 21%), despite an immediate recovery of pulmonary vascular resistance in all groups. The hypoxia-reoxygenated (21%-100%) hearts had similarly increased MMP-2 activity and decreased glutathione levels (P < 0.05, 100% vs. controls), which correlated significantly with cardiac index and stroke volume during reoxygenation, and similar features of early myocardial necrosis. In neonatal resuscitation, if used with caution because of a slower resolution of pulmonary hypertension, 21% reoxygenation results in similar cardiac function and early myocardial injury as 50% or 100%. The significance of higher oxidative stress with high oxygen concentration is unknown, at least in the acute recovery period.

Developing a long-term surviving piglet model of neonatal hypoxic-ischemic encephalopathy

OBJECTIVE

This study was designed to develop a piglet model of neonatal hypoxic-ischemic encephalopathy, which would allow for serial assessments of long-term neurodevelopmental impairment.

METHODS

In 12 newborn piglets, we produced hypoxia by 8% oxygen breathing for 5-91 minutes. We combined ischemia by reversible bilateral common carotid artery occlusion for varying times. Outcome measures were clinical neurological evaluation, magnetic resonance spectroscopy studies and brain histology.

RESULTS

Those animals which received intravenous sedation and no mechanical ventilation showed poor tolerance to hypoxia-ischemia and died early in the course of the experiments. The use of inhalation anesthesia during surgical procedures and mechanical ventilation during hypoxia-ischemia was associated with long-term survival. Seven of eight animals that survived > or = 48 hr showed clinical neurological abnormalities, that later resolved. Magnetic resonance spectroscopy measurements did not change significantly following hypoxia-ischemia. None of the animals had histopathological brain lesions.

CONCLUSION

When subjected to acute hypoxia-ischemia, piglets were likely to survive only if they were given such supportive measures as anesthesia and mechanical ventilation. Even with hypoxic-ischemic injury sufficient to produce acute signs of neurological dysfunctions, longterm, stable survival with no evident brain histopathological abnormalities was possible.

Oxygen for newborns: how much is too much?

International guidelines for newborn resuscitation recommend the use of 100% oxygen. However, high concentrations of oxygen after asphyxiation activate reactive oxygen species that may contribute to a number of morbidities. Animal models have been useful in describing their mechanisms, but only large-scale clinical trials can provide evidence that may be used to alter clinical practice. It has been demonstrated that neonates recover faster when resuscitated with room air as opposed to pure oxygen and neonatal mortality rates are improved. Increases in saturation are equal with oxygen and room air resuscitation. Studies of normal oxygen saturation immediately after birth suggest that clinicians may unnecessarily be rushing to high saturations. In the first weeks of life, lower saturation targets in preterm infants reduce retinopathy of prematurity and pulmonary complications and may improve growth. The neonatologist would be well served to think of oxygen as a medication, and use it sparingly.

Inflammation increases vulnerability to hypoxia in newborn piglets: effect of reoxygenation with 21% and 100% O2

OBJECTIVE

The purpose of this study was to assess whether inflammation increases vulnerability to hypoxia, and influences the effect of 100% O(2) and 21% O 2 reoxygenation on brain.

STUDY DESIGN

Newborn piglets (n = 31) were randomized to 4 interventional groups: pretreatment with saline or endotoxin. After hypoxia they were reoxygenated with 21% or 100% oxygen for 30 minutes, followed by 21% oxygen for all groups. To assess brain injury we measured extracellular brain tissue glucose, glycerol, and lactate/pyruvate by microdialysis, brain tissue oxygen tension, and laser Doppler flow.

RESULTS

Administration of endotoxin reduced the time to reach base excess (BE) -20 mmol/L by median 32 minutes compared with saline ( P < .05). We found no differences in changes in biochemical markers, brain tissue microcirculation, or oxygen tension between piglets in the 4 groups.

CONCLUSION

Endotoxin and hypoxia acted synergistically in inducing metabolic acidosis. In the presence of experimental inflammation, 21% oxygen seems as effective as 100% O(2) in reoxygenating piglets.

Air or 100% oxygen for asphyxiated babies? Time to decide

Both experimental and clinical studies have demonstrated that room air is as efficient as 100% oxygen for newborn resuscitation and improves short-term recovery. The recent meta-analysis by Davis and colleagues in the Lancet includes five studies from the past 10 years where asphyxiated infants were randomised or pseudo-randomised to be resuscitated in room air or in 100% oxygen. A significant reduction in mortality was seen when infants were resuscitated in room air compared to 100% oxygen. It is astonishing that a brief exposure of only a few minutes to 100% oxygen may be so toxic to the newborn infant; this finding, however, is supported by increasing evidence from experimental work emphasising that resuscitation in 100% oxygen may be associated with an aggravation of cellular injury when compared with resuscitation in air. It is imperative that these findings are reflected in the new newborn resuscitation guidelines and that further research continues in this area of neonatal medicine. Key areas include defining the best resuscitation practice for the preterm infant, designing adequate multicentre, randomised and blinded studies of term newborn resuscitation with adequate outcome data, and pursuing intense experimental research into the mechanisms and prevention of injury from oxygen free radicals.

Room air resuscitation-two decades of neonatal research

Experimental as well as clinical studies have demonstrated that room air is as efficient as pure oxygen for newborn resuscitation. Recent data even indicate that outcome is improved if pure oxygen is avoided. Thus, in a meta-analysis, neonatal mortality was significantly lower in those newly born infants resuscitated with 21% than with 100% oxygen. Short-term recovery is also improved in the room air group since time to first breath is shorter, heart rate at 90 s and 5 min Apgar score are higher. Animal data indicate that injury in a number of organs, including the brain, is aggravated by giving pure oxygen to newly born depressed infants even for a brief period. Although the optimal oxygen concentration probably is not known for newborn infants in need of resuscitation, pure oxygen should be avoided. These data should be reflected in new guidelines that are under way.

Resuscitation with 100% O2 increases cerebral injury in hypoxemic piglets

Perinatal asphyxia is a major cause of immediate and postponed brain injury in the newborn. We hypothesized that resuscitation with 100% O2 compared with ambient air is detrimental to the cerebral tissue. We assessed cerebral injury in newborn piglets that underwent global hypoxia and subsequent resuscitation with 21 or 100% O2 by extracellular glycerol, matrix metalloproteinase (MMP) expression levels, and oxidative stress. Extracellular glycerol was sampled by cerebral microdialysis. MMP levels were analyzed in cerebral tissue by gelatin zymography, broad matrix degrading capacity, and real-time PCR. Total endogenous antioxidant capacity was measured by the oxygen radical absorbance capacity assay. Extracellular glycerol increased 50% after resuscitation with 100% O2 compared with 21% O2. Total MMP activity was doubled in resuscitated animals at endpoint compared with baseline (p=0.018), and the MMP-2 activity was significantly increased in piglets that were resuscitated with 21% O(2) (p=0.003) and 100% O2 (p=0.001) compared with baseline. MMP-2 mRNA level was 100% increased in piglets that were resuscitated with 100% O2 as compared with 21% O2 (p < 0.05). Oxygen radical absorbance capacity values in piglets that were resuscitated with 100% O2 were considerably reduced compared with both baseline (p=0.001) and piglets that were resuscitated with 21% O2 (p=0.001). In conclusion, our data show increased MMP-2 activity at both gene and protein levels, accompanied with cerebral leakage of glycerol, presumably triggered by augmented oxidative stress. Our findings suggest that resuscitation of asphyxiated piglets with 100% O2 is detrimental to the piglet brain compared with resuscitation with 21% O2.

Comparison of short- and long-duration oxygen treatment after cerebral asphyxia in newborn piglets

We tested whether reoxygenation with 100% O(2) for 5 min after experimental asphyxia in newborn piglets was as efficient as 100% O(2) for 20 min compared with room air. Forty-one anesthetized piglets, 1-3 d old, were randomized to cerebral hypoxemia-ischemia-hypercapnia (HIH) or control (n = 5). HIH was achieved by ventilation with 8% O(2), temporary occlusion of the common carotid arteries, and adding of CO(2). After 25 min, reoxygenation-reperfusion was started with 100% O(2) for 20 min (group 1, n = 12), 100% O(2) for 5 min (group 2, n = 12), or 21% O(2) (group 3, n = 12). All piglets were observed for 2 h. During reoxygenation-reperfusion, significantly higher blood pressure and more complete restoration of microcirculation (laser Doppler flow) in the cerebral cortex was found in both groups reoxygenated with 100% O(2) compared with 21% O(2) (regional cerebral blood flow >or=100% versus 70% of baseline, p = 0.04). Reoxygenation with 100% O(2) for 5 min was as efficient as 20 min. Oxygen delivery in cortex was significantly higher in groups 1 and 2 compared with group 3 (p = 0.03), but there were no significant differences in cerebral metabolic rate for oxygen. In the striatum, no significant differences in flow or extracellular glutamate, glycerol, and lactate/pyruvate ratio were found between the groups. In conclusion, after experimental asphyxia, newborn piglets can be reoxygenated as efficiently with 100% O(2) for only 5 min as 100% O(2) for 20 min compared with room air.

The mismatch between evidence and practice. Common therapies in search of evidence

Many therapies in neonatology persist without supportive evidence: some common therapies may actually be harmful. Evidence-based medicine is the "conscientious, explicit and judicious use of current best evidence in making decisions about the care of individual patients". The best available evidence, however, is not always sound or valid evidence. Sometimes, when faced with a collection of reports that do not constitute good evidence, attempts to choose the best evidence become pointless; in this case, a statement of no good evidence is preferable. There is a continuing problem with the place of usual practice in the hierarchy of evidence; usual practice generates experience with a particular practice but no reliable information regarding how the practice compares with alternative strategies. Although clinical and institutional inertia combined with a litigious practice environment tend to uphold current practice, the field of neonatology is ripe with examples of established therapies that were subsequently shown to be harmful. It is important to focus on important long-term outcomes and as much on the possibility of harm as on the chance of benefit, especially for new therapies, before they become routine practice. In the face of inadequate evidence, it is particularly important to avoid the temptation to institute treatment guidelines that inhibit further research. Patients are better served by guidelines that recommend only strategies that are supported by strong evidence and recommend further research when the evidence is inadequate.

Cerebral intracellular calcium concentrations in asphyxiated rat fetuses resuscitated with oxygen

OBJECTIVE

To investigate the effects of resuscitation with three different oxygen concentrations on cerebral intra- and extra-cellular calcium, sodium and potassium changes in asphyxiated rat fetuses.

METHODS

Fifty-six fetal rats of gestational age of 20 days were randomly assigned into five study groups: sham operation group (control, n = 11), room-air resuscitation group (n = 10), and 3 oxygen-resuscitated groups (n = 14, 11, and 10 respectively). Different inhaled oxygen concentrations and different timings of oxygen delivery were assigned. Except for control all fetal rats were rendered ischemic and hypoxic in utero by interrupting the placental circulation. After re-circulation, intra- and extra-cellular concentrations of calcium, sodium, and potassium in the brains were measured for each individual group.

RESULTS

The mean intracellular free calcium concentration of fetal rat brains was similar for the room-air resuscitation group (552.1 +/- 93.5 nmol/L) and the group resuscitated with 92.8% oxygen (520.6 +/- 79.1 nmol/L) and both were significantly higher than in the control (315.3 +/- 86.9 nmol/L) (P < 0.001). After resuscitation with 65% oxygen, be it instituted before or immediately after hypoxia, their mean intracellular free calcium concentrations in the brain cells (441.5 +/- 47.9 and 452.9 +/- 36.4 nmol/L respectively) were significantly lower than those in the room-air resuscitation (P < 0.01) and 92.8% oxygen group (P < 0.05), though still higher than in the control (P > 0.05). There was no difference in the total concentrations of calcium, sodium, or potassium among all groups.

CONCLUSIONS

Resuscitation with 92.8% oxygen or room air exerted a similar effect on the parameters measured, indicating that resuscitation of asphyxiated neonates using 100% oxygen might not be superior to using room air. Resuscitation with 65% oxygen resulted in lower cerebral intracellular calcium concentrations and might produce a better outcome than using 100% oxygen or room air.

Oxygen toxicity in premature infants

Oxygen causes tissue injury through the formation of reactive oxygen intermediates and peroxidation of membrane lipids. Premature infants, who have severely reduced antioxidant defenses, are particularly sensitive to the toxic effects of oxygen. Supplemental oxygen in premature infants contributes to the development of chronic lung disease (bronchopulmonary dysplasia), characterized by dysregulated inflammation and altered expression of proteases and growth factors. This can result in fibrosis, asymmetric aeration, and respiratory insufficiency. Oxygen also induces aberrant physiologic responses that can be damaging in premature infants. For example, vasoconstriction in the retina is an early response to oxygen that can lead to vasoobliteration, neovascularization, and retinal traction (retinopathy of prematurity). Increasing knowledge of the mechanisms underlying oxygen toxicity in premature infants has suggested strategies to minimize tissue injury and to optimize long-term medical outcomes. These include limiting oxygen supplementation and light exposure, the use of antiinflammatory agents or antioxidants, and the use of room air in neonatal resuscitation.

Neonatal resuscitation guidelines 2000: framework for practice

Based on extensive evaluation of the existing evidence and consensus development, there are new recommendations in the Neonatal Resuscitation Program (NRP) guidelines 2000. These guidelines are described along with a brief review of the supporting evidence. The goal is to establish a framework for practice of neonatal resuscitation with a more scientific, evidence-based approach.

An improved survival model of hypoxia/ischaemia in the piglet suitable for neuroprotection studies

The purpose of this study was to develop a newborn piglet model of hypoxia/ischaemia which would better emulate the clinical situation in the asphyxiated human neonate and produce a consistent degree of histopathological injury following the insult. One-day-old piglets (n=18) were anaesthetised with a mixture of propofol (10 mg/kg/h) and alfentinal (55.5 microg/kg/h) i.v. The piglets were intubated and ventilated. Physiological variables were monitored continuously. Hypoxia was induced by decreasing the inspired oxygen (FiO(2)) to 3-4% and adjusting FiO(2) to maintain the cerebral function monitor peak amplitude at < or =5 microV. The duration of the mild insult was 20 min while the severe insult was 30 min which included 10 min where the blood pressure was allowed to fall below 70% of baseline. Control piglets (n=4 of 18) were subjected to the same protocol except for the hypoxic/ischaemic insult. The piglets were allowed to recover from anaesthesia then euthanased 72 h after the insult. The brains were perfusion-fixed, removed and embedded in paraffin. Coronal sections were stained by haematoxylin/eosin. A blinded observer examined the frontal and parietal cortex, hippocampus, basal ganglia, thalamus and cerebellum for the degree of damage. The total mean histology score for the five areas of the brain for the severe insult was 15.6+/-4.4 (mean +/-S.D., n=7), whereas no damage was seen in either the mild insult (n=4) or control groups. This 'severe damage' model produces a consistent level of damage and will prove useful for examining potential neuroprotective therapies in the neonatal brain.

Cerebral hypoxemia-ischemia and reoxygenation with 21% or 100% oxygen in newborn piglets: effects on extracellular levels of excitatory amino acids and microcirculation

OBJECTIVE

To determine whether reoxygenation with 21% oxygen is preferable to 100% oxygen in normalizing extracellular levels of excitatory amino acids in the brains of hypoxic-ischemic newborn piglets and to compare this model of combined hypoxemia-ischemia to a previously used model of global hypoxemia.

DESIGN

Prospective, randomized animal study.

SETTING

Surgical research laboratory.

SUBJECTS

Twenty-four anesthetized piglets, 1-3 days old.

INTERVENTIONS

Hypoxemia-ischemia was achieved by normoventilation with 8% oxygen and temporary occlusion of the common carotid arteries. After 20 mins, reoxygenation-reperfusion was started with 21% oxygen (HI 21% group, n = 12) or 100% oxygen (HI 100% group, n = 12) for 30 mins followed by 21% oxygen. All piglets were observed for 2 hrs.

MEASUREMENTS AND MAIN RESULTS

We measured extracellular concentrations of amino acids in striatum and hypoxanthine in cerebral cortex (microdialysis), microcirculation in cerebral cortex (laser Doppler), plasma hypoxanthine, and mean arterial pressure. During the 2-hr reoxygenation-reperfusion period, levels of amino acids were significantly higher in the HI 21% group compared with the HI 100% group (glutamate, p = 0.02; aspartate, p = 0.03). Mean arterial pressure was significantly lower in the HI 21% group (p = 0.04). Microcirculation decreased to <10% of baseline during hypoxemia-ischemia and normalized during reoxygenation-reperfusion in the HI 100% group, but it remained at a significantly lower level in the HI 21% group (p = 0.03).

CONCLUSIONS

Significantly higher levels of excitatory amino acids in striatum, significantly lower mean arterial pressure, and a significantly greater degree of hypoperfusion in cerebral cortex were found after reoxygenation with 21% oxygen compared with 100% oxygen in normocapnic, hypoxemic-ischemic newborn piglets. This suggests a less favorable outcome in the group receiving room air.

Newborn piglets with meconium aspiration resuscitated with room air or 100% oxygen

We investigated whether newborn piglets exposed to hypoxemia and severe meconium aspiration could be reoxygenated with room air as efficiently as with 100% O(2). Twenty-one 2- to 5-d-old piglets were randomly divided into three groups: 1) the room air group: hypoxemia, meconium aspiration, and reoxygenation with room air (n = 8); 2) the O(2) group: hypoxemia, meconium aspiration, and reoxygenation with 100% O(2) (n = 8); and 3) the control group: meconium aspiration, and reoxygenation with room air (n = 5). Hypoxemia was induced by ventilation with 8% O(2) until the mean blood pressure reached <20 mm Hg or the base excess reached <-20 mM. At this point, reoxygenation was started with either room air or 100% O(2). Three milliliters per kilogram of meconium 110 mg/mL was instilled into the trachea immediately before the start of reoxygenation. The O(2) tension in arterial blood was significantly lower in the room air group; at 5 min of reoxygenation it was 9.1 +/- 0.5 kPa versus 43.5 +/- 6 kPa in the O(2) group (p < 0.05). At 5 min of reoxygenation the tidal volume per kilogram was 12.1 +/- 0.7 mL/kg in the room air group and 13.1 +/- 0.9 mL/kg in the O(2) group (NS). There were no significant differences between the room air and the O(2) groups during 120 min of reoxygenation in mean arterial blood pressure, pulmonary arterial pressure, cardiac index, base excess, or plasma hypoxanthine. In conclusion, hypoxic newborn piglets with meconium aspiration were found to be reoxygenated as efficiently with room air as with 100% O(2).

Hydrogen peroxide production in leukocytes during cerebral hypoxia and reoxygenation with 100% or 21% oxygen in newborn piglets

The aim of this study was to investigate whether reoxygenation with 21% O2 rather than 100% O2 results in reduced hydrogen peroxide (H2O2) concentrations in neutrophils (PMN). Piglets (2-4 d old) exposed to severe hypoxia (inspired fraction of oxygen, 0.08) were randomized to resuscitation with 21 (n = 13) or 100% O2 (n = 12). Five animals served as controls. H2O2 concentrations in PMN in terms of rhodamine 123 (Rho 123) fluorescence intensity from arterial and superior sagittal sinus blood were quantified by flow cytometry. Laser Doppler flowmetry (LDF) was used to assess cortical blood perfusion. During hypoxia, Rho 123 increased in arterial PMN in both study groups by 15 and 32%, respectively (p < 0.05). In cerebral venous PMN, the increase was less dominant (p = 0.06). Reoxygenation with 100 or 21% O2 had no different effect on Rho 123 in arterial PMN. In cerebral venous PMN, Rho 123 was approximately 40% higher after 60 min and 30% higher after 120 min compared with corresponding data in the 21% O2 group (p < 0.05), which were close to baseline levels. Further, O2 treatment in both groups induced PMN accumulation in arterial blood (p < 0.05). Laser Doppler flowmetry signals increased during transient hypoxia (p < 0.0001 compared with baseline) and were normalized after reoxygenation in both study groups. In conclusion, arterial and cerebral venous H2O2 concentration in PMN tended to increase during hypoxia. During reoxygenation, H2O2 concentration in PMN in the cerebral circulation was low with 21% O2 but remained high with 100% O2 ventilation. We speculate that oxygen should be reintroduced with more caution during neonatal resuscitation.

Resuscitation of newborn infants with room air or oxygen

Oxygen is a toxic agent and a critical approach regarding its use during resuscitation at birth is developing. Animal data indicate that room air is efficient for newborn resuscitation. Three clinical studies have established that normal ventilation is delayed after oxygen resuscitation. Oxidative stress is augmented for several weeks in infants exposed to oxygen at birth -- the long-term implications of these observations remain unclear. There are limited data regarding the use of room air during complicated resuscitations, i.e. in meconium aspiration, the severely asphyxiated infant and in the preterm infant. Thus, it is necessary to continue ongoing rigorous examination of the long-accepted practice of oxygen administration during neonatal resuscitation.

Impaired early neurologic outcome in newborn piglets reoxygenated with 100% oxygen compared with room air after pneumothorax-induced asphyxia

Birth asphyxia is a serious problem worldwide, resulting in 1 million deaths and an equal number of neurologic sequelae annually. It is therefore important to develop new and better ways to treat asphyxia. In the present study we tested the effects of reoxygenation with room air or with 100% oxygen (O2) after experimental pneumothorax-induced asphyxia on the blood oxidative stress indicators, early neurologic outcome, and cerebral histopathology of newborn piglets. Twenty-six animals were studied in three experimental groups: 1) sham-operated animals (SHAM, n = 6), 2) animals reoxygenated with room air after pneumothorax (R21, n = 10), and 3) animals reoxygenated with 100% O2 after pneumothorax (R100, n = 10). In groups R21 and R100, asphyxia was induced under anesthesia with bilateral intrapleural room air insufflation. Gasping, bradyarrhythmia, arterial hypotension, hypoxemia, hypercarbia, and combined acidosis occurred 62 +/- 6 min (R21) or 65 +/- 7 min (R100; mean +/- SD) after the start of the experiments; then pneumothorax was relieved, and a 10-min reoxygenation period was started with mechanical ventilation with room air (R21) or with 100% O2 (R100). The newborn piglets then breathed room air spontaneously during the next 3 h. Blood oxidative stress indicators (oxidized and reduced glutathione, plasma Hb, and malondialdehyde concentrations) were measured at different stages of the experiments. Early neurologic outcome examinations (neurologic score of 20 indicates normal, 5 indicates brain-dead) were performed at the end of the study. The brains were next fixed, and various regions were stained for cerebral histopathology. In the SHAM group, the blood gas and acid-base status differed significantly from those measured in groups R21 and R100. In group R100, arterial PO2 was significantly higher after 5 (13.8 +/- 5.6 kPa) and 10 min (13.2 +/- 6.3 kPa) of reoxygenation than in group R21 (8.7 +/- 2.8 kPa and 9.2 +/- 3.1 kPa). The levels of all oxidative stress indicators remained unchanged in the study groups (SHAM, R21, and R100). The neurologic examination score in the SHAM group was 18 +/- 0, in group R21 it was 13.5 +/- 3.1, and in group R100 it was 9.5 +/- 4.1 (significant differences between SHAM and R21 or R100, and between R21 and R100). Cerebral histopathology revealed marked damage of similar severity in both asphyxiated groups. We conclude that the blood oxidative stress indicators and cerebral histopathology did not differ significantly after a 10-min period of reoxygenation with room air or with 100% O2 after pneumothorax-induced asphyxia, but reoxygenation with 100% O2 might impair the early neurologic outcome of newborn piglets.

Repetitive intermittent hypoxia-ischemia and brain damage in neonatal rats

OBJECTIVE

To know the effect of brief-repetitive intermittent hypoxia-ischemia on the development of perinatal brain damage.

STUDY DESIGN

Seven-day-old Wistar rats underwent ligation of the unilateral common carotid artery. The animals were allocated to three groups (n=12 in each group) and exposed to 8% oxygen as follows: group A: continuous exposure for 180 min; group B: continuous exposure for 90 min; and group C: 10 min of exposure repeated at 10-min intervals over a period of 180 min (total exposure time, 90 min). Seventy-two hours after exposure to hypoxia, the cerebral cortex was examined to assess the degree of neuronal necrosis and brain damage was classified into four grades of severity, 0-3. To evaluate the extent of brain damage, we used immunohistochemical staining with TIB-128 antibody, which reacts to MAC-1 antigen specific to microglia, and observed the glial reaction in the cerebral cortex, hippocampus, thalamus, and striatum.

RESULTS

All the brain damage observed in groups A-C occurred on the side where the ligation was performed. The most severe damage was found in group A animals, of which seven showed significant neuronal necrosis, having a grade 2 or more advanced lesion. In group B, neuronal necrosis was modest, with only one animal having a grade 2 lesion. In group C, a significant neuronal necrosis was found in six animals despite having the same period of hypoxic exposure as those in group B. MAC-1 positive cells appeared in the cerebral cortex of histologically damaged animals and extended to the hippocampus, thalamus, and striatum in severely damaged animals from groups A, B, and C.

CONCLUSION

Examination of the neonatal rat model suggested that repetitive and intermittent, rather than continuous hypoxia-ischemia, causes pronounced damage in the immature brain.