Effects of hypoxia and reoxygenation with 21% and 100%-oxygen on cerebral nitric oxide concentration and microcirculation in newborn piglets

Novel interventions to reduce oxidative-stress related brain injury in neonatal asphyxia

Perinatal asphyxia-induced brain injury may present as hypoxic-ischemic encephalopathy in the neonatal period, and disability including cerebral palsy in the long term. The brain injury is secondary to both the hypoxic-ischemic event and the reoxygenation-reperfusion following resuscitation. Early events in the cascade of brain injury can be classified as either inflammation or oxidative stress through the generation of free radicals. The objective of this paper is to present efforts that have been made to limit the oxidative stress associated with hypoxic-ischemic encephalopathy. In the acute phase of ischemia/hypoxia and reperfusion/reoxygenation, the outcomes of asphyxiated infants can be improved by optimizing the initial delivery room stabilization. Interventions include limiting oxygen exposure, and shortening the time to return of spontaneous circulation through improved methods for supporting hemodynamics and ventilation. Allopurinol, melatonin, noble gases such as xenon and argon, and magnesium administration also target the acute injury phase. Therapeutic hypothermia, N-acetylcysteine2-iminobiotin, remote ischemic postconditioning, cannabinoids and doxycycline target the subacute phase. Erythropoietin, mesenchymal stem cells, topiramate and memantine could potentially limit injury in the repair phase after asphyxia. To limit the injurious biochemical processes during the different stages of brain injury, determination of the stage of injury in any particular infant remains essential. Currently, therapeutic hypothermia is the only established treatment in the subacute phase of asphyxia-induced brain injury. The effects and side effects of oxidative stress reducing/limiting medications may however be difficult to predict in infants during therapeutic hypothermia. Future neuroprotection in asphyxiated infants may indeed include a combination of therapies. Challenges include timing, dosing and administration route for each neuroprotectant.

Inhaled Gases for Neuroprotection of Neonates: A Review

Importance: Hypoxic-ischemic encephalopathy (HIE) is a significant cause of morbidity and mortality in neonates. The incidence of HIE is 1-8 per 1,000 live births in developed countries. Whole-body hypothermia reduces the risk of disability or death, but 7 infants needed to be treated to prevent death or major neurodevelopmental disability. Inhalational gases may be promising synergistic agents due to their rapid onset and easy titratability. Objective: To review current data on different inhaled gases with neuroprotective properties that may serve as adjunct therapies to hypothermia. Evidence review: Literature review was performed using the PubMed database, google scholar, and ClinicalTrials.Gov. Results focused on articles published from January 1, 2005, through December 31, 2017. Articles published earlier than 2005 were included when appropriate for historical perspective. Our review emphasized preclinical and clinical studies relevant to the use of inhaled agents for neuroprotection. Findings: Based on the relevance to our topic, 111 articles were selected pertaining to the incidence of HIE, pathophysiology of HIE, therapeutic hypothermia, and emerging therapies for hypoxic-ischemic encephalopathy in preclinical and clinical settings. Supplemental tables summarizes highly relevant 49 publications that were included in this review. The selected publications emphasize the emergence of promising inhaled gases that may improve neurologic survival and alleviate neurodevelopmental disability when combined with therapeutic hypothermia in the future. Conclusions: Many inhaled agents have neuroprotective properties and could serve as an adjunct therapy to whole-body hypothermia. Inhaled agents are ideal due to their easy administration, titrability, and rapid onset and offset.

Cyclosporine treatment reduces oxygen free radical generation and oxidative stress in the brain of hypoxia-reoxygenated newborn piglets

Oxygen free radicals have been implicated in the pathogenesis of hypoxic-ischemic encephalopathy. It has previously been shown in traumatic brain injury animal models that treatment with cyclosporine reduces brain injury. However, the potential neuroprotective effect of cyclosporine in asphyxiated neonates has yet to be fully studied. Using an acute newborn swine model of hypoxia-reoxygenation, we evaluated the effects of cyclosporine on the brain, focusing on hydrogen peroxide (H(2)O(2)) production and markers of oxidative stress. Piglets (1-4 d, 1.4-2.5 kg) were block-randomized into three hypoxia-reoxygenation experimental groups (2 h hypoxia followed by 4 h reoxygenation) (n = 8/group). At 5 min after reoxygenation, piglets were given either i.v. saline (placebo, controls) or cyclosporine (2.5 or 10 mg/kg i.v. bolus) in a blinded-randomized fashion. An additional sham-operated group (n = 4) underwent no hypoxia-reoxygenation. Systemic hemodynamics, carotid arterial blood flow (transit-time ultrasonic probe), cerebral cortical H(2)O(2) production (electrochemical sensor), cerebral tissue glutathione (ELISA) and cytosolic cytochrome-c (western blot) levels were examined. Hypoxic piglets had cardiogenic shock (cardiac output 40-48% of baseline), hypotension (mean arterial pressure 27-31 mmHg) and acidosis (pH 7.04) at the end of 2 h of hypoxia. Post-resuscitation cyclosporine treatment, particularly the higher dose (10 mg/kg), significantly attenuated the increase in cortical H(2)O(2) concentration during reoxygenation, and was associated with lower cerebral oxidized glutathione levels. Furthermore, cyclosporine treatment significantly attenuated the increase in cortical cytochrome-c and lactate levels. Carotid blood arterial flow was similar among groups during reoxygenation. Conclusively, post-resuscitation administration of cyclosporine significantly attenuates H(2)O(2) production and minimizes oxidative stress in newborn piglets following hypoxia-reoxygenation.

Oxygen resuscitation after hypoxia ischemia stimulates prostaglandin pathway in rat cortex

Exposure to hypoxia and hyperoxia in a rodent model of perinatal ischemia results in delayed cell death and inflammation. Hyperoxia increases oxidative stress that can trigger inflammatory cascades, neutrophil activation, and brain microvascular injury. Here we show that 100% oxygen resuscitation in our rodent model of perinatal ischemia increases cortical COX-2 protein levels, S-nitrosylated COX-2cys526, PGE2, iNOS and 5-LOX, all components of the prostaglandin and leukotriene inflammatory pathway.

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.

Neonatal resuscitation: Current issues

The following guidelines are intended for practitioners responsible for resuscitating neonates. They apply primarily to neonates undergoing transition from intrauterine to extrauterine life. The updated guidelines on Neonatal Resuscitation have assimilated the latest evidence in neonatal resuscitation. Important changes with regard to the old guidelines and recommendations for daily practice are provided. Current controversial issues concerning neonatal resuscitation are reviewed and argued in the context of the ILCOR 2005 consensus.

Effects of postresuscitation N-acetylcysteine on cerebral free radical production and perfusion during reoxygenation of hypoxic newborn piglets

Hydrogen peroxide (H2O2) and nitric oxide (NO) contribute to the pathogenesis of cerebral hypoxic-ischemic injury. We evaluated the neuroprotective effect of N-acetyl-l-cysteine (NAC, a free radical scavenger) against oxidative stress and perfusion in a model of neonatal hypoxia-reoxygenation (H-R). Piglets (1-3 d, 1.6-2.3 kg) were randomized into a sham-operated group (without H-R) (n = 5) and two H-R experimental groups (2 h normocapnic alveolar hypoxia followed by 4 h reoxygenation) (n = 7/group). Five minutes after reoxygenation, piglets were given either i.v. saline (H-R controls) or NAC (30 mg/kg bolus then 20 mg/kg/h infusion) in a blinded-randomized fashion. Heart rate, mean arterial pressure, carotid arterial blood flow (transit-time ultrasonic probe), cerebral cortical H2O2 and NO production (electrochemical sensor), cerebral tissue glutathione and nitrotyrosine levels (enzyme-linked immunosorbent assay) were examined. Hypoxic piglets were acidotic (pH 6.88-6.90), which recovered similarly in the H-R groups (p > 0.05 versus shams). Postresuscitation NAC treatment significantly attenuated the increase in cortical H2O2, but not NO, concentration during reoxygenation, with lower cerebral oxidized glutathione levels. NAC-treated piglets had significantly higher carotid oxygen delivery and lower cerebral lactate levels than that of H-R controls with corresponding changes in carotid arterial flow and vascular resistance. In newborn piglets with H-R, postresuscitation administration of NAC reduced cerebral oxidative stress and improved cerebral perfusion.

N-acetylcysteine improves the hemodynamics and oxidative stress in hypoxic newborn pigs reoxygenated with 100% oxygen

Neonatal asphyxia may lead to cardiac and renal complications perhaps mediated by oxygen free radicals. Using a model of neonatal hypoxia-reoxygenation, we tested the hypothesis that N-acetylcysteine (NAC) would improve cardiac function and renal blood flow. Eighteen piglets (aged 1-4 days old, weighing 1.4-2.2 kg) were anesthetized and acutely instrumented for continuous monitoring of pulmonary and renal artery flow (cardiac index [CI] and renal artery flow index [RAFI], respectively) and mean blood pressure. Alveolar hypoxia was induced for 2 h, followed by resuscitation with 100% oxygen for 1 h and 21% oxygen for 3 h. Animals were randomized to sham-operated, hypoxic control, and NAC treatment (i.v. bolus of 150 mg/kg given at 10 min of reoxygenation followed by 100 mg/kg per h infusion) groups. Myocardial and renal tissue glutathione content and lipid hydroperoxide levels were assayed, and histology was examined. After 2 h of hypoxia, all animals were acidotic (pH 6.96 +/- 0.04) and in cardiogenic shock with depressed renal blood flow. Upon reoxygenation, CI and RAFI increased but gradually deteriorated later. The NAC treatment prevented the decreased CI, stroke volume, mean blood pressure, systemic oxygen delivery, RAFI, and renal oxygen delivery at 2 to 4 h of reoxygenation observed in hypoxic controls (versus shams, all P < 0.05). The myocardial and renal tissue glutathione content was significantly higher in the NAC treatment group (versus controls). The CI and RAFI at 4 h of reoxygenation correlated with the tissue glutathione redox ratio (r = 0.5 and 0.6, respectively, P < 0.05). There were no significant differences in heart rate, pulmonary artery pressure, systemic oxygen uptake, and tissue lipid hydroperoxide levels between groups. No histologic injury was found in the heart or kidney. In this porcine model of neonatal hypoxia and 100% reoxygenation, NAC improved cardiac function and renal perfusion, with improved tissue glutathione content.

Effects of hyperoxia and nitric oxide on endogenous nitric oxide production in polymorphonuclear leukocytes

BACKGROUND

Exposure to hyperoxia and nitric oxide (NO) occur frequently during the treatment of neonatal hypoxic pulmonary failure.

OBJECTIVE

The aim of the study was to quantify the endogenous synthesis of NO in neonatal polymorphonuclear neutrophils following exposure to hyperoxia and NO in vitro.

METHODS

Neonatal cord blood was exposed to room air, 25, 30 and 100% oxygen and 10 or 20 ppm NO added to the different oxygen concentrations for up to 30 min. 4,5-Diaminofluorescein diacetate (DAF-2 DA) is an intracellular dye used to measure real-time changes in NO levels in vivo. The molecular structure of DAF-2 DA changes upon contact with NO to its oxidized and fluorescent form diaminofluorescein-triazol (DAF-2T) and after being hydrolyzed by intracellular esterases cannot leave the cell. DAF-2 DA signals following equilibration with room air were used as controls.

RESULTS

Exposure to 100% oxygen increased NO production significantly when compared to 20 ppm NO plus 100% oxygen (p = 0.031) and to 20 ppm NO alone (p = 0.006). 10 ppm NO produced a similar effect. Significant increases in NO production were also noticed following exposure to 25% oxygen. This increase was already present after 10 min of oxygen exposure.

CONCLUSION

These findings support the propagated avoidance of hyperoxia not only in preterm infants, but also in term neonates.

A dose-response study of graded reoxygenation on the carotid haemodynamics, matrix metalloproteinase-2 activities and amino acid concentrations in the brain of asphyxiated newborn piglets

PURPOSE

It is controversial to choose an appropriate oxygen concentration to resuscitate asphyxiated newborns regarding the clinical and biochemical oxidative effects. We examined the vasomotor response to reoxygenation with graded reoxygenation and the effects on matrix metalloproteinases and amino acids of the immature brain.

METHODS

Thirty-two piglets (1-3 days, 1.5-2.1 kg) were instrumented for continuous monitoring of left common carotid and pulmonary arterial flows (Transonic). Piglets were randomized to a sham-operated control group (without hypoxia/reoxygenation) or 2 h hypoxia induced by decreasing the inspired oxygen concentration to 10-15%, followed by reoxygenation with 21, 50 or 100% oxygen for 1 h and then 21% oxygen for 3 h (n=8 each). The brains were then flash frozen and analyzed for matrix metalloproteinases and amino acid levels by zymography and HPLC, respectively.

RESULTS

After 2 h oxygen deprivation, the absolute carotid flow remained similar but accounted for 38% of cardiac output (increased from 17% at baseline, p=0.001). During early reoxygenation, the flow rose in the piglets resuscitated with air (p<0.05), but not in those with supplemental oxygen. Carotid vascular resistance correlated significantly with the arterial partial pressure of oxygen (r=0.7). There was an oxygen-dependent increase in global cerebral activity of matrix metalloproteinase-2 with specific increases in the basal ganglia of all hypoxic-reoxygenated brains. There were no significant differences in glutamate and other amino acids in any brain regions.

CONCLUSIONS

Although using high oxygen concentration to resuscitate asphyxiated newborn piglets increased carotid vascular resistance and cerebral matrix metalloproteinase-2 activity, there is no detrimental effect observed in this acute model of hypoxia-reoxygenation.

Changes of neuronal activity in areas CA1 and CA3 during anoxia and normoxic or hyperoxic reoxygenation in juvenile rat organotypic hippocampal slice cultures

In neonates, asphyxia is usually followed by hyperoxic treatment. In order to study whether hyperoxic reoxygenation might cause additional impairment of neuronal function, we subjected organotypic hippocampal slice cultures of juvenile rats (7 DIV, P6-8) to 30 min anoxia followed by 60 min hyperoxic or normoxic reoxygenation (95% or 19% O2, respectively). Spontaneous and evoked field potentials as well as [Ca2+]o were recorded in the pyramidal layer of area CA1 or area CA3. In area CA1, 30 min of anoxia led to decline of evoked field potential amplitudes by on average 67% and to profound changes in field potential characteristics and Ca2+ homeostasis which were not related to outcome after reoxygenation. Hyperoxic reoxygenation resulted first in a fast recovery of the field potential amplitude to 82% of the control value and then, in 75% of slice cultures, in a large negative field potential shift accompanied by a prolonged decrease of [Ca2+]o and loss of excitability outlasting the experiment. Recovery of field potential amplitude under normoxic conditions stayed poor, with a first increase to 51% and a second decrease to 22%. In contrast, field potential amplitude in area CA3 recovered to 80% of the initial amplitude, irrespective of the reoxygenation mode. The selective loss of function during hyperoxic reoxygenation in area CA1 might be a first sign of neuronal injury that we observed 1 h after end of hyperoxic reoxygenation in a previous study. Whether the poor outcome after normoxic reoxygenation would favour long-term recovery remains to be determined.

Physiologically relevant measurements of nitric oxide in cardiovascular research using electrochemical microsensors

Nitric oxide (NO) plays an important role in the regulation of blood flow. Pharmacological tools and a series of other techniques have been developed for studying the NO/L-arginine pathway, but it has proved difficult to make a quantitative link between effect and tissue NO concentration. NO microsensors have been applied with success for the measurement of NO in suspensions of mitochondria and cells, such as platelets and leukocytes, and in cell cultures, which together with other interventions or measurements are particularly useful for the examination of cell signalling related to the NO/L-arginine pathway. In isolated vascular segments, studies using the NO microsensor have defined the relationship between NO concentration and relaxation and revealed residual NO release in the presence of NO synthase inhibitors. Moreover, simultaneous measurements of NO concentration and vasorelaxation in isometric preparations have shown that agonist-induced relaxation is L-arginine dependent and NO release is reduced in hypertension. By placing NO microsensors in catheters, it is possible to measure NO in the living animal and man. This approach has been applied for the measurements of NO concentration in relation to increases in flow, erection, in conditions of hypoxia, and in endotoxemia. However, further methodological development of NO microsensors is necessary to avoid the influence of changes in temperature, pH and oxygen on the measurements.

The role of oxygen in neonatal resuscitation

New knowledge has accumulated in recent years making it prudent to ask questions regarding current oxygenation policies and guidelines. Because new-born resuscitation affects so many individuals, and because resuscitation procedures may have dramatic consequences on infant and child health, intensified discussion and research in this field are not only necessary but are a requirement. In particular, there is a lack of data on infants born before term. It is difficult to give absolute recommendations on which oxygen concentration should be applied for newborn resuscitation; however, it seems that ambient air is safe. It is easy to handle, is always at hand, and is inexpensive. Conversely, regarding 100% O2, I believe we have sufficient data to conclude that this should not be given routinely at birth to depressed infants; however, whether it is beneficial or harmful to start out resuscitation with 30%, 40%, or 60% O2 is not known. No data exist to answer this question. A call for more research in this area is timely. The effect of pure oxygen on cell growth and cell death, gene activation, and possibly DNA damage should be carefully investigated. Even before such data are collected, it is known that pure oxygen at birth triggers long-term and poorly understood effects. Oxygen obviously is more toxic than previously thought, and oxygen given to small infants has a 50-year history of uncertain benefits. Table 1 summarizes the pros and cons of using 21%versus 100% 02 for newborn resuscitation. Brain circulation as assessed by microspheres is restored as quickly with 21% O2 as it is with 100% O2; however, microcirculation is somewhat slower. Metabolism, pulmonary flow, and myocardial performance are normalized just as quickly by 21% and 100% O2. Brain injury as assessed by glycerol augmentation, matrix injury, and neonatal mortality is less in infants given 21% versus 100% 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.

A consideration of neonatal resuscitation

There are currently two major areas of resuscitation of the newborn which have come into question: the use of intermittent positive pressure ventilation and the use of oxygen. There is evolving evidence that volutrauma associated with IPPV, especially in the premature infant, may induce changes in the lung which can lead to chronic lung disease. There is reason to believe that the use of continuous positive airway pressure in premature infants who are making respiratory efforts may be less harmful than the use of IPPV. With regard to the use of oxygen, it is clear that most infants can be successfully resuscitated with room air. Although we can identify markers for oxidative stress in newborns when resuscitated with 100% oxygen, the clinical importance of these markers remain an open issue. If the presence of these markers after resuscitation is shown to relate to clinical problems, then the use of oxygen may need to be considered.

Nitric oxide synthesis inhibition during cerebral hypoxemia and reoxygenation with 100% oxygen in newborn pigs

The objective of our study was to evaluate the effects of N(sigma)-nitro-L-arginine methyl ester (L-NAME), an inhibitor of the nitric oxide (NO) pathway, on cerebral microcirculation during hypoxemia and reoxygenation with 100% oxygen in newborn pigs. Twenty-two pigs were randomized to hypoxemia [inspired fraction of oxygen (FIO(2)) 0.08; 20 min] and reoxygenation (FIO(2) 1.0; 60 min) or normoxia. The hypoxemic animals were further randomized to receive either an intravenous bolus injection of 5 mg/kg L-NAME (n = 8) or a corresponding volume of isotonic saline (n = 8) 30 min before the onset of hypoxemia. The normoxemic group (n = 6) received the same pretreatment with L-NAME. Cerebral hemodynamics were assessed by laser Doppler flowmetry and intracranial pressure monitoring. The cerebral NO concentration was continuously measured using an electrochemical sensor. Pretreatment with L-NAME resulted in a more severe systemic hypotension and reduced cerebral microcirculation during the period of hypoxemia compared with the saline/hypoxemia group. NO synthesis inhibition during reoxygenation with 100% oxygen, however, blunted the increase in NO concentration (p < 0.05) without reduction of cerebral blood flow and cerebral perfusion pressure. In conclusion, in this newborn pig model, pretreatment with a bolus infusion of L-NAME induced severe hypotension and reduced cerebral microcirculation during hypoxemia. However, it appears to have no significant adverse effect on cerebral hemodynamics during the period of reoxygenation with 100% oxygen. This deleterious effect during hypoxemia limits the use of L-NAME as a preventive drug but suggests beneficial effects during reoxygenation with 100% oxygen.

Fetal lamb cerebral blood flow (CBF) and oxygen tensions during hypoxia: a comparison of laser Doppler and microsphere measurements of CBF

This study was undertaken to compare microsphere and laser Doppler flowmetry techniques for the measurement of cerebral blood flow, to assess the effect of probe implantation at the tip of the sensing probe and to measure brain tissue P(O2) (tP(O2)) in response to acute hypoxia. Fetal sheep of ~131 days gestation (n = 8) were chronically instrumented with bilateral laser Doppler probes in the parietal cortices and catheters for injection of fluorescent microspheres. Five days after surgery fetuses were subjected to 1 h periods of baseline control breathing, hypoxia and recovery. Microspheres were injected 10 min prior to and 10, 30, 50 and 120 min after initiation of hypoxia. Microspheres were counted in four 12 mm(3) tissue samples from each hemisphere, the tip of the laser Doppler probe being positioned in the centre of one of the cubes. The cube containing the probe tip was also subdivided into 4 mm(3) pieces of tissue. In response to hypoxia, fetal arterial P(O2) declined from 21 +/- 2 to 12 +/- 1 Torr and brain tissue P(O2) fell from 10 +/- 1 to a nadir of 1 +/- 1 Torr. Each method detected a significant increase in CBF that reached a maximum after 30-45 min, although the increase of flow measured by laser Doppler flowmetry was less than that measured by spheres after 10 and 30 min (P < 0.05). Microspheres did not detect altered flow at the probe tip or heterogeneity of flow in surrounding volumes of cortical tissue. In summary, laser Doppler flowmetry is a useful measure of continuous relative changes of CBF in the chronically instrumented fetal sheep. Flow compensations in acute hypoxia are not adequate to sustain O(2) delivery, and other compensations, including reduced metabolic rate, are possible.

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.

Acute neuronal injury after hypoxia is influenced by the reoxygenation mode in juvenile hippocampal slice cultures

In neonates asphyxia is usually followed by hyperoxia due to resuscitation procedures. In order to study whether hyperoxic reoxygenation might cause additional cell injury we subjected organotypic hippocampal slice cultures of juvenile rats to normoxic or hyperoxic reoxygenation (19 or 85% oxygen, respectively) following hypoxia (3% oxygen) for 30, 60, and 120 min. Cell injury was quantified by lactate dehydrogenase (LDH) release and propidium iodide (PI) fluorescence 1 h after end of the reoxygenation period. In both experimental groups, LDH activity was significantly enhanced by hypoxia as compared to normoxic controls. However, hyperoxic reoxygenation caused a larger increase in LDH activity than normoxic reoxygenation (e.g., by a factor of 1.60 vs. 1.29 following 120 min hypoxia). PI fluorescence increased after hypoxia in all principal cell layers of the hippocampus but again showed a larger enhancement after hyperoxic reoxygenation as compared to normoxic reoxygenation (e.g., by a factor of 3.9 vs. 1.7 for CA1 and 120 min of hypoxia). After normoxic reoxygenation, PI fluorescence intensity was lower in the dentate gyrus as compared to CA1 and CA3, while it reached similar values like CA1 after high oxygen supply. In conclusion, juvenile hippocampal slice cultures subjected to hyperoxic reoxygenation display a greater amount of acute neuronal injury than slice cultures undergoing normoxic reoxygenation.

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.

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.