Effects Of Resuscitation With Room Air Or Pure Oxygen On Blood Gas And Cerebral SOD Concentration In Asphyxial Neonatal Rats

BackgroundPerinatal asphyxia is still a major clinical problem, leading to neonatal deaths and subsequent neurological handicaps among survivors. Resuscitation after perinatal asphyxia is a common procedure. It is important to develop optimal resuscitation methods to prevent further injury. International consensus statements on resuscitation of the newborn infant emphasize that adequate ventilation is the key to success, and that if assisted ventilation is required, 100% oxygen should be delivered by positive pressure ventilation. Concerns have been raised about the potential adverse effects of 100% oxygen. High concentrations of oxygen lead to generation of radical oxygen species (ROS), which have a role in reperfusion injury after asphyxia. In addition, hyperoxia will contribute to the development of chronic lung disease and retinopathy of prematurity. Thus it is time to doubt that neonatal resuscitation with 100% oxygen is the best choice.However, there were only a few studies focused on the choice of resuscitation gas. From 1980's, the choice of resuscitation gas has been raised. Most of studies have been done in western countries, but rare researches in China. Most of our current understanding concerning the choice of resuscitation gas has come from animal studies over the past two decades. More and more people believe that resuscitation with room airis as good as, or even better than pure oxygen. But recent researches done by Solas were on the other side. Therefore, the answer to the question is still uncertain. The prior studies, comparing effects of different resuscitation gas on the base of postnatal hypoxia-ischemia animal model, might not be suitable to reflect the human condition, because the transition process from fetus to newborn was ignored in such models. So setting a more appropriate animal model is necessary to study the issue on the choice of neonatal resuscitation gas.A number of researches announced that ROS plays the key role in reperfusion injury after asphyxia. ROS can react with the polyunsaturated fatty acid, side chains of membrane lipids, initiating lipid peroxidation. The resulting lipid hydroperoxide products are potent inhibitors of cellular enzymes and can directly damage proteins or membranes. ROS can also directly damage DNA and cause protein sulfhydryl oxidation. Superoxide dimutase (SOD) is one of the main enzymes, which can transfer ROS to less toxic materials. It is considered that the amount of SOD is an indicator of ROS level in tissue.In our studies, we established the model of fetal asphyxial rats, which was more suitable for the research of neonatal resuscitation with room air or pure oxygen We compared the effects of resuscitation with room air or pure oxygen in neonatal rats with intrauterine asphyxia, including early neonatal mortality, blood gas and cerebral SOD concentration. The aim of the study was to find out whether resuscitation with pure oxygen would lead to hyperoxemia and increase the generation of ROS in perinatal asphyxial neonatal rats compared with room air.Materials and methodsPart one: Establishment of fetal asphyxia model in ratsTo compare the effects of resuscitation gas with room air or pure oxygen, fetal asphyxia model, described previously by Loidl et al. (1994, 2000) was established. Within the last day of gestation, eight pregnant SD rats, supplied by Experimental Animal Center of Zhejiang Academy of Medical Science, were sacrificed by neck dislocation and hysterectomized. Neonatal rats used as controls were taken out of uterusas soon as possible, and uterus containing the rest fetal rats was placed in a water bath at 37℃ for 15 minutes. Then uterus horns were rapidly opened and the asphyxial fetal rats were taken out of uterus. We observed the appearance of the neonatal rats soon after birth, and did blood gas analysis within 30 seconds after they were born. The total of neonatal rats in control group is 12, and the number of neonatal rats in fetal asphyxia group is 14. Part two: Effects of resuscitation with room air or oxygen in neonatal rats with intrauterine asphyxiaThe model of neonatal rats with intrauterine asphyxia was established according to the methods described in part one, neonatal rats were randomly divided into three groups: control group, room air resuscitation group (RAR) and pure oxygen resuscitation group (POR). After they were born, asphyxial neonatal rats were dried and placed into an oxyhood filled with room air or pure oxygen, and resuscitated by tactile stimulation to the skin. The duration of resuscitation lasted for 30 minutes. Neonatal rats in control group were dried only. All neonatal rats were kept in the warm condition by heating lamps after they were born. If they had no spontaneous breath with pale skin and had no response to stimulation at the end of resuscitation, the neonatal rats were considered dead and excluded from further studies. The mortality of neonatal rats at 0 hours after resuscitation, blood gas and cerebral SOD at 0,6,24 hours after resuscitation in each group were compared.All data were evaluated by the statistical programs(SPSS 11.0 for windows). The level of P<0.05 was considered statistically significant.ResultsPart one: Assessment of the fetal asphyxia model85.7% (12/14) of neonatal rats in asphyxia group had no spontaneous breath and 14.3% (2/14) had weak breath, all (14/14) neonatal rats were with pale skin. In control group, all (12/12) neonatal rats had vigorous breath with pink skin. The results of blood gas of neonatal rats with intrauterine asphyxia showed hypoxemia with serious mixed acidosis compared to those of control group.Part two: Effects of resuscitation with room air or pure oxygen on blood gas and cerebral SOD concentration in neonatal rats with intrauterine asphyxia1 There was no difference in mortality of neonatal rats among each group. Mortality of neonatal rats in control group, room air resuscitation group and pure oxygen group were 0(0/24), 0 (0/26) and 3.7%(1/27) respectively.2 Blood PO₂ level of neonatal rats in POR at 0 hours after resuscitation was much higher than that of RAR (P=0.004). The averages of Blood PO₂ level in POR and RAR were 69.2 ± 8.2mmHg and 55.5 ± 10.3 mmHg, respectively. Blood pH and PCO₂ and BE levels in each group were similar at 0 hours after resuscitation (P>0.05). Blood pH, PO₂, PCO₂ and BE levels were also similar between POR and RAR at 6, 24 hours after resuscitation.3 Cerebral SOD concentrations of neonatal rats in POR at 0,6 hours after resuscitation were lower than those in RAR (P<0.05). The averages of cerebral SOD concentration in POR at 0,6 hours after resuscitation were 38.3±9.8 U/mgprot and 8.6 ± 3.6U/mgprot, those in RAR were 53.8 ± 10.6 U/mgprot and 13.0±4.6 U/mgprot respectively. Cerebral SOD concentration in POR at 24 hours after resuscitation was not significantly different from that in RAR (P>0.05). Cerebral SOD concentrations in control group at 0,6,24 hours after resuscitation were much higher than those in POR and RAR (P<0.01).Conclusion1 The model of neonatal rats with fetal asphyxia is more suitable than prior models for the study of gas selection in neonatal resuscitation.2 Room air is as good as oxygen in neonatal resuscitation, in respect of early mortality and improvement of acidosis in asphyxial neonatal rats.3 Resuscitation with room air will generate less radical oxygen species than pure oxygen in asphyxial neonatal rats.