Regulation of cerebral blood flow after asphyxia in neonatal lambs

Meconium Aspiration Syndrome, Hypoxic-Ischemic Encephalopathy and Therapeutic Hypothermia-A Recipe for Severe Pulmonary Hypertension?

Hypoxic-ischemic encephalopathy (HIE) is the leading cause of mortality among term newborns globally. Infants born through meconium-stained amniotic fluid are at risk of developing meconium aspiration syndrome (MAS) and HIE. Simultaneous occurrence of MAS and HIE is a perilous combination for newborns due to the risk of persistent pulmonary hypertension of the newborn (PPHN). Moreover, therapeutic hypothermia (TH), which is the current standard of care for the management of HIE, may increase pulmonary vascular resistance (PVR) and worsen PPHN. Infants with MAS and HIE require close cardiorespiratory and hemodynamic monitoring for PPHN. Therapeutic strategies, including oxygen supplementation, ventilation, use of surfactant, inhaled nitric oxide and other pulmonary vasodilators, and systemic vasopressors, play a critical role in the management of PPHN in MAS, HIE, and TH. While TH reduces death or disability in infants with HIE, infants with MAS and HIE undergoing TH need close hemodynamic monitoring for PPHN.

Ultrasound Diagnosis and Near-Infrared Spectroscopy in the Study of Encephalopathy in Neonates Born under Asphyxia: Narrative Review

Brain injury resulting from adverse events during pregnancy and delivery is the leading cause of neonatal morbidity and disability. Surviving neonates often suffer long-term motor, sensory, and cognitive impairments. Birth asphyxia is among the most common causes of neonatal encephalopathy. The integration of ultrasound, including Doppler ultrasound, and near-infrared spectroscopy (NIRS) offers a promising approach to understanding the pathology and diagnosis of encephalopathy in this special patient population. Ultrasound diagnosis can be very helpful for the assessment of structural abnormalities associated with neonatal encephalopathy such as alterations in brain structures (intraventricular hemorrhage, infarcts, hydrocephalus, white matter injury) and evaluation of morphologic changes. Doppler sonography is the most valuable method as it provides information about blood flow patterns and outcome prediction. NIRS provides valuable insight into the functional aspects of brain activity by measuring tissue oxygenation and blood flow. The combination of ultrasonography and NIRS may produce complementary information on structural and functional aspects of the brain. This review summarizes the current state of research, discusses advantages and limitations, and explores future directions to improve applicability and efficacy.

Oxygen Targets in Neonatal Pulmonary Hypertension: Individualized, "Precision-Medicine" Approach

Oxygen is a specific pulmonary vasodilator. Hypoxemia causes pulmonary vasoconstriction, and normoxia leads to pulmonary vasodilation. However, hyperoxia does not enhance pulmonary vasodilation but causes oxidative stress. There are no clinical trials evaluating optimal oxygen saturation or Pao2 in pulmonary hypertension. Data from translational studies and case series suggest that oxygen saturation of 90% to 97% or Pao2 between 50 and 80 mm Hg is associated with the lowest pulmonary vascular resistance.

Creatine supplementation reduces the cerebral oxidative and metabolic stress responses to acute in utero hypoxia in the late-gestation fetal sheep

Abstract

Prophylactic creatine treatment may reduce hypoxic brain injury due to its ability to sustain intracellular ATP levels thereby reducing oxidative and metabolic stress responses during oxygen deprivation. Using microdialysis, we investigated the real‐time in vivo effects of fetal creatine supplementation on cerebral metabolism following acute in utero hypoxia caused by umbilical cord occlusion (UCO). Fetal sheep (118 days’ gestational age (dGA)) were implanted with an inflatable Silastic cuff around the umbilical cord and a microdialysis probe inserted into the right cerebral hemisphere for interstitial fluid sampling. Creatine (6 mg kg⁻¹ h⁻¹) or saline was continuously infused intravenously from 122 dGA. At 131 dGA, a 10 min UCO was induced. Hourly microdialysis samples were obtained from −24 to 72 h post‐UCO and analysed for percentage change of hydroxyl radicals (^•OH) and interstitial metabolites (lactate, pyruvate, glutamate, glycerol, glycine). Histochemical markers of protein and lipid oxidation were assessed at post‐mortem 72 h post‐UCO. Prior to UCO, creatine treatment reduced pyruvate and glycerol concentrations in the microdialysate outflow. Creatine treatment reduced interstitial cerebral ^•OH outflow 0 to 24 h post‐UCO. Fetuses with higher arterial creatine concentrations before UCO presented with reduced levels of hypoxaemia (PO2 and SO2) during UCO which associated with reduced interstitial cerebral pyruvate, lactate and ^•OH accumulation. No effects of creatine treatment on immunohistochemical markers of oxidative stress were found. In conclusion, fetal creatine treatment decreased cerebral outflow of ^•OH and was associated with an improvement in cerebral bioenergetics following acute hypoxia.

Key points

Fetal hypoxia can cause persistent metabolic and oxidative stress responses that disturb energy homeostasis in the brain. Creatine in its phosphorylated form is an endogenous phosphagen; therefore, supplementation is a proposed prophylactic treatment for fetal hypoxia.

Fetal sheep instrumented with a cerebral microdialysis probe were continuously infused with or without creatine‐monohydrate for 10 days before induction of 10 min umbilical cord occlusion (UCO; 131 days’ gestation). Cerebral interstitial fluid was collected up to 72 h following UCO.

Prior to UCO, fetal creatine supplementation reduced interstitial cerebral pyruvate and glycerol concentrations.

Fetal creatine supplementation reduced cerebral hydroxyl radical efflux up to 24 h post‐UCO. Fetuses with higher arterial creatine concentrations before UCO and reduced levels of systemic hypoxaemia during UCO were associated with reduced cerebral interstitial pyruvate, lactate and ^•OH following UCO.

Creatine supplementation leads to some improvements in cerebral bioenergetics following in utero acute hypoxia.

Early postnatal color Doppler changes in neonates receiving delivery room resuscitation with low 5 min Apgar score-a pilot study

AIMS AND OBJECTIVES

To evaluate the Doppler changes in the intracranial arteries of neonates exposed to perinatal hypoxic insult and compare it with normal neonates.

MATERIALS AND METHODS

Color Doppler of bilateral anterior and middle cerebral arteries was performed within 6 h of birth in 26 healthy neonates and 50 neonates who received delivery room resuscitation (DRR) for perinatal depression and had a 5 min Apgar score <7. Comparisons of resistive index (RI) and peak systolic velocity (PSV) were made between the (a) control group (b) patients with low 5 min Apgar score <7 who without clinical features of neonatal encephalopathy at 24 h (c) neonates with perinatal depression with a clinical evidence of disturbed neurological function at 24 h of birth and examination consistent with mild, moderate, or severe encephalopathy using modified Sarnat and Sarnat's classification.

RESULTS

Significantly higher RI was observed in the neonates with to perinatal depression compared to the normal neonates. Significantly higher RI was seen in the patients with clinical features of neonatal encephalopathy (Group C) compared to group B. RI <0.6 and >0.82 was associated with severe neonatal encephalopathy. Differences in PSV were not statistically significant in the various groups.

CONCLUSION

The study presents the changes in early cerebral Doppler parameters observed in neonates with low 5 min Apgar score following DRR compared to the normal neonates. We also present the relations of Doppler parameters with increasing severity of neonatal encephalopathy according to Sarnat classification.

Pathophysiology of Cerebral Hyperperfusion in Term Neonates With Hypoxic-Ischemic Encephalopathy: A Systematic Review for Future Research

Worldwide neonatal hypoxic-ischemic encephalopathy (HIE) is a common cause of mortality and neurologic disability, despite the implementation of therapeutic hypothermia treatment. Advances toward new neuroprotective interventions have been limited by incomplete knowledge about secondary injurious processes such as cerebral hyperperfusion commonly observed during the first 1-5 days after asphyxia. Cerebral hyperperfusion is correlated with adverse neurodevelopmental outcome and it is a process that remains poorly understood. In order to provide an overview of the existing knowledge on the pathophysiology and highlight the gaps in current understanding of cerebral hyperperfusion in term animals and neonates with HIE, we performed a systematic research. We included papers scoping for study design, population, number of participants, study technique and relevant findings. Methodological quality was assessed using the checklist for cohort studies from The Joanna Briggs Institute. Out of 2,690 results, 34 studies were included in the final review-all prospective cohort studies. There were 14 studies of high, 17 moderate and 3 of low methodological quality. Data from the literature were analyzed in two main subjects: (1) Hemodynamic Changes subdivided into macro- and microscopic hemodynamic changes, and (2) Endogenous Pathways which was subdivided into N-methyl-D-aspartate/Mitogen activated protein kinase (NDMA/MAPK), Nitric Oxide (NO), prostanoids and other endogenous studies. Cerebral hyperperfusion in term neonates with HIE was found to be present 10-30 min after the hypoxic-ischemic event and was still present around day 10 and up to 1 month after birth. Cerebral hyperperfusion was also characterized by angiogenesis and cerebral vasodilation. Additionally, cerebral vasodilation was mediated by endogenous pathways such as MAPK through urokinase Plasminogen Activator (uPA), by neuronal NO synthase following NMDA and by prostanoid synthesis. Future research should elucidate the precise role of NMDA, MAPK and prostanoids in cerebral hyperperfusion. Moreover, research should focus on possible interventions and the effect of hypothermia on hyperperfusion. These findings should be taken into account simultaneously with brain imagining techniques, becoming a valuable asset in assessing the impact in neurodevelopmental outcome.

Protection from systemic pyruvate at resuscitation in newborn lambs with asphyxial cardiac arrest

BACKGROUND

Infants with hypoxic-ischemic injury often require cardiopulmonary resuscitation. Mitochondrial failure to generate adenosine triphosphate (ATP) during hypoxic-ischemic reperfusion injury contributes to cellular damage. Current postnatal strategies to improve outcome in hypoxic-ischemic injury need sophisticated equipment to perform servo-controlled cooling. Administration of intravenous pyruvate, an antioxidant with favorable effects on mitochondrial bioenergetics, is a simple intervention that can have a global impact. We hypothesize that the administration of pyruvate following the return of spontaneous circulation (ROSC) would improve cardiac function, systemic hemodynamics, and oxygen utilization in the brain in newborn lambs with cardiac arrest (CA).

METHODS

Term lambs were instrumented, delivered by C-section and asphyxia induced by umbilical cord occlusion along with clamping of the endotracheal tube until asystole; Lambs resuscitated following 5 min of CA; upon ROSC, lambs were randomized to receive pyruvate or saline infusion over 90 min and ventilated for 150 min postinfusion. Pulmonary and systemic hemodynamics and arterial gases monitored. We measured plasma pyruvate, tissue lactate, and ATP levels (heart and brain) in both groups.

RESULTS

Time to ROSC was not different between the two groups. Systolic and diastolic blood pressures, stroke volume, arterial oxygen content, and cerebral oxygen delivery were similar between the two groups. The cerebral metabolic rate of oxygen was higher following pyruvate infusion; higher oxygen consumption in the brain was associated with lower plasma levels but higher brain ATP levels compared to the saline group.

CONCLUSIONS

Pyruvate promotes energy generation accompanied by efficient oxygen utilization in the brain and may facilitate additional neuroprotection in the presence of hypoxic-ischemic injury.

Neonates with hypoxic-ischemic encephalopathy treated with hypothermia: Observations in a large Canadian population and determinants of death and/or brain injury

BACKGROUND

Birth asphyxia in term neonates remains a serious condition that causes significant mortality and long-term neurodevelopmental sequelae despite hypothermia treatment. The objective of this study was to review therapeutic hypothermia practices in a large population of neonates with hypoxic-ischemic encephalopathy (HIE) across Canada and to identify determinants of adverse outcome.

METHODS

Our retrospective observational cohort study examined neonates≥36 weeks, admitted to the Canadian Neonatal Network NICUs between 2010 and 2014, diagnosed with HIE, and treated with hypothermia. Adverse outcome was defined as death and/or brain injury. Maternal, birth, and postnatal characteristics were compared between neonates with adverse outcome and those without. The association between the variables which were significantly different (p < 0.05) between the two groups and adverse outcome were further tested, while adjusting for gestational age, birth weight, gender, and initial severity of encephalopathy.

RESULTS

A total of 2187 neonates were admitted for HIE; 52% were treated with hypothermia and 40% developed adverse outcome. Initial severity of encephalopathy (moderate, p = 0.006; severe, p < 0.0001), hypotension treated with inotropes (p = 0.001), and renal failure (p = 0.007) were significantly associated with an increased risk of death and/or brain injury.

CONCLUSIONS

In asphyxiated neonates treated with hypothermia, not only their initial severity of encephalopathy on admission, but also their cardiac and renal complications during the first days after birth were significantly associated with risk of death and/or brain injury. Careful monitoring and cautious management of these complications is warranted.

Optimal Chest Compression Rate and Compression to Ventilation Ratio in Delivery Room Resuscitation: Evidence from Newborn Piglets and Neonatal Manikins

Cardiopulmonary resuscitation (CPR) duration until return of spontaneous circulation (ROSC) influences survival and neurologic outcomes after delivery room (DR) CPR. High quality chest compressions (CC) improve cerebral and myocardial perfusion. Improved myocardial perfusion increases the likelihood of a faster ROSC. Thus, optimizing CC quality may improve outcomes both by preserving cerebral blood flow during CPR and by reducing the recovery time. CC quality is determined by rate, CC to ventilation (C:V) ratio, and applied force, which are influenced by the CC provider. Thus, provider performance should be taken into account. Neonatal resuscitation guidelines recommend a 3:1 C:V ratio. CCs should be delivered at a rate of 90/min synchronized with ventilations at a rate of 30/min to achieve a total of 120 events/min. Despite a lack of scientific evidence supporting this, the investigation of alternative CC interventions in human neonates is ethically challenging. Also, the infrequent occurrence of extensive CPR measures in the DR make randomized controlled trials difficult to perform. Thus, many biomechanical aspects of CC have been investigated in animal and manikin models. Despite mathematical and physiological rationales that higher rates and uninterrupted CC improve CPR hemodynamics, studies indicate that provider fatigue is more pronounced when CC are performed continuously compared to when a pause is inserted after every third CC as currently recommended. A higher rate (e.g., 120/min) is also more fatiguing, which affects CC quality. In post-transitional piglets with asphyxia-induced cardiac arrest, there was no benefit of performing continuous CC at a rate of 90/min. Not only rate but duty cycle, i.e., the duration of CC/total cycle time, is a known determinant of CC effectiveness. However, duty cycle cannot be controlled with manual CC. Mechanical/automated CC in neonatal CPR has not been explored, and feedback systems are under-investigated in this population. Evidence indicates that providers perform CC at rates both higher and lower than recommended. Video recording of DR CRP has been increasingly applied and observational studies of what is actually done in relation to outcomes could be useful. Different CC rates and ratios should also be investigated under controlled experimental conditions in animals during perinatal transition.

Cardiopulmonary Bypass-Induced Inflammatory Response: Pathophysiology and Treatment

OBJECTIVES

The objectives of this review are to discuss the pathophysiology of the pro-inflammatory response to the cardiopulmonary bypass circuit, the impact of ischemia reperfusion injury on post-operative organ function, the compensatory anti-inflammatory response and the evidence for immune-modulatory strategies and their impact on outcomes.

DATA SOURCE

MEDLINE, PubMed.

CONCLUSION

Innovations such as the development of more biocompatible surfaces and miniaturized circuits, as well as the increasing expertise of surgeons, anesthesiologists and perfusionists has transformed cardiac surgery and the use of cardiopulmonary bypass into a relatively routine procedure with favorable outcomes. Despite these refinements, the attendant inflammatory response to bypass, ischemia reperfusion injury and the compensatory anti-inflammatory response contribute to post-operative morbidity and mortality. Additional studies are needed to further delineate the impact of immunomodulatory strategies on outcomes.

Secondary Increase of Lactate Levels in Asphyxiated Newborns during Hypothermia Treatment: Reflect of Suboptimal Hemodynamics (A Case Series and Review of the Literature)

Objective To evaluate whether a secondary increase of serum lactate levels in asphyxiated newborns during hypothermia treatment may reflect suboptimal dynamics. Methods-Retrospective case series and review of the literature. We present the clinical course of four asphyxiated newborns treated with hypothermia who presented with hypotension requiring inotropic support, and who displayed a secondary increase of serum lactate levels during hypothermia treatment. Serial serum lactate levels are correlated with blood pressure and inotropic support within the first 96 hours of life. Results Lactate levels initially decreased in the four patients. However, each of them started to present lower blood pressure, and lactate levels started to increase again. Inotropic support was started to raise blood pressure. The introduction of an epinephrine drip consistently worsened the increase of lactate levels in these newborns, whereas dopamine and dobutamine enabled the clearance of lactate in addition to raising the blood pressure. Rewarming was associated with hemodynamics perturbations (a decrease of blood pressure and/or an increase of lactate levels) in the three newborns who survived. Conclusions Lactate levels during the first 4 days of life should be followed as a potential marker for suboptimal hemodynamic status in term asphyxiated newborns treated with hypothermia, for whom the maintenance of homeostasis during hypothermia treatment is of utmost importance to alleviate brain injury.

Cerebrovascular injury in premature infants: current understanding and challenges for future prevention

Cerebrovascular insults are a leading cause of brain injury in premature infants, contributing to the high prevalence of motor, cognitive, and behavioral deficits. Understanding the complex pathways linking circulatory immaturity to brain injury in premature infants remains incomplete. These mechanisms are significantly different from those causing injury in the mature brain. The gaps in knowledge of normal and disturbed cerebral vasoregulation need to be addressed. This article reviews current understanding of cerebral perfusion, in the sick premature infant in particular, and discusses challenges that lie ahead.

Post-hypoxic hypoperfusion is associated with suppression of cerebral metabolism and increased tissue oxygenation in near-term fetal sheep

Secondary cerebral hypoperfusion is common following perinatal hypoxia-ischaemia. However, it remains unclear whether this represents a true failure to provide sufficient oxygen and nutrients to tissues, or whether it is simply a consequence of reduced cerebral metabolic demand. We therefore examined the hypothesis that cerebral oxygenation would be reduced during hypoperfusion after severe asphyxia, and further, that the greater neural injury associated with blockade of the adenosine A(1) receptor during the insult would be associated with greater hypoperfusion and deoxygenation. Sixteen near-term fetal sheep received either vehicle or 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) for 1 h, followed by 10 min of severe asphyxia induced by complete occlusion of the umbilical cord. Infusions were discontinued at the end of the occlusion and data were analysed for the following 8 h. A transient, secondary fall in carotid artery blood flow and laser Doppler flow was seen from approximately 1-4 h after occlusion (P < 0.001), with no significant differences between vehicle and DPCPX. Changes in laser Doppler blood flow were highly correlated with carotid blood flow (r(2)= 0.81, P < 0.001). Cortical metabolism was suppressed, reaching a nadir 1 h after occlusion and then resolving. Cortical tissue P(O(2)) was significantly increased at 1, 2 and 3 h after occlusion compared to baseline, and inversely correlated with carotid blood flow (r(2)= 0.69, P < 0.001). In conclusion, contrary to our initial hypothesis, delayed posthypoxic hypoperfusion was associated with suppression of cerebral metabolism and increased tissue P(O(2)), and was not significantly affected by preceding adenosine A1 blockade. These data suggest that posthypoxic hypoperfusion is actively mediated and reflects suppressed cerebral metabolism.

Hypoxic regulation of the fetal cerebral circulation

Fetal cerebrovascular responses to acute hypoxia are fundamentally different from those observed in the adult cerebral circulation. The magnitude of hypoxic vasodilatation in the fetal brain increases with postnatal age although fetal cerebrovascular responses to acute hypoxia can be complicated by age-dependent depressions of blood pressure and ventilation. Acute hypoxia promotes adenosine release, which depresses fetal cerebral oxygen consumption through action of adenosine on neuronal A1 receptors and vasodilatation through activation of A2 receptors on cerebral arteries. The vascular effect of adenosine can account for approximately half the vasodilatation observed in response to hypoxia. Hypoxia-induced release of nitric oxide and opioids can account for much of the adenosine-independent cerebral vasodilatation observed in response to hypoxia in the fetus. Direct effects of hypoxia on cerebral arteries account for the remaining fraction, although the vascular endothelium contributes relatively little to hypoxic vasodilatation in the immature cerebral circulation. In contrast to acute hypoxia, fetal cerebral blood flow tends to normalize during acclimatization to chronic hypoxia even though cardiac output is depressed. However, uncompensated chronic hypoxia in the fetus can produce significant changes in brain structure and function, alteration of respiratory drive and fluid balance, and increased incidence of intracranial hemorrhage and periventricular leukomalacia. At the level of the fetal cerebral arteries, chronic hypoxia increases protein content and depresses norepinephrine release, contractility, and receptor densities associated with contraction but also attenuates endothelial vasodilator capacity and decreases the ability of ATP-sensitive and calcium-sensitive potassium channels to promote vasorelaxation. Overall, fetal cerebrovascular adaptations to chronic hypoxia appear prioritized to conserve energy while preserving basic contractility. Many gaps remain in our understanding of how the effects of acute and chronic hypoxia are mediated in fetal cerebral arteries, but studies of adult cerebral arteries have produced many powerful pharmacological and molecular tools that are simply awaiting application in studies of fetal cerebral artery responses to hypoxia.

Transient NMDA receptor-mediated hypoperfusion following umbilical cord occlusion in preterm fetal sheep

Exposure to severe hypoxia leads to delayed cerebral and peripheral hypoperfusion. There is evidence in the very immature brain that transient abnormal glutaminergic receptor activity can occur during this phase of recovery. We therefore examined the role of N-methyl-D-aspartate (NMDA) receptor activity in mediating secondary hypoperfusion in preterm fetal sheep at 70% of gestation. Fetuses received either sham asphyxia or asphyxia and were studied for 12 h recovery. The specific, non-competitive NMDA receptor antagonist dizocilpine maleate (2 mg kg-1 bolus plus 0.07 mg kg h-1i.v.) or saline (vehicle) was infused from 15 min after asphyxia until 4 h. In the asphyxia-vehicle group abnormal epileptiform EEG transients were observed during the first 4 h of reperfusion, the peak of which corresponded approximately to the nadir in peripheral and cerebral hypoperfusion. Dizocilpine significantly suppressed this activity (2.7+/-1.3 versus 11.2+/-2.7 counts min-1 at peak frequency, P<0.05) and markedly delayed and attenuated the rise in vascular resistance in both peripheral and cerebral vascular beds observed after asphyxia, effectively preventing the initial deep period of hypoperfusion in carotid blood flow and femoral blood flow (P<0.01). However, while continued infusion did attenuate subsequent transient tachycardia, it did not prevent the development of a secondary phase of persistent but less profound hypoperfusion. In conclusion, the present studies suggest that in the immature brain the initial phase of delayed cerebral and peripheral hypoperfusion following exposure to severe hypoxia is mediated by NMDA receptor activity. The timing of this effect in the cerebral circulation corresponds closely to abnormal EEG activity, suggesting a pathological glutaminergic activation that we speculate is related to evolving brain injury.

Length of stay after infant heart surgery is related to cognitive outcome at age 8 years

OBJECTIVE

To test the hypothesis that postoperative length of stay (LOS) after infant heart surgery might be an easily measured surrogate marker for various events that culminate in later adverse cognitive outcome.

METHODS

Among 160 eligible patients with D-transposition of the great arteries undergoing reparative surgery in infancy, 155 (97%) were reevaluated at age 8 years with IQ and achievement testing. We explored whether LOS quartiles were associated with these outcomes when adjusting for perioperative and sociodemographic variables.

RESULTS

Longer cardiac intensive care unit (CICU) LOS quartiles were associated at age 8 years with lower full-scale IQ (P=.02), lower verbal IQ (P=.02), and with tendencies toward lower performance IQ (P=.08) and math achievement (P=.08) in adjusted models. Compared with patients in the first quartile of CICU LOS, those in the fourth quartile had mean scores for full-scale IQ that were lower by 7.2 points (P=.01); verbal IQ, 7.3 points (P=.02); performance IQ, 5.8 points (P=.05); and math achievement, 6.0 points (P=.07). Analyses on hospital LOS quartile were similar.

CONCLUSIONS

Longer postoperative LOS is associated with worse later cognitive function, even when adjusted for perioperative events, perfusion times, and sociodemographic variables. Further research is necessary to determine the mechanisms underlying this relation.

Asphyxia aggravates systemic hypotension but not pulmonary hypertension in piglets with meconium aspiration

Meconium aspiration and birth asphyxia are both separately connected to significant pulmonary and systemic hemodynamic changes in newborns, but, although these insults frequently coexist, their combined effects on the neonatal circulation are still controversial. To determine the pulmonary and systemic circulatory changes induced by pulmonary meconium contamination with concurrent asphyxia, 41 anesthetized and ventilated newborn piglets (10-12 d) were studied for 6 h. Eleven piglets were instilled with a bolus of human meconium intratracheally, and 10 piglets had meconium instillation with immediate induction of an asphyxic insult. Eight piglets had only asphyxia and 12 ventilated piglets served as controls. Meconium instillation (with and without asphyxia) resulted in a sustained decrease in the oxygenation, which remained, however, on the control level in the asphyxic group. Although meconium insufflation (with and without asphyxia) increased pulmonary artery pressure and vascular resistance progressively during the study period, the meconium-induced hypertensive effect was actually diminished by additional asphyxia. Asphyxia alone did not have any effect on these pulmonary hemodynamic parameters. On the other hand, whereas systemic arterial pressure and vascular resistance remained on the control level after meconium instillation alone, asphyxia (with and without pulmonary meconium insult) resulted in a sustained fall in systemic pressure already by 4 h. Our data thus indicate that although the coexisting asphyxia seems to moderate the meconium aspiration-induced pulmonary hypertensive response, this additional asphyxic insult does not affect the associated hypoxemia, but rather significantly exacerbates systemic hypotension.

Effects of partial liquid ventilation on cerebral blood flow and cerebral metabolism in neonatal lambs

BACKGROUND/PURPOSE

Liquid ventilation is a promising therapy for respiratory failure. The effects of perfluorochemical on cardiac output have not been well described. The purpose of this study was to compare cerebral blood flow (Q(CAROTID)) and cerebral metabolic rates (CMR) during conventional ventilation (CV) and partial liquid ventilation (PLV).

METHODS

Five 2-week-old lambs underwent tracheostomy and central venous, aortic, and postcerebral venous catheter placement. Doppler flow probes were placed around the common ovine trunk, and the lambs underwent CV for 1 hour. Ventilation was adjusted to maintain physiologic blood gases. Pre- and postcerebral blood gas, glucose, and lactate samples were obtained every 15 minutes. Perfluorodecalin then was instilled endotracheally. The lambs underwent 1 hour of PLV with similar sampling. Data were analyzed using the Wilcoxon matched pairs test, significance at P </=.05.

RESULTS

The authors observed no difference in mean Q(CAROTID) or carotid vascular resistance between CV and PLV (P =.35 and.34, respectively). The CMR of oxygen, glucose, and lactate were calculated using the Fick principle. CMR was unchanged between modes (P =.5).

CONCLUSIONS

PLV did not adversely alter Q(CAROTID) or CMR in lambs, implying that this mode of ventilation should be safe in neonatal patients at risk for neurologic injury.

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.

The effect of asphyxia on superior mesenteric artery blood flow in the premature sheep fetus

BACKGROUND/PURPOSE

The aim of this study was to determine superior mesenteric artery blood flow changes during and after an asphyxial insult in utero in chronically instrumented unanaesthetised premature fetal sheep.

METHODS

Fetal sheep at 0.7 gestation (103 to 104 days) underwent 25 minutes of complete umbilical cord occlusion (n = 6) or sham occlusion (n = 6). Fetal heart rate, blood pressure, superior mesenteric artery (SMA) blood flow and vascular resistance, electroencephalographic activity, and nuchal electromyographic activity were measured from 6 hours before occlusion until 3 days after occlusion. Fetal gastrointestinal tissue was taken for histological assessment.

RESULTS

During occlusion, cardiovascular response was characterised by 3 phases: initial redistribution of blood flow away from the gut to maintain vital organ function, subsequently partial failure of this redistribution, and finally near terminal cardiovascular collapse with profound hypotension and gastrointestinal hypoperfusion. Postasphyxia there was a secondary period of hypoperfusion that was mediated by increased vascular resistance, not hypotension. There was no evidence of injury on standard histological assessment after 3 days of recovery.

CONCLUSIONS

SMA blood flow is not only significantly reduced during asphyxia, but also for several hours after an asphyxial insult. The authors speculate that these perturbations of gastrointestinal blood flow could compromise gut wall integrity potentially leading to increased vulnerability to necrotising enterocolitis.

The cardiovascular and cerebrovascular responses of the immature fetal sheep to acute umbilical cord occlusion

1. In premature fetal sheep (89-93 days gestation) we examined the fetal response to asphyxia induced by 30 min of complete umbilical cord occlusion. Fetuses were also studied during the first 3 days after asphyxia. We measured heart rate, blood pressure, carotid and femoral blood flows, vascular resistance, electroencephalographic activity and cerebral changes in haemoglobin concentration by near infrared spectroscopy (NIRS). 2. Fetuses tolerated 30 min of asphyxia and the cardiovascular response was characterized by three phases: initial redistribution of blood flow away from the periphery to maintain vital organ function, partial failure of this redistribution and near terminal cardiovascular collapse, with profound hypotension and cerebral and peripheral hypoperfusion. 3. Post-asphyxia carotid blood flow and NIRS data demonstrated that between 3-5 h there was a significant secondary reduction in cerebral blood flow, blood volume and oxygenation despite normal perfusion pressure and heart rate. There was also a secondary fall in femoral blood flow which persisted throughout recovery. 4. These data demonstrate that the immature fetus can survive a prolonged period of asphyxia, but paradoxically the capacity to survive exposes the fetus to profound hypotension and hypoperfusion. A secondary period of significant cerebral hypoperfusion and reduced oxygen delivery also occurred post-asphyxia. These cardiovascular and cerebrovascular responses may contribute to the patterns of cerebral injury seen in the human preterm fetus.

Effects of hypoxia and reoxygenation on nitric oxide production and cerebral blood flow in developing rat striatum

We investigated the role of nitric oxide (NO) in the regulation of regional cerebral blood flow (rCBF) during hypoxia and reoxygenation in developing rat striatum. The subjects were urethane-anesthetized 7- and 14-d-old rats. After 120 min of baseline measurements, the rats received an i.p. injection of either saline (as a control) or an NO synthase inhibitor, N-nitro-L-arginine methyl ester (L-NAME, 30 mg/kg) 30 min before hypoxia. Then they were subjected to a 60-min hypoxia in 8% O2, followed by a 60-min recovery in 21% O2. rCBF and NO concentration in the striatum were measured by laser Doppler flowmetry and an NO electrode throughout the experimental period. In the controls, rCBF decreased to 93 +/- 3% of baseline during hypoxia and increased to 124 +/- 3% of baseline during reoxygenation in 7-d-old rats (n = 13), whereas rCBF increased during both hypoxia and reoxygenation in 14-d-old rats to 125 +/- 6% and 168 +/- 6% of baseline, respectively (n = 17). L-NAME attenuated the hyperemic response to hypoxia/reoxygenation in both ages (n = 11, in each age). Striatal NO production increased during hypoxia and reoxygenation in both ages, but the increase was significantly less in 7-d-old than in 14-d-old rats. The NO increase was associated with the increase in rCBF, and both were attenuated by L-NAME. We speculate that NO release during hypoxia/reoxygenation modulates rCBF. The immature young rat brain may have less capacity to activate NO production than the more developed brain.

Adenosine depletion alters postictal hypoxic cerebral vasodilation in the newborn pig

Altered postictal cerebral blood flow dilatory responses may contribute to brain injury following neonatal seizures. We developed an initial series of experiments to characterize the effects of seizure activity on cerebral vascular dilatory responses during the immediate postictal period. Significant attenuation of postictal hypoxic cerebral vasodilation was noted. We hypothesize that this diminished cerebral dilator response to hypoxia involves depletion of adenosine (Ado) activity resulting from seizure ictus. Additional experiments were designed to evaluate whether the altered postictal responses were related to a depletion of Ado stores or a decreased response to Ado in the postictal state. Farm-bred piglets were equipped with closed cranial windows. Responses to hypercapnia (10% CO2), hypoxia (fractional inspired O2 = 0.10), and topical sodium nitroprusside (10(-6) M) were compared before and after bicuculline-induced seizures (1 mg/kg). Hypoxia-induced cerebral vasodilation was significantly attenuated in the first 90 min postictal (control: 56.5 +/- 6%, 10 min postictal: 6.3 +/- 2%, 60 min postictal: 21.7 +/- 6%, and 90 min postictal: 21.6 +/- 5%; P < 0.01), whereas the dilator responses to hypercapnia and topical sodium nitroprusside remained intact. In a separate group of piglets, both a dilating (10(-5) M) and a nondilating concentration of Ado (10(-11) M) were topically administered postictally to measure their effects on pial vessel dilatory response to hypoxia. Dilation to topical Ado (10(-5) M) was not altered postictally compared with control. Ado (10(-11) M) restored hypoxia-induced vasodilation to preseizure control values in the immediate postictal period (control: 51.0 +/- 8%, postictal: 46.7 +/- 8%, P > 0.05). Postictal administration of Ado will restore hypoxia-induced cerebral vasodilation in piglets even when a nondilating concentration is employed. This suggests that depletion of Ado with seizure activity is a mechanism for the loss of postictal cerebral vasodilation to hypoxia, and the role of Ado in hypoxic cerebral vasodilation is permissive.

Longitudinal follow-up of a cohort of newborn infants treated with inhaled nitric oxide for persistent pulmonary hypertension

OBJECTIVE

To describe the outcome of a group of term newborn infants treated with inhaled nitric oxide for severe persistent pulmonary hypertension.

STUDY DESIGN

We performed a prospective longitudinal medical and neurodevelopmental follow-up of 51 infants treated as neonates for persistent pulmonary hypertension of the newborn with inhaled nitric oxide. The original number of treated infants was 87, of whom 25 died in the neonatal period; of 62 infants who survived, 51 were seen at 1 year of age and 33 completed a 2-year evaluation. Statistical analysis used population medians, means, and standard deviations for parameters assessed. Paired t tests and chi-square analysis were used to compare outcomes measured at 1 year with assessment at 2 years for the 32 infants seen at both 1- and 2-year visits.

RESULTS

At 1-year follow-up median growth percentiles were 20%, 72.5%, and 50% for weight, length, and occipitofrontal circumference, respectively. Thirteen of 51 infants (25.5%) were < 5th percentile in weight. Nine of 51 infants (17.6%) had feeding problems (need for gastrostomy feeding or gastroesophageal reflux), and 14 (27.5%) had a clinical diagnosis of reactive airways disease. Infant development as measured by the Bayley Scales of Infant Development was 104 +/- 16 for the mental development index and 97 +/- 20 for the psychomotor index. Six of 51 infants (11.8%) were found to have severe neurologic handicaps, defined as a Bayley score on either the mental development or psychomotor index of < 68, abnormal findings on neurologic examination, or both. Fewer children (6.1% vs 15.7%) required supplemental oxygen at 2 years compared with 1 year, and performance on the psychomotor index of the Bayley Scales improved significantly.

CONCLUSIONS

One- and 2-year follow-up of a cohort of infants with persistent pulmonary hypertension of the newborn who were treated with inhaled nitric oxide had an 11.8% (1 year) and 12.1% (2-year) rate of severe neurodevelopmental disability. There are ongoing medical problems in these infants including reactive airways disease and slow growth that merit continued close longitudinal follow-up.

Cerebral hemodynamics and distribution of left ventricular output during inhalation of nitric oxide

OBJECTIVES

Inhaled nitric oxide is being utilized as a selective pulmonary vasodilator in the treatment of persistent pulmonary hypertension of the newborn. However, the effects of inhaled nitric oxide on cerebral hemodynamics and distribution of left ventricular output in newborn subjects have not been studied. This study was designed to measure quantitatively the effect of inhaled nitric oxide on the distribution of left ventricular output and on cerebral hemodynamics in a perinatal animal model.

DESIGN

Prospective, controlled, experimental study.

SETTING

Research laboratory.

SUBJECTS

Eight fetal sheep.

INTERVENTIONS

Each animal was exposed to three separate study periods: a) mechanical ventilation with low FIO2 (maintaining fetal levels of PaO2); b) inhalation of nitric oxide (20 parts per million) during mechanical ventilation and low FIO2; and c) mechanical ventilation with an FIO2 of 1.0.

MEASUREMENTS AND MAIN RESULTS

Left ventricular output and cerebral blood flow were measured with radiolabeled microspheres. Cerebral oxygen delivery and consumption variables were calculated using measurements of arterial and cerebral venous (sagittal sinus) oxygen content. Total left ventricular output did not differ among the three treatment groups: 235 +/- 16 mL/min/kg with hypoxic ventilation; 283 +/- 13 mL/min/kg with nitric oxide inhalation; and 242 +/- 17 mL/min/kg with an FIO2 of 1.0. Lung blood flow increased 2.7-fold with inhaled nitric oxide and 1.6-fold during mechanical ventilation with an FIO2 of 1.0. With a left ventricle microsphere injection, increased lung blood flow is indicative of increased systemic-to-pulmonary shunt across the ductus arteriosus. Whole brain blood flow did not differ between the three groups: 49.6 +/- 6.7 mL/min/100 g with hypoxic ventilation; 46.4 +/- 7.4 mL/min/100 g with nitric oxide inhalation; and 36.4 +/- 3.8 mL/min/100 g with an FIO2 of 1.0. Cerebral oxygen delivery increased during inhalation of an FIO2 of 1.0 when compared with nitric oxide inhalation (p < .007); fractional extraction of oxygen decreased (p < .004 compared with hypoxic ventilation, p < .0005 compared with nitric oxide inhalation). Cerebral oxygen consumption did not differ between the three groups (1.11 +/- 0.12 microns/min/100 g with hypoxic ventilation, 0.95 +/- 0.12 microns/min/100 g with nitric oxide inhalation, and 0.96 +/- 0.08 microns/min/100 g with an FIO2 of 1.0).

CONCLUSION

Acute pulmonary vasodilation caused by inhalation of nitric oxide does not change left ventricular output, cerebral blood flow, or cerebral oxygen consumption, despite an increased systemic-to-pulmonary shunt across the ductus arteriosus.

Influence of end expiratory pressure on cerebral blood flow in preterm infants

The effect of interruption of positive and expiratory pressure (PEEP) on cerebral blood flow velocity (CBFV) and CBF fluctuation (CBFF) in the internal carotid arteries and on heart rate, restlessness and wakefulness has been studied in 17 mechanically ventilated neonates with RDS. A decrease in CBFV was found, but no significant change in CBFF. Multiple regression analysis showed that the decrease in CBFV is less pronounced if the PEEP interruption is accompanied by restlessness. It further appeared that the decrease in CBFV is more pronounced if CBFV is high, the ductus arteriosus is patent, or RDS follows a complicated course. These findings indicate that PEEP supports CBF, probably by a decrease in ductal stealing from the brain. Therewith PEEP protects against cerebral hypoperfusion which is one of the major risks in RDS and immaturity. Furthermore, our findings suggest that the decrease in CBF during PEEP interruption is moderated by restlessness and accentuated by brain damage.

Antenatal phenobarbital to prevent or minimize intraventricular hemorrhage in the low-birthweight neonate

OBJECTIVE

To review the evidence that antenatal phenobarbital can reduce the incidence or severity of periventricular-intraventricular hemorrhage (PIVH) in low-birthweight neonates.

DATA SOURCES

MEDLINE searches were conducted with fan searches of all papers.

STUDY SELECTION

Emphasis was placed on human data supplemented by relevant animal data.

DATA SYNTHESIS

The barbiturates have been used to reduce hypoxic-ischemic cerebral events. Giving phenobarbital to high-risk pregnant women allows the drug to be in therapeutic concentrations during the critical period when PIVH occurs in low-birthweight infants. Current data suggest that antenatal phenobarbital can decrease the severity of PIVH; fewer data are available stating that it can decrease the incidence of PIVH.

CONCLUSIONS

Evidence supports the hypothesis that antenatal phenobarbital is effective in decreasing the severity of PIVH in low-birthweight neonates. Further data are necessary regarding the incidence of low Apgar scores and respiratory depression in neonates given antenatal phenobarbital.

Congenital diaphragmatic hernia, extracorporeal membrane oxygenation, and death: a spectrum of etiologies

Extracorporeal Life Support Organization (ELSO) registry data show increased mortality in congenital diaphragmatic hernia (CDH) infants compared with other extracorporeal membrane oxygenation (ECMO) indications. To test the hypothesis that death might be related to various clinical parameters, retrospective data collection was solicited on 175 ECMO-related CDH deaths from 41 American ECMO centers (ELSO Registry 1980 through 1989). Data capture forms were received on 100 of 175 infants representing 29 of 41 centers. After review of all available material, a predominant cause of death was assigned. Other diagnoses were given secondary status. We analyzed arterial blood gas values at 6, 3, and 1 hour pre-ECMO, as well as at the time of highest recorded PO2 (preductal and postductal) and lowest recorded PCO2, and correlated these findings with predominant cause of death. The relationship between individual variables and cause of death was assessed by t test. Multivariate analysis was performed by using a stepwise discriminate procedure. The most common predominant causes of death were brain death (29%), pulmonary hypertension (25%), and pulmonary hypoplasia (17%). Correlation of arterial blood gas values at 6, 3, and 1 hour pre-ECMO with predominant causes of death established the following statistically significant associations (P less than .05): (1) pulmonary hypoplasia and low PO2 at 6 hours pre-ECMO; (2) brain death and low pH at 1 hour pre-ECMO; and (3) pulmonary hypertension and high HCO3- at 1 hour pre-ECMO.(ABSTRACT TRUNCATED AT 250 WORDS)

Control of cerebral circulation in the high-risk neonate

A knowledge of neonatal cerebrovascular physiology is essential to the understanding of diseases that frequently affect the subsequent development of the newborn brain. Recent observations indicate that the cerebral vessels of the healthy newborn infant, even the very preterm, respond to physiological stimuli in the same manner as in the mature organism. Thus, cerebral blood flow changes with changes in arterial carbon dioxide tension (PaCO2), oxygen concentration (CaO2), or glucose concentration, whereas cerebral blood flow remains constant at minor fluctuations in arterial blood pressure. In pathological states, pressure autoregulation may become impaired, and in severe cases the vessels do not react to chemical or metabolic stimuli. These infants are at high risk for developing cerebral lesion, and they may be candidates for new "brain-protecting regimens."

Fetal cerebral blood flow and metabolism during oligemia and early postoligemic reperfusion

The early time period following ischemia may be of pathogenetic importance in hypoxic-ischemic brain injury. Global cerebral oligemia was induced in ten late gestation fetal sheep by inflation of a balloon occluder around the brachiocephalic artery. Cerebral blood flow, oxygen, glucose, and lactate net flux, and oxygen delivery were measured by the Fick principle following 1 h of oligemia and at 5, 30, and 60 min of postoligemic reperfusion. During oligemia, cerebral blood flow decreased by 74 +/- 10% (mean +/- SD) and oxygen consumption decreased by 34 +/- 24%. The glucose:oxygen quotient was elevated throughout the oligemic period. In the early (5 min) reperfusion period, blood flow and oxygen delivery were not different from control but oxygen consumption was persistently depressed by 27 +/- 32%; fractional extraction of oxygen was 0.38 +/- 0.10 during control and 0.24 +/- 0.09 during early reperfusion. The venous oxygen tension increased modestly from 15.2 +/- 2.4 to 18.0 +/- 1.7 mm Hg; the postoligemic venous pO2 was limited by the lack of reactive hyperemia combined with the low arterial pO2 of the intrauterine environment. Postoligemic carbohydrate fluxes could not be differentiated from control possibly due to blood-brain barrier limitations. These factors may be related to the relative resistance of the fetal brain to hypoxic-ischemic injury.

Antenatal phenobarbital for the prevention of periventricular and intraventricular hemorrhage: a double-blind, randomized, placebo-controlled, multihospital trial

To determine whether the neuroprotective properties of phenobarbital would alter the incidence and severity of intracranial hemorrhage in premature infants, we randomly assigned 110 women at less than 31 weeks of gestation to receive 10 mg/kg phenobarbital or placebo in a blinded fashion before delivery. Infants were examined postnatally with real-time ultrasonography for evidence of intracranial hemorrhage. Maternal demographics, pregnancy complications, antenatal management, and route of delivery did not differ between the phenobarbital group (n = 50) and the placebo group (n = 60). The total incidence of periventricular-intraventricular hemorrhage did not differ between the phenobarbital-treated (n = 54) and the placebo-treated (n = 67) infants. However, the frequency of grade 3 and grade 4 hemorrhages was 15% (10 infants) in the placebo group and 3.7% (2 infants) in the phenobarbital group (p less than 0.05). There were no differences in the severity of associated conditions in the babies to explain the difference in the incidence of severe hemorrhage between the study groups. We conclude that antenatal administration of phenobarbital appears to be effective in decreasing the severity of periventricular-intraventricular hemorrhage in infants delivered at less than 31 weeks of gestation.

Extracorporeal membrane oxygenation causes significant changes in intracranial pressure and carotid artery blood flow in newborn lambs

The effects of ECMO on cerebral dynamics, particularly in the face of asphyxia, are largely unknown. We inquired as to whether carotid artery blood flow (CABF) and intracranial pressure (ICP) were affected by carotid artery/jugular vein ligation, asphyxia, ECMO, and ECMO with asphyxia. Lightly sedated newborn lambs (two to four days old, 3 to 4 kg) in four groups were monitored for mean ICP by an epidural sensor, mean CABF by a flow probe, and mean arterial pressure. Mean values were determined for the duration of each step of the experiment. ECMO was venoarterial at 100 to 120 mL/kg/min. CABF and ICP were measured in group 1 before and after CA/JV ligation; in group 2 during normoxia/normocapnia followed by hypoxia (30 to 40 torr)/hypercapnia (70 to 90 torr); in group 3 before, during, after ECMO while normoxic/normocapnic throughout; and in group 4 as ECMO was begun while hypoxic/hypercapnic. Vessel ligation alone caused no significant CABF/ICP changes. Asphyxia caused physiologic increases in CABF (P less than .03) and ICP (P less than .01). ECMO alone caused a significant decrease in ICP (P less than .003). ECMO with asphyxia caused an even more severe decrease in ICP (P less than .001) combined with augmented CABF (P less than .03). The ICP decrease was limited to the duration of ECMO. Possible explanations include loss of cerebral autoregulation induced by hypoxia/hypercarbia and alterations in cerebral venous drainage necessitated by this method of cardiopulmonary bypass.