The effect of focal cerebral cooling on perinatal hypoxic-ischemic brain damage

Administration of selective brain hypothermia using a simple cooling device in neonatal rats

BACKGROUND

The interruption of oxygen and blood supply to the newborn brain around the time of birth is a risk factor for hypoxic-ischemic encephalopathy and may lead to infant mortality or lifelong neurological impairments. Currently, therapeutic hypothermia, the cooling of the infant's head or entire body, is the only treatment to curb the extent of brain damage.

NEW METHOD

In this study, we designed a focal brain cooling device that circulates cooled water at a steady state temperature of 19 ± 1 °C through a coil of tubing fitted onto the neonatal rat's head. We tested its ability to selectively decrease brain temperature and offer neuroprotection in a neonatal rat model of hypoxic-ischemic brain injury.

RESULTS

Our method cooled the brain to 30-33 °C in conscious pups, while keeping the core body temperature approximately 3.2 °C warmer. Furthermore, the application of the cooling device to the neonatal rat model demonstrated a reduction in brain volume loss compared to pups maintained at normothermia and achieved a level of brain tissue protection the same as that of whole-body cooling.

COMPARISON WITH EXISTING METHODS

Prevailing methods of selective brain hypothermia are designed for adult animal models rather than for immature animals such as the rat as a conventional model of developmental brain pathology. Contrary to existing methods, our method of cooling does not require surgical manipulation or anaesthesia.

CONCLUSION

Our simple, economical, and effective method of selective brain cooling is a useful tool for rodent studies in neonatal brain injury and adaptive therapeutic interventions.

Neuroprotection Strategies for Term Encephalopathy

Brain injury in the full-term and near-term neonates is a significant cause of mortality and long-term morbidity, resulting in injury patterns distinct from that seen in premature infants and older patients. Therapeutic hypothermia improves long-term outcomes for many of these infants, but there is a continued search for therapies to enhance the plasticity of the newborn brain, resulting in long-term repair. It is likely that a combination strategy utilizing both early and late interventions may have the most benefit, capitalizing on endogenous mechanisms triggered by hypoxia or ischemia. Optimizing care of these critically ill newborns in the acute setting is also vital for improving both short- and long-term outcomes.

Neurodevelopmental Follow Up After Therapeutic Hypothermia for Perinatal Asphyxia

Introduction:

Neuroprotective benefit of therapeutic hypothermia in term newborns with hypoxic-ischemic encephalopathy (HIE) was assessed by analyzing survival and neurodevelopmental outcome of neonates subjected to this procedure.

Material and methods:

Newborns with gestational age > 36 weeks and < 6 hours of age with moderate to severe asphyxial encephalopathy underwent cooling protocol at a temperature of 33.5 °C for 72 hours and rewarming period of 6 hours. Outcome measures assessed were death and neurodevelopmental characteristics, which were compared at the different age using ASQ-3. Twenty-five children were assessed at age 3-6, 12-18 and 24-36 months. Median gestational age was 40 weeks, birth weight 3470 g, Apgar score 2/4 and pH on admission to the hospital 7.02. Four (16%) children died.

Results:

At the first assessment developmental categories of communication were normal in 78.9%, problem solving in 63.2%, personal-social in 68.4%, gross motor in 68.4%, and fine motor in 42.1% with a high need of retesting in this area. Second assessment was done in 17 patients: developmental categories of communication normal in 58.8%, problem solving in 70.6%, personal-social in 64.7%, gross motor in 64.7%, and fine motor in 35.3%. Third evaluation was done in 14 patients: developmental categories of communication were normal in 64.3%, problem solving in 71.4%, personal-social in 57.1%, gross motor in 64.3%, and fine motor in 42.9%.

Conclusion:

There was no correlation between baseline parameters and outcome. Results of the study are showing that therapeutic hypothermia in term newborns can provide better survival and less neurologic sequels in HIE patients.

Has Quality Improvement Really Improved Outcomes for Babies in the Neonatal Intensive Care Unit?

During the past decade, the emergence of outcome measurement and quality improvement in the neonatal intensive care unit, far more than the introduction of new research approaches or novel therapies, has had a profound effect on improving outcomes for premature neonates. Collection of outcome data, review of those data, and strategies to identify and resolve problems using continuous quality improvement methods can dramatically improve patient outcomes. It is likely that further initiatives in quality improvement will continue to have additional beneficial effects for the neonate.

Pathophysiology of an hypoxic–ischemic insult during the perinatal period

Hypoxia-ischemia is a leading cause of morbidity and mortality in the perinatal period with an incidence of 1/4000 live births. Biochemical events such as energy failure, membrane depolarization, brain edema, an increase of neurotransmitter release and inhibition of uptake, an increase of intracellular Ca(2+), production of oxygen-free radicals, lipid peroxidation, and a decrease of blood flow are triggered by hypoxia-ischemia and may lead to brain dysfunction and neuronal death. These abnormalities can result in mental impairments, seizures, and permanent motor deficits, such as cerebral palsy. The physical and emotional strain that is placed on the children affected and their families is enormous. The care that these individuals need is not only confined to childhood, but rather extends throughout their entire life span, so it is very important to understand the pathophysiology that follows a hypoxic-ischemic insult. This review will highlight many of the mechanisms that lead to neuronal death and include the emerging area of white matter injury as well as the role of inflammation and will provide a summary of therapeutic strategies. Hypothermia and oxygen will also be discussed as treatments that currently lack a specific target in the hypoxic/ischemic cascade.

Neuroprotection in the newborn infant

Neonatal brain injury is an important cause of death and disability, with pathways of oxidant stress, inflammation, and excitotoxicity that lead to damage that progresses over a long period of time. Therapies have classically targeted individual pathways during early phases of injury, but more recent therapies such as growth factors may also enhance cell proliferation, differentiation, and migration over time. More recent evidence suggests combined therapy may optimize repair, decreasing cell injury while increasing newly born cells.

Induced hypothermia for infants with hypoxic- ischemic encephalopathy using a servo-controlled fan: an exploratory pilot study

OBJECTIVE

Several trials suggest that hypothermia is beneficial in selected infants with hypoxic-ischemic encephalopathy. However, the cooling methods used required repeated interventions and were either expensive or reported significant temperature variation. The objective of this pilot study was to describe the use, efficacy, and physiologic impact of an inexpensive servo-controlled cooling fan blowing room-temperature air.

PATIENTS AND METHODS

A servo-controlled fan was manufactured and used to cool 10 infants with hypoxic-ischemic encephalopathy to a rectal temperature of 33 degrees C to 34 degrees C. The infants were sedated with phenobarbital, but clonidine was administered to some infants if shivering or discomfort occurred. A servo-controlled radiant warmer was used simultaneously with the fan to prevent overcooling. The settings used on the fan and radiant warmer differed slightly between some infants as the technique evolved.

RESULTS

A rectal temperature of 34 degrees C was achieved in a median time of 58 minutes. Overcooling did not occur, and the mean temperature during cooling was 33.6 degrees C +/- 0.2 degrees C. Inspired oxygen requirements increased in 6 infants, and 5 infants required inotropic support during cooling, but this was progressively reduced after 1 to 2 days. Dehydration did not occur. Five infants shivered when faster fan speeds were used, but 4 of the 5 infants had hypomagnesemia. Shivering was controlled with clonidine in 4 infants, but 1 infant required morphine.

CONCLUSIONS

Servo-controlled fan cooling with room-temperature air, combined with servo-controlled radiant warming, was an effective, simple, and safe method of inducing and maintaining rectal temperatures of 33 degrees C to 34 degrees C in sedated infants with hypoxic-ischemic encephalopathy. After induction of hypothermia, a low fan speed facilitated accurate temperature control, and warmer-controlled rewarming at 0.2 degrees C increments every 30 minutes resulted in more appropriate rewarming than when 0.5 degrees C increments every hour were used.

Hypothermia for hypoxic-ischemic encephalopathy

We are entering an era in which hypothermia will be used in combination with other novel neuroprotective interventions. The targeting of multiple sites in the cascade leading to brain injury may prove to be a more effective treatment strategy after hypoxic-ischemic encephalopathy in newborn infants than hypothermia alone.

Therapeutics for neonatal brain injury

Neonatal brain injury is an important cause of death and neurodevelopmental delay. Multiple pathways of oxidant stress, inflammation, and excitotoxicity lead to both early and late phases of cell damage and death. Therapies targeting these different pathways have shown potential in protecting the brain from ongoing injury. More recent therapies, such as growth factors, have demonstrated an ability to increase cell proliferation and repair over longer periods of time. Even though hypothermia, which decreases cerebral metabolism and possibly affects other mechanisms, may show some benefit in particular cases, no widely effective therapeutic interventions for human neonates exist. In this review, we summarize recent findings in neuroprotection and neurogenesis for the immature brain, including combination therapy to optimize repair.

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

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

Intervention strategies for neonatal hypoxic-ischemic cerebral injury

BACKGROUND

Accumulating evidence points to an evolving process of brain injury after intrapartum hypoxia-ischemia that initiates in utero and extends into a recovery period. It is during this recovery period that the potential for neuroprotection exists.

OBJECTIVE

This discussion briefly reviews the cellular characteristics of hypoxic-ischemic cerebral injury and the current and future therapeutic strategies aimed at ameliorating ongoing brain injury after intrapartum hypoxia-ischemia.

METHODS

As part of the Newborn Drug Development Initiative, the National Institute of Child Health and Human Development and the US Food and Drug Administration cosponsored a workshop held March 29 and 30, 2004, in Baltimore, Maryland. Information for this article was gathered during that workshop. Literature searches of MEDLINE (Ovid) and EMBASE (1996-2005) were also conducted; search terms included newborn, infant, hypoxia-ischemia, hypoxic-ischemic encephalopathy, asphyxia, pathogenesis, treatment, reperfusion injury, and mechanisms, as well as numerous interventions (ie, therapeutic hypothermia, magnesium, and barbiturates).

RESULTS

The acute brain injury results from the combined effects of cellular energy failure, acidosis, glutamate release, intracellular calcium accumulation, lipid peroxidation, and nitric oxide neurotoxicity that serve to disrupt essential components of the cell, resulting in death. Many factors, including the duration or severity of the insult, influence the progression of cellular injury after hypoxia-ischemia. A secondary cerebral energy failure occurs from 6 to 48 hours after the primary event and may involve mitochondrial dysfunction secondary to extended reactions from primary insults (eg, calcium influx, excitatory neurotoxicity, oxygen free radicals, or nitric oxide formation). Some evidence suggests that circulatory and endogenous inflammatory cells/mediators also contribute to ongoing brain injury. The goals of management of a newborn infant who has sustained a hypoxic-ischemic insult and is at risk for injury should include early identification of the infant at highest risk for evolving injury, supportive care to facilitate adequate perfusion and nutrients to the brain, attempts to maintain glucose homeostasis, and consideration of interventions to ameliorate the processes of ongoing brain injury. Recent evidence suggests a potential role for modest hypothermia (ie, a reduction in core body temperature to -34 degrees C) administered to high-risk term infants within 6 hours of birth. Either selective (head) or systemic (body) cooling reduces the incidence of death and/or moderate to severe disability at 18-month follow-up. Additional strategies-including the use of oxygen free radical inhibitors and scavengers, excitatory amino acid antagonists, and growth factors; prevention of nitric oxide formation; and blockage of apoptotic pathways-have been evaluated experimentally but have not been replicated in a systematic manner in the human neonate. Other avenues of potential neuroprotection that have been studied in immature animals include platelet-activating factor antagonists, adenosinergic agents, monosialoganglioside GM1, insulin-like growth factor-1, and erythropoietin.

CONCLUSIONS

Much progress has been made toward understanding the mechanisms contributing to ongoing brain injury after intrapartum hypoxia-ischemia. This should facilitate more specific pharmacologic intervention strategies that might provide neuroprotection during the reperfusion phase of injury.

Modest hypothermia as a neuroprotective strategy in high-risk term infants

This article briefly reviews the pathogenesis of hypoxic-ischemic cerebral injury, the experimental data, and clinical studies that have evaluated the potential therapeutic benefit of modest selective or whole body hypothermia in reducing the subsequent development of irreversible brain injury without untoward side effects. Data are insufficient to recommend routine use of either modest selective or whole body hypothermia after resuscitation of infants with suspected asphyxia. Further clinical trials are needed to determine which infants would benefit most and which method of cooling would be most effective.

Whole-body hypothermia for neonates with hypoxic-ischemic encephalopathy

BACKGROUND

Hypothermia is protective against brain injury after asphyxiation in animal models. However, the safety and effectiveness of hypothermia in term infants with encephalopathy is uncertain.

METHODS

We conducted a randomized trial of hypothermia in infants with a gestational age of at least 36 weeks who were admitted to the hospital at or before six hours of age with either severe acidosis or perinatal complications and resuscitation at birth and who had moderate or severe encephalopathy. Infants were randomly assigned to usual care (control group) or whole-body cooling to an esophageal temperature of 33.5 degrees C for 72 hours, followed by slow rewarming (hypothermia group). Neurodevelopmental outcome was assessed at 18 to 22 months of age. The primary outcome was a combined end point of death or moderate or severe disability.

RESULTS

Of 239 eligible infants, 102 were assigned to the hypothermia group and 106 to the control group. Adverse events were similar in the two groups during the 72 hours of cooling. Primary outcome data were available for 205 infants. Death or moderate or severe disability occurred in 45 of 102 infants (44 percent) in the hypothermia group and 64 of 103 infants (62 percent) in the control group (risk ratio, 0.72; 95 percent confidence interval, 0.54 to 0.95; P=0.01). Twenty-four infants (24 percent) in the hypothermia group and 38 (37 percent) in the control group died (risk ratio, 0.68; 95 percent confidence interval, 0.44 to 1.05; P=0.08). There was no increase in major disability among survivors; the rate of cerebral palsy was 15 of 77 (19 percent) in the hypothermia group as compared with 19 of 64 (30 percent) in the control group (risk ratio, 0.68; 95 percent confidence interval, 0.38 to 1.22; P=0.20).

CONCLUSIONS

Whole-body hypothermia reduces the risk of death or disability in infants with moderate or severe hypoxic-ischemic encephalopathy.

Antenatal events causing neonatal brain injury in premature infants

Understanding possible causes of neonatal brain injury is important for perinatal nurses because neonatal brain injury predicts subsequent infant mortality and morbidity in the premature infant. The pathogenesis of the brain injury, germinal matrix/intraventricular hemorrhage and white matter damage, is usually related to a hypoxic event. The hypoxic event may occur in utero, resulting from various conditions, such as maternal infection, maternal alcohol consumption, maternal smoking, placental bleeding disorders, maternal hypercoagulability conditions, metabolic disorders (diabetes and hyperthyroidism), and oligohydramnios. Strategies for prevention beginning before and in pregnancy are needed.

Brain injury in the term infant

Perinatal brain injury in the term infant is a relatively uncommon event. The principal lesions are intracranial hemorrhage including subarachnoid, subdural, intraparenchymal, intraventricular, focal cerebral infarction and hypoxic ischemic cerebral injury secondary to intrapartum hypoxia-ischemia. Both intracranial hemorrhage and focal cerebral infarction are invariably identified at the time of clinical symptoms, ie, seizures or apnea. This clearly limits the potential for prevention. The mechanisms contributing to brain injury secondary to intrapartum hypoxia-ischemia have become more clearly defined. Secondary or reperfusion injury is potentially amenable to neuroprotective strategies. Modest hypothermia is one such therapy that has been studied in high-risk newborn infants with some initial success. Future studies need to focus on additional neuroprotective strategies.

Hypoxic-ischemic brain injury in the term infant-current concepts

An enhanced understanding of the cellular characteristics contributing to ongoing brain injury following intrapartum hypoxia-ischemia has resulted in the implementation of targeted neuroprotective strategies in the newborn period. This review briefly covers the pathogenesis of hypoxic-ischemic injury with an emphasis on reperfusion injury; the role of magnetic resonance imaging in the detection of such injury, and focuses on potential strategies both supportive and neuroprotective to prevent ongoing injury with a specific emphasis on modest hypothermia.

Traumatic injury in the developing brain–effects of hypothermia

Little is known about the underlying mechanisms of head trauma in the developing brains, despite considerable social and economic impact following such injuries. Age has been shown to substantially influence morbidity and mortality. Children younger than 4 years of age had worse cognitive, motor, and brain atrophy outcomes than children 6 years of age and older. Younger children tend to more frequently suffer from diffuse cerebral swelling compared to adults. Typical autoptic findings also include axonal injury and ischemic neurodegeneration. These differences impact not only the primary response of the brain to injury but the secondary response as well. The complexity of damaging mechanisms in traumatic brain injury contributes to the problem of determining effective therapy. As an alternative/ adjunct to pharmacological approaches, hypothermia has been shown to be cerebroprotective in traumatized adult brains. Although a large number of animal studies have shown protective effects of hypothermia in a variety of damaging mechanisms after TBI, little data exist for young, developing brains. The injury mechanisms of TBI in the immature, effects of hypothermia following resuscitation on adult and immature traumatized brains, and some possible mechanisms of action of hypothermia in the immature traumatized brain are discussed in this review.

Protective effects of brief intra- and delayed postischemic hypothermia in a transient focal ischemia model in the neonatal rat

Hypothermia provides neuroprotection in virtually all animal models of ischemia, including adult stroke models and the neonatal hypoxic-ischemic (HI) model. In these studies, brief periods of hypothermia are examined in a neonatal model employing transient focal ischemia in a 7-day-old rat pup. Pups underwent permanent middle cerebral artery (MCA) occlusion coupled with a temporary (1 h) occlusion of the ipsilateral common carotid artery (CCA). This study included five treatment groups: (1) normothermic (Normo)-brain temperature was maintained at 37 degrees C; (2) intraischemic hypothermia (IntraH)-28 degrees C during the 1-h ischemic period only; (3) postischemic hypothermia (PostH)-28 degrees C for the second hour of reperfusion only; (4) late-onset postischemic hypothermia (LPostH) cooled to 28 degrees C for the fifth and sixth hours of reperfusion only; and (5) Shams. After various times (3 days-6 weeks), the lesion was assessed using 2,3,5-triphenyltetrazolium chloride (TTC) or hematoxylin and eosin (H&E) stains. Intraischemic hypothermia resulted in significant protection in terms of survival, lesion size, and histology. Postischemic hypothermia was not effective in reducing lesion size early after ischemia, but significantly reduced the eventual long-term damage (2-6 weeks). Late-onset postischemic hypothermia did not reduce infarct volume. Therefore, both intraischemic and postischemic hypothermia provided neuroprotection in the neonatal rat, but with different effects on the degenerative time course. While there were no observable differences in simple behaviors or growth, all hypothermic conditions significantly reduced mortality rates. While the protection resulting from intraischemic hypothermia is similar to what is observed in other models, the degree of long-term ischemic protection observed after 1 h of postischemic hypothermia was remarkable and distinct from what has been observed in other adult or neonatal models.

Combination effect of systemic hypothermia and caspase inhibitor administration against hypoxic-ischemic brain damage in neonatal rats

Caspases are believed to play a key role in the delayed neuronal cell death observed in the rat brain after hypoxic-ischemic (HI) insult. Caspase inhibitors have been developed as antiapoptotic agents. Hippocampal damage after HI insult is strongly related to tissue temperature, and systemic hypothermia has been introduced clinically for brain protection. In this study, we examined the effects of a caspase inhibitor and systemic hypothermia on neuronal protection in the developing rat brain. Postnatal d 7 rat pups were subjected to the Rice model of hypoxia for 1 h. Systemic hypothermia was induced with a water bath at 29 degrees C. Before HI insult, a pan-caspase inhibitor, boc-aspartyl-(OMe)-fluoromethyl-ketone (BAF), was injected into the cerebral ventricle. The ipsilateral hippocampus was subjected to caspase assays and histologic assessment. The HI group at 37 degrees C (HI-37 degrees C) showed a peak of caspase-3 activity 16 h after insult. This activity was significantly reduced in the presence of BAF or hypothermia (HI-29 degrees C group, p < 0.05) or by the combination of HI-29 degrees C + BAF (p < 0.01 versus HI-37 degrees C). The number of neuronal cells in the ipsilateral hippocampal CA1 region in the HI-37 degrees C group was significantly decreased (62.9% versus control). The number of neuronal cells was maintained in the HI-37 degrees C + BAF group (82.7%), the HI-29 degrees C group (78.7%), and the combination group (95.2%) (p < 0.05 versus HI-37 degrees C). A combination of systemic hypothermia and BAF produced a strong protective effect against neuronal damage in the developing rat brain, along with a reduction in caspase-3 activity.

Differences in brain temperature and cerebral blood flow during selective head versus whole-body cooling

OBJECTIVE

To compare brain temperature and cerebral blood flow (CBF) during head and body cooling, with and without systemic hypoxemia.

METHODS

Seventeen newborn swine were studied for either measurement of brain temperature alone (n = 9) or measurement of brain temperature and CBF (n = 8). All animals were ventilated and instrumented, and temperature probes were inserted into the rectum, into the brain at depths of 2 and 1 cm from the cortical surface, and on the dural surface. Blood flow was measured with microspheres. The protocol consisted of a control period, an interval of either head or body cooling, and cooling with 15 minutes of superimposed hypoxia. After a 1-hour recovery period, animals were exposed to the same sequence except that the alternate mode of cooling was evaluated.

RESULTS

Head cooling with a constant rectal temperature resulted in an increase in the temperature gradient across the brain from the warmer central structures to the cooler periphery (brain 2 cm - dura temperature: 1.3 +/- 1.1 degrees C at control to 7.5 +/- 3.5 degrees C during cooling). Hypoxia superimposed on head cooling decreased the temperature gradient by at least 50%. In contrast, body cooling was associated with an unchanged temperature gradient across the brain (brain 2 cm - dura temperature: 1.5 +/- 1.2 degrees C at control to 1.1 +/- 0.9 degrees C during cooling). Hypoxia superimposed on body cooling did not change brain temperature. Both modes of brain cooling resulted in similar reductions of global CBF ( approximately 40%) and O(2) uptake.

CONCLUSION

Brain hypothermia achieved through head or body cooling results in different brain temperature gradients. Alterations in systemic variables (ie, hypoxemia) alters brain temperature differently in these 2 modes of brain cooling. The mode of brain cooling may affect the efficacy of modest hypothermia as a neuroprotective therapy.

Is temperature important in delivery room resuscitation?

The possibility that temperature may affect the outcome of resuscitation from severe perinatal asphyxia has been a long-standing focus of research. Experimentally it is now well established that even small changes in temperature during severe hypoxia-ischemia critically modulate outcome. Clinical and experimental studies have now shown that hypoxic-ischemic injury continues to evolve after resuscitation. Experimentally, prolonged mild to moderate hypothermia can dramatically reduce this delayed injury, while mild hyperthermia over the same period worsens injury. Indeed there are data indicating that moderate post-ischemic hyperthermia can be deleterious as late as 24 h after reperfusion. Hypothermia has significant potential adverse effects, and at present its clinical use is restricted to large randomized controlled trials. The present paper reviews evidence suggesting that both primary prevention of maternal pyrexia during labour, and secondary prevention of hyperthermia after neonatal resuscitation, have the potential to significantly reduce the consequences of perinatal hypoxia-ischemia.

The use of hypothermia: a role in the treatment of neonatal asphyxia?

Perinatal asphyxia remains one of the most devastating neurologic processes. Although the understanding of the pathophysiology after perinatal asphyxia is extensive, there are few therapeutic interventions available to prevent or even mitigate the devastating process that unfolds after injury. The search for a safe and efficacious therapy has prompted scientists and clinicians to consider various promising therapies. One such therapy is therapeutic hypothermia. On the basis of adult, pediatric, and animal research, there is increasing evidence to suggest that therapeutic hypothermia may be an effective intervention to lessen the secondary neuronal injury that ensues after a hypoxic-ischemic insult. In this article the historic and modern-day uses of therapeutic hypothermia are first reviewed. The pathophysiology of neonatal asphyxia is examined next, with emphasis on the changes that occur when therapeutic hypothermia is implemented. Potential side-effects of the therapy in the neonate and the debate over systemic vs selective hypothermia are discussed. Lastly, although hypothermia as a potential treatment modality for neonates with hypoxic-ischemic encephalopathy is supported by numerous studies, the need for well-designed multicenter trials with detailed patient entry criteria and therapeutic conditions is emphasized.

Postinjury magnesium sulfate treatment is not markedly neuroprotective for striatal medium spiny neurons after perinatal hypoxia/ischemia in the rat

Hypoxic/ischemic (H/I) brain injury is thought to be mediated via the N-methyl-D-aspartate receptor complex, which can be blocked by the magnesium ion. Striatal medium spiny neurons abundantly express N-methyl-D-aspartate receptors and are known to be injured after H/I. Thus, the aim of this study was to investigate the effect of postinjury magnesium treatment on the total number of medium spiny neurons in the striatum after perinatal H/I injury in the rat. Anesthetized postnatal day (PN) 7 rats underwent common carotid artery ligation followed 2 h later by exposure to hypoxia for 1.5 h. Contralateral hemispheres served as controls as did animals exposed to normoxia. Immediately after hypoxia or normoxia, the magnesium groups received s.c. injections of 300 mg/kg MgSO4. Control, hypoxic or normoxic animals received NaCl injections. This continued daily until PN13. Eleven matched-for-weight H/I pups were injected in total. A power calculation showed that 11 pups per treatment group would permit detection of a treatment difference of 32% or more. Animals were killed on PN18, and 40-micron serial sections were cut through each entire striatum. The total number of the predominant medium spiny neurons within each striatum was stereologically determined via the use of an unbiased optical dissector/Cavalieri combination. It was found that postinjury magnesium treatment did not improve neuronal survival by 32% or more in the striatum. The results suggest that magnesium treatment after perinatal H/I damage in the rat is not markedly neuroprotective for striatal medium spiny neurons.

The 'pharmacology' of neuronal rescue with cerebral hypothermia

The neuroprotective effects of hypothermia during cerebral ischaemia or asphyxia are well known. Although, in view of this, the possibility of a therapeutic role for hypothermia during or after resuscitation from such insults has been a long standing focus of research, early studies had limited and contradictory results. Clinically and experimentally severe perinatal asphyxial injury is associated with a latent phase after reperfusion, with initial recovery of cerebral energy metabolism but EEG suppression, followed by a secondary phase with seizures, cytotoxic edema, accumulation of cytotoxins, and cerebral energy failure from 6 to 15 h after birth. Recent studies have led to the hypothesis that changes in post-ischaemic cerebral temperature can critically modulate encephalopathic processes which are initiated during the primary phase of hypoxia-ischaemia, but which extend into the secondary phase of cerebral injury. This conceptual framework allows a better understanding of the 'pharmacological' parameters that determine effective hypothermic neuroprotection, including the timing of initiation of cooling, its duration and the depth of cooling attained. Moderate cerebral hypothermia initiated in the latent phase, between one and as late as 6 hours after reperfusion, and continued for a sufficient duration in relation to the severity of the cerebral injury, has been associated with potent, long-lasting neuroprotection in both adult and perinatal species. These encouraging results must be balanced against the adverse systemic effects of hypothermia. Randomised clinical trials are in progress to establish the safety and efficacy of prolonged cerebral hypothermia.

Selective head cooling in newborn infants after perinatal asphyxia: a safety study

AIMS

To determine the practicality and safety of head cooling with mild or minimal systemic hypothermia in term neonates with moderate to severe hypoxic-ischemic encephalopathy.

METHODS

Study group infants >/=37 weeks' gestation, who had an umbilical artery pH </=7. 09 or Apgars </=6 at 5 minutes, plus evidence of encephalopathy. Infants with major congenital abnormalities were excluded. TRAIL DESIGN: Infants were randomized to either no cooling (controls; rectal temperature = 37.0 +/- 0.2 degreesC, n = 10) or sequentially, either minimal systemic cooling (rectal temperature = 36.3 +/- 0.2 degreesC, n = 6) or mild systemic cooling (rectal temperature = 35.7 +/- 0.2 degreesC, n = 6). Head cooling was accomplished by circulating water at 10 degreesC through a coil of tubing wrapped around the head for up to 72 hours. All infants were warmed by servo-controlled overhead heaters to maintain the allocated rectal temperature. The rectal, fontanelle, and nasopharyngeal temperatures were continuously monitored.

RESULTS

From January 1996 to October 1997, 22 term infants were randomized from 2 to 5 hours after birth. All infants showed a metabolic acidosis at delivery, with similar umbilical artery pH in the control group (mean +/- standard deviation, 6.79 +/- 0.25), minimal cooling group (6.98 +/- 0.21), and mild cooling group (6.93 +/- 0.11), and depressed Apgar scores at 5 minutes in the control group (4.5 +/- 2), minimal cooling group, (4.7 +/- 2) and mild cooling group (6.0 +/- 1). In the mild-cooled infants, the nasopharyngeal temperature was 34.5 degreesC during cooling, 1.2 degreesC lower than the rectal temperature. This gradient narrowed to 0.5 degreesC after cooling was stopped. No adverse effects because of cooling were observed. No infants developed cardiac arrhythmias, hypotension, or bradycardia during cooling. Thrombocytopenia occurred in 2 out of 10 controls, 2 out of 6 minimal cooling infants, and 1 out of 6 mild cooling infants. Hypoglycemia (glucose <2.6 mM) was seen on at least one occasion in 2 out of 10 controls, 4 out of 6 minimal cooling infants, and 1 out of 6 mild cooling infants. Acute renal failure occurred in all infants. The metabolic acidosis present in all infants at the time of enrollment into the study progressively resolved despite cooling, even in the mild hypothermia group.

CONCLUSIONS

Mild selective head cooling combined with mild systemic hypothermia in term newborn infants after perinatal asphyxia is a safe and convenient method of quickly reducing cerebral temperature with an increased gradient between the surface of the scalp and core temperature. The safety of mild hypothermia with selective head cooling is in contrast with the historical evidence of adverse effects with greater depths of whole-body hypothermia. This safety study and the strong experimental evidence for improved cerebral outcome justify a multicenter trial of selective head cooling for neonatal encephalopathy in term infants.

Fetal brain injury following prolonged hypoxemia and placental insufficiency: a review

It is well-established that severe, acute episodes of hypoxemia can damage the brain before birth, but the effects of more sustained hypoxemia are less well understood. We have used fetal fetal sheep in a series of studies aimed at determining the effects of prolonged hypoxemia, induced by placental insufficiency of differing severity and duration, on fetal brain structure. Restriction of placental, and hence fetal, growth by carunclectomy caused impaired development of neural processes and connections in the hippocampus, cerebellum, and visual cortex; neuronal migration and neuronal numbers did not appear to be affected. Twenty days of placental insufficiency during late gestation induced by umbilicoplacental embolisation also caused abnormalities in brain structure; the cerebellum, which develops late in gestation, was particularly affected. In the cortex, there was evidence of white matter lesions, an increase in the size of capillaries and a proliferation of astroglia. We also examined the effects of shorter periods of hypoxemia (6-12 hr) near mid-gestation on brain structure; fetuses were allowed to recover for 7 or 35 days after the hypoxemic challenge. The major changes were mild focal damage in the cortical white matter, a reduction in the number of Purkinje cells, a delay in the growth of neural processes in the cerebellum and proliferation of blood vessels. The hippocampus was also affected, in particular the areal density of pyramidal cells was reduced. The use of several classes of pharmacological agents with the potential to protect neurons from hypoxemic injury is discussed in relation to the developing brain.

Intracerebroventricular prostaglandin administration increases the neural damage evoked by global hemispheric hypoxic ischemia

This study was designed to determine if central (intracerebroventricular, i.c.v.) administration of prostaglandin E2 (PGE2, mediator of core temperature elevation following exogenous or endogenous pyrogen administration) worsens the neural damage of anesthetized rats to global hemispheric hypoxic-ischemia (GHHI) from damage seen in normothermic, i.c.v. saline control groups. The first study (no GHHI) showed that 10 or 50 ng PGE2 given i.c.v. to groups of anesthetized Long-Evans rats evoked dose-related increases in colonic (systemic core) temperature but no neural damage. In the second study anesthetized rats were given an i.c.v. injection of sterile saline or PGE2 plus GHHI (ligation of the right common carotid artery plus 35 min of 12% O2) at the peak of the temperature response. Thermal response indices (TRI, degrees C x min), determined from brain (temporalis muscle, ipsilateral and contralateral to ligation) and core (colonic) temperatures, showed significant increases in the 50-ng PGE2 group compared to the TRIs of the 10-ng PGE2 or saline control group. The 50-ng PGE2, GHHI group had a higher mortality rate and showed greater ipsilateral hemispheric neural damage than the saline-treated group given the same insult, especially due to increased damage to the cortex. The results show that i.c.v. PGE2 administration significantly increases the neural damage caused by GHHI, possibly due to the associated rise in core temperature.

Modest hypothermia preserves cerebral energy metabolism during hypoxia-ischemia and correlates with brain damage: a 31P nuclear magnetic resonance study in unanesthetized neonatal rats

Recent studies have shown that mild to moderate (modest) hypothermia decreases the damage resulting from hypoxic-ischemic insult (HI) in the immature rat. To determine whether suppression of oxidative metabolism during HI is central to the mechanism of neuroprotection, 31P nuclear magnetic resonance (NMR) spectroscopy was used to measure high energy metabolites in 7-d postnatal rats under conditions of modest hypothermia during the HI. The rats underwent unilateral common carotid artery ligation followed by exposure to hypoxia in 8% oxygen for 3 h. Environmental temperature was decreased by 3 or 6 degrees C from the control temperature, 37 degrees C, which reliably produces hemispheric damage in over 90% of pups. The metabolite parameters and tissue swelling (edema) at 42 h recovery varied very significantly with the three temperatures. Tissue swelling was 26.9, 5.3, and 0.3% at 37, 34, and 31 degrees C, respectively. Core temperature and swelling were also measured, with similar results, in parallel experiments in glass jars. Multislice magnetic resonance imaging, histology, and triphenyltetrazolium chloride staining confirmed the fairly uniform damage, confined to the hemisphere ipsilateral to the ligation. The NMR metabolite levels were integrated over the last 2.0 h out of 3.0 h of HI, and were normalized to their baseline for all surviving animals (n = 25). ATP was 47.9, 69.0, and 83.0% of normal, whereas the estimator of phosphorylation potential (phosphocreatinine/inorganic phosphorus) was 16.9, 27.8, and 42.6% of normal at 37, 34, and 31 degrees C, respectively. There was a significant correlation of both phosphocreatinine/inorganic phosphorus (p < 0.0001) and ATP levels (p < 0.0001) with brain swelling. Abnormal brain swelling and thus damage can be reliably predicted from a threshold of these metabolite levels (p < 0.0001). Thus for all three temperatures, a large change in integrated high energy metabolism during HI is a prerequisite for brain damage. With a moderate hypothermia change of 6 degrees C, where there is an insufficient change in metabolites, there is no subsequent HI brain damage. In general, treatment for HI in our 7-d-old rat model should be aimed at preserving energy metabolism.

Carotid artery and jugular vein ligation with and without hypoxia in the rat

A continuing concern about the use of extracorporeal membrane oxygenation (ECMO) is the cannulation of the common carotid artery or the internal jugular vein. The authors investigated the changes that might occur in the brain with neck vessel ligation in the normal and the hypoxic rat. Two groups of 60 rats each were studied. The first group was divided into three subgroups of 20 animals each. Subgroup 1 (HH) was hypoxic both 24 hours before and 24 hours after operation. Subgroup 2 (HN) (the ECMO model) was hypoxic before operation and recovered for 24 hours in room air. Subgroup 3 (NN) underwent the entire procedure in room air. For each oxygen environment, four different operations were performed: carotid artery ligation, jugular vein ligation, carotid artery and jugular vein ligation, and dissection of the vessels without ligation (sham). Thus each subgroup was further divided into four sub-subgroups based on the operation performed. Rats were again anesthetized after a 24-hour recovery period and killed using low, blunt cervical dislocation. In the first group of 60 rats, the skull was opened and the brain was carefully removed from the cranial vault and placed in a fixative. The brains were placed in a small magnetic resonance imaging (MRI) head coil in groups of five and scans were obtained to provide T1 and T2 images that correlated with histological sections. MRI scans were reviewed in random, blinded fashion by an imager unaware of how these animals had been treated. The brains were then sectioned coronally at six corresponding levels: frontal, mid and posterior cerebrum, midbrain, pons, and medulla. Histological examination was performed in blinded fashion. The number of lesions (usually ischemic as noted by a decrease in the number of neurons) was totaled for each area of the brain. There were no differences that were consistent or statistically significant in the MR images of brains removed from the head, although it would appear that rats with jugular vein and carotid artery ligation were relatively protected. In the HN group jugular vein ligation was worst, and adding carotid artery ligation was best. In the histological studies the NN group had significantly more lesions than the HH group (P < .01). The second group of 60 rats was divided and treated as the first group in all respects except that MRI was conducted immediately after death on intact heads, and no histological studies were performed. This was done to control for lesions that might have been produced by removal of the brains from the skulls. In this group all findings were right sided. One animal in the HN group showed midcerebral white matter edema after jugular and carotid ligation. Focal anterior cerebral edema was seen in another animal (HH) after isolated carotid ligation. An occipital infarct was found in one animal (HH) after both carotid and jugular ligation. The authors conclude that neck vessel ligation in the hypoxic or normoxic rat causes only occasional and sporadic brain injury much as is seen clinically in newborn ECMO patients.

Animal models for the study of perinatal hypoxic-ischemic encephalopathy: a critical analysis

We critically evaluated various design features from 292 animal studies related to perinatal hypoxic-ischemic encephalopathy (HIE). Rodents were the most frequently used animals in HIE research (26%), followed by piglets (23%) and sheep (22%). Asphyxia with or without ischemia was the most predominant method of producing experimental brain damage, but there were significant variations in specific details, particularly regarding the method and duration of brain insult. In 71% (207/292) of studies the CNS outcomes were tested within 24 h of experimental insult and in 29% (85/292) they were tested 24 h or more after the insult. Acute CNS metabolic end-points were assessed in 82-100% of all studies. In 90% of studies the chronological age of the animal was equivalent to that of human term newborn infant. However, in only 23% (67/292) were clinical neurological, developmental or behavioral outcomes evaluated, and in only 26% (76/292) was neuropathology assessed. While no single animal model was found to be ideal for all HIE research, some models were distinctly superior to others, depending upon the specific research question. The fetal sheep, newborn lamb and piglet models are well suited for the study of acute and subacute metabolic and physiologic endpoints, whereas the rodent and primate models could be used for long-term neurological and behavioral outcome experiments as well. We also feel that standardizing the study design features, including an HI insult method that produces consistent and predictable brain damage is urgently needed. Studies in neuro-ethology should explore how well brains of various animals compare with that of the newborn human infant. There is also a need for developing animal models that mimic clinical entities in which long-term neuro-developmental and behavioral outcomes can be assessed.

The effect of age on susceptibility to brain damage in a model of global hemispheric hypoxia-ischemia

Stroke occurs in all age groups, ranging from the newborn to the elderly. The immature brain is generally believed to be more resistant to the damaging effects of cerebrovascular compromise compared to the more mature brain. However, recent experiments suggest that the correlation between brain damage and age is not linear. To determine the effects of age and development on hypoxic-ischemic brain damage, we developed a model whereby rats of increasing age received identical cerebrovascular insults, and assessed neuropathologic outcome. Male Wistar rats of 1, 3, 6, and 9 weeks and 6 months underwent unilateral common carotid artery ligation and exposure to 12% oxygen for 35 min. Animals were all spontaneously breathing under light halothane anesthesia (0.5%). Core temperatures were maintained at 37 degrees C. Blood pressures were monitored via indwelling carotid artery catheters on the side ipsilateral to the carotid artery ligation. Cerebral blood flow was assessed in separate groups utilizing Laser Doppler flowmetry. Physiologic monitoring revealed that under these experimental conditions, mean arterial blood pressure and cerebral blood flow decreased to the same extent in each of the age groups, verifying that all animals experienced an identical insult. Neuropathologic assessment at 7 days of recovery showed that brain damage was most severe in the 1 and 3 week old animals followed by those that were 6 months. The 6 and 9 week old groups had significantly less injury than the other 3 age groups. Hippocampal damage was most severe in the 3 week and 6 month old rats compared to all other age groups. Our findings contrast previously held beliefs regarding the enhanced tolerance of the immature brain to hypoxic-ischemic damage and demonstrates that, in a physiologically controlled in vivo model of hemispheric global ischemia, (1) the immature brain is, in fact, less resistant to hypoxic-ischemic brain damage than its adult counterpart, (2) the brain damaging effects of hypoxic-ischemia are age dependent, but do not increase linearly with advancing age and development, and (3) the intermediate age groups are more tolerant to hypoxic-ischemic brain injury than either very young or more mature ages.

Effect of mild hypothermia on cerebral energy metabolism during the evolution of hypoxic-ischemic brain damage in the immature rat

BACKGROUND AND PURPOSE

Intraischemic hypothermia (34 degrees C and 31 degrees C) has a profound neuroprotective effect on the brain of the immature rat. Hypothermia immediately after hypoxia-ischemia is not beneficial. To determine the mechanisms by which mild to moderate hypothermia affects cerebral energy metabolism of the brain of the newborn rat pup, we examined alterations in cerebral glycolytic intermediates and high-energy phosphate compounds during intraischemic and postischemic hypothermia and correlated these findings with known neuropathologic injury.

METHODS

Seven-day-old rat pups underwent unilateral common carotid artery ligation and exposure to hypoxia in 8% oxygen at either 37 degrees C, 34 degrees C, or 31 degrees C for 3.0 hours. Separate groups were exposed to hypoxia-ischemia at 37 degrees C for 3 hours but recovered at either 37 degrees C, 34 degrees C, or 31 degrees C. At 60, 120, and 180 minutes of intraischemic hypothermia and at 10, 30, 60, and 240 minutes of postischemic hypothermia, individual rat pups were quick-frozen in liquid nitrogen for later determination of cerebral concentrations of glucose, lactate, ATP, and phosphocreatine.

RESULTS

Cerebral glucose was significantly higher and lactate significantly lower in the 31 degrees C animals during hypoxia-ischemia than either the 34 degrees C or 37 degrees C groups. Brain ATP concentrations were completely preserved during hypoxia-ischemia at 31 degrees C, whereas 34 degrees C of hypothermia had no effect on preserving high-energy phosphate compounds compared with those animals in the 37 degrees C group. Postischemic hypothermia of either 34 degrees C or 31 degrees C had no effect on the rate or extent of recovery of glycolytic intermediates or high-energy phosphate compounds compared with the normothermic 37 degrees C rat pups.

CONCLUSIONS

Moderate hypothermia of 31 degrees C completely inhibits the depletion of ATP during hypoxia-ischemia, a mechanism that likely accounts for its neuroprotective effect. No preservation of ATP was seen, however, during intraischemic mild hypothermia of 34 degrees C despite the relatively profound neuroprotective effect of this degree of temperature reduction. Thus, the mechanisms by which mild hypothermia is neuroprotective are temperature dependent and may act at more than one point along the cascade of events eventually leading to hypoxic-ischemic brain damage in the immature rat.