Keeping a cool head, post-hypoxic hypothermia--an old idea revisited

The Role of Oxidative Stress in the Progression of Secondary Brain Injury Following Germinal Matrix Hemorrhage

Germinal matrix hemorrhage (GMH) can be a fatal condition responsible for the death of 1.7% of all neonates in the USA. The majority of GMH survivors develop long-term sequalae with debilitating comorbidities. Higher grade GMH is associated with higher mortality rates and higher prevalence of comorbidities. The pathophysiology of GMH can be broken down into two main titles: faulty hemodynamic autoregulation and structural weakness at the level of tissues and cells. Prematurity is the most significant risk factor for GMH, and it predisposes to both major pathophysiological mechanisms of the condition. Secondary brain injury is an important determinant of survival and comorbidities following GMH. Mechanisms of brain injury secondary to GMH include apoptosis, necrosis, neuroinflammation, and oxidative stress. This review will have a special focus on the mechanisms of oxidative stress following GMH, including but not limited to inflammation, mitochondrial reactive oxygen species, glutamate toxicity, and hemoglobin metabolic products. In addition, this review will explore treatment options of GMH, especially targeted therapy.

Adaptations to a hypoxic lifestyle in naked mole-rats

Hypoxia is one of the strongest environmental drivers of cellular and physiological adaptation. Although most mammals are largely intolerant of hypoxia, some specialized species have evolved mitigative strategies to tolerate hypoxic niches. Among the most hypoxia-tolerant mammals are naked mole-rats (Heterocephalus glaber), a eusocial species of subterranean rodent native to eastern Africa. In hypoxia, naked mole-rats maintain consciousness and remain active despite a robust and rapid suppression of metabolic rate, which is mediated by numerous behavioural, physiological and cellular strategies. Conversely, hypoxia-intolerant mammals and most other hypoxia-tolerant mammals cannot achieve the same degree of metabolic savings while staying active in hypoxia and must also increase oxygen supply to tissues, and/or enter torpor. Intriguingly, recent studies suggest that naked mole-rats share many cellular strategies with non-mammalian vertebrate champions of anoxia tolerance, including the use of alternative metabolic end-products and potent pH buffering mechanisms to mitigate cellular acidification due to upregulation of anaerobic metabolic pathways, rapid mitochondrial remodelling to favour increased respiratory efficiency, and systemic shifts in energy prioritization to maintain brain function over that of other tissues. Herein, I discuss what is known regarding adaptations of naked mole-rats to a hypoxic lifestyle, and contrast strategies employed by this species to those of hypoxia-intolerant mammals, closely related African mole-rats, other well-studied hypoxia-tolerant mammals, and non-mammalian vertebrate champions of anoxia tolerance. I also discuss the neotenic theory of hypoxia tolerance – a leading theory that may explain the evolutionary origins of hypoxia tolerance in mammals – and highlight promising but underexplored avenues of hypoxia-related research in this fascinating model organism.

Free Radicals and Neonatal Brain Injury: From Underlying Pathophysiology to Antioxidant Treatment Perspectives

Free radicals play a role of paramount importance in the development of neonatal brain injury. Depending on the pathophysiological mechanisms underlying free radical overproduction and upon specific neonatal characteristics, such as the GA-dependent maturation of antioxidant defenses and of cerebrovascular autoregulation, different profiles of injury have been identified. The growing evidence on the detrimental effects of free radicals on the brain tissue has led to discover not only potential biomarkers for oxidative damage, but also possible neuroprotective therapeutic approaches targeting oxidative stress. While a more extensive validation of free radical biomarkers is required before considering their use in routine neonatal practice, two important treatments endowed with antioxidant properties, such as therapeutic hypothermia and magnesium sulfate, have become part of the standard of care to reduce the risk of neonatal brain injury, and other promising therapeutic strategies are being tested in clinical trials. The implementation of currently available evidence is crucial to optimize neonatal neuroprotection and to develop individualized diagnostic and therapeutic approaches addressing oxidative brain injury, with the final aim of improving the neurological outcome of this population.

Variability and sex-dependence of hypothermic neuroprotection in a rat model of neonatal hypoxic-ischaemic brain injury: a single laboratory meta-analysis

Therapeutic hypothermia (HT) is standard care for term infants with hypoxic–ischaemic (HI) encephalopathy. However, the efficacy of HT in preclinical models, such as the Vannucci model of unilateral HI in the newborn rat, is often greater than that reported from clinical trials. Here, we report a meta-analysis of data from every experiment in a single laboratory, including pilot data, examining the effect of HT in the Vannucci model. Across 21 experiments using 106 litters, median (95% CI) hemispheric area loss was 50.1% (46.0–51.9%; n = 305) in the normothermia group, and 41.3% (35.1–44.9%; n = 317) in the HT group, with a bimodal injury distribution. Median neuroprotection by HT was 17.6% (6.8–28.3%), including in severe injury, but was highly-variable across experiments. Neuroprotection was significant in females (p < 0.001), with a non-significant benefit in males (p = 0.07). Animals representing the median injury in each group within each litter (n = 277, 44.5%) were also analysed using formal neuropathology, which showed neuroprotection by HT throughout the brain, particularly in females. Our results suggest an inherent variability and sex-dependence of the neuroprotective response to HT, with the majority of studies in the Vannucci model vastly underpowered to detect true treatment effects due to the distribution of injury.

Hypothermia Used in Medical Applications for Brain and Spinal Cord Injury Patients

Despite more than 80 years of animal experiments and clinical practice, efficacy of hypothermia in improving treatment outcomes in patients suffering from cell and tissue damage caused by ischemia is still ongoing. This review will first describe the history of utilizing cooling in medical treatment, followed by chemical and biochemical mechanisms of cooling that can lead to neuroprotection often observed in animal studies and some clinical studies. The next sections will be focused on current cooling approaches/devices, as well as cooling parameters recommended by researchers and clinicians. Animal and clinical studies of implementing hypothermia to spinal cord and brain tissue injury patients are presented next. This section will review the latest outcomes of hypothermia in treating patients suffering from traumatic brain injury (TBI), spinal cord injury (SCI), stroke, cardiopulmonary surgery, and cardiac arrest, followed by a summary of available evidence regarding both demonstrated neuroprotection and potential risks of hypothermia. Contributions from bioengineers to the field of hypothermia in medical treatment will be discussed in the last section of this review. Overall, an accumulating body of clinical evidence along with several decades of animal research and mathematical simulations has documented that the efficacy of hypothermia is dependent on achieving a reduced temperature in the target tissue before or soon after the injury-precipitating event. Mild hypothermia with temperature reduction of several degrees Celsius is as effective as modest or deep hypothermia in providing therapeutic benefit without introducing collateral/systemic complications. It is widely demonstrated that the rewarming rate must be controlled to be lower than 0.5 °C/h to avoid mismatch between local blood perfusion and metabolism. In the past several decades, many different cooling methods and devices have been designed, tested, and used in medical treatments with mixed results. Accurately designing treatment protocols to achieve specific cooling outcomes requires collaboration among engineers, researchers, and clinicians. Although this problem is quite challenging, it presents a major opportunity for bioengineers to create methods and devices that quickly and safely produce hypothermia in targeted tissue regions without interfering with routine medical treatment.

Neuroprotective body hypothermia among newborns with hypoxic ischemic encephalopathy: three-year experience in a tertiary university hospital. A retrospective observational study

CONTEXT AND OBJECTIVE

Neonatal hypoxic-ischemic encephalopathy is associated with high morbidity and mortality. Studies have shown that therapeutic hypothermia decreases neurological sequelae and death. Our aim was therefore to report on a three-year experience of therapeutic hypothermia among asphyxiated newborns.

DESIGN AND SETTING

Retrospective study, conducted in a university hospital.

METHODS

Thirty-five patients with perinatal asphyxia undergoing body cooling between May 2009 and November 2012 were evaluated.

RESULTS

Thirty-nine infants fulfilled the hypothermia protocol criteria. Four newborns were removed from study due to refractory septic shock, non-maintenance of temperature and severe coagulopathy. The median Apgar scores at 1 and 5 minutes were 2 and 5. The main complication was infection, diagnosed in seven mothers (20%) and 14 newborns (40%). Convulsions occurred in 15 infants (43%). Thirty-one patients (88.6%) required mechanical ventilation and 14 of them (45%) were extubated within 24 hours. The duration of mechanical ventilation among the others was 7.7 days. The cooling protocol was started 1.8 hours after birth. All patients showed elevated levels of creatine phosphokinase, creatine phosphokinase- MB and lactate dehydrogenase. There was no severe arrhythmia; one newborn (2.9%) presented controlled coagulopathy. Four patients (11.4%) presented controlled hypotension. Twenty-nine patients (82.9%) underwent cerebral ultrasonography and 10 of them (34.5%) presented white matter hyper-echogenicity. Brain magnetic resonance imaging was performed on 33 infants (94.3%) and 11 of them (33.3%) presented hypoxic-ischemic changes. The hospital stay was 23 days. All newborns were discharged. Two patients (5.8%) needed gastrostomy.

CONCLUSION

Hypothermia as therapy for asphyxiated newborns was shown to be safe.

Safety and efficacy of topiramate in neonates with hypoxic ischemic encephalopathy treated with hypothermia (NeoNATI)

BACKGROUND

Despite progresses in neonatal care, the mortality and the incidence of neuro-motor disability after perinatal asphyxia have failed to show substantial improvements. In countries with a high level of perinatal care, the incidence of asphyxia responsible for moderate or severe encephalopathy is still 2-3 per 1000 term newborns. Recent trials have demonstrated that moderate hypothermia, started within 6 hours after birth and protracted for 72 hours, can significantly improve survival and reduce neurologic impairment in neonates with hypoxic-ischemic encephalopathy. It is not currently known whether neuroprotective drugs can further improve the beneficial effects of hypothermia. Topiramate has been proven to reduce brain injury in animal models of neonatal hypoxic ischemic encephalopathy. However, the association of mild hypothermia and topiramate treatment has never been studied in human newborns. The objective of this research project is to evaluate, through a multicenter randomized controlled trial, whether the efficacy of moderate hypothermia can be increased by concomitant topiramate treatment.

METHODS/DESIGN

Term newborns (gestational age ≥ 36 weeks and birth weight ≥ 1800 g) with precocious metabolic, clinical and electroencephalographic (EEG) signs of hypoxic-ischemic encephalopathy will be randomized, according to their EEG pattern, to receive topiramate added to standard treatment with moderate hypothermia or standard treatment alone. Topiramate will be administered at 10 mg/kg once a day for the first 3 days of life. Topiramate concentrations will be measured on serial dried blood spots. 64 participants will be recruited in the study. To evaluate the safety of topiramate administration, cardiac and respiratory parameters will be continuously monitored. Blood samplings will be performed to check renal, liver and metabolic balance. To evaluate the efficacy of topiramate, the neurologic outcome of enrolled newborns will be evaluated by serial neurologic and neuroradiologic examinations. Visual function will be evaluated by means of behavioural standardized tests.

DISCUSSION

This pilot study will explore the possible therapeutic role of topiramate in combination with moderate hypothermia. Any favourable results of this research might open new perspectives about the reduction of cerebral damage in asphyxiated newborns.

[Neuroprotection with hypothermia in the newborn with hypoxic-ischaemic encephalopathy. Standard guidelines for its clinical application]

Standardisation of hypothermia as a treatment for perinatal hypoxic-ischaemic encephalopathy is supported by current scientific evidence. The following document was prepared by the authors on request of the Spanish Society of Neonatology and is intended to be a guide for the proper implementation of this therapy. We discuss the difficulties that may arise when moving from the strict framework of clinical trials to clinical daily care: early recognition of clinical encephalopathy, inclusion and exclusion criteria, hypothermia during transport, type of hypothermia (selective head or systemic cooling) and side effects of therapy. The availability of hypothermia therapy has changed the prognosis of children with hypoxic-ischaemic encephalopathy and our choices of therapeutic support. In this sense, it is especially important to be aware of the changes in the predictive value of the neurological examination and the electroencephalographic recording in cooled infants. In order to improve neuroprotection with hypothermia we need earlier recognition of to recognise earlier the infants that may benefit from cooling. Biomarkers of brain injury could help us in the selection of these patients. Every single infant treated with hypothermia must be included in a follow up program in order to assess neurodevelopmental outcome.

Filling the evidence gap: how can we improve the outcome of neonatal encephalopathy in the next 10 years?

Neonatal encephalopathy associated with perinatal hypoxia-ischaemia is one of the most common causes of death and permanent disability worldwide. However, of a wide range of "experimentally neuroprotective treatments" invented so far, only therapeutic hypothermia has been promoted into a standard clinical practice. Such a wide gap in the efficacy of neuroprotective treatments between the experimental setting and clinical practice may be attributed to the strategic flaw in translating basic knowledge into clinical care. When previous clinical studies are carefully reviewed, one may notice that few therapeutic options were chosen based on their track records in experimental studies; protective effects of some drugs had been assumed only based on their pharmacokinetics in adult species; several therapies were chosen merely because clinicians were familiar to these treatments for other purpose; some other therapies were imported too preliminarily from laboratory to clinical practice, potentially ignoring the difference in physiological and pathological backgrounds between rodent models and human patients. When further clinical trials are planned, it is important to ask whether (i) the treatment is supported by pharmacokinetics specific to immature brain, and (ii) the neuroprotective effect of the treatment has consistently been demonstrated using clinically relevant models and study designs. The use of translational large animal models allows the practical simulation and fine-tuning of clinical protocols, which may further assist successful translation of basic knowledge. In addition to the effort to develop alternative therapeutic options, it is important to maximise the effect of the current only neuroprotective option, or therapeutic hypothermia. Independent variables which influence the efficacy of hypothermia have to be elucidated to improve its therapeutic protocol, and to increase the number of patients who will benefit from this treatment.

Whole body hypothermia and oxidative stress in babies with hypoxic-ischemic brain injury

According to increasing evidence, hypothermia can significantly improve outcomes in term neonates manifesting asphyxic insult and hypoxic-ischemic encephalopathy. Oxidative stress plays a key role in hypoxic-ischemic and inflammatory brain injuries. We investigated the impact of hypothermia on oxidative stress in babies with hypoxic-ischemic encephalopathy. Term infants were randomly selected for treatment with moderate whole body hypothermia or standard care on normothermia, after perinatal asphyxia. Total hydroperoxides as biochemical markers of oxidative stress, and C-reactive protein as a marker of inflammation, were assayed in blood samples drown at 6, 12, 24, 48, and 72 postnatal hours. In both hypothermic and normothermic groups, total hydroperoxides and C-reactive protein exhibited a continuous increase in the first days after birth. Nevertheless, a tendency was evident for slower and smaller elevations of total hydroperoxides and C-reactive protein in hypothermic compared with normothermic infants. A significant correlation was observed between total hydroperoxides and C-reactive protein in all patients, indicating an association between inflammation and oxidative stress during asphyxia. The slower increase and lower peaks of total hydroperoxides in the hypothermic group support the hypothesis that postasphyxic oxidative stress may be reduced by hypothermia.

Surveillance during labour

The technology of intrapartum surveillance made rapid strides from the 1960s through the 1980s but then stagnated as increasing resort to caesarean section was made rather than improving measures of fetal condition and labour progress. However, despite caesarean section rates commonly over 30%, medicolegally expensive mistakes continue to be made because it is difficult to teach clinicians to make reliable use of existing technology. It may be that as with aircraft navigation, the safest solution is to replace human judgement with the obstetric equivalent of automatic pilots.

Effects of acute hypoxia and hyperthermia on the permeability of the blood-brain barrier in adult rats

Acute mountain sickness (AMS) develops within a few hours after arrival at high altitude and includes headache, anorexia, nausea, vomiting, and malaise. This afflicts 15-25% of the general tourist population at moderate altitudes. High-altitude cerebral edema (HACE) is considered to be the end stage of severe AMS and has been suggested to be a vasogenic edema, raising the possibility that acute hypoxia may increase blood-brain barrier (BBB) permeability. At present, there are no good small-animal models to study this syndrome. We hypothesize 1) that acute hypoxia can damage the BBB and 2) that rat can be used as a model to study hypoxia-induced changes in BBB permeability, especially if hypoxia-induced hypothermia could be minimized with high ambient temperature (HAT). Male Wistar rats were exposed to 1, 2, and 7 days of hypobaric hypoxia (equivalent to 0.5 atm), and changes in the temperature and BBB permeability were studied. The extravasation of endogenous immunoglobulin G, a large molecule, did not increase during room temperature hypoxia but did increase when hypoxia was combined with HAT. Hypoxia caused a significant increase in the leakage of sodium fluorescein (mol wt 376 Da). The expression of endothelial barrier antigen (EBA), a protein associated with the BBB, was reduced to 50% between 24 and 48 h after exposure to hypoxia, and the loss was exacerbated by HAT. The values almost returned to control levels by 7 days, showing adaptation to hypoxia. Hypoxic rats exhibited sodium fluorescein leakage mainly in focal areas in the brain parenchyma. In conclusion, it is possible to have transient BBB damage through exposure to acute hypoxia, and this damage is exacerbated by increasing body temperature to more of a normothermic value.

Post-resuscitation management of asphyxiated neonates

Inspite of major advances in monitoring technology and knowledge of fetal and perinatal medicine, perinatal asphyxia is one of the significant causes of mortality and long term morbidity. Data from National Neonatal Perinatal Database suggests that perinatal asphyxia contributes to almost 20% of neonatal deaths in India. "Failure to initiate or sustain respiration after birth" has been defined as criteria for the diagnosis of asphyxia by WHO. Perinatal asphyxia results in hypoxic injury to various organs including kidneys, lungs and liver but the most serious effects are seen on the central nervous system. Levene's classification is a useful clinical tool for grading the severity of hypoxic ischemic encephalopathy. Good supportive care is essential in the first 48 hours after asphyxia to prevent ongoing brain injury in the penumbra region. Strict monitoring and prompt correction is needed for common problems including temperature maintenance, blood sugars, blood pressure and oxygenation. Phenobarbitone is the drug of choice for the treatment of convulsions.

Resuscitative hypothermia protects the neonatal rat brain from hypoxic-ischemic injury

The effect of 24 h of hypothermic recovery on moderate hypoxic-ischemic brain damage in P7-rats was investigated for 42 d after the insult, using magnetic resonance and histopathology. Occlusion of right common carotid artery and 90 min exposure to 8% O2 at 37 degrees C body temperature produced cytotoxic edema of 51(+/-11)% brain volume (BV) and depression of brain energy metabolism (PCr/Pi) from 1.43(+/-0.21) to 0.14(+/-0.11). During recovery, the body temperature was reduced to 30 degrees C for 24 h in 36 animals, but was kept at 37 degrees C in 34 animals. The edema waned upon reoxygenation leaving only the core lesion at 2 h, but reappeared reaching a maximal extent of 11+/-8% BV under hypothermia compared to 45(+/-10)% under normothermia at around 24 h. PCr/Pi recovered transiently within 13 h and declined again to 1.07(+/-0.19) under hypothermia and to 0.48(+/-0.22) under normothermia at around 24 h. Hypothermia led to significant long term brain protection, leaving permanent tissue damage of 12(+/-6)% BV compared to 35(+/-12)% BV under normothermia. However, animals with severe initial injury developed large infarctions, despite hypothermic treatment. Even then, the time to develop infarction was significantly prolonged, leaving the opportunity for additional therapeutic intervention.

Early head cooling in newborn piglets is neuroprotective even in the absence of profound systemic hypothermia

BACKGROUND

Selective head cooling in the newborn infant has been proposed as a neuroprotective treatment with a lower level of systemic adverse effect than that of systemic hypothermia. However, the efficacy is not confirmed as well as that of systemic hypothermia. In order to analyze the safety and efficacy of selective head cooling, 25 newborn piglets were randomly selected for either normothermic or hypothermic treatment.

METHODS

Global hypoxic insult was induced by lowering the oxygen concentration to the maximal level to maintain the background electroencephalogram (EEG) voltage under 7 microV for 45 min. The core temperature of normothermic piglets was maintained between 38.5 degrees C and 39 degrees C, while prophylactic cooling was applied to the hypothermic piglets at the same time of the insult. Very mild systemic hypothermia by 1 degrees C was induced in addition to selective head cooling with 10 degrees C coolant temperature. Animals were killed for histopathological examination seven hours after the end of the insult.

RESULTS

Two normothermic piglets died while all hypothermic piglets survived. Neuropathological findings were significantly severer in the normothermic group than in the hypothermic group. Intracranial pressure was significantly lower, and EEG recovery was significantly better in the hypothermic piglets. There was no significant difference in the lowest oxygen concentration, degrees of acidosis, blood lactate, and blood pressure between the groups, although heart rate was significantly lower in the hypothermic group.

CONCLUSIONS

We have demonstrated that early head cooling was effective in preventing some of the earliest brain damage due to hypoxic insult even in the absence of profound systemic hypothermia.

Extreme marginal donor: severe hypothermia as a rare preservation condition for explantable organs--a case report

The progressive increase in patients with end stage liver disease has lengthend the waiting- list for liver transplantation. Unfortunately this has not been followed by a suitable increase in the number of donors. The expanding "donor pool" has required use of "marginal" donors (ICU stay > 10 days, sepsi; steatosis > 30-40%, hypernatremia > 155 mmol/L, inotropic drugs). We report the case of a skier who remained for more than 1 hour in cardio-respiratory arrest under the snow; the 49-year-old women was extracted from the snow after 1 hour and 12 minutes and found to be asystolic, fixed pupils and deep hypothermia (27.2 degrees C). After cardiopulmonary resuscitation, partial cardio-respiratory activity was re-established. In the ICU severe hypothermia (26.7 degrees C) was treated with extracorporeal circulation until a re-establishment of satisfactory cardio-circulatory conditions was obtained. Unfortunately cerebral anoxic cerebral death was established and multiorgan procurement performed 3 days later. After liver transplantation into a 59 year-old patient with PNC-C was performed. The course was uneventful and the patient was discharged on the 19th postoperative day.

CONCLUSIONS

Organ procurement from donors involved in accidental traumatic events with cardio-respiratory arrest and hypothermia, is similar to the non-heart-beating donor (NHBD) condition. Correct cardiopulmonary resuscitation and the use of extracorporeal circulation for gradual restoration of body temperature are necessary for optimal organ perfusion. In the present case the anoxic insult induced by the cessation of the cardio-respiratory function, was probably mitigated (if not even annulled) by the hypothermia.

Brain temperature in newborn piglets under selective head cooling with minimal systemic hypothermia

BACKGROUND

Although selective brain hypothermia is expected to be a promising neuroprotective treatment, the thermal distribution under hypothermia is not fully investigated. We applied selective head cooling to seven newborn piglets under general anesthesia in order to investigate the mechanism of cooling.

METHODS

Seven healthy, large white piglets aged within 5 days after birth were studied. Temperatures were monitored at the superficial brain (0.5 cm), deep brain (2.0 cm), scalp skin, nasopharynx, tympanum, esophagus, and rectum. A radiant heater and a warmer blanket were used to maintain the normal rectal temperature (38.5-39 degrees C). For the first piglet, the coolant temperature was widely changed from 15 degree C to - 20 degree C in order to define the practical range. Subsequently, the coolant temperature was set at 10 degree C, 0 degree C, and - 10 degree C for the remaining six piglets. The target deep brain temperature was set at 35 degree C, as the same reduction of brain temperature might provide moderate brain hypothermia in the human neonate.

RESULTS

With 0 degree C coolant temperature, the deep brain temperature was cooled to 35 degree C; however, the scalp skin attached to the cooling cap became broadly blotchy and injured in all animals. When we induced minimal systemic hypothermia by 1C for a cohort of three piglets, the deep brain temperature decreased in parallel with the rectal temperature, which enabled us to achieve the target temperature with 10 degrees C coolant without injuring the scalp skin. The scalp skin and nasopharyngeal temperatures were good predictors of both superficial and deep-brain temperatures throughout the experiment.

CONCLUSIONS

Our results suggest that moderate brain hypothermia may be applied to newborn infants without inducing moderate systemic hypothermia.

Hypothermia attenuates circulatory shock and cerebral ischemia in experimental heatstroke

We tested the hypothesis in a rat model that body cooling suppresses circulatory shock and cerebral ischemia in heatstroke. Animals under urethane anesthesia were exposed to water blanket temperature (Tblanket) of 42 degrees C until mean arterial pressure (MAP) and local cerebral blood flow (CBF) in the hippocampus began to decrease from their peak levels, which was arbitrarily defined as the onset of heatstroke. Control rats were exposed to 26 degrees C. Extracellular concentrations of glutamate, glycerol, lactate, and lactate/pyruvate in the hippocampus were assessed by microdialysis methods. Cooling was accomplished by decreasing Tblanket from 42 degrees C to 16 degrees C. The values of MAP and CBF after the onset of heat stroke in heatstroke rats received no cooling were all significantly lower than those in control rats. However, the neuronal damage score and extracellular levels of ischemia and damage markers in the hippocampus were greater. Cooling immediately after the onset of heatstroke reduced the heatstroke-induced circulatory shock, cerebral ischemia, neuronal damage, and surge of tissue ischemia and damage markers in the hippocampus, and resulted in prolongation of survival time. Delaying the onset of cooling reduced the therapeutic efficiency. The results suggest that body cooling attenuates circulatory shock and cerebral ischemia insults in heatstroke.

Hypothermia throughout intestinal ischaemia-reperfusion injury attenuates lung neutrophil infiltration

BACKGROUND/PURPOSE

Secondary organ damage to the lungs is an important consequence of intestinal ischaemia reperfusion (IIR) injury. Moderate hypothermia ameliorates gut necrosis and liver energy failure after IIR but potential beneficial effects on lung neutrophil infiltration after reperfusion of ischaemic bowel have not been investigated.

METHODS

Adult Sprague-Dawley rats underwent 60 minutes intestinal ischaemia followed by 120 minutes of reperfusion. The animals were maintained at either normothermia (36 degrees to 38 degrees C) or moderate hypothermia (30 degrees to 32 degrees C). Four groups were studied: (A) sham normothermia; (B) IIR normothermia; (C) sham hypothermia; and (D) IIR hypothermia. Lungs and terminal ileum were removed for measurement of myeloperoxidase activity (a marker of neutrophil infiltration). Results are expressed as milliunits per milligrams protein, mean +/- SEM, and one-way analysis of variance (ANOVA) with Tukey post-test was used for group comparisons.

RESULTS

Lungs: IIR at normothermia significantly increased lung neutrophil infiltration assessed by myeloperoxidase activity compared with sham-operated controls (normothermia sham 4.6 +/- 1.0, n = 8; normothermia IIR 37.7 +/- 13.8, n = 8; P =.011). Moderate hypothermia during IIR significantly attenuated lung neutrophil infiltration (7.2 +/- 2.1, n = 9) compared with normothermia IIR (P =.016) such that myeloperoxidase activity was similar to that found in sham normothermia (4.6 +/- 1.0, n = 8) and sham hypothermia (3.1 +/- 1.3, n = 8). Intestine: Gut myeloperoxidase activity was 0.9 +/- 0.5 in sham normothermia (n = 9) and 2.3 +/- 0.6 after normothermic IIR (n = 8). After IIR at hypothermia gut myeloperoxidase activity (0.5 +/- 0.2; n = 8) was significantly less than normothermic IIR (P =.035) and higher than sham hypothermia (0.2 +/- 0.1, n = 9; P =.01).

CONCLUSIONS

These results indicate that moderate hypothermia may prevent damage to another distant organ, ie the lungs, by preventing recruitment of neutrophils. This may be of benefit in decreasing distal organ damage in diseases in which intestinal ischaemia-reperfusion is implicated in the pathogenesis.

Cold water submersion and cardiac arrest in treatment of severe hypothermia with cardiopulmonary bypass

In the paediatric population, submersion injury with drowning or near-drowning represents a significant cause of morbidity and mortality. This study reviews retrospectively our own experiences and the literature on the use of cardiopulmonary bypass (CPB) to rewarm paediatric victims of cold water submersion who suffer severe hypothermia (<28 degrees C) and cardiac arrest (asystole or ventricular fibrillation). In addition to three children treated at our institution, nine other victims were found in the literature. In this cohort of 12 children aged between 2 and 12 years, there was a tendency to better outcome with lower core temperature at the beginning of extracorporeal circulation (mean temperature in nine survivors, 20 degrees C; in three non-survivors, 25.5 degrees C). The lowest temperature survived was 16 degrees C. Neither base excess, pH nor serum potassium levels were reliable prognostic factors. The lowest base excess in a survivor was -36.5 mmol/l, the lowest pH 6.29. We consider CPB as the method of choice for resuscitation and rewarming of children with severe accidental hypothermia and cardiac arrest (asystole or ventricular fibrillation). Compared with adults, children, especially smaller ones, require special consideration with regard to intravenous cannulation as drainage can be inadequate using femoral-femoral cannulation. In hypothermic children we advocate, therefore, emergency median sternotomy. Until more information regarding prognostic factors are available, children who are severely hypothermic and clinically dead after submersion in cold water--even if for an unknown length of time--should receive cardiopulmonary resuscitation (CPR) and be transported without delay to a facility with capabilities for CPB instituted via a median sternotomy.

Cranial ultrasound as a predictor of outcome in term newborn encephalopathy

As part of a patient-based case-control study of newborn encephalopathy, we examine the cranial ultrasound results of 212 patients to determine the validity of ultrasound in predicting an adverse outcome. Forty-six (22%) patients died or developed cerebral palsy (adverse outcome) by 2 years of age. On the basis of clinical decision, 125 (60%) patients had an ultrasound before 72 hours of age; of these, 29% had an adverse outcome. The resistive index is the primary measure of interest, with a value of 0.55 or less considered abnormal. Infants with an abnormal resistive index are 8.8 times (P < 0.001) more likely to have an adverse outcome than those with a normal result. The positive predictive value of an abnormal resistive index was 71%. The results are similar for the subgroup with intrapartum hypoxia and the subgroup that had ultrasound performed before 24 hours of age. It is clear that resistive index results cannot be used in isolation, although they may have a place, in combination with other factors, in the counseling of parents and, cautiously, in the clinical management of patients.

Post-resuscitation management of asphyxiated neonates

Perinatal asphyxia is one of the common causes of neonatal mortality. Data from National Neonatal Perinatal database suggest that perinatal asphyxia contributes to almost 20% of neonatal deaths in India. Failure to initiate or sustain respiration after birth has been defined as criteria for the diagnosis of asphyxia by WHO. Perinatal asphyxia results in hypoxic injury to various organs including kidneys, lungs and liver but the most serious effects are seen on the central nervous system. Levene's classification is a useful clinical tool for grading the severity of hypoxic ischemic encephalopathy. Good supportive care is essential in the first 48 hours after asphyxia to prevent ongoing brain injury in the penumbra region. Strict monitoring and prompt correction is needed for common problems including temperature maintenance, blood sugars, blood pressure and oxygenation. Phenobarbitone is the drug of choice for the treatment of convulsions.

Theoretical simulation of temperature distribution in the brain during mild hypothermia treatment for brain injury

Mild or moderate hypothermia (>30 degrees C) has been proposed for clinical use as a therapeutic option for achieving protection from cerebral ischaemia in brain injury patients. In this research, a theoretical model was developed to examine the brain temperature gradients during selective cooling of the brain surface after head injury. The head was modelled as a hemisphere consisting of several layers, representing the scalp, skull and brain tissue, respectively. The dimensions, physical properties and physiological characteristics for each layer, as well as the arterial blood temperature, were used as the input to the Pennes bioheat transfer equation to simulate the steady-state temperature distribution within the brain. Depending on the head surface temperature, a temperature gradient of up to 13 degrees C exists in the brain tissue. The results have shown that the volumetric-averaged brain tissue temperature Tbt,avg for adults and infants can be 1.7 and 4.3 degrees C, respectively, lower than the temperature of the arterial blood supplied to the brain tissue. The location where the probe should be placed to measure Tbt,avg was also determined by the simulation. The calculation suggests that the temperature sensor should be placed 7.5mm and 5.9 mm beneath the brain tissue surface for adults and infants, respectively, to monitor Tbt,avg continuously.

Rapid (0.5 degrees C/min) minimally invasive induction of hypothermia using cold perfluorochemical lung lavage in dogs

OBJECTIVE

Demonstrate minimally invasive rapid body core and brain cooling in a large animal model.

DESIGN

Prospective controlled animal trial.

SETTING

Private research laboratory.

SUBJECTS

Adult dogs, anesthetized, mechanically ventilated.

INTERVENTIONS

Cyclic lung lavage with FC-75 perfluorochemical (PFC) was administered through a dual-lumen endotracheal system in the new technique of 'gas/liquid ventilation' (GLV). In Trial-I, lavage volume (V-lav) was 19 ml/kg, infused and withdrawn over a cycle period (tc) of 37 s. (effective lavage rate V'-lav=31 ml/kg/min.) Five dogs received cold (approximately 4 degrees C) PFC; two controls received isothermic PFC. In Trial-II, five dogs received GLV at V-lav=8.8 ml/kg, tc=16 s, V'-lav=36 ml/kg/min.

MEASUREMENTS AND MAIN RESULTS

Trial-I tympanic temperature change was -3.7+/-0.6 degrees C (SD) at 7.5 min, reaching -7.3+/-0.6 degrees C at 18 min. Heat transfer efficiency was 60%. In Trial-II, efficiency fell to 40%, but heat-exchange dead space (VDtherm) remained constant. Lung/blood thermal equilibration half-time was <8 s. Isothermic GLV caused hypercapnia unless gas ventilation was increased. At necropsy after euthanasia (24 h), modest lung injury was seen.

CONCLUSIONS

GLV cooling times are comparable to those for cardiopulmonary bypass. Heat and CO(2) removal can be independently controlled by changing the mix of lavage and gas ventilation. Due to VDtherm of approximately 6 ml/kg in dogs, efficient V-lav is >18 ml/kg. GLV cooling power appears more limited by PFC flows than lavage residence times. Concurrent gas ventilation may mitigate heat-diffusion limitations in liquid breathing, perhaps via bubble-induced turbulence.

Perinatal brain injury: from pathogenesis to neuroprotection

Brain injury secondary to hypoxic-ischemic disease is the predominant form of all brain injury encountered in the perinatal period. The focus of this article is the most recent research developments in this field and especially those developments that should lead to the most profound effects on interventions in the first years of the new millennium. Neuronal injury is the predominant form of cellular injury in the term infant. The principal mechanisms leading to neuronal death after hypoxia-ischemia/reperfusion are initiated by energy depletion, accumulation of extracellular glutamate, and activation of glutamate receptors. The cascade of events that follows involves accumulation of cytosolic calcium and activation of a variety of calcium-mediated deleterious events. Notably this deleterious cascade, which evolves over many hours, may be interrupted even if interventions are instituted after termination of the insult, an important clinical point. Of the potential interventions, the leading candidates for application to the human infant in the relative short-term are mild hypothermia, inhibitors of free radical production, and free radical scavengers. Promising clinical data are available for the use of mild hypothermia.

Ambient temperature modulates hypoxic-induced changes in rat body temperature and activity differentially

When rats, acclimated to an ambient temperature (T(a)) of 29 degrees C, are exposed to 10% O(2) for 63 h, the circadian rhythms of body temperature (T(b)) and level of activity (L(a)) are abolished, T(b) falls to a hypothermic nadir followed by a climb to a hyperthermic peak, L(a) remains depressed (Bishop B, Silva G, Krasney J, Salloum A, Roberts A, Nakano H, Shucard D, Rifkin D, and Farkas G. Am J Physiol Regulatory Integrative Comp Physiol 279: R1378-R1389, 2000), and overt brain pathology is detected (Krasney JA, Farkas G, Shucard DW, Salloum AC, Silva G, Roberts A, Rifkin D, Bishop B, and Rubio A. Soc Neurosci Abstr 25: 581, 1999). To determine the role of T(a) in these hypoxic-induced responses, T(b) and L(a) data were detected by telemetry every 15 min for 48 h on air, followed by 63 h on 10% O(2) from rats acclimated to 25 or 21 degrees C. Magnitudes and rates of decline in T(b) after onset of hypoxia were inversely proportional to T(a), whereas magnitudes and rates of T(b) climb after the hypothermic nadir were directly proportional to T(a). No hyperthermia, so prominent at 29 degrees C, occurred at 25 or 21 degrees C. The hypoxic depression of L(a) was least at 21 degrees C and persisted throughout the hypoxia. In contrast, T(a) was a strong determinant of the magnitudes and time courses of the initial fall and subsequent rise in T(b). We propose that the absence of hyperthermia at 21 and 25 degrees C as well as a persisting hypothermia may protect the brain from overt pathology.

Perinatal asphyxia: a clinical review, including research with brain hypothermia

Perinatal asphyxia may occur in utero, during labor and delivery, or in the postnatal period. There are numerous causes, and the clinical manifestations vary. Infants who experience mild asphyxia may show no neurologic injury. Severe asphyxia may be fatal in utero, or immediately after birth, with survivors showing extensive neurologic sequelae, with or without cognitive deficits. Mild brain hypothermia appears promising in the prevention of further neurologic damage in encephalopathic infants following asphyxia. Recent research on newborn animal models has focused on the timing, duration, and depth of hypothermia. Promising new research is now under way in nurseries in the U.S. in an attempt to establish clinical protocols for use of hypothermia in human neonates.

Moderate hypothermia ameliorates liver energy failure after intestinal ischaemia-reperfusion in anaesthetised rats

BACKGROUND/PURPOSE

Intestinal ischaemia-reperfusion (IR) can cause liver failure. The aims of this work were to study the effects of intestinal IR on liver energy metabolism and to evaluate the effects of moderate hypothermia.

METHODS

Intestinal IR (90-minute intestinal ischaemia plus 60-minute or 240-minute reperfusion) was achieved by clamping and unclamping the superior mesenteric artery in rats. Normothermia or moderate hypothermia (30 degrees to 33 degrees C) was maintained by adjusting the environmental temperature. The ratio of hepatic inorganic phosphate to adenosine triphosphate (ATP) was monitored continuously during intestinal IR using in vivo phosphorus ((31)P) magnetic resonance spectroscopy. Phosphorus metabolites also were measured in extracts prepared from freeze-clamped liver and intestine.

RESULTS

Mortality occurred exclusively during normothermic intestinal IR. A progressive increase in the hepatic inorganic phosphate to ATP ratio after normothermic intestinal IR was observed. Moderate hypothermia delayed this effect. Analysis of liver extracts confirmed above findings. However, there was no difference in intestinal phosphocreatine or ATP between normothermic and hypothermic rats undergoing intestinal IR.

CONCLUSIONS

Intestinal IR at normothermia was associated with liver energy failure and high mortality rate. Moderate hypothermia ameliorated liver energy failure but did not attenuate intestinal energy failure after intestinal IR. Hypothermia may prove to be useful in the management of patients with intestinal IR injuries in the future.

Circadian rhythms of body temperature and activity levels during 63 h of hypoxia in the rat

The hypothermic response of rats to only brief ( approximately 2 h) hypoxia has been described previously. The present study analyzes the hypothermic response in rats, as well as level of activity (L(a)), to prolonged (63 h) hypoxia at rat thermoneutral temperature (29 degrees C). Mini Mitter transmitters were implanted in the abdomens of 10 adult Sprague-Dawley rats to continuously record body temperature (T(b)) and L(a). After habituation for 7 days to 29 degrees C and 12:12-h dark-light cycles, 48 h of baseline data were acquired from six control and four experimental rats. The mean T(b) for the group oscillated from a nocturnal peak of 38.4 +/- 0.18 degrees C (SD) to a diurnal nadir of 36.7 +/- 0.15 degrees C. Then the experimental group was switched to 10% O(2) in N(2). The immediate T(b) response, phase I, was a disappearance of circadian rhythm and a fall in T(b) to 36.3 +/- 0.52 degrees C. In phase II, T(b) increased to a peak of 38.7 +/- 0.64 degrees C. In phase III, T(b) gradually decreased. At reoxygenation at the end of the hypoxic period, phase IV, T(b) increased 1.1 +/- 0.25 degrees C. Before hypoxia, L(a) decreased 70% from its nocturnal peak to its diurnal nadir and was entrained with T(b). With hypoxia L(a) decreased in phase I to essential quiescence by phase II. L(a) had returned, but only to a low level in phase III, and was devoid of any circadian rhythm. L(a) resumed its circadian rhythm on reoxygenation. We conclude that 63 h of sustained hypoxia 1) completely disrupts the circadian rhythms of both T(b) and L(a) throughout the hypoxic exposure, 2) the hypoxia-induced changes in T(b) and L(a) are independent of each other and of the circadian clock, and 3) the T(b) response to hypoxia at thermoneutrality has several phases and includes both hypothermic and hyperthermic components.

Cooling the newborn after asphyxia - physiological and experimental background and its clinical use

Many years of experimental work on hypoxic-ischaemic injury have supported the hypothesis that cooling the body and brain after the primary injury offers permanent neuroprotection. Clinically, the question of how late cooling can start after the insult and still have a protective effect is important and not fully investigated. Pilot studies in human adults initiated cooling after 10-18 h (trauma, stroke), however animal data suggest cooling is not effective if started later than 6 h. There might be a threshold for 'cooling dose' - by depth or duration - to achieve permanent protection. Hypothermia must be administered with understanding of the extensive physiological effects. Different enzymes have different sensitivity to changes in temperature, hence some effects may be beneficial and some deleterious. Hypothermia and cardiovascular responses and coagulation needs careful monitoring.

Idazoxan does not prevent but worsens focal hypoxic-ischemic brain damage in neonatal Wistar rats

We examined the neuroprotective efficacy of a post-treatment with idazoxan (Idaz): an alpha2-adrenoceptor antagonist with activity at the I1- and I2-subtypes of the imidazoline receptor (I-receptor), in an experimental model of perinatal hypoxic-ischemic (HI) brain damage. Seventy-two, 7-day-old Wistar rats were subjected to permanent unilateral ligation of the common carotid artery and transient (2 hr) hypoxia (8% O(2)). The surviving animals were sub-divided into 3 groups: one "control" group received intraperitoneal (i.p.) injection of saline (Sigma; n = 21) and two "treated" groups received, 10 min post-HI, i.p. treatments with Idaz (I3: 3 mg/kg; n = 19) or (I8: 8 mg/kg; n = 20). Idaz effects were assessed by TTC-staining 72 hr post-HI for Sigma (n = 13), I3 (n = 11), and I8 (n = 12) groups and by MRI-examination 5 weeks post-HI for Sigma (n = 8), I3 (n = 8), and I8 (n = 6) groups. Total ratio of brain infarct areas were significantly (P < 0.01) different between Sigma and Idaz-treated rats: 20.9 +/- 4.0%, 35.6 +/- 5.9 % and 36.8 +/- 5.8% for Sigma, I3 and I8, respectively, when determined with TTC-staining and; 23.3 +/- 3.7%, 39.8 +/- 4.2%, and 43.2 +/- 10.1%, for Sigma, I3, and I8, respectively, when assessed by MRI. Our results suggest that Idaz, given as a post-HI treatment, does not exert neuroprotective effects but enhances the brain injury induced by focal neonatal cerebral HI. The deleterious mechanism may result from an overactivity of sympathetic tone and/or the immaturity of central I-receptors in newborn rats.

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.

Neonatal tolerance to hypoxia: a comparative-physiological approach

Newborn mammals exhibit a number of physiological reactions which differ from normal adult physiology and are often regarded as signs of immaturity. However, when looked upon from a comparative point of view, it becomes obvious that some of these 'physiological peculiarities' bear striking similarity to adaptation mechanisms known from hypoxia-tolerant animals and may thus contribute to the well-established, yet poorly understood, phenomenon of neonatal hypoxia tolerance. As the mammalian fetus lives at oxygen partial pressures corresponding to 8000 m altitude, the first line of perinatal hypoxia defense consists of long-term adaptations to limited intrauterine oxygen supply: (1) improved O2 transport by fetal acclimatization to high altitude, (2) reduced metabolic rate by hibernation-like deviation from metabolic size allometry, (3) diminished cerebral vulnerability by functional analogies to diving turtle brain, and (4) enhanced metabolic flexibility by optional repartitioning of energy supply from growth to maintenance metabolism. In the case of birth asphyxia, these background mechanisms are complemented by short-term responses to acute oxygen lack: (1) reduction of body temperature as in natural torpor, (2) reduction of heart rate and redistribution of circulation as in diving mammals, (3) reduction of respiration rate typical of 'hypoxic hypometabolism', and (4) reduction of blood pH according to the concept of 'acidotic torpidity'. Although anaerobic metabolism is improved in neonatal mammals by increased glycogen stores, reduced metabolic demands, and sustained wash-out of acid metabolites, neonatal hypoxia tolerance seems to be primarily based on the ability to maintain tissue aerobiosis as long as possible. This is even reflected by isoenzyme patterns which do not consistently favour anaerobic glycolysis and, thus, are reminiscent of the 'lactate paradox' found in high altitude adaptation. Altogether, from a biological point of view, the perinatal period appears as a source of adaptive mechanisms that can be refound, in varying combinations, in many survival strategies. From a clinical point of view, the interplay of long- and short-term mechanisms offers a novel approach to estimation of the newborn's ability to withstand temporary oxygen lack. However, most of these mechanisms are not unambiguous and, above all, not unlimited in their protective effect so that they do not release obstetricians or neonatologists from their obligation to counteract fetal or neonatal hypoxia without delay.

Hypoxic-ischemic brain injury: advancements in the understanding of mechanisms and potential avenues for therapy

Hypoxic-ischemic brain injury occurs frequently in infancy and childhood. Events such as perinatal asphyxia, near drowning, respiratory arrest, and near sudden infant death syndrome cause significant mortality and morbidity. Despite current critical care practices, the outcomes from such injuries may be life-long neurologic deficits. This review discusses findings from laboratory investigations into such injuries--in particular the roles of excitotoxic amino acids, proteolytic enzymes, free radicals, nitric oxide, and leukocytes. Understanding of the two distinct forms of neuronal death, necrosis and apoptosis, provides additional insights into mechanisms of injury. The development of new therapies for hypoxic-ischemic brain injury depends on such understanding. To date, the results of preclinical therapeutic trials have not demonstrated a "magic bullet." Nevertheless, the understanding of injury mechanisms has uncovered potential avenues for new therapies, particularly combination therapies or single interventions that have multiple effects. Clinical trials, using these strategies, are planned or have been recently begun and offer hope for advancements in treatment.

Neuroprotective strategies: voltage-gated Na+-channel down-modulation versus presynaptic glutamate release inhibition

Insufficient ATP production relative to cellular requirements is the key factor detrimental to neurons in neurological disorders associated with deficient oxygen/glucose supply or mitochondrial dysfunction. As a large part of the energy consumed by brain cells is used to maintain the Na+ gradient across the cellular membrane, reduction of energy demand by down-modulation of voltage-gated Na+-channels is a rational strategy for neuroprotection against these conditions. Preservation of the inward Na+ gradient is likely to be also beneficial as it is an essential driving force for vital ion exchanges and transport mechanisms (e.g. Ca2+-homeostasis and cell volume regulation). From these elements, I propose that use-dependent Na+-channel blockers increase the resilience of nerve cells to the primary insult and/or subsequent deleterious events, and that reduced efflux of glutamate and other compounds is only a consequence of cellular stress attenuation. The widespread hypothesis that down-modulation of Na+-channels is neuroprotective primarily through reduction of presynaptic glutamate release conflicts with strong experimental evidence.