Prolonged mild hypothermia therapy protects the brain against permanent focal ischemia

The efficacy of hypothermia combined with thrombolysis or mechanical thrombectomy on acute ischemic stroke: a systematic review and meta-analysis

Background

Therapeutic hypothermia improves outcomes in experimental stroke models, especially after ischemia-reperfusion injury. In recent years, the safety and efficacy of hypothermia combining thrombolysis or mechanical thrombectomy have attracted widespread attention. The primary objective of the study was to evaluate the effectiveness and safety of hypothermia by combining reperfusion therapy in acute ischemic stroke patients.

Methods

A systematic search was performed in PubMed, EMBASE, Cochrane Library, and the Clinical Trial Registries on articles published until May 2024. The full-text articles were thoroughly reviewed, and relevant information regarding study characteristics and outcomes was extracted. Mantel-Haenszel (M-H) random-effects model was used to calculate pooled risk ratios (RR) with 95% confidence intervals (CI). In addition, subgroup analyses were performed focusing on the different hypothermia modalities and duration.

Results

After screening 2,265 articles, 10 studies were included in the present analysis with a total sample size of 785. Forest plots of clinical outcomes were as follows: modified Rankin Scale (mRS) ≤2 at 3 months (RR = 1.28, 95% CI 1.01-1.61, p = 0.04), mortality within 3 months (RR = 0.95, 95% CI 0.69-1.29, p = 0.73), total complications (RR = 1.02, 95% CI 0.89-1.16, p = 0.77) and pneumonia (RR = 1.35, 95% CI 0.76-2.40, p = 0.31). Subgroup analyses indicated a mild protective effect of selective cerebral hypothermia; however, the difference in mortality between the hypothermia and control groups was not statistically significant (RR = 0.88, 95% CI 0.57-1.35, p = 0.55). Patients undergoing hypothermia for 24-48 h experienced a higher rate of overall complications (RR = 1.37, 95% CI 1.01-1.86, p = 0.04) and pneumonia (RR = 2.84, 95% CI 1.05-7.66, p = 0.04).

Conclusion

The preliminary evidence supports the safety and feasibility of hypothermia combined with reperfusion therapy, which should be further investigated in randomized controlled studies.

Systematic review registration

https://www.crd.york.ac.uk/prospero/, identifier CRD42024556625.

The 100 most-cited articles in hypothermic brain protection journals: a bibliometric and visualized analysis

Introduction

A bibliometric analysis is used to assess the impact of research in a particular field. However, a specialized bibliometric analysis focused on hypothermic brain protection has not yet been conducted. This study aimed to identify the 100 most-cited articles published in the field of hypothermic brain protection and analyze their bibliometric characteristics.

Methods

After screening articles from the Web of Science citation database, complete bibliographic records were imported into Python for data extraction. The following parameters were analyzed: title, author's name and affiliation, country, publication year, publication date, first author, corresponding author, study design, language, number of citations, journal impact factors, keywords, Keywords Plus®, and research topic.

Results

The 100 articles were published between 1990 and 2016. The citation frequency for each publication ranged from 86 to 470. Among the 100 articles, 73 were original articles, 18 were review articles, 8 were clinical articles, and 1 was editorial material. These papers were published in 37 journals, with the Journal of Cerebral Blood Flow and Metabolism being the most prolific with 15 papers. Eighteen countries contributed to the 100 publications, 51 of which were from United States institutions. In addition, the keywords in the Sankey plot indicated that research in the field of hypothermic brain protection is growing deeper and overlapping with other disciplines.

Discussion

The results provide an overview of research on hypothermic brain protection, which may help researchers better understand classical research, historical developments, and new discoveries, as well as providing ideas for future research.

An Assessment of Physical and N6-Cyclohexyladenosine-Induced Hypothermia in Rodent Distal Focal Ischemic Stroke

Therapeutic hypothermia (TH) mitigates damage in ischemic stroke models. However, safer and easier TH methods (e.g., pharmacological) are needed to circumvent physical cooling complications. This study evaluated systemic and pharmacologically induced TH using the adenosine A1 receptor agonist, N6-cyclohexyladenosine (CHA), with control groups in male Sprague-Dawley rats. CHA was administered intraperitoneally 10 minutes following a 2-hour intraluminal middle cerebral artery occlusion. We used a 1.5 mg/kg induction dose, followed by three 1.0 mg/kg doses every 6 hours for a total of 4 doses, causing 20-24 hours of hypothermia. Animals assigned to physical hypothermia and CHA-hypothermia had similar induction rates and nadir temperatures, but forced cooling lasted ∼6 hours longer compared with CHA-treated animals. The divergence is likely attributable to individual differences in CHA metabolism, which led to varied durations at nadir, whereas physical hypothermia was better regulated. Physical hypothermia significantly reduced infarction (primary endpoint) on day 7 (mean reduction of 36.8 mm3 or 39% reduction; p = 0.021 vs. normothermic animals; Cohen's d = 0.75), whereas CHA-induced hypothermia did not (p = 0.33). Similarly, physical cooling improved neurological function (physical hypothermia median = 0, physical normothermia median = 2; p = 0.008) and CHA-induced cooling did not (p > 0.99). Our findings demonstrate that forced cooling was neuroprotective compared with controls, but prolonged CHA-induced cooling was not neuroprotective.

Rat model of asphyxia-induced cardiac arrest and resuscitation

Neurologic injury after cardiopulmonary resuscitation is the main cause of the low survival rate and poor quality of life among patients who have experienced cardiac arrest. In the United States, as the American Heart Association reported, emergency medical services respond to more than 347,000 adults and more than 7,000 children with out-of-hospital cardiac arrest each year. In-hospital cardiac arrest is estimated to occur in 9.7 per 1,000 adult cardiac arrests and 2.7 pediatric events per 1,000 hospitalizations. Yet the pathophysiological mechanisms of this injury remain unclear. Experimental animal models are valuable for exploring the etiologies and mechanisms of diseases and their interventions. In this review, we summarize how to establish a standardized rat model of asphyxia-induced cardiac arrest. There are four key focal areas: (1) selection of animal species; (2) factors to consider during modeling; (3) intervention management after return of spontaneous circulation; and (4) evaluation of neurologic function. The aim was to simplify a complex animal model, toward clarifying cardiac arrest pathophysiological processes. It also aimed to help standardize model establishment, toward facilitating experiment homogenization, convenient interexperimental comparisons, and translation of experimental results to clinical application.

Low-Dose Activated Protein C Suppresses the Development of Cerebral Infarction and Neurological Deficits in Mice

BACKGROUND

A large prospective study previously reported that a higher plasma level of protein C (PC) was associated with a lower incidence of ischemic stroke.

OBJECTIVE

To investigate the neuroprotective properties of activated PC (APC) against acute ischemic stroke using the 3-vessel occlusion model.

METHODS

Male C57BL/6J mice received APC (human APC) at 0.25, 0.5, or 1.0 (low dose) or 2.0, 4.0, or 8.0 mg/kg (high dose). Edaravone (Eda) (1.0, 3.0, or 10 mg/kg, a neuroprotectant approved for use in Japan), albumin (2.0 mg/kg), heparin (100 or 600 U/kg), or saline was used as the control. The drug or control was administered intravenously twice in the initial 24 h or 5 times in 3 d, starting 5 min after the induction of ischemia.

RESULTS

Low-dose APC significantly reduced lesion volumes, not affecting the depth of ischemia. High-dose APC did not significantly reduce lesion volumes, causing hemorrhagic transformation in some cases. In the chronic phase, lesion volumes were significantly suppressed in the APC or Eda group, and only the APC group showed a significant attenuation of neurological deficits. The protease-activated receptor (PAR)-1 antagonist SCH79797, administered during preischemia, completely abolished APC-induced neuroprotection. The overshoot-like abrupt recovery in regional cerebral blood flow observed in the control in the initial reperfusion phase was significantly suppressed by the APC treatment, indicating that the cerebral autoregulation system, consisting of endothelial cells and blood-brain barrier functions, was preserved.

CONCLUSION

Low-dose APC, potentially via the PAR-1-dependent anti-inflammatory cascade, protects the brain against ischemic stroke without increasing the risk of hemorrhagic transformation or death.

Hypothermia augments stress response in mammalian cells

Mild hypothermia (32 °C) is routinely used in medical practice to alleviate hypoxic ischemic damage, however, the mechanisms that underlie its protective effects remain uncertain. Using a systems approach based on genome-wide expression screens, reporter assays and biochemical studies, we find that cellular hypothermia response is associated with the augmentation of major stress-inducible transcription factors Nrf2 and HIF1Α affecting the antioxidant system and hypoxia response pathways, respectively. At the same time, NF-κB, a transcription factor involved in the control of immune and inflammatory responses, was not induced by hypothermia. Furthermore, mild hypothermia did not trigger unfolded protein response. Lower temperatures (27 °C and 22 °C) did not activate Nrf2 and HIF1A pathways as efficiently as mild hypothermia. Current findings are discussed in the context of the thermodynamic hypothesis of therapeutic hypothermia. We argue that the therapeutic effects are likely to stem both from metabolic suppression (inhibitory component) and augmentation of stress tolerance (activating component). We argue that systems coping with cellular stressors are plausible targets of therapeutic hypothermia and deserve more attention in clinical hypothermia research.

Safety and Efficacy of Hypothermia (34°C) after Hemicraniectomy for Malignant MCA Infarction

OBJECTIVE

The beneficial effect of hypothermia after hemicraniectomy in malignant middle cerebral artery (MCA) infarction has been controversial. We aim to investigate the safety and clinical efficacy of hypothermia after hemicraniectomy in malignant MCA infarction.

METHODS

From October 2012 to February 2016, 20 patients underwent hypothermia (Blanketrol III, Cincinnati Sub-Zero, Cincinnati, OH, USA) at 34°C after hemicraniectomy in malignant MCA infarction (hypothermia group). The indication of hypothermia included acute cerebral infarction >2/3 of MCA territory and a Glasgow coma scale (GCS) score <11 with a midline shift >10 mm or transtentorial herniation sign (a fixed and dilated pupil). We retrospectively collected 27 patients, as the control group, who had undergone hemicraniectomy alone and simultaneously met the inclusion criteria of hypothermia between January 2010 and September 2012, before hypothermia was implemented as a treatment strategy in Dong-A University Hospital. We compared the mortality rate between the two groups and investigated hypothermia-related complications, such as postoperative bleeding, pneumonia, sepsis and arrhythmia.

RESULTS

The age, preoperative infarct volume, GCS score, National institutes of Health Stroke Scale score, and degree of midline shift were not significantly different between the two groups. Of the 20 patients in the hypothermia group, 11 patients were induced with hypothermia immediately after hemicraniectomy and hypothermia was initiated in 9 patients after the decision of hypothermia during postoperative care. The duration of hypothermia was 4±2 days (range, 1 to 7 days). The side effects of hypothermia included two patients with arrhythmia, one with sepsis, one with pneumonia, and one with hypotension. Three cases of hypothermia were discontinued due to these side effects (one sepsis, one hypotension, and one bradycardia). The mortality rate of the hypothermia group was 15.0% and that of the control group was 40.7% (p=0.056). On the basis of the logistic regression analysis, hypothermia was considered to contribute to the decrease in mortality rate (odds ratio, 6.21; 95% confidence interval, 1.04 to 37.05; p=0.045).

CONCLUSION

This study suggests that hypothermia after hemicraniectomy is a viable option when the progression of patients with malignant MCA infarction indicate poor prognosis.

Prospects of modeling poststroke epileptogenesis

This Review describes the current status of poststroke epilepsy (PSE) with an emphasis on poststroke epileptogenesis modeling for testing new therapeutic agents. Stroke is a leading cause of epilepsy in an aging population. Late-onset "epileptic" seizures have been reported in up to 30% cases after stroke. Nevertheless, the overall prevalence of PSE is 2-4%. Rodent models of stroke have contributed to our understanding of the relationship between seizures and the underlying ischemic damage to neurons. To understand whether acutely generated stroke events lead to a chronic phenotype more closely resembling PSE with recurrent seizures, a limited variety of approaches emerged in early 2000s. These limited methods of causing an occlusion in mice and rats show different infarct size and neurological deficits. The most often employed procedure for inducing focal ischemia is the middle cerebral artery occlusion. This mimics the pathophysiology seen in humans in terms of extent of damage to cortex and striatum. Photothrombosis and endothelin-1 models can similarly evoke episodes of ischemic stroke. These models are well suited to studying mechanisms and biomarkers of epileptogenesis or optimizing novel drug discoveries. However, modeling of PSE is tedious, is highly variable, and lacks validity; therefore, it is not widely implemented in epilepsy research. Moreover, the relevance of ischemic models to specific forms of human stroke remains unclear. Stroke modeling in young male rodents lacks clinical relevance to elderly populations and especially to women, likely as a result of sex differences. Nevertheless, because of the neuronal damage and epileptogenic insult that these models trigger, they are helpful tools in studying acquired epilepsy and prophylactic drug therapy. © 2016 Wiley Periodicals, Inc.

Therapeutic hypothermia protocols

The application of targeted temperature management has become common practice in the neurocritical care setting. It is important to recognize the pathophysiologic mechanisms by which temperature control impacts acute neurologic injury, as well as the clinical limitations to its application. Nonetheless, when utilizing temperature modulation, an organized approach is required in order to avoid complications and minimize side-effects. The most common clinically relevant complications are related to the impact of cooling on hemodynamics and electrolytes. In both instances, the rate of complications is often related to the depth and rate of cooling or rewarming. Shivering is the most common side-effect of hypothermia and is best managed by adequate monitoring and stepwise administration of medications specifically targeting the shivering response. Due to the impact cooling can have upon pharmacokinetics of commonly used sedatives and analgesics, there can be significant delays in the return of the neurologic examination. As a result, early prognostication posthypothermia should be avoided.

Hypothermia inhibits the propagation of acute ischemic injury by inhibiting HMGB1

Acute ischemic stroke causes significant chronic disability worldwide. We designed this study to clarify the mechanism by which hypothermia helps alleviate acute ischemic stroke. In a middle cerebral artery occlusion model (4 h ischemia without reperfusion), hypothermia effectively reduces mean infarct volume. Hypothermia also prevents neurons in the infarct area from releasing high mobility group box 1 (HMGB1), the most well-studied damage-associated molecular pattern protein. By preventing its release, hypothermia also prevents the typical middle cerebral artery occlusion-induced increase in serum HMGB1. We also found that both glycyrrhizin-mediated inhibition of HMGB1 and intracerebroventricular neutralizing antibody treatments before middle cerebral artery occlusion onset diminish infarct volume. This suggests a clear neuroprotective effect of HMGB1 inhibition by hypothermia in the brain. We next used real-time polymerase chain reaction to measure the levels of pro-inflammatory cytokines in peri-infarct regions. Although middle cerebral artery occlusion increases the expression of interleukin-1β and tissue necrosis factor-α, this elevation is suppressed by both hypothermia and glycyrrhizin treatment. We show that hypothermia reduces the production of inflammatory cytokines and helps salvage peri-infarct regions from the propagation of ischemic injury via HMGB1 blockade. In addition to suggesting a potential mechanism for hypothermia’s therapeutic effects, our results suggest HMGB1 modulation may lengthen the therapeutic window for stroke treatments.

Therapeutic Hypothermia for Acute Stroke

Hypothermia is the most potent neuroprotective therapy available. Clinical use of hypothermia is limited by technology and homeostatic mechanisms that maintain core body temperature. Recent advances in intravascular cooling catheters and successful trials of hypothermia for cardiac arrest revivified interest in hypothermia for stroke, resulting in Phase 1 clinical trials and plans for further development. Given the recent spate of neuroprotective therapy failures, we sought to clarify whether clinical trials of therapeutic hypothermia should be mounted in stroke patients. We reviewed the preclinical and early clinical trials of hypothermia for a variety of indications, the putative mechanisms for neuroprotection with hypothermia, and offer several hypotheses that remain to be tested in clinical trials. Therapeutic hypothermia is promising, but further Phase 1 and Phase 2 development efforts are needed to ensure that cooling of stroke patients is safe, before definitive efficacy trials.

Temperature Control in Rodent Neuroprotection Studies: Methods and Challenges

Extensive animal research facilitated the clinical translation of therapeutic hypothermia for cardiac arrest in adults and hypoxic-ischemic injury in infants. Similarly, clinical interest in hypothermia for other brain injuries, such as stroke, has been greatly supported by positive findings in preclinical work. The reliability, validity, and utility of animal models, among many research practices (blinding, randomization, etc.), are key to successful clinical translation. Here, we review methods used to induce and maintain hypothermia in animal models. These include physical and pharmacological methods. We emphasize the advantages and limitations of each approach, and the importance of using clinically relevant cooling protocols and appropriate monitoring and reporting approaches. Moreover, we performed a literature survey of ischemic stroke studies published in 2015 to highlight the continuing risk of temperature confounds in neuroprotection studies. For example, many still do not accurately monitor and report temperature during surgery (23.5%), even though almost half of these studies (46.0%) use pharmaceutical agents that likely influence temperature. We hope this review stimulates awareness and discussion of the importance of temperature in neuroprotective studies.

Ischemia reperfusion injury as a modifiable therapeutic target for cardioprotection or neuroprotection in patients undergoing cardiopulmonary resuscitation

AIMS

We sought to review cellular changes that occur with reperfusion to try to understand whether ischemia-reperfusion injury (RI) is a potentially modifiable therapeutic target for cardioprotection or neuroprotection in patients undergoing cardiopulmonary resuscitation.

DATA SOURCES

Articles written in English and published in PubMed.

RESULTS

Remote ischemic conditioning (RIC) involves brief episodes of non-lethal ischemia and reperfusion applied to an organ or limb distal to the heart and brain. Induction of hypothermia involves cooling an ischemic organ or body. Both have pluripotent effects that reduce the potential harm associated with RI in the heart and brain by reduced opening of the mitochondrial permeability transition pore. Recent trials of RIC and induced hypothermia did not demonstrate these treatments to be effective. Assessment of the effect of these interventions in humans to date may have been modified by use of concurrent medications including propofol.

CONCLUSIONS

Ongoing research is necessary to assess whether reduction of RI improves patient outcomes.

Experimental Models Combining Traumatic Brain Injury and Hypoxia

Traumatic brain injury (TBI) is one of the most common causes of death and disability, and cerebral hypoxia is a frequently occurring harmful secondary event in TBI patients. The hypoxic conditions that occur on the scene of accident, where the airways are often obstructed or breathing is in other ways impaired, could be reproduced using animal TBI models where oxygen delivery is strictly controlled throughout the entire experimental procedure. Monitoring physiological parameters of the animal is of utmost importance in order to maintain an adequate quality of the experiment. Peripheral oxygen saturation, O2 pressure (pO2) in the blood, or fraction of inhaled O2 (FiO2) could be used as goals to validate the hypoxic conditions. Different models of traumatic brain injury could be used to inflict desired injury type, whereas effects then could be studied using radiological, physiological and functional tests. In order to confirm that the brain has been affected by a hypoxic injury, appropriate substances in the affected cerebral tissue, cerebrospinal fluid, or serum should be analyzed.

Delayed localized hypothermia reduces intracranial pressure following collagenase-induced intracerebral hemorrhage in rat

Brain injury, such as from intracerebral hemorrhage (ICH), causes edema and raises intracranial pressure (ICP)--a potentially life-threatening complication. Clinical studies suggest that therapeutic hypothermia (TH) reduces edema and ICP after ICH. Similarly, animal studies show that TH can sometimes reduce edema, but whether ICP would be attenuated is not known. Here we tested whether 24-h delayed TH reduces edema and ICP in rats with severe striatal ICH (collagenase model). First, we showed that ICH increased epidural ICP (mean of 18 vs. 6.5mm Hg in controls), measured via telemetry. Second, we confirmed that delayed TH did not affect hematoma size at 7d ay (~65 vs. ~61 µL in controls). A cranial cooling device lowered striatal temperature to ~33 °C from 24 to 72 h after ICH. Third, we compared normothermic rats to those with TH that were rewarmed immediately or over 6h. Both TH protocols significantly reduced average and peak ICP by the second treatment day, and benefits persisted after rewarming. However, TH with slow rewarming failed to mitigate edema at 96 h (83.2% vs. 83.6% in controls) whereas rapid rewarming worsened edema (85.7%). Finally, we compared normothermic and TH rats without rewarming and found no impact on edema at 72 h (~81%). In summary, it appears that 24-h delayed local TH lowers ICP by a mechanism other than edema. Rapid rewarming worsens edema after local cooling, but this did not markedly impact ICP. Thus, TH should reduce ICP in patients with severe ICH, but not necessarily through mitigating edema.

The macrosphere model-an embolic stroke model for studying the pathophysiology of focal cerebral ischemia in a translational approach

The main challenge of stroke research is to translate promising experimental findings from the bench to the bedside. Many suggestions have been made how to achieve this goal, identifying the need for appropriate experimental animal models as one key issue. We here discuss the macrosphere model of focal cerebral ischemia in the rat, which closely resembles the pathophysiology of human stroke both in its acute and chronic phase. Key pathophysiological processes such as brain edema, cortical spreading depolarizations (CSD), neuroinflammation, and stem cell-mediated regeneration are observed in this stroke model, following characteristic temporo-spatial patterns. Non-invasive in vivo imaging allows studying the macrosphere model from the very onset of ischemia up to late remodeling processes in an intraindividual and longitudinal fashion. Such a design of pre-clinical stroke studies provides the basis for a successful translation into the clinic.

ERV enhances spatial learning and prevents the development of infarcts, accompanied by upregulated BDNF in the cortex

PURPOSES

An anti-allergic and analgesic drug, "an extract derived from the inflamed cutaneous tissue of rabbits inoculated with vaccinia virus (ERV)", has been used in medical practice in Japan and some other countries. We examined the effect of ERV, prior to induction of ischemia, on the development of cerebral infarction, on learning and memory, or on brain-derived neurotrophic factor (BDNF) levels in C57BL/6J mice.

METHODS

Following oral administration of ERV (the same in humans: ×1) or vehicle, daily for three consecutive weeks, temporary focal ischemia was induced by the three vessel occlusion technique. In the other group of animals, after daily ERV (Low: ×1; Med: ×3, or High dose: ×9) or vehicle administration for three weeks, we performed a quantitative assessment of spatial learning or intracerebral BDNF levels.

RESULTS

The volumes of infarcted lesions, brain edema and the extent of the neurological deficits were significantly reduced in the ERV-treated group. ERV treatment also enhanced spatial learning, accompanied by upregulated BDNF in the cortex.

CONCLUSIONS

Daily oral intake of ERV, at a clinically relevant dose, protects the brain from ischemic stroke, and also enhances the learning function in normal mice. As millions of people are currently taking the drug safely, and have been for many years in some cases, there is a need to test the inhibitory actions of the drug on progressive dementia encountered in humans with recurrent ischemic attacks or Alzheimer's disease.

Mild hypothermia as a treatment for central nervous system injuries: Positive or negative effects

Besides local neuronal damage caused by the primary insult, central nervous system injuries may secondarily cause a progressive cascade of related events including brain edema, ischemia, oxida-tive stress, excitotoxicity, and dysregulation of calcium homeostasis. Hypothermia is a beneficial strategy in a variety of acute central nervous system injuries. Mild hypothermia can treat high intra-cranial pressure following traumatic brain injuries in adults. It is a new treatment that increases sur-vival and quality of life for patients suffering from ischemic insults such as cardiac arrest, stroke, and neurogenic fever following brain trauma. Therapeutic hypothermia decreases free radical produc-tion, inflammation, excitotoxicity and intracranial pressure, and improves cerebral metabolism after traumatic brain injury and cerebral ischemia, thus protecting against central nervous system dam-age. Although a series of pathological and physiological changes as well as potential side effects are observed during hypothermia treatment, it remains a potential therapeutic strategy for central nervous system injuries and deserves further study.

Systemic oxygen delivery by peritoneal perfusion of oxygen microbubbles

Severe hypoxemia refractory to pulmonary mechanical ventilation remains life-threatening in critically ill patients. Peritoneal ventilation has long been desired for extrapulmonary oxygenation owing to easy access of the peritoneal cavity for catheterization and the relative safety compared to an extracorporeal circuit. Unfortunately, prior attempts involving direct oxygen ventilation or aqueous perfusates of fluorocarbons or hemoglobin carriers have failed, leading many researchers to abandon the method. We attribute these prior failures to limited mass transfer of oxygen to the peritoneum and have designed an oxygen formulation that overcomes this limitation. Using phospholipid-coated oxygen microbubbles (OMBs), we demonstrate 100% survival for rats experiencing acute lung trauma to at least 2 h. In contrast, all untreated rats and rats treated with peritoneal oxygenated saline died within 30 min. For rats treated with OMBs, hemoglobin saturation and heart rate were at normal levels over the 2-h timeframe. Peritoneal oxygenation with OMBs was therefore shown to be safe and effective, and the method requires less equipment and technical expertise than initiating and maintaining an extracorporeal circuit. Further translation of peritoneal oxygenation with OMBs may provide therapy for acute respiratory distress syndrome arising from trauma, sepsis, pneumonia, aspiration, burns and other pulmonary diseases.

Triggering Receptor Expressed on Myeloid Cells-2 Correlates to Hypothermic Neuroprotection in Ischemic Stroke

Hypothermia is neuroprotective against many acute neurological insults, including ischemic stroke. We and others have previously shown that protection by hypothermia is partially associated with an anti-inflammatory effect. Phagocytes are thought to play an important role in the clearance of necrotic debris, paving the way for endogenous repair mechanisms to commence, but the effect of cooling and phagocytosis has not been extensively studied. Triggering receptor expressed on myeloid cells-2 (TREM2) is a newly identified surface receptor shown to be involved in phagocytosis. In this study, we examined the effect of therapeutic hypothermia on TREM2 expression. Mice underwent permanent middle cerebral artery occlusion (MCAO) and were treated with one of the two cooling paradigms: one where cooling (30°C) began at the onset of MCAO (early hypothermia [eHT]) and another where cooling began 1 hour later (delayed hypothermia [dHT]). In both groups, cooling was maintained for 2 hours. A third group was maintained at normothermia (NT) as a control (37°C). Mice from the NT and dHT groups had similar ischemic lesion sizes and neurological performance, but the eHT group showed marked protection as evidenced by a smaller lesion size and less neurological deficits up to 30 days after the insult. Microglia and macrophages increased after MCAO as early as 3 days, peaked at 7 days, and decreased by 14 days. Both hypothermia paradigms were associated with decreased numbers of microglia and macrophages at 3 and 7 days, with greater decreases in the early paradigm. However, the proportion of the TREM2-positive microglia/macrophages was actually increased among the eHT group at day 7. eHT showed a long-term neurological benefit, but neuroprotection did not correlate to immune suppression. However, hypothermic neuroprotection was associated with a relative increase in TREM2 expression, and suggests that TREM2 may serve a beneficial role in brain ischemia.

Neuroprotective efficacy of selective brain hypothermia induced by a novel external cooling device on permanent cerebral ischemia in rats

OBJECTIVES

This study was aimed at examining whether hypothermia is neuroprotective against permanent cerebral ischemia in rats.

METHODS

A total of 32 male Sprague--Dawley rats were subjected to a middle cerebral artery occlusion. In the hypothermic group, rats (n=10) underwent selective brain hypothermia for 5 hours with the use of a novel surface coil with coolant circulating inside. In the control (n=13) and sham groups (n=9), the rats were maintained at normothermia. After a period of 168 hours ischemia, animals were killed to measure the infarction volume of the brain stained with hematoxylin-eosin.

RESULTS

There were no significant differences in physiological parameters except for the temperature. The present style of hypothermia significantly reduced infarction volume in the cortex and caudoputamen.

DISCUSSION

The present results endorse the neuroprotective effect of our method of hypothermia in permanent focal cerebral ischemia at an endpoint of 1 week under the following two conditions: (1) reduction of muscle and caudoputamen temperature to 29 and 31 degrees C, respectively; (2) maintenance of the mean arterial blood pressure above 90 mmHg during hypothermia.

DEcompressive surgery Plus hypoTHermia for Space-Occupying Stroke (DEPTH-SOS): a protocol of a multicenter randomized controlled clinical trial and a literature review

RATIONALE

Although decompressive hemicraniectomy clearly reduces mortality in severe space-occupying middle cerebral artery infarction (so-called malignant middle cerebral artery infarction), every fifth patient still dies in the acute phase and every third patient is left with moderate to severe disability. Therapeutic hypothermia is a neuroprotective and antiedematous treatment option that has shown promising effects in severe stroke. A combination of both treatment strategies may have the potential to further reduce mortality and morbidity in malignant middle cerebral artery infarction, but needs evaluation of its efficacy within the setting of a randomized clinical trial.

AIMS

The DEcompressive surgery Plus hypoTHermia for Space-Occupying Stroke (DEPTH-SOS) trial aims to investigate safety and feasibility of moderate therapeutic hypothermia (33°C ± 1) over at least 72 h in addition to early decompressive hemicraniectomy (≤48 hours after symptom onset) in patients with malignant middle cerebral artery infarction.

DESIGN

The DEcompressive surgery Plus hypoTHermia for Space-Occupying Stroke is a prospective, multicenter, open, two-arm (1:1) comparative, randomized, controlled trial.

STUDY OUTCOMES

The primary end-point is mortality at day 14. The secondary end-points include functional outcome at day 14 and at 12 months follow-up, and complications related to hypothermia.

DISCUSSION

The results of this trial will provide data on safety and feasibility of moderate hypothermia in addition to decompressive hemicraniectomy in malignant middle cerebral artery infarction. Furthermore, efficacy data on early mortality and long-term functional outcome will be obtained, forming the basis of subsequent trials.

Hypothermia for acute ischaemic stroke

Ischaemic stroke is one of the leading causes of death and disability worldwide, and intravenous alteplase is the only proven effective treatment in the acute setting. Hypothermia has been shown to improve neurological outcomes after global ischaemia-hypoxia in comatose patients who have had cardiac arrest, and is one of the most extensively studied and powerful therapeutic strategies in acute ischaemic stroke. The protective mechanisms of therapeutic hypothermia affect the ischaemic cascade across several parallel pathways and, when coupled with reperfusion strategies, might yield synergistic benefits for patients who have had a stroke. Technological advances have allowed hypothermia to be induced rapidly, and the treatment has been used safely in acute stroke patients. Conclusive efficacy trials assessing therapeutic hypothermia combined with reperfusion therapies in acute ischaemic stroke are ongoing.

Alogliptin, a dipeptidylpeptidase-4 inhibitor, for patients with diabetes mellitus type 2, induces tolerance to focal cerebral ischemia in non-diabetic, normal mice

Effective interventions that provide obvious neuroprotection are currently fairly limited. Glucagon-like peptide-1 (GLP-1), an enhancer of insulin production with a trophic effect on β cells in the islets, has been found to be trophic for neuronal cells. Alogliptin benzoate (AGL), a selective inhibitor of dipeptidylpeptidase-4 (DPP-4) functioning as a long-acting agonist of GLP-1, is in clinical use worldwide for patients with diabetes mellitus type 2. To clarify whether administration of AGL, independent of the insulinotropic effect, protects the brain against focal ischemia, we investigated the effect of AGL on the development of cerebral infarction in non-diabetic normal mice. Male C57BL/6J mice were administered AGL (7.5, 15, or 30μg) once a day for three weeks by intragastric gavage. After the induction of temporary focal ischemia, volumes of infarcted lesions and neurological deficits were analyzed at 24h (acute phase) and seven days (chronic phase). In the acute phase, significant reductions were observed in the volumes of infarcted lesions (p=0.009), and in the severity of neurological deficits (p=0.004), in the group treated with 15μg of alogliptin benzoate, but not the 7.5 or 30μg-treated groups. This significant reduction in volumes of infarcted lesions persisted into the chronic phase. At the end of the AGL treatment; before the induction of ischemia, the levels of brain-derived neurotrophic factor (BDNF), a potent neuroprotectant in the brain, were elevated in the cortex (p=0.008), or in the whole forebrain (p=0.023). AGL could be used as a daily neuroprotectant or an enhancer of BDNF production aiming to attenuate cerebral injuries, for the growing number of people who have the risk of ischemic stroke.

Magnolol reduces glutamate-induced neuronal excitotoxicity and protects against permanent focal cerebral ischemia up to 4 hours

Neuroprotective efficacy of magnolol, 5,5′-dially-2,2′-dihydroxydiphenyl, was investigated in a model of stroke and cultured neurons exposed to glutamate-induced excitotoxicity. Rats were subjected to permanent middle cerebral artery occlusion (pMCAO). Magnolol or vehicle was administered intraperitoneally, at 1 hr pre-insult or 1–6 hrs post-insult. Brain infarction was measured upon sacrifice. Relative to controls, animals pre-treated with magnolol (50–200 mg/kg) had significant infarct volume reductions by 30.9–37.8% and improved neurobehavioral outcomes (P<0.05, respectively). Delayed treatment with magnolol (100 mg/kg) also protected against ischemic brain damage and improved neurobehavioral scores, even when administered up to 4 hrs post-insult (P<0.05, respectively). Additionally, magnolol (0.1 µM) effectively attenuated the rises of intracellular Ca²⁺ levels, [Ca²⁺](i), in cultured neurons exposed to glutamate. Consequently, magnolol (0.1–1 µM) significantly attenuated glutamate-induced cytotoxicity and cell swelling (P<0.05). Thus, magnolol offers neuroprotection against permanent focal cerebral ischemia with a therapeutic window of 4 hrs. This neuroprotection may be, partly, mediated by its ability to limit the glutamate-induced excitotoxicity.

Therapeutic applications of hypothermia in cerebral ischaemia

There is considerable experimental evidence that hypothermia is neuroprotective and can reduce the severity of brain damage after global or focal cerebral ischaemia. However, despite successful clinical trials for cardiac arrest and perinatal hypoxia-ischaemia and a number of trials demonstrating the safety of moderate and mild hypothermia in stroke, there are still no established guidelines for its use clinically. Based upon a review of the experimental studies we discuss the clinical implications for the use of hypothermia as an adjunctive therapy in global cerebral ischaemia and stroke and make some suggestions for its use in these situations.

NDRG4 protein-deficient mice exhibit spatial learning deficits and vulnerabilities to cerebral ischemia

The N-myc downstream-regulated gene (NDRG) family consists of four related proteins, NDRG1-NDRG4, in mammals. We previously generated NDRG1-deficient mice that were unable to maintain myelin sheaths in peripheral nerves. This condition was consistent with human hereditary motor and sensory neuropathy, Charcot-Marie-Tooth disease type 4D, caused by a nonsense mutation of NDRG1. In contrast, the effects of genetic defects of the other NDRG members remain unknown. In this study, we focused on NDRG4, which is specifically expressed in the brain and heart. In situ mRNA hybridization on the brain revealed that NDRG4 was expressed in neurons of various areas. We generated NDRG4-deficient mice that were born normally with the expected Mendelian frequency. Immunochemical analysis demonstrated that the cortex of the NDRG4-deficient mice contained decreased levels of brain-derived neurotrophic factor (BDNF) and normal levels of glial cell line-derived neurotrophic factor, NGF, neurotrophin-3, and TGF-β1. Consistent with BDNF reduction, NDRG4-deficient mice had impaired spatial learning and memory but normal motor function in the Morris water maze test. When temporary focal ischemia of the brain was induced, the sizes of the infarct lesions were larger, and the neurological deficits were more severe in NDRG4-deficient mice compared with the control mice. These findings indicate that NDRG4 contributes to the maintenance of intracerebral BDNF levels within the normal range, which is necessary for the preservation of spatial learning and the resistance to neuronal cell death caused by ischemic stress.

Molecular mechanisms underlying hypothermia-induced neuroprotection

Stroke is a dynamic event in the brain involving heterogeneous cells. There is now compelling clinical evidence that prolonged, moderate cerebral hypothermia initiated within a few hours after severe ischemia can reduce subsequent neuronal death and improve behavioral recovery. The neuroprotective role of hypothermia is also well established in experimental animals. However, the mechanism of hypothermic neuroprotection remains unclear, although, presumably involves the ability of hypothermia to suppress a broad range of injurious factors. In this paper, we addressed this issue by utilizing comprehensive gene and protein expression analyses of ischemic rat brains. To predict precise target molecules, we took advantage of the therapeutic time window and duration of hypothermia necessary to exert neuroprotective effects. We proposed that hypothermia contributes to protect neuroinflammation, and identified candidate molecules such as MIP-3α and Hsp70 that warrant further investigation as targets for therapeutic drugs acting as “hypothermia-like neuroprotectants.”

Monitoring intracranial pressure in patients with malignant middle cerebral artery infarction: is it useful?

Object

Intracranial pressure (ICP) monitoring is increasingly used in the treatment of patients with malignant middle cerebral artery (MCA) infarction. However, neurological deterioration may exist independent from intracranial hypertension. This study aimed to present the findings of continuous ICP monitoring in a cohort of patients with malignant MCA infarction and to correlate these findings with clinical and radiological features.

Methods

The authors studied a prospective cohort of 25 patients with malignant MCA infarction consecutively admitted to the neurotrauma intensive care unit of the Vall d'Hebron University Hospital between March 2002 and September 2006. The patients were treated using a combined protocol of initial moderate hypothermia and hemicraniectomy. The latter was performed when patients showed a midline shift (MLS) ≥ 5 mm or ICP > 20 mm Hg. Six patients had an MLS ≥ 5 mm on the first CT scan and underwent surgery without prior ICP monitoring. This study focuses on the subgroup of 19 patients who underwent intraparenchymatous ICP monitoring before surgery.

Results

Intracranial pressure readings were evaluated and correlated with pupillary abnormalities, MLS, and ischemic tissue volume. In 12 of the 19 patients, ICP values were always ≤ 20 mm Hg, despite a mean (± SD) MLS of 6.7 ± 2 mm and a mean ischemic tissue volume of 241.3 ± 83 cm3. In 2 patients with anisocoria, ICP values were also normal.

Conclusions

In patients with a malignant MCA infarction, pupillary abnormalities and severe brainstem compression may be present despite normal ICP values. Therefore, continuous ICP monitoring cannot substitute for close clinical and radiological follow-up in the management of these patients.

Prevention of hypoglycemia-induced neuronal death by hypothermia

Hypothermia reduces neuronal damage after cerebral ischemia and traumatic brain injury, while hyperthermia exacerbates damage from these insults. Previously we have shown that temperature-dependent modulation of excitotoxic neuronal death is mediated in part by temperature-dependent changes in the synaptic release/translocation of Zn(2+). In this study, we hypothesize that brain temperature also affects hypoglycemia-induced neuronal death by modulation of vesicular Zn(2+) release from presynaptic terminals. To test our hypothesis, we used a rat model of insulin-induced hypoglycemia. Here we found that hypoglycemia-induced neuronal injury was significantly affected by brain temperature, that is, hypothermia inhibited while hyperthermia aggravated neuronal death. To investigate the mechanism of temperature-dependent neuronal death after hypoglycemia, we measured zinc release/translocation, reactive oxygen species (ROS) production, and microglia activation. Here we found that hypoglycemia-induced Zn(2+) release/translocation, ROS production, and microglia activation were inhibited by hypothermia but aggravated by hyperthermia. Even when the insult was accompanied by hyperthermic conditions, zinc chelation inhibited ROS production and microglia activation. Zinc chelation during hyperthermia reduced neuronal death, superoxide production, and microglia activation, which was comparable to the protective effects of hypothermia. We conclude that neuronal death after hypoglycemia is temperature-dependent and is mediated by increased Zn(2+) release, superoxide production, and microglia activation.

Effects of citicoline used alone and in combination with mild hypothermia on apoptosis induced by focal cerebral ischemia in rats

The effects of citicoline used either alone or in combination with hypothermia on the suppression of apoptotic processes after transient focal cerebral ischemia were investigated. Middle cerebral artery occlusion (MCAo) was performed for 2 hours on Sprague-Dawley (SD) rats using intraluminal thread insertion. The treatment groups were as follows: Group 1, sham-operated; Group 2, saline; Group 3, citicoline (400mg/kg intraperitoneal.); Group 4, hypothermia (34+/-1 degrees C); Group 5, citicoline+hypothermia. All rats were reperfused for 24 hours, and after sacrifice and transcardiac perfusion, immunohistochemical studies were performed for markers of apoptosis. In Group 2, the Bcl-2 immunostaining score (mean+/-standard deviation, 0.71+/-0.75) was lower compared to Groups 3, 4 and 5 (2.33+/-0.81; 3.00+/-0.00; 2.20+/-0.83; p<0.05). There was higher expression of caspase-3 proteins in Group 2 (2.28+/-0.95) compared to Group 5 (1.50+/-0.83; p<0.05). Bax proteins were also increased in Group 2 (1.85+/-1.06) compared to Group 5 (0.40+/-0.54) and in Group 4 (2.00+/-0.00) compared to Group 5 (0.40+/-0.54; p<0.05). Significant differences in caspase-9 immunostaining scores were found in Group 2 (2.29+/-0.96) compared to Group 5 (0.20+/-0.44) (p<0.05); Group 3 (1.00+/-0.70) compared to Group 5 (0.20+/-0.44; p<0.05); and Group 4 (3.00+/-0.00; p<0.05) compared to Group 5 (0.40+/-0.54; p<0.05). Thus by suppressing apoptotic processes citicoline with hypothermia is more effective than either used alone in ameliorating cerebral damage after transient focal ischemia.

Protection in animal models of brain and spinal cord injury with mild to moderate hypothermia

For the past 20 years, various laboratories throughout the world have shown that mild to moderate levels of hypothermia lead to neuroprotection and improved functional outcome in various models of brain and spinal cord injury (SCI). Although the potential neuroprotective effects of profound hypothermia during and following central nervous system (CNS) injury have long been recognized, more recent studies have described clinically feasible strategies for protecting the brain and spinal cord using hypothermia following a variety of CNS insults. In some cases, only a one or two degree decrease in brain or core temperature can be effective in protecting the CNS from injury. Alternatively, raising brain temperature only a couple of degrees above normothermia levels worsens outcome in a variety of injury models. Based on these data, resurgence has occurred in the potential use of therapeutic hypothermia in experimental and clinical settings. The study of therapeutic hypothermia is now an international area of investigation with scientists and clinicians from every part of the world contributing to this important, promising therapeutic intervention. This paper reviews the experimental data obtained in animal models of brain and SCI demonstrating the benefits of mild to moderate hypothermia. These studies have provided critical data for the translation of this therapy to the clinical arena. The mechanisms underlying the beneficial effects of mild hypothermia are also summarized.

Use of prolonged hypothermia to treat ischemic and hemorrhagic stroke

Therapeutic (induced) hypothermia (TH) has been extensively studied as a means to reduce brain injury following global and focal cerebral ischemia, intracerebral hemorrhage (ICH), and subarachnoid hemorrhage (SAH). Here, we briefly review the clinical and experimental evidence supporting the use of TH in each condition. We emphasize the importance of systematically evaluating treatment parameters, especially the duration of cooling, in each condition. We contend that TH provides considerable protection after global and focal cerebral ischemia, especially when cooling is prolonged (e.g., >24 h). However, there is presently insufficient evidence to support the clinical use of TH for ICH and SAH. In any case, further animal work is needed to develop optimized protocols for treating cardiac arrest (global ischemia), and to maximize the likelihood of successful clinical translation in focal cerebral ischemia.

[Hypothermia for intracranial hypertension]

There is a large body of experimental evidence showing benefits of deliberate mild hypothermia (33-35 degrees C) on the injured brain as well as an improvement of neurological outcome after cardiac arrest in humans. However, the clinical evidence of any benefit of hypothermia following stroke, brain trauma and neonatal asphyxia is still lacking. Controversial results have been published in patients with brain trauma or neonatal asphyxia. Hypothermia can reduce the elevation of intracranial pressure, through mechanisms not completely understood. Hypothermia-induced hypocapnia should have a role on the reduction of intracranial pressure. The temperature target is unknown but no additional benefit was found below 34 degrees C. The duration of deliberate hypothermia for the treatment of elevated intracranial pressure might be at least 48 hours, and the subsequent rewarming period must be very slow to prevent adverse effects.

Combination therapy with hypothermia for treatment of cerebral ischemia

Mild hypothermia is an established neuroprotectant in the laboratory, showing remarkable and consistent effects across multiple laboratories and models of brain injury. At the clinical level, mild hypothermia has shown benefits in patients who have suffered cardiac arrest and in some pediatric populations suffering hypoxic brain insults. However, a review of the literature has demonstrated that in order to appreciate the maximum benefits of hypothermia, brain cooling needs to begin soon after the insult, maintained for relatively long period periods of time, and, in the case of ischemic stroke, should be applied in conjunction with the re-establishment of cerebral perfusion. Translating this to the clinical arena can be challenging, especially rapid cooling and the re-establishment of perfusion. The addition of a second neuroprotectant could potentially (1) enhance overall protection, (2) prolong the temporal therapeutic window for hypothermia, or (3) provide protection where hypothermic treatment is only transient. Combination therapies resulting in recanalization following ischemic stroke would improve the likelihood of a good outcome, as the experimental literature suggests more consistent neuroprotection against ischemia with reperfusion, than ischemia without. Since recombinant tissue plasiminogen activator (rt-PA) is the only FDA approved treatment for acute ischemic stroke, and acts to recanalize occluded vessels, it is an obvious initial strategy to combine with hypothermia. However, the effects of thrombolytics are also temperature dependent, and the risk of hemorrhage is significant. The experimental data nevertheless seem to favor a combinatorial approach. Thus, in order to apply hypothermia to a broader range of patients, combination strategies should be further investigated.

Comparison of 12, 24 and 48 h of systemic hypothermia on outcome after permanent focal ischemia in rat

Mild hypothermia reduces injury in models of global and focal cerebral ischemia even when initiated after the insult. Neuroprotection depends critically upon the duration of hypothermia with longer treatments often being more efficacious. However, the ideal treatment duration is not known for most insults and this knowledge would facilitate clinical studies. Thus, we compared 12, 24 and 48 h of systemic hypothermia (33 degrees C vs. normothermia) initiated 1 h after permanent middle cerebral artery occlusion (pMCAO), which was produced by permanent occlusion of the carotid arteries and cauterization of the distal MCA in rat. Behavioral recovery and lesion volume were determined 7 days after pMCAO. All three treatments significantly and equally attenuated neurological deficits (e.g., forelimb placing response). Conversely, stepping error rate in the horizontal ladder test was significantly reduced only by the 24-h (18.7%) and 48-h treatments (11.7%) compared to normothermic rats (34.4%), and the 48-h treatment was significantly better than the 12-h treatment (28.8%). Similarly, brain injury was significantly reduced by 24-h (78.8 mm(3) lesion volume) and 48-h (66.8 mm(3)) treatments compared to normothermia (142.6 mm(3)), and the 48-h treatment was significantly better than the 12-h duration (114.6 mm(3)). In separate experiments cerebral edema was measured via wet-dry weight measurements and significantly reduced by hypothermia (e.g., from 83.7% water in the injured cortex of normothermic rats to 81.4% in rats cooled for one day), but for this there were no significant duration effects. In summary, prolonged hypothermia treatment provides superior protection overall, but this is not explained by reductions in edema.

Therapeutic hypothermia for global and focal ischemic brain injury--a cool way to improve neurologic outcomes

BACKGROUND

Therapeutic hypothermia (TH) has been employed as a neuroprotective strategy for a wide array of clinical problems since the late 1940s. Animal studies have determined that the neuroprotective effect of hypothermia is pleiotropic, impacting many steps in both the ischemic cascade and reperfusion injury. Interest in the neuroprotective effects of TH for ischemic brain injury has been resurgent, fueled by both recent positive and negative clinical trials. A review of preclinical and clinical reports on TH in adult patients is provided in this article.

REVIEW SUMMARY

Animal data and several large clinical studies of mild to moderate TH (32 degrees C-34 degrees C) for global cerebral ischemia describe favorable neurologic outcomes, with few adverse effects. However, clinical implementation for global ischemia remains poor. Some animal data support a role for TH in focal cerebral ischemia, if instituted soon after the onset of ischemia, and in the setting of reperfusion. Clinical studies of TH for focal cerebral ischemia have so far been equivocal. The available data suggest that, despite sharing some common components in the ischemic cascade, focal and global cerebral ischemia are pathophysiologically disparate, and may respond to different neuroprotective strategies.

CONCLUSION

TH is a safe, effective neuroprotective strategy for global cerebral ischemia. Because of the neuroprotective efficacy of TH in adult comatose survivors of cardiac arrest, neurologists should advocate the implementation of this strategy. TH for focal ischemia is a promising therapeutic option, but requires more basic and clinical investigation.

Transient cooling during early reperfusion attenuates delayed edema and infarct progression in the Spontaneously Hypertensive Rat. Distribution and time course of regional brain temperature change in a model of postischemic hypothermic protection

The temperature threshold for protection by brief postischemic cooling was evaluated in a model of transient focal ischemia in the Spontaneously Hypertensive Rat, using an array of epidural probes to monitor regional brain temperatures. Rats were subjected to 90 mins tandem occlusion of the right middle cerebral artery (MCA) and common carotid artery. Systemic cooling to 32 degrees C was initiated 5 mins before recirculation, with simultaneous brain cooling to temperatures ranging from 28 degrees C to 32 degrees C within the MCA territory by means of a temperature-controlled saline drip. Rewarming was initiated at 2 h recirculation and was complete within 30 mins. Tissue damage and edema volume showed clear temperature-dependent reductions when evaluated at 3 days survival, with no protection evident in the group at 32 degrees C but progressive effects on both parameters after deeper cooling. A particularly striking effect was the essentially complete elimination of edema progression between 1 and 3 days. Temperature at distal sites within the MCA territory better predicted reductions in lesion volume, indicating that protection required effective cooling of the penumbral regions destined to be spared. These results show that even brief cooling can be highly protective when initiated at the time of recirculation after focal ischemia, but indicate a substantially lower temperature threshold for hypothermic protection than has been reported for other strains, occlusion methods, and cooling durations.

General versus specific actions of mild-moderate hypothermia in attenuating cerebral ischemic damage

Mild or moderate hypothermia is generally thought to block all changes in signaling events that are detrimental to ischemic brain, including ATP depletion, glutamate release, Ca(2+) mobilization, anoxic depolarization, free radical generation, inflammation, blood-brain barrier permeability, necrotic, and apoptotic pathways. However, the effects and mechanisms of hypothermia are, in fact, variable. We emphasize that, even in the laboratory, hypothermic protection is limited. In certain models of permanent focal ischemia, hypothermia may not protect at all. In cases where hypothermia reduces infarct, some studies have overemphasized its ability to maintain cerebral blood flow and ATP levels, and to prevent anoxic depolarization, glutamate release during ischemia. Instead, hypothermia may protect against ischemia by regulating cascades that occur after reperfusion, including blood-brain barrier permeability and the changes in gene and protein expressions associated with necrotic and apoptotic pathways. Hypothermia not only blocks multiple damaging cascades after stroke, but also selectively upregulates some protective genes. However, most of these mechanisms are addressed in models with intraischemic hypothermia; much less information is available in models with postischemic hypothermia. Moreover, although it has been confirmed that mild hypothermia is clinically feasible for acute focal stroke treatment, no definite beneficial effect has been reported yet. This lack of clinical protection may result from suboptimal criteria for patient entrance into clinical trials. To facilitate clinical translation, future efforts in the laboratory should focus more on the protective mechanisms of postischemic hypothermia, as well as on the effects of sex, age and rewarming during reperfusion on hypothermic protection.

Conditions of protection by hypothermia and effects on apoptotic pathways in a rat model of permanent middle cerebral artery occlusion

OBJECT

Hypothermia is protective in stroke models, but findings from permanent occlusion models are conflicting. In this article the authors induced focal ischemia in rats by permanent distal middle cerebral artery (MCA) occlusion plus transient occlusion of the common carotid arteries (CCAs). This models a scenario in which the MCA remains occluded but partial reperfusion occurs through collateral vessels. The authors also determined whether hypothermia mediates ischemic damage by blocking apoptotic pathways.

METHODS

The left MCA was occluded permanently and the CCAs were reopened after 2 hours, leading to partial reperfusion in rats maintained at 37 degrees C, 33 degrees C (mild hypothermia), or 30 degrees C (moderate hypothermia) for 2 hours during and/or after CCA occlusion (that is, for a total of 2 or 4 hours of hypothermia or normothermia). Infarct size was measured 2 days after the stroke. Immunofluorescence staining and Western blot analysis were used to detect cytochrome c and apoptosis inducing factor (AIF) translocation.

RESULTS

Four hours of prolonged mild hypothermia (33 degrees C) reduced the infarct size 22% in the model of permanent MCA occlusion, whereas 2 hours of such mild hypothermia maintained either during CCA occlusion or after CCA release did not attenuate ischemic damage. However, moderate hypothermia (30 degrees C) during CCA occlusion was significantly more protective than 4 hours of 33 degrees C (46% decrease in infarct size). Four hours of mild or moderate hypothermia reduced cytosolic cytochrome c release and both nuclear and cytosolic AIF translocation in the penumbra 2 days after stroke.

CONCLUSIONS

These findings suggest that hypothermic neuroprotection might be achieved by blocking AIF and cytochrome c-mediated apoptosis.