Mild hypothermia improves recovery of cortical extracellular potassium ion activity and excitability after middle cerebral artery occlusion in the rat

Serum potassium changes during hypothermia and rewarming: a case series and hypothesis on the mechanism

Introduction

Hypokalemia is known to occur in association with therapeutically induced hypothermia and is usually managed by the administration of potassium (K+).

Methods

We reviewed data from 74 patients who underwent a therapeutic hypothermia protocol at our medical institution.

Results

In four patients in whom data on serum K+ and temperature were available, a strong positive correlation between serum K+ and body temperature was found. Based on the close positive relationship between serum K+ and total body temperature, we hypothesize that serum K+ decreases during hypothermia owing to decreased activity of temperature-dependent K+ exit channels that under normal conditions are sufficiently active to match cellular K+ intake via sodium/K+/adenosine triphosphatase. Upon rewarming, reactivation of these channels results in a rapid increase in serum K+ as a result of K+ exit down its concentration gradient.

Conclusion

Administration of K+ during hypothermia should be done cautiously and avoided during rewarming to avoid potentially life-threatening hyperkalemia. K+ exit via temperature-dependent K+ channels provides a logical explanation for the rebound hyperkalemia. K+ exit channels may play a bigger role than previously appreciated in the regulation of serum K+ during normal and pathophysiological conditions.

Reperfusion Changes After Stroke and Practical Approaches for Neuroprotection

Reperfusion is the first line of care in a growing number of eligible acute ischemic stroke patients. Early reperfusion with thrombolytic drugs and endovascular mechanical devices is associated with improved outcome and lower mortality rates compared with natural history. Reperfusion is not without risk, however, and may result in reperfusion injury, which manifests in hemorrhagic transformation, brain edema, infarct progression, and neurologic worsening. In this article, the functional and structural changes and underlying molecular mechanisms of ischemia and reperfusion are reviewed. The pathways that lead to reperfusion injury and novel neuroprotective strategies with endogenous properties are discussed.

Selective brain hypothermia

Selective brain hypothermia is a powerful concept for neuroprotection that has been successfully investigated in a variety of animal models of global and focal ischemia. Its major advantages over systemic hypothermia include rapid induction of cooling, ability to achieve profound target brain temperatures, organ-selective cooling, and temperature control. Clinical systems and devices are available or are currently under development that utilize conductive (surface-cooling pads, closed-loop catheters), convective (transnasal coolant delivery), or mass and energy transport (cold intra-arterial infusion) methods to achieve and maintain selective brain hypothermia. The "ideal" brain-cooling system that is characterized by rapid cooling to profound hypothermia, its ability to maintain selective cooling over several days, and is noninvasive in nature, remains unrealistic. Instead, systems may be identified by their distinct advantages to meet a specific need in the care of a patient. This involves the consideration of the timing of ischemic injury (preischemic, intraischemic, postischemic), extent of ischemic damage (excitotoxicity, inflammation, necrosis, edema), and type and setting of therapeutic intervention (intensive care, interventional therapy, surgery). The successful translation of these systems into clinical practice will depend on smart engineering, safety and efficacy, and usability in current clinical work flow.

The impact of hypothermia on serum potassium concentration: A systematic review

BACKGROUND

Blood potassium is the main prognostic biomarker used for triage in hypothermic cardiac arrest. The aim of this review was to assess the impact of hypothermia on blood potassium levels and compare the underlying pathophysiological theories.

METHODS

The Medline electronic database was searched via PubMed for articles published from January 1970 to December 2016. The search strategy included studies related to hypothermia and potassium levels. The relevant literature on clinical studies and experimental studies was reviewed by the authors.

RESULTS

Among the 50 studies included in the review, 39 (78%) reported a decrease in blood potassium levels upon hypothermia onset. Hypothermic hypokalaemia is linked to an intracellular shift rather than an actual net loss. The intracellular shift is caused by a variety of factors such as enhanced functioning of Na+K+ATPase, beta-adrenergic stimulation, pH and membrane stabilisation in deep hypothermia. In contrast, hypothermia can act as an aggravating factor in severe trauma with hyperkalaemia being an indicator of an irreversible state of cell death. An increase in the blood potassium level during hypothermia may result from a lack of enzyme functioning at cold temperatures and blocked active transport.

CONCLUSION

Hypothermia causes an initial decrease of potassium levels; however, the final stage of hypothermic cardiac arrest can induce hyperkalaemia due to cell lysis and final depolarisation. Better understanding the physiopathology of potassium levels during accidental hypothermia could be critically important to better select patients who could benefit from aggressive resuscitation therapy such as extracorporeal cardiopulmonary resuscitation.

Fatty acid methyl esters and Solutol HS 15 confer neuroprotection after focal and global cerebral ischemia

We previously showed that palmitic acid methyl ester (PAME) and stearic acid methyl ester (SAME) are simultaneously released from the sympathetic ganglion and PAME possesses potent vasodilatory properties which may be important in cerebral ischemia. Since PAME is a potent vasodilator simultaneously released with SAME, our hypothesis was that PAME/SAME confers neuroprotection in rat models of focal/global cerebral ischemia. We also examined the neuroprotective properties of Solutol HS15, a clinically approved excipient because it possesses similar fatty acid compositions as PAME/SAME. Asphyxial cardiac arrest (ACA, 6 min) was performed 30 min after PAME/SAME treatment (0.02 mg/kg, IV). Solutol HS15 (2 ml/kg, IP) was injected chronically for 14 days (once daily). Histopathology of hippocampal CA1 neurons was assessed 7 days after ACA. For focal ischemia experiments, PAME, SAME, or Solutol HS15 was administered following reperfusion after 2 h of middle cerebral artery occlusion (MCAO). 2,3,5-Triphenyltetrazolium staining of the brain was performed 24 h after MCAO and the infarct volume was quantified. Following ACA, the number of surviving hippocampal neurons was enhanced by PAME-treated (68%), SAME-treated (69%), and Solutol-treated HS15 (68%) rats as compared to ACA only-treated groups. Infarct volume was decreased by PAME (83%), SAME (68%), and Solutol HS15 (78%) as compared to saline (vehicle) in MCAO-treated animals. PAME, SAME, and Solutol HS15 provide robust neuroprotection in both paradigms of ischemia. This may prove therapeutically beneficial since Solutol HS15 is already administered as a solublizing agent to patients. With proper timing and dosage, administration of Solutol HS15 and PAME/SAME can be an effective therapy against cerebral ischemia.

Genetics and genomics of ischemic tolerance: focus on cardiac and cerebral ischemic preconditioning

A subthreshold ischemic insult applied to an organ such as the heart and/or brain may help to reduce damage caused by subsequent ischemic episodes. This phenomenon is known as ischemic tolerance mediated by ischemic preconditioning (IPC) and represents the most powerful endogenous mechanism against ischemic injury. Various molecular pathways have been implicated in IPC, and several compounds have been proposed as activators or mediators of IPC. Recently, it has been established that the protective phenotype in response to ischemia depends on a coordinated response at the genomic, molecular, cellular and tissue levels by introducing the concept of 'genomic reprogramming' following IPC. In this article, we sought to review the genetic expression profiles found in cardiac and cerebral IPC studies, describe the differences between young and aged organs in IPC-mediated protection, and discuss the potential therapeutic application of IPC and pharmacological preconditioning based on the genomic response.

Therapeutic hypothermia in acute ischemic stroke

Induced hypothermia has been used for neuroprotection in cardiac and neurovascular procedures. Experimental and translational studies provide evidence for its utility in the treatment of ischemic cerebrovascular disease. Over the past decade, these principles have been applied to the clinical management of acute stroke. Varying induction methods, time windows, clinical indications, and adjuvant therapies have been studied. In this article the authors review the mechanisms and techniques for achieving therapeutic hypothermia in the setting of acute stroke, and they outline pertinent side effects and complications. The manuscript summarizes and examines the relevant clinical trials to date. Despite a reasonable amount of existing data, this review suggests that additional trials are warranted to define the optimal time window, temperature regimen, and precise clinical indications for induction of therapeutic hypothermia in the setting of acute stroke.

Neurobehavioral Grand Rounds introduction: Does near drowning in ice water prevent anoxic induced brain injury?

Cold water near-drowning is often thought to be neuroprotective in individuals with anoxia of a longer duration than that usually required to produce irreversible neurologic damage. There is a paucity of data in adults with cold water near-drowning that assess neuropsychological outcomes. Information regarding long-term effects of near cold water near-drowning on neuropathology, neuropsychological and neurobehavioral outcomes are uncommon. This paper provides an introduction to two cases of cold water near-drowning reported in this issue of JINS by Sameulson and colleagues and provides background information for interpretation of the findings of these cases in the context of outcomes following anoxia.

Bioenergetics of cerebral ischemia: a cellular perspective

In cerebral ischemia survival of neurons, astrocytes, oligodendrocytes and endothelial cells is threatened during energy deprivation and/or following re-supply of oxygen and glucose. After a brief summary of characteristics of different cells types, emphasizing the dependence of all on oxidative metabolism, the bioenergetics of focal and global ischemia is discussed, distinguishing between events during energy deprivation and subsequent recovery attempt after re-circulation. Gray and white matter ischemia are described separately, and distinctions are made between mature and immature brains. Next comes a description of bioenergetics in individual cell types in culture during oxygen/glucose deprivation or exposure to metabolic inhibitors and following re-establishment of normal aerated conditions. Due to their expression of NMDA and non-NMDA receptors neurons and oligodendrocytes are exquisitely sensitive to excitotoxicity by glutamate, which reaches high extracellular concentrations in ischemic brain for several reasons, including failing astrocytic uptake. Excitotoxicity kills brain cells by energetic exhaustion (due to Na(+) extrusion after channel-mediated entry) combined with mitochondrial Ca(2+)-mediated injury and formation of reactive oxygen species. Many (but not all) astrocytes survive energy deprivation for extended periods, but after return to aerated conditions they are vulnerable to mitochondrial damage by cytoplasmic/mitochondrial Ca(2+) overload and to NAD(+) deficiency. Ca(2+) overload is established by reversal of Na(+)/Ca(2+) exchangers following Na(+) accumulation during Na(+)-K(+)-Cl(-) cotransporter stimulation or pH regulation, compensating for excessive acid production. NAD(+) deficiency inhibits glycolysis and eventually oxidative metabolism, secondary to poly(ADP-ribose)polymerase (PARP) activity following DNA damage. Hyperglycemia can be beneficial for neurons but increases astrocytic death due to enhanced acidosis.

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.

Technology insight: noninvasive brain stimulation in neurology-perspectives on the therapeutic potential of rTMS and tDCS

In neurology, as in all branches of medicine, symptoms of disease and the resulting burden of illness and disability are not simply the consequence of the injury, inflammation or dysfunction of a given organ; they also reflect the consequences of the nervous system's attempt to adapt to the insult. This plastic response includes compensatory changes that prove adaptive for the individual, as well as changes that contribute to functional disability and are, therefore, maladaptive. In this context, brain stimulation techniques tailored to modulate individual plastic changes associated with neurological diseases might enhance clinical benefits and minimize adverse effects. In this Review, we discuss the use of two noninvasive brain stimulation techniques--repetitive transcranial magnetic stimulation and transcranial direct current stimulation--to modulate activity in the targeted cortex or in a dysfunctional network, to restore an adaptive equilibrium in a disrupted network for best behavioral outcome, and to suppress plastic changes for functional advantage. We review randomized controlled studies, in focal epilepsy, Parkinson's disease, recovery from stroke, and chronic pain, to illustrate these principles, and we present evidence for the clinical effects of these two techniques.

Hypothermia for neuroprotection after cardiac arrest: mechanisms, clinical trials and patient care

Therapeutic hypothermia is a proven part of cardio-cerebral resuscitation after cardiac arrest as it improves neurologic outcomes after hypoxic brain injury. This article reviews the mechanisms of hypothermic neuroprotection, the clinical trials that support its use after cardiac arrest, as well as the impact of hypothermia on patient management and prognosis. In caring for patients suffering hypoxic brain injury after cardiac arrest, the role of the neurologist is no longer limited to prognosis but is now to become actively involved in clinical management which includes the use of therapeutic hypothermia.

Early excitability changes in a novel acute model of transient focal ischemia and reperfusion in the in vitro isolated guinea pig brain

The study of the early events that characterize cerebral ischemia is limited in available experimental models. The study of neurophysiological network changes that occur in brain tissue during the early minutes that follow focal ischemia induction is restricted in the in vivo condition. Very simplified systems, such as in vitro brain slices and in isolated neurons, have been utilized for this type of studies. We describe here a new model of transient focal ischemia and reperfusion developed in the isolated guinea pig brain, maintained in vitro by arterial perfusion with a complex saline solution without blood cells. In this preparation, that combines the advantage of an in vitro preparation with the functional preservation of both vascular and neuronal compartments, the arteries of the Willis circle are directly accessible by visual control. To induce transitory focal ischemia, one medial cerebral artery (MCA) was transiently tied for 30 min, while brain activity was recorded with multiple electrodes positioned in brain areas within and outside MCA territory. Anoxic depression in ischemic areas propagated to the surrounding tissue and was associated with the abolition of evoked responses due to both functional impairment of afferent olfactory input and tissue depression. Recovery of evoked responses was obtained after MCA reperfusion. The spatial distribution of hypoxic depressions was characterized and was correlated with the extension of brain damage, defined by immunohistochemical analysis with antibodies against microtubule-associated protein (MAP-2). We propose that the present model can be utilized to analyze brain activity changes that occur in early stages of focal brain ischemia and reperfusion.

Increased seizure duration and slowed potassium kinetics in mice lacking aquaporin-4 water channels

The glial water channel aquaporin-4 (AQP4) has been hypothesized to modulate water and potassium fluxes associated with neuronal activity. In this study, we examined the seizure phenotype of AQP4 -/- mice using in vivo electrical stimulation and electroencephalographic (EEG) recording. AQP4 -/- mice were found to have dramatically prolonged stimulation-evoked seizures after hippocampal stimulation compared to wild-type controls (33 +/- 2 s vs. 13 +/- 2 s). In addition, AQP4 -/- mice were found to have a higher seizure threshold (167 +/- 17 microA vs. 114 +/- 10 microA). To assess a potential effect of AQP4 on potassium kinetics, we used in vivo recording with potassium-sensitive microelectrodes after direct cortical stimulation. Although there was no significant difference in baseline or peak [K(+)](o), the rise time to peak [K(+)](o) (t(1/2), 2.3 +/- 0.5 s) as well as the recovery to baseline [K(+)](o) (t(1/2), 15.6 +/- 1.5 s) were slowed in AQP4 -/- mice compared to WT mice (t(1/2), 0.5 +/- 0.1 and 6.6 +/- 0.7 s, respectively). These results implicate AQP4 in the expression and termination of seizure activity and support the hypothesis that AQP4 is coupled to potassium homeostasis in vivo.

K+ waves in brain cortex visualized using a long-wavelength K+-sensing fluorescent indicator

We synthesized a water-soluble, long-wavelength K(+) sensor, TAC-Red, consisting of triazacryptand coupled to 3,6-bis(dimethylamino)xanthylium, whose fluorescence increased 14-fold at 0-50 mM K(+) with K(+)-to-Na(+) selectivity >30. We visualized K(+) waves in TAC-Red-stained brain cortex in mice during spreading depression, with velocity 4.4 +/- 0.5 mm/min, and K(+) release and reuptake half-times (t(1/2)) of 12 +/- 2 and 32 +/- 4 s, respectively. Aquaporin-4 (AQP4) deletion slowed K(+) reuptake about twofold, suggesting AQP4-dependent K(+) uptake by astroglia.

Biphasic cytochrome c release after transient global ischemia and its inhibition by hypothermia

Hypothermia is effective in preventing ischemic damage. A caspase-dependent apoptotic pathway is involved in ischemic damage, but how hypothermia inhibits this pathway after global cerebral ischemia has not been well explored. It was determined whether hypothermia protects the brain by altering cytochrome c release and caspase activity. Cerebral ischemia was produced by two-vessel occlusion plus hypotension for 10 mins. Body temperature in hypothermic animals was reduced to 33 degrees C before ischemia onset and maintained for 3 h after reperfusion. Western blots of subcellular fractions revealed biphasic cytosolic cytochrome c release, with an initial peak at about 5 h after ischemia, which decreased at 12 to 24 h, and a second, larger peak at 48 h. Caspase-3 and -9 activity increased at 12 and 24 h. A caspase inhibitor, Z-DEVD-FMK, administered 5 and 24 h after ischemia onset, protected hippocampal CA1 neurons from injury and blocked the second cytochrome c peak, suggesting that caspases mediate this second phase. Hypothermia (33 degrees C), which prevented CA1 injury, did not inhibit cytochrome c release at 5 h, but reduced cytochrome c release at 48 h. Caspase-3 and -9 activity was markedly attenuated by hypothermia at 12 and 24 h. Thus, biphasic cytochrome c release occurs after transient global ischemia and mild hypothermia protects against ischemic damage by blocking the second phase of cytochrome c release, possibly by blocking caspase activity.

Astrocytic contributions to bioenergetics of cerebral ischemia

Astrocytes are multifunctional cells that interact with neurons and other astrocytes in signaling and metabolic functions, and their resistance to pathophysiological conditions can help restrict loss of tissue after an ischemic event provided adequate nutrients are supplied to support their requirements. Astrocytes have substantial oxidative capacity and mechanisms to upregulate glycolytic capability when respiration is impaired. An astrocytic enzyme that synthesizes a powerful activator of glycolysis is not present in neurons, endowing astrocytes with the ability to sustain ATP production under restrictive conditions. The monocarboxylic acid transporter (MCT) isoforms predominating in astrocytes are optimized to facilitate very large increases in lactate flux as lactate concentration increases within (1-3 mM) and above (>3 mM) the normal range. In sharp contrast, the major neuronal MCT serves as a barrier to increased transmembrane transport as lactate rises above 1 mM, restricting both entry and efflux. Lactate can serve as fuel during recovery from ischemia but direct evidence that lactate is oxidized by neurons (vs. astrocytes) to maintain synaptic function is lacking. Astrocytes have critical roles in regulation of ionic homeostasis and control of extracellular glutamate levels, and spreading depression associated with ischemia places high demands on energy supplies in astrocytes and contributes to metabolic exhaustion and demise. Disruption of Ca2+ homeostasis, generation of oxygen free radicals and nitric oxide, and mitochondrial depolarization contribute to astrocyte death during and after a metabolic insult. Novel pharmaceutical agents targeted to astrocytes and hyperoxic therapy that restores penumbral oxygen level during energy failure might improve postischemic outcome.

Clinical trials for cytoprotection in stroke

To date, many cytoprotective drugs have reached the stage of pivotal phase 3 efficacy trials in acute stroke patients. (Table 1) Unfortunately, throughout the neuroprotective literature, the phrase "failure to demonstrate efficacy" prevails as a common thread among the many neutral or negative trials, despite the largely encouraging results encountered in preclinical studies. The reasons for this discrepancy are multiple, and have been discussed by Dr. Zivin in his review. Many of the recent trials have addressed deficiencies of the previous ones with more rigorous trial design, including more specific patient selection criteria (ensure homogeneity of stroke location and severity), stratified randomization algorithms (time-to-treat), narrowed therapeutic time-window and pharmacokinetic monitoring. Current trials have also incorporated biologic surrogate markers of toxicity and outcome such as drug levels and neuroimaging. Lastly, multi-modal therapies and coupled cytoprotection/reperfusion strategies are being investigated to optimize tissue salvage. This review will focus on individual therapeutic strategies and we will emphasize what we have learned from these trials both in terms of trial design and the biologic effect (or lack thereof) of these agents.

Hypothermic protection in rat focal ischemia models: strain differences and relevance to "reperfusion injury"

Hypothermic protection was compared in Long-Evans and spontaneously hypertensive rat (SHR) strains using transient focal ischemia, and in Wistar and SHR strains using permanent focal ischemia. Focal ischemia was produced by distal surgical occlusion of the middle cerebral artery and tandem occlusion of the ipsilateral common carotid artery (MCA/CCAO). Moderate hypothermia of 2 hours' duration was produced by systemic cooling to 32 degrees C, with further cooling of the brain achieved by reducing to 30 degrees C the temperature of the saline drip superfusing the exposed occlusion site. Infarct volume was determined from serial hematoxylin and eosin-stained frozen sections obtained routinely at 24 hours, or in some cases after 3 days' survival. In the SHR, moderate hypothermia was only effective when initiated before recirculation after a 90-minute occlusion period. In contrast, the same intervention was strikingly effective in the Long-Evans rat even when initiated after as long as 30-minute reperfusion after a 3-hour occlusion. This magnitude and duration of cooling was not protective in permanent MCA/CCAO in the SHR, but such transient hypothermia did effectively reduce infarct volume after permanent occlusions in Wistar rats. These results show striking differences in the temporal window for hypothermic protection among rat focal ischemia models. As expected, "reperfusion injury" in the Long-Evans strain is particularly responsive to delayed cooling. The finding that the SHR can be protected by hypothermia initiated immediately before recirculation suggests a rapidly evolving component of injury occurs subsequent to reperfusion in this model as well. Hypothermic protection after permanent occlusion in Wistar rats identifies a transient, temperature-sensitive phase of infarct evolution that is not evident in the unreperfused SHR. These observations confirm that distinct mechanisms can underlie the temporal progression of injury in rat stroke models, and emphasize the critical importance of considering model and strain differences in extrapolating results of hypothermic protection studies in animals to the design of interventions in clinical stroke.

Effect of intra-ischemic hypothermia on the expression of c-Fos and c-Jun, and DNA binding activity of AP-1 after focal cerebral ischemia in rat brain

It is unknown whether immediate early gene (IEG) induction and subsequent late gene regulation after ischemia is beneficial or deleterious. The aim of this study was to examine the effect of hypothermia on expression of c-Fos and c-Jun, and AP-1 DNA binding activity, after transient focal cerebral ischemia in rat brain, and clarify the role of IEGs and AP-1 after insults. Male Wistar rats underwent right middle cerebral artery occlusion for 1 h with the intraluminal suture method. During ischemia, animals were assigned to either normothermic (NT) or hypothermic (HT) groups. In the NT group, brain temperature was observed to spontaneously increase to 40 degrees C during ischemia. In the HT group, brain temperature decreased to 30 degrees C. Infarct volume in cortex was decreased in the HT group, compared with that in the NT group (P<0.001). Increased c-Fos immunoreactivity in the cortex was observed at 3 h after reperfusion in the HT, but not the NT group, while c-Jun expression was not affected by HT treatment. There was also a significant increase in AP-1 DNA binding activity at 3 h in the HT group when compared to the NT group (P<0.01). In conclusion, hypothermia decreased cerebral infarction in association with early increases in c-Fos expression and AP-1 DNA binding activity in peri-infarct cortex. It remains to be established whether such responses are a cause or consequence of cell survival, but these results clearly establish that altered transcription is a key feature of tissue spared following hypothermic focal ischemia.

Mild hypothermia during hemorrhagic shock in rats improves survival without significant effects on inflammatory responses

OBJECTIVE

To explore the hypothesis that the survival benefit of mild, therapeutic hypothermia during hemorrhagic shock is associated with inhibition of lipid peroxidation and the acute inflammatory response.

DESIGN

Prospective and randomized.

SETTING

Animal research facility.

SUBJECTS

Male Sprague-Dawley rats.

INTERVENTIONS

Rats underwent pressure-controlled (mean arterial pressure 40 mm Hg) hemorrhagic shock for 90 mins. They were randomized to normothermia (38.0 +/- 0.5 degrees C) or mild hypothermia (33-34 degrees C from hemorrhagic shock 20 mins to resuscitation time 12 hrs). Rats were killed at resuscitation time 3 or 24 hrs.

MEASUREMENTS AND MAIN RESULTS

All seven rats in the hypothermia group and seven of 15 rats in the normothermia group survived to 24 hrs (p <.05). Hypothermic rats had lower serum potassium and higher blood glucose concentrations at 90 mins of hemorrhagic shock (p <.05). At resuscitation time 24 hrs, the hypothermia group had less liver injury (based on serum concentrations of ornithine carbamolytransferase and liver histology) and higher blood glucose than the normothermia group (p <.05). There were no differences in serum free 8-isoprostane (a marker of lipid peroxidation by free radicals) between the two groups at either baseline or resuscitation time 1 hr. Serum concentrations of interleukin- 1 beta, interleukin-6, and tumor necrosis factor-alpha peaked at resuscitation time 1 hr. Tumor necrosis factor-alpha concentrations were higher (p <.05) at resuscitation time 1 hr in the hypothermia group compared with the normothermic group. Serum cytokine concentrations were not different between survivors and nonsurvivors in the normothermia group. Serum cytokine concentrations returned to baseline values in both groups by 24 hrs. There were no differences in the number of neutrophils in the lungs or the small intestine between the groups. More neutrophils were found in the lungs at resuscitation time 3 hrs than at resuscitation time 24 hrs in both groups (p <.01).

CONCLUSIONS

These data suggest that lipid peroxidation and systemic inflammatory responses to hemorrhagic shock are minimally influenced by mild hypothermia, although liver injury is mitigated and survival improved. Other mechanisms of benefit from mild hypothermia need to be explored.

Systemic hypothermia, but not regional gut hypothermia, improves survival from prolonged hemorrhagic shock in rats

BACKGROUND

Extracorporeal blood perfusion of the gut or enterectomy can improve survival during hemorrhagic shock (HS), suggesting that the gut may be of primary importance in resuscitation. We hypothesized that cooling the gut alone could improve survival in a rat HS model and avoid potential deleterious effects of systemic hypothermia.

METHODS

Thirty-two Sprague-Dawley rats were anesthetized with halothane. The gut (small intestine, cecum, and colon) was exteriorized. The right atrial (T ), rectal, and gut (T ) intraluminal temperatures were monitored. HS was induced by withdrawal of 2 mL of blood per 100 g body weight over 10 minutes. Mean arterial pressure was then maintained at 35 to 40 mm Hg to HS 90 min. From HS 20 min to resuscitation time 1 h, rats were randomized into four groups (n = 8 each): normothermia (T and T approximately 38.0 degrees C), gut-25 degrees C (T approximately 38 degrees C, T approximately 25 degrees C, induced by rinsing the gut with cooled saline), gut-33 degrees C (T approximately 38 degrees C, T approximately 33 degrees C), and systemic hypothermia (T approximately 33 degrees C, T approximately 25 degrees C). At HS 90 min, shed blood and Ringer's solution were infused to restore normotension. Survival, metabolism, and tissue damage were observed to 72 hours.

RESULTS

Blood pressure was not different between groups. Compared with the normothermia group, the systemic hypothermia group had lower base deficit and lactate, and needed less fluid during resuscitation for normotension (p < 0.05), but these values were not different in the gut hypothermia groups. In addition, there were no significant improvements in tissue protection induced by regional gut hypothermia, whereas the systemic hypothermia group had lower plasma potassium, lower ornithine carbamoyltransferase (marker of liver injury), and higher glucose levels after HS (all p < 0.05). All rats in the systemic hypothermia group survived to 72 hours, whereas there was only one survivor in the normothermia group, two in the gut-33 degrees C group, and none in the gut-25 degrees C group (all p < 0.05 vs. systemic hypothermia).

CONCLUSION

Cooling the gut alone does not improve acute survival from HS, suggesting that early deaths are not secondary to gut ischemia. Mild systemic hypothermia allowed 100% survival from prolonged HS.

Cerebral ischemia and trauma-different etiologies yet similar mechanisms: neuroprotective opportunities

Cerebral ischemia leads to brain damage caused by pathogenetic mechanisms that are also activated by neurotrauma. These mechanisms include among others excitotoxicity, over production of free radicals, inflammation and apoptosis. Furthermore, cerebral ischemia and trauma both trigger similar auto-protective mechanisms including the production of heat shock proteins, anti-inflammatory cytokines and endogenous antioxidants. Neuroprotective therapy aims at minimizing the activation of toxic pathways and at enhancing the activity of endogenous neuroprotective mechanisms. The similarities in the damage-producing and endogenous auto-protective mechanisms may imply that neuroprotective compounds found to be active against one of these conditions may indeed be also protective in the other. This review summarizes the pathogenetic events of ischemic and traumatic brain injury and reviews the neuroprotective strategies employed thus far in each of these conditions with a special emphasize on their clinical relevance and on future directions in the field of neuronal protection.

Effect of hypothermia on brain multi-parametric activities in normoxic and partially ischemic rats

Hypothermia, as well as anesthesia, are known to protect the brain against ischemia, hypoxia and other pathological damages. One of the mechanisms of this improvement could be by lowering brain function, and thereby lowering oxygen demand. We examined the effect of hypothermia on brain function and blood supply in awake and anesthetized rats and studied the interaction between partial ischemia and the responses to hypothermia. The brain function multiprobe (BFM) used enabled simultaneous measurements of cerebral blood flow (CBF), mitochondrial NADH redox state, extracellular K(+) concentration, DC potential and ECoG from the cerebral cortex in rats whose brain temperature was lowered by 5 degrees C. Hypothermia was induced in awake, anesthetized and brain ischemic-anesthetized rats. In anesthetized and ischemic-anesthetized rats, the time required for lowering the brain temperature by 5 degrees C was five times less than in the normal awake animals. No significant changes in CBF and NADH levels were found in response to hypothermia in the awake animals. In contrast, a significant decrease in extracellular K(+) concentration was recorded under hypothermia, probably due to the lower rate of depolarization. Hypothermia in anesthetized and in ischemic-anesthetized rats did not significantly affect the levels of mitochondrial NADH, CBF and extracellular K(+). Hypothermia under ischemia was expected to be more effective.

Mild hypothermia reduces apoptosis of mouse neurons in vitro early in the cascade

Recent experimental work has shown that hypothermia with even small decreases in temperature is broadly neuroprotective, but the mechanism of this protection remains unclear. Although reduction of metabolism could explain protection by deep hypothermia, it does not explain the robust protection found with mild hypothermia. Several reports have suggested that ischemic apoptosis is reduced by hypothermia. The authors examined the effects of hypothermia on neuronal apoptosis using serum deprivation, a well-accepted model that induces neuronal apoptosis. Mild hypothermia (33 degrees C) significantly reduced the number of morphologically apoptotic neurons to less than half the number seen in normothermic culture temperatures (37 degrees C) after 48 hours. They examined the effect of hypothermia on several steps in the cascade. Caspase-3, -8, and -9 activity was significantly increased after 24 hours at 37 degrees C, and was significantly lower in cultures deprived of serum at 33 degrees C. Cytochrome c translocation was reduced by hypothermia. Western blot analysis failed to detect significant changes in Bax, bcl -2, or hsp -70 at early time points, whereas hypothermia significantly reduced cJun N-terminal kinase activation. The authors conclude that small decreases in temperature inhibit apoptosis very early, possibly at the level of the initiation of apoptosis, as suggested by reduced cJun N-terminal kinase activation and before the translocation of cytochrome c, with subsequent prevention of caspase activation.

Mild hypothermia decreases the incidence of transient ADC reduction detected with diffusion MRI and expression of c-fos and hsp70 mRNA during acute focal ischemia in rats

The effects of mild hypothermia on the apparent diffusion coefficient of water (ADC) and expression of c-fos and hsp70 mRNA were examined during acute focal cerebral ischemia. Young adult rats were subjected to 60-min middle cerebral artery occlusion under either normothermia (37.5 degrees C) or hypothermia (33 degrees C). Diffusion-weighted echo-planar magnetic resonance imaging was used to monitor changes in ADC throughout the ischemic period. Perfusion MRI with dysprosium contrast was used at the end of the ischemic period to verify that the occlusion was successful. C-fos and hsp70 mRNA expression were examined with in situ hybridization at the end of the ischemic period. The results indicate that the size of the region that exhibited reduced ADC was smaller during hypothermia than during normothermia. Hypothermia also decreased the frequency of occurrence of transient ADC reductions, especially in dorsal aspects of cortex. Expression of both c-fos and hsp70 mRNA were markedly reduced by hypothermia. Transient ADC reduction and c-fos expression are associated with spreading depression, which is believed to contribute to lesion expansion during acute focal ischemia. The results suggest that part of the neuroprotective effect of hypothermia may be due to a reduced incidence of spreading depression.