The effect of mild hypothermia on permanent focal ischemia in the rat

Temperature management for deliberate mild hypothermia during neurosurgical procedures

Deliberate mild hypothermia is safer from the point of view of adverse events and does not require a specific technique, unlike deep or moderate hypothermia. Therefore, mild hypothermia was widely used for neurosurgical procedures. Unfortunately, the neuroprotective efficacy of intraoperative mild hypothermia has not yet been proven; however, temperature management for intraoperative deliberate mild hypothermia has been improved over the past two decades. It is very important to achieve mild hypothermia before the commencement of the main surgery, and to maintain the patient's body temperature until the procedure is completed. In addition, it is also important to complete rewarming by the end of the surgery so that an accurate neurological evaluation can be made. Regarding the effects of mild hypothermia on outcomes, a large randomized controlled study reported that unfavorable outcomes did not differ between participants with or without hypothermia. Apart from these unfavorable outcomes, it is known that temperature management during deliberate intraoperative mild hypothermia has contributed to improvement of anesthesia practice. The accumulation of experience in this field is important. Clinical interest in deliberate mild hypothermia is currently low; however, anesthesiologists should be prepared for the time when it is required again in the future.

Review : Neuroprotection in Brain Ischemia: An Update (Part II

Ischemic brain injury produced by stroke or cardiac arrest is a major cause of human neurological disability. Steady advances in the neurosciences have elucidated the pathophysiological mechanisms of brain ischemia and have suggested many therapeutic approaches to achieve neuroprotection of the acutely ischemic brain that are directed at specific injury mechanisms. In the second portion of this two-part review, the following potential therapeutic approaches to acute ischemic injury are considered: 1) modulation of nonglutamatergic neurotransmission, including monoaminergic systems (dopamine, norepinephrine, serotonin), γ-aminobutyric acid, and adenosine; 2) mild-to-moderate therapeutic hypothermia; 3) calcium channel antagonism; 4) an tagonism of oxygen free radicals; 5) modulation of the nitric oxide system; 6) antagonism of cytoskeletal proteolysis; 7) growth factor administration; 8) therapy directed at cellular mediators of injury; and 9) the rationale for combination pharmacotherapy. The Neuroscientist 1:164-175, 1995

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.

Asphyxia and therapeutic hypothermia modulate plasma nitrite concentrations and carotid vascular resistance in preterm fetal sheep

In this study, we tested the hypothesis that cerebral hypoperfusion after asphyxia and induced hypothermia is associated with reduced circulating nitrite levels as an index of nitric oxide synthase (NOS) activity. The preterm fetal sheep at 0.7 gestation (103-104 days, term = 147 days) received 25-minute umbilical cord occlusion, followed by mild whole-body cooling from 30 minutes to 72 hours after occlusion. Occlusion and induced hypothermia were independently associated with reduced carotid vascular conductance (CaVC) from 2 to 72 hours, and with transiently suppressed plasma nitrite levels at 6 hours. There was a significant within-subjects correlation (r(2) = 0.33, P = .002) between CaVC and plasma nitrite values in the first 24 hours after occlusion but not after sham occlusion. These findings suggest that in preterm fetal sheep, changes in NOS activity are an important mediator of changes in carotid vascular tone in the early recovery phase after asphyxia and may help mediate some of the vascular effects of induced hypothermia.

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.

A magnetic resonance (MR) compatible selective brain temperature manipulation system for preclinical study

There is overwhelming evidence that hypothermia can improve the outcome of an ischemic stroke. However, the most widely used systemic cooling method could lead to multiple side effects, while the incompatibility with magnetic resonance imaging of the present selective cooling methods highly limit their application in preclinical studies. In this study, we developed a magnetic resonance compatible selective brain temperature manipulation system for small animals, which can regulate brain temperature quickly and accurately for a desired period of time, while maintaining the normal body physiological conditions. This device was utilized to examine the relationship between T1 relaxation, cerebral blood flow, and temperature in brain tissue during magnetic resonance imaging of ischemic stroke. The results showed that this device can be an efficient brain temperature manipulation tool for preclinical studies needing local hypothermic or hyperthermic conditions.

Time-dependent effects of hypothermia on microglial activation and migration

Background

Therapeutic hypothermia is one of the neuroprotective strategies that improve neurological outcomes after brain damage in ischemic stroke and traumatic brain injury. Microglial cells become activated following brain injury and play an important role in neuroinflammation and subsequent brain damage. The aim of this study was to determine the time-dependent effects of hypothermia on microglial cell activation and migration, which are accompanied by neuroinflammation.

Methods

Microglial cells in culture were subjected to mild (33 °C) or moderate (29 °C) hypothermic conditions before, during, or after lipopolysaccharide (LPS) or hypoxic stimulation, and the production of nitric oxide (NO), proinflammatory cytokines, reactive oxygen species, and neurotoxicity was evaluated. Effects of hypothermia on microglial migration were also determined in in vitro as well as in vivo settings.

Results

Early-, co-, and delayed-hypothermic treatments inhibited microglial production of inflammatory mediators to varying degrees: early treatment was the most efficient, and delayed treatment showed time-dependent effects. Delayed hypothermia also suppressed the mRNA levels of proinflammatory cytokines and iNOS, and attenuated microglial neurotoxicity in microglia-neuron co-cultures. Furthermore, delayed hypothermia reduced microglial migration in the Boyden chamber assay and wound healing assay. In a stab injury model, delayed local hypothermia reduced migration of microglia toward the injury site in the rat brain.

Conclusion

Taken together, our results indicate that delayed hypothermia is sufficient to attenuate microglial activation and migration, and provide the basis of determining the optimal time window for therapeutic hypothermia. Delayed hypothermia may be neuroprotective by inhibiting microglia-mediated neuroinflammation, indicating the therapeutic potential of post-injury hypothermia for patients with brain damages exhibiting some of the inflammatory components.

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.

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.

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.

Induced hypothermia in acute ischemic stroke

BACKGROUND

Induced hypothermia is a promising neuroprotective treatment for acute ischemic stroke. Data from both global and focal ischemia animal models have been encouraging. However, only a few small clinical studies have investigated its use in humans.

OBJECTIVE

To review the background, possible mechanisms of action, and the preclinical and clinical data supporting the neuroprotective role of induced hypothermia following acute ischemic stroke.

METHODS

A literature search was performed using the PubMed database. Only papers in English were reviewed.

RESULTS/CONCLUSIONS

Induced hypothermia is effective as a neuroprotectant in animal models of acute ischemic stroke. Its multimodal mechanism of action makes it a very attractive method of neuroprotection. Although human studies suggest it is safe and feasible, larger randomized controlled trials are necessary to address clinical efficacy and to refine the methods and parameters of induced hypothermia protocols.

Therapeutic effect of post-ischemic hypothermia duration on cerebral ischemic injury

OBJECTIVE

To study the efficacy of post-ischemic mild brain hypothermia lasting for different time intervals on cerebral ischemic reperfusion injury.

METHOD

Male Sprague-Dawley rats were divided into a sham-operated group, normothermia (37-38 degrees C) ischemia group and mild hypothermia (31-32 degrees C) group. The last group was subdivided into four groups: 30 minute hypothermia plus 210 minute normothermia, 60 minute hypothermia plus 180 minute nomothermia,120 minute hypothermia plus 120 minute normothermia, and 240 minute hypothermia (n=8). Global cerebral ischemia was established using the Pulsinelli four-vessel occlusion model. Brain tissue was collected following a 20 minute cerebral ischemia and 240 minute reperfusion, and was used to measure the levels of glutamate (Glu), aspartate (Asp), glycine (Gly), gamma-aminobutyric acid (GABA), dopamine (DA), norepinephrine (NE), serotonin(5-HT) and hydroxyindoleacetic acid (5-HIAA), nitrite (NO(2)), endothelin-1 (ET(1)), tumor necrosis factor alpha(TNFalpha) and interleukin-1beta (IL-1beta). Serum was collected to measure the levels of lactate dehydrogenase (LDH), aspartate aminotransferase (AST), creatine kinase (CK) and its brain band isoenzyme (CK-BB).

RESULTS

Hypothermia lasting for 60-240 minutes delayed the decrease in these amino acids, postponed the decrease in DA, NE and 5-HT and increase in hydroxyindoleacetic acid (5-HIAA), and decreased the levels of IL-1beta, TNFalpha, ET(1) and NO(2) in brain tissue. Hypothermia also decreased the levels of LDH, AST, CK and CK-BB in serum as compared to normothermia group (p<0.05 or p<0.01). Hypothermia lasting for 30 minutes delayed the decreases in these amino acids and 5-HT and increase in 5-HIAA in brain tissue (p<0.05), but failed to influence the levels of IL-1beta, TNFalpha, ET(1) and NO(2) in brain tissue and the amounts of LDH, AST, CK and CK-BB in serum as compared to normothermia ischemia group (p>0.05).

CONCLUSIONS

Post-ischemic mild brain hypothermia can significantly suppress the excessive release of amino acids, monoamine neurotransmitters and inflammation response in ischemic tissue. It can also stabilize the function of the cell membrane, which is associated with the mechanism of cerebral protection by mild hypothermia. These results suggest that mild hypothermia should be applied immediately after ischemia and last for more than 60 minutes in order to obtain neuroprotective effects.

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.

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.

Focal epidural cooling reduces the infarction volume of permanent middle cerebral artery occlusion in swine

BACKGROUND

The protective effect against ischemic stroke by systemic hypothermia is limited by the cooling rate and it has severe complications. This study was designed to evaluate the effect of SBH induced by epidural cooling on infarction volume in a swine model of PMCAO.

METHODS

Permanent middle cerebral artery occlusion was performed in 12 domestic swine assigned to groups A and B. In group A, the cranial and rectal temperatures were maintained at normal range (37 degrees C-39 degrees C) for 6 hours after PMCAO. In group B, cranial temperature was reduced to moderate (deep brain, <30 degrees C) and deep (brain surface, <20 degrees C) temperature and maintained at that level for 5 hours after 1 hour after PMCAO, by the epidural cooling method. All animals were euthanized 6 hours after MCAO; their brains were sectioned and stained with 2,3,5-triphenyltetrazolium chloride and their infarct volumes were calculated.

RESULTS

The moderate and deep brain temperature (at deep brain and brain surface) can be induced by rapid epidural cooling, whereas the rectal temperature was maintained within normal range. The infarction volume after PMCAO was significantly reduced by epidural cooling compared with controls (13.73% +/- 1.82% vs 5.62% +/- 2.57%, P < .05).

CONCLUSIONS

The present study has demonstrated, with histologic confirmation, that epidural cooling may be a useful strategy for reducing infarct volume after the onset of ischemia.

Decompressive craniectomy and mild hypothermia reduces infarction size and counterregulates Bax and Bcl-2 expression after permanent focal ischemia in rats

Both mild hypothermia (MH) and decompressive craniectomy (CE) have been shown to have neuroprotective effects in brain ischemia. We investigated a possible effect of MH and a combination of CE and MH (CE + MH) on the changes of infarction size, DNA fragmentation, and immunoreactivities for Bcl-2 and Bax after 24 h of permanent middle cerebral artery occlusion (MCAO) in rats. For the estimation of ischemic brain injury, we calculated the infarct size of the MCA region at 24 h after the MCAO. Terminal deoxynucleotidyl transferase-mediated dUTP-biotin in situ nick labeling (TUNEL) staining was performed for the detection of DNA fragmentation. Immunoreactivities for Bcl-2 and Bax were stained. Infarction size after permanent MCAO was significantly reduced by CE+MH treatment (P < 0.01). Infarction size did not change significantly by application of MH alone (P > 0.05). TUNEL staining was remarkably reduced both in MH-treated animals and in CE + MH-treated animals. Immunoreactivity for Bcl-2 was greatly induced both in MH-treated animals and in CE + MH-treated animals. Induction of immunoreactivity for Bcl-2 was obviously inhibited both in MH-treated animals and in CE + MH-treated animals. It suggests that temporary MH delays infarct evolution and ameliorates neuron apoptosis but does not significantly reduce definite infarction size. CE + MH not only ameliorates neuron apoptosis but also remarkably reduces infarction size.

Effect of mild hypothermia on the thrombolytic efficacy of 120 kHz ultrasound enhanced thrombolysis in an in-vitro human clot model

INTRODUCTION

Ischemic stroke causes substantial death and disability. Currently, recombinant tissue plasminogen activator (rt-PA) is the only FDA approved therapy. However, there are dangerous side effects and therapy must start within 3 h of onset. Therefore, there is interest in adjunctive therapies such as ultrasound enhanced thrombolysis (UET) and hypothermia. Recently, transcranial ultrasound during rt-PA therapy was shown to improve vessel recanalization. Also hypothermia (32-34 degrees C) was shown to be safe and possibly improve outcome. This suggests combining UET and hypothermia to treat ischemic stroke. Little is known about the effects of hypothermia on UET, and in-vitro rt-PA efficacy is reduced for T<37 degrees C. Here, the effects of hypothermia on UET in in-vitro human clot are presented. It is hypothesized that UET efficacy at 33 degrees C is less than at 37 degrees C.

MATERIALS AND METHODS

Whole blood was drawn from volunteers. Clots were made, incubated at 37 degrees C and aged for 2 days for maximal lytic resistance. Clots were exposed to human fresh-frozen plasma (control), hFFP and rt-PA ([rt-PA]=3.2 microg/ml), hFFP and 120 kHz ultrasound (US), and hFFP, rt-PA and ultrasound (UET) at 33 degrees C and 37 degrees C. Clot percent mass loss (Deltam) was measured to determine thrombolytic efficacy. Data were analyzed using mixed-model analysis of variance.

RESULTS AND CONCLUSIONS

US and rt-PA independently increased Deltam (3.5+/-1.0% and 5.1+/-0.9% respectively; p<0.01) over control. UET increased Deltam an additional 8.1+/-1.3% (p=0.026) The effect of temperature on Deltam (-1.6+/-0.7%) was not significant (p=0.09). Hypothermia did not reduce UET efficacy in this in-vitro model.

Mild hypothermia enhances the neuroprotective effects of FK506 and expands its therapeutic window following transient focal ischemia in rats

FK506 (tacrolimus), an immunosuppressant, reportedly reduces ischemic brain injury following transient middle cerebral artery occlusion (MCAO) in rats. The authors previously reported that the therapeutic window of FK506 in this model is more than 1 h, but less than 2 h. The aim of the present study is to determine whether mild hypothermia (35 degrees C) enhances the neuroprotective effects of FK506 and expands its therapeutic window. Sprague-Dawley rats were subjected to 2 h MCAO followed by 24 h reperfusion. Animals were randomly divided into four groups: (I) vehicle-treated normothermic group; (II) FK506-treated normothermic group; (III) vehicle-treated hypothermic group; (IV) FK506-treated hypothermic group. Animals received a single injection of FK506 (0.3 mg/kg) or vehicle intravenously at 2 h after ischemic induction. During ischemia, temporal muscle and rectal temperatures were maintained at 37 degrees C in the normothermic animals and at 35 degrees C in the hypothermic animals. Infarct volumes and neurological performance were evaluated at 24 h after reperfusion. The combination of FK506 and mild hypothermia significantly reduced infarct volume (cortex, -61%; striatum, -31%) and edema volume (cortex, -57%; striatum, -41%), while mild hypothermia or FK506 alone failed to improve ischemic brain damage. Furthermore, this combination also provided for the best functional outcome. These results demonstrate that the combination of FK506 and mild hypothermia significantly reduces ischemic brain damage following transient MCAO in rats, and expands the therapeutic window for FK506. This therapy may be a new approach for treatment of acute stroke.

The neuroprotective effect of a free radical scavenger and mild hypothermia following transient focal ischemia in rats

Edaravone, a novel free radical scavenger, has been reported to reduce ischemic damage in rats subjected to transient focal ischemia. The aim of this study is, therefore, to investigate the effect of a combined therapy with edaravone and mild hypothermia of 35 degrees C. Sprague-Dawley rats were subjected to MCA occluding an intraluminal suture technique for 2 hrs. The rats were reperfused for 24 h and decapitated for infarct and edema analysis. Animals were randomly devided into four groups: (I) vehicle + normothermia (control) (II) vehicle + mild hypothermia (III) Edaravone + normothermia (IV) Edaravone + mild hypothermia. Mild hypothermia alone had no reduction of the brain damage. The edaravone alone significantly reduced edema volume. The combined treatment with edaravone and mild hypothermia reduced both infarct and edema volume. In addition, this treatment provided for the best functional outcome. These results demonstrate that free radical scavenger, edaravone attenuates brain edema and that the combined therapy with edaravone and mild hypothermia significantly reduces not only edema but also infarct on transient focal cerebral ischemia in rats. The neuroprotective effects seen in this study may be due to the combined interaction of antiedema activity between edaravone and mild hypothermia, suppressing free radical production.

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.

Hypothermia and stroke: the pathophysiological background

Hypothermia to mitigate ischemic brain tissue damage has a history of about six decades. Both in clinical and experimental studies of hypothermia, two principal arbitrary patterns of core temperature lowering have been defined: mild (32-35 degrees C) and moderate hypothermia (30-33 degrees C). The neuroprotective effectiveness of postischemic hypothermia is typically viewed with skepticism because of conflicting experimental data. The questions to be resolved include the: (i) postischemic delay; (ii) depth; and (iii) duration of hypothermia. However, more recent experimental data have revealed that a protected reduction in brain temperature can provide sustained behavioral and histological neuroprotection, especially when thermoregulatory responses are suppressed by sedation or anesthesia. Conversely, brief or very mild hypothermia may only delay neuronal damage. Accordingly, protracted hypothermia of 32-34 degrees C may be beneficial following acute cerebral ischemia. But the pathophysiological mechanism of this protection remains yet unclear. Although reduction of metabolism could explain protection by deep hypothermia, it does not explain the robust protection connected with mild hypothermia. A thorough understanding of the experimental data of postischemic hypothermia would lead to a more selective and effective clinical therapy. For this reason, we here summarize recent experimental data on the application of hypothermia in cerebral ischemia, discuss problems to be solved in the experimental field, and try to draw parallels to therapeutic potentials and limitations.

Effect of mild hypothermia on focal cerebral ischemia. Review of experimental studies

The purposes of this review are to clarify the effect of hypothermia therapy on focal cerebral ischemia in rats, and to consider the relevancy of its application to human focal cerebral ischemia. Since 1990, 26 reports confirming the brain-protecting effect of hypothermia in rat focal cerebral ischemia models have been published. Seventy-four experimental groups in these 26 reports were classified as having transient middle cerebral arterial occlusion (MCAO) with mild hypothermia (group A; 43 groups), permanent MCAO with mild hypothermia (group B; 14 groups), permanent MCAO with deep hypothermia (group C; 8 groups) and transient or permanent MCAO with mild hyperthermia (group D; 9 groups). The results were evaluated as the % infarct volume change caused by hypothermia or hyperthermia compared with the infarct volume in normothermic animals. The effectiveness was confirmed in 36 (83%) of the 43 groups in group A, 10 (71%) of the 14 in group B, and six (75%) of the eight in group C. The infarct volume of eight of the nine groups in group D was markedly aggravated. The percent infarct volume change was 55.3% +/- 27.1% in group A, 57.6% +/- 24.7% in group B, 60.8% +/- 45.5% in group C, and 189.7% +/- 89.4% in group D. For effective reduction of the infarct volume, hypothermia should be started during ischemia or within 1 h, at latest, after the beginning of reperfusion in the rat transient MCAO model. However, it is not clear whether this neuroprotective effect of hypothermia can also be observed in the chronic stage, such as several months later. Keeping the body temperature normothermic in order to avoid mild hyperthermia seems to be rather important for not aggravating cerebral infarction. Clinical randomized studies on the efficacy of mild hypothermia for focal cerebral ischemia and sophisticated mild hypothermia therapy techniques are mandatory.

Treatment of space-occupying cerebral infarction

OBJECTIVE

Patients with a hemispheric infarct accompanied by massive edema have a poor prognosis; the case fatality rate may be as high as 80%, and most survivors are left severely disabled. Various treatment strategies have been proposed to limit brain tissue shifts and to reduce intracranial pressure, but their use is controversial. We performed a systematic search of the literature to review the evidence of efficacy of these therapeutic modalities.

DATA SOURCES

Literature searches were carried out on MEDLINE and PubMed.

STUDY SELECTION

Studies were included if they were published in English between 1966 and February 2002 and addressed the effect of osmotherapy, hyperventilation, barbiturates, steroids, hypothermia, or decompressive surgery in supratentorial infarction with edema in animals or humans.

DATA SYNTHESIS

Animal studies of medical treatment strategies in focal cerebral ischemia produced conflicting results. If any, experimental support for these strategies is derived from studies with animal models of moderately severe focal ischemia instead of severe space-occupying infarction. None of the treatment options have improved outcome in randomized clinical trials. Two large nonrandomized studies of decompressive surgery yielded promising results in terms of reduction of mortality and improvement of functional outcome.

CONCLUSIONS

There is no treatment modality of proven efficacy for patients with space-occupying hemispheric infarction. Decompressive surgery might be the most promising therapeutic option. For decisive answers, randomized, controlled clinical trials are needed.

Mild hypothermia reduces ICAM-1 expression, neutrophil infiltration and microglia/monocyte accumulation following experimental stroke

Mild hypothermia is an established neuroprotectant against cerebral ischemic injury. Studies have shown that inflammation potentiates cerebral ischemic injury, particularly in the setting of reperfusion. To further elucidate the mechanism by which mild hypothermia attenuates the inflammatory response, we assessed endothelial intercellular adhesion molecule-1 (ICAM-1) expression, neutrophil and monocyte infiltration, and microglial activation following 2 h of transient focal cerebral ischemia under normothermic and mildly hypothermic conditions. Ischemia was induced using the intraluminal suture method in Sprague-Dawley rats. Immunohistochemistry was used to detect endothelial ICAM-1, infiltrating neutrophils and monocytes, and microglia at 1, 3, and 7 days post-ischemia. Immunopositive cell and vessel densities were measured in the peri-infarct region. Mild hypothermia was associated with decreased neutrophils at 1 and 3 days post-ischemia, decreased ICAM-1-positive vessels at 1, 3, and 7 days, and decreased monocytes/activated microglia at 3 and 7 days, but not at 1 day. These data demonstrate that mild hypothermia significantly reduces endothelial adhesion molecule expression, acute (neutrophil) and subacute (monocyte) leukocyte infiltration, and microglial activation up to 7 days following insult in a rodent model of transient focal cerebral ischemia.

Natural hypothermia immediately after transient global cerebral ischemia induced by spontaneous subarachnoid hemorrhage

OBJECT

Spontaneous subarachnoid hemorrhage (SAH) has an aspect of graded transient global cerebral ischemia. The purpose of the present study was the documentation of sequential changes in body temperature immediately after SAH-induced transient global cerebral ischemia in humans.

METHODS

Patients admitted within 12 hours after the initial onset of SAH were examined retrospectively (426 patients). Patients with unruptured cerebral aneurysms served as a control group (73 patients). Body temperature measured at the axilla on admission was analyzed. The grade of SAH was established according to the Glasgow Coma Scale (GCS): Grade I, GCS Score 15; Grade II, GCS Score 11 to 14; Grade III, GCS Score 8 to 10; Grade IV, GCS Score 4 to 7; and Grade V, GCS Score 3. The mean body temperature of patients in the control group was 36.49 +/- 0.45 degrees C (mean +/- standard deviation). The mean body temperature of patients in the SAH group who had been admitted within 4 hours of onset for Grades I to V were significantly different (p < 0.001, analysis of variance [ANOVA]): 36.26 +/- 0.7 degrees C, 59 patients; 35.98 +/-0.85 degrees C, 73 patients; 35.52 +/- 0.79 degrees C, 25 patients; 35.9 +/- 1.09 degrees C, 108 patients; and 35.56 +/- 1.14 degrees C, 73 patients, respectively. These values were significantly lower than those in control volunteers, except for patients with Grade I SAH. The reduction in body temperature was unrelated to the location of the cerebral aneurysm and was not the product of circadian rhythm. The temperatures of patients in the SAH group who were admitted beyond 4 hours after onset for each grade were significantly different (p < 0.01, ANOVA): 36.8 +/- 0.91 degrees C, 36 patients; 36.74 +/- 0.68 degrees C, 31 patients; 36.73 +/- 0.38 degrees C, three patients; 37.41 +/- 1.37 degrees C, 17 patients; and 38.9 degrees C, one patient, respectively. These values were significantly higher than those in patients admitted within 4 hours of SAH onset for all grades except Grade V, and significantly higher than control values in patients with Grades I and IV SAH.

CONCLUSIONS

These results indicate that body temperature falls and then rises immediately after the SAH-induced transient global cerebral ischemia without cardiac arrest in humans. The reduction in temperature may be a natural cerebral protection mechanism that is activated shortly after ischemic insult.

Mild hypothermia can enhance pial arteriolar vasodilation induced by isoflurane and sevoflurane in cats

OBJECTIVE

Volatile anesthetics have been shown to dilate cerebral vessels. Recent evidence suggests that mild hypothermia can alter vascular reactivity of the cerebral vessels. However, the effect of mild hypothermia on volatile anesthetic-induced vasodilation of cerebral vessels is unknown. In the present study, we investigated the effect of mild hypothermia on pial arteriolar vasodilation induced by isoflurane and sevoflurane in cats.

DESIGN

Prospective, randomized, experimental study with repeated measures.

SETTING

Investigational animal laboratory.

SUBJECTS

Forty cats were used for the study of systemic administration of volatile anesthetics, and 22 cats were used for the study of topical administration of volatile anesthetics.

INTERVENTIONS

This study was approved by the Animal Experiment Committee of Nara Medical University. Animals were anesthetized with pentobarbital to maintain suppressive electroencephalographic patterns, which were introduced to measure direct effects of anesthetic agents after removing metabolic effects. The cranial window technique, combined with microscopic video recording, was used for the measurement of small (50-100 microm) and large (100-200 microm) pial arteriolar diameter in an experiment. Animals were randomly assigned to either a normothermic (37 degrees C) or a hypothermic group (33 degrees C). Desired temperatures were maintained by using a water blanket. In the first phase of the study, the effect of hypothermia on pial arteriolar vasodilation induced by systemic administration of isoflurane or sevoflurane was assessed. Each cat received isoflurane or sevoflurane at 0.5, 1.0, 1.5, and 2.0 minimum alveolar anesthetic concentrations, and the diameter of pial arterioles was measured. In the second group of animals, the direct effect of isoflurane and sevoflurane on pial vessels was evaluated. The artificial cerebrospinal fluid bubbled with isoflurane or sevoflurane (minimum alveolar anesthetic concentrations of 1 or 3) was topically administered in the cranial window.

MEASUREMENTS AND MAIN RESULTS

Systemic and topical administration of isoflurane and sevoflurane produced significant dilation of both small and large pial arterioles in a dose-dependent manner during normothermia. In the hypothermic group, vasodilation of small pial arterioles by systemic administration of isoflurane and sevoflurane at a high concentration was significantly larger than in the normothermic group (p <.05). Vasodilation of both small and large pial arterioles by topical administration of isoflurane and sevoflurane was significantly greater in the hypothermic group than in the normothermic group (p <.05).

CONCLUSIONS

These results suggest that pial arteriolar vasodilation induced by isoflurane and sevoflurane can be enhanced by mild hypothermia in cats anesthetized with pentobarbital.

Effect of mild hypothermia on nicorandil-induced vasodilation of pial arterioles in cats

OBJECTIVE

Nicorandil is characterized as hybrid between nitrates and potassium channel activators. Recent evidence suggested that mild hypothermia may alter cerebral vasodilation induced by a nitrate agent and potassium channel opener. However, the effect of mild hypothermia on nicorandil-induced vasodilation is not known. The present study was conducted to investigate whether mild hypothermia could alter nicorandil-induced cerebral vasodilation. In addition, the effects of mild hypothermia on cerebral vasodilation induced by nitroglycerin, a nitrate agent, and cromakalim, a selective adenosine 5'-triphosphate-sensitive potassium channel opener, were assessed in the same model.

DESIGN

Prospective, randomized, experimental study with repeated measures.

SETTING

Investigational animal laboratory.

SUBJECTS

Twenty-four cats.

INTERVENTIONS

Animals were anesthetized with pentobarbital. The cranial window technique, combined with microscopic video recording, was used to measure small (50-100 microm) and large (100-200 microm) pial arteriolar diameter in an experiment. Animals were assigned randomly to either a normothermic (37 degrees C) or a hypothermic (33 degrees C) group. Nicorandil, nitroglycerin, or cromakalim at concentrations of 10(-8), 10(-6), or 10(-4) mol/L was applied topically in the cranial window, and the diameter of pial arterioles was measured.

MEASUREMENTS AND MAIN RESULTS

Topical administration of nicorandil, nitroglycerin, and cromakalim significantly dilated both small and large pial arterioles in a dose-dependent manner during normothermia. Nicorandil-induced vasodilation of either large or small pial arterioles was not affected by hypothermia. However, hypothermia significantly attenuated nitroglycerine-induced vasodilation in both large and small pial arterioles and enhanced cromakalim-induced vasodilation in both large and small pial arterioles.

CONCLUSIONS

Nicorandil-induced vasodilation of cerebral pial arterioles was not affected by mild hypothermia. By contrast, mild hypothermia significantly attenuated nitroglycerin-induced vasodilation and enhanced cromakalim-induced vasodilation.

Neuroprotective effect of hypothermia at defined intraischemic time courses in cortical cultures

Many experimental and clinical studies have shown that hypothermia confers cerebroprotective benefits against ischemic insults. Because of the many conflicting reports on hypothermic neuroprotection, we undertook this cellular study to identify the optimal temperature or a range of temperatures for maximal neuroprotection at different times (6-24 hr) during ischemic insults. Cultured Wistar rat cortical neurons were exposed to oxygen deprivation at defined times and temperatures (37 degrees C normothermia, 32 degrees C mild hypothermia, 27 degrees C moderate hypothermia, 22 degrees C deep hypothermia, and 17 degrees C profound hypothermia). The survival rate of neurons was evaluated by assessing viable neurons on photomicrographs. The normothermic group demonstrated a significantly lower survival rate of cultured neurons (6 hr, 80.3% +/- 2.7%; 12 hr, 56.1% +/- 2.1%; 18 hr, 34.2% +/- 1%; 24 hr, 18.1% +/- 2.2%) compared to hypothermic groups (P < 0.001). The survival rate for the profound hypothermic group was significantly reduced (P < 0.01) compared to other hypothermic groups (at 17 degrees C: 12 hr, 85.9% +/- 2.5%, 18 hr, 74.7% +/- 3.7%, 24 hr, 58.7% +/- 2.7%). Almost equal survival rates were observed among mild, moderate, and deep hypothermic groups following <18 hr exposure to hypoxia, but the deep hypothermic group showed a significantly higher survival rate (84.1% +/- 1.6%; P < 0.001) when subjected to hypoxia for 24 hr. In conclusion, hypothermia offers marked neuroprotection against hypoxia, but attenuation of neuronal cell death was less with profound hypothermia compared to mild, moderate, and deep hypothermia. Deep hypothermia affords maximal protection of neurons compared to mild and moderate hypothermia during long-lasting hypoxia (>18 hr).

Post-ischemic administration of DY-9760e, a novel calmodulin antagonist, reduced infarct volume in the permanent focal ischemia model of spontaneously hypertensive rat

We assessed the effect of a novel calmodulin antagonist, DY-9760e (3-[2-[4-(3-chloro-2-methylphenyl)-1-piperazinyl]ethyl]-5,6-dimethoxy-1-(4-imidazolylmethyl)-1H-indazole dihydrochloride 3.5 hydrate) in a spontaneously hypertensive rat (SHR) permanent focal cerebral ischemia. In experiment I, the left middle cerebral artery was permanently occluded in 62 SHRs. DY-9760e (0.5 mg kg(-1) h(-1)) or vehicle alone were administered continuously i.v. for 6 h, beginning 0, 30, or 60 min after the arterial occlusion. The infarct volume was measured 24 h of ischemia. In experiment II, the effect of DY-9760e on CBF was assessed in 10 SHRs. Administration without a delay resulted in a mean infarct volume of 166.7 +/- 21.0 mm3 (vehicle; n = 10) and 125.1 +/- 31.8 mm3 (DY-9760e; n = 9). Administration with a 30 min delay resulted in a mean infarct volume of 173.2 +/- 32.4 mm3 (vehicle; n = 12) and 143.3 +/- 35.3 mm3 (DY-9760e; n = 11). Dy-9760e significantly reduced the infarct under these conditions (p < 0.05). The administration with a 60 min delay failed to reduce the infarct. DY-9760e had no effect on the CBF. Continuous i.v. administration of DY-9760e reduced infarct volume in a SHR permanent focal ischemia without affecting ischemic CBF.

Avaliação da isquemia cerebral pela respiração mitocondrial: modelo experimental

A isquemia cerebral acontece em várias doenças. Um dos fatores críticos para a recuperação de um paciente é a duração do processo isquêmico. A atividade cerebral depende do suprimento de energia, isto sugere que o estudo da função mitocondrial pode ser utilizado para a avaliação do dano neuronal. O objetivo deste trabalho foi o de estudar a respiração mitocondrial pela oclusão da artéria cerebral média esquerda pela técnica do fio intraluminal. Ratos da raça Wistar foram subdivididos em 4 grupos: controle e 15, 30 e 60 minutos de oclusão. Os resultados mostraram que não há diferença estatisticamente significativa entre o grupo de 15 minutos e o grupo controle. O grupo de 30 minutos teve diminuição do estado III da respiração mitocondrial comparado com o grupo controle. O grupo de 60 minutos teve diminuição dos estados III e IV comparados com o grupo controle. A respiração mitocondrial permitiu uma avaliação efetiva e precoce do processo isquêmico focal no cérebro do rato.

Intra-ischemic hypothermia attenuates intercellular adhesion molecule-1 (ICAM-1) and migration of neutrophil

Adhesion of neutrophil to the endothelium and subsequent transmigration has been reported to contribute to progression of focal ischemia. Hypothermia has been known to attenuate ischemic insult through various mechanisms of action. The authors evaluated the effect of hypothermia on expression of intercellular adhesion molecule-1 (ICAM-1) protein and on transmigration of neutrophil with immunohistochemical method. Transient focal ischemia model in rats was employed, and animals received 2 h of either normothermic or hypothermic ischemia. To confirm the effectiveness of hypothermia on neuroprotection, cortical infarct area was compared between the two groups. Our results demonstrated that hypothermia reduced both the number of microvessels expressing ICAM-1 and that of neutrophils migrating into ischemic tissue. Comparison of cortical infarct area showed persistent protective effect. This study indicates that reduction of ICAM-1 expression and subsequent reduction of migrating neutrophil in hypothermia can contribute to attenuation of ischemic damage.

Temporal changes in sensitivity of rats to cerebral ischemic insult

OBJECT

Experimental rat models are often used to study cerebral ischemia, yet rats are nocturnal animals that have activity cycles that are the opposite of those of humans. In the following study the authors examined the circadian rhythm of sensitivity to an ischemic insult in rats by using an intraluminal thread technique to produce reversible middle cerebral artery occlusion.

METHODS

Ischemia (2 hours of blockage followed by 22 hours of reperfusion) was induced in rats according to the 24-hour clock at either 100, 400, 700, 1,000, 1,300, 1,600, 1,900, or 2,200 hours (11-14 rats per time period). The rat brains were removed, coronally sectioned, stained with 2,3,5-triphenyltetrazolium chloride and analyzed using commercially available software. Analysis of variance and cosinor-rhythmometry statistical tests were used for analysis of data. The time of day when the ischemic infarct was induced had a significant (p = 0.011) influence on the volume of the lesion. The volume of total brain infarct produced at 400 hours (7.65 +/- 1.31%) was more than three times greater than the volume produced at 1600 hours (2.1 +/- 0.34%). Cosinor-rhythm analysis indicated a peak occurrence of infarct volume at 6:02 (95% confidence interval 5:49-6:16). The size of the infarct correlated with core body temperature rhythms, which varied by 1.3 +/- 0.62 degrees C (mean +/- standard deviation).

CONCLUSIONS

Circadian rhythms, as well as the reversed natural body rhythms of the rat compared with humans, should be considered when extrapolating data to human or other animal studies. Temporal rhythms may also provide information concerning the cascading disease processes associated with cerebral ischemia.

Effect of selective brain hypothermia on regional cerebral blood flow and tissue metabolism using brain thermo-regulator in spontaneously hypertensive rats

To investigate the effect of selective hypothermia of the brain (brain cooling) on regional cerebral blood flow and tissue metabolism, we have developed a brain thermo-regulator. Brain temperature was modulated by a water-cooled metallic plate placed on the surface of the rats' scalp to get the appropriate brain temperature precisely with ease. Regional cerebral blood flow and brain temperature were measured simultaneously using a Teflon-coated platinum electrode and thermocouple probe inserted stereotaxically into the parietal cortex and thalamus in spontaneously hypertensive rats. Experimental forebrain ischemia was induced by the occlusion of bilateral common carotid artery under normo- and hypothermic brain condition, and the supratentorial brain tissue metabolites were measured enzymatically after 60 min of forebrain ischemia. When cortical temperature was set to hypothermia, cortical blood flow was significantly lowered by 40% at 30 degree C and 20% at 33 degree C as compared with that at 36 degree C (p < 0.0001 and p < 0.05, respectively). Thalamic blood flow was also significantly reduced by 20% when cortical temperature was set to 30 degree C as compared with 36 degree C (p < 0.05). There were no significant differences in arterial blood pressure and gas parameters throughout these experiments. In the rats with selective brain hypothermia (30 degree C) immediately after the induction of cerebral ischemia, the level of brain ATP concentration after 60 min of ischemia was significantly higher than that in normothermia rats (36 degree C) (p < 0.05). Our findings indicate that: 1) the metallic plate brain thermo-regulator is useful in small animal experiments; 2) regional brain temperature regulates regional cerebral blood flow; and 3) selective brain hypothermia, even started after the forebrain ischemia, ameliorates the derangement of brain metabolism, suggesting its effectiveness as a cytoprotective strategy.

Comparative neuroprotective efficacy of prolonged moderate intraischemic and postischemic hypothermia in focal cerebral ischemia

OBJECT

The purpose of this study was to compare the effects of prolonged hypothermia on ischemic injury in a highly reproducible model of middle cerebral artery (MCA) occlusion in rats.

METHODS

Male Sprague-Dawley rats were anesthetized with halothane and subjected to 120 minutes of temporary MCA occlusion by retrograde insertion of an intraluminal nylon suture coated with poly-L-lysine through the external carotid artery into the internal carotid artery and the MCA. Two levels of prolonged postischemic cranial hypothermia (32 degrees C and 27 degrees C) and one level of intraischemic cranial hypothermia (32 degrees C) were compared with the ischemic normothermia (37 degrees C) condition. Target cranial temperatures were maintained for 3 hours and then gradually restored to 35 degrees C over an additional 2-hour period. The animals were evaluated using a quantitative neurobehavioral battery of tests before inducing MCA occlusion, during occlusion (at 60 minutes postonset in all rats except those in the intraischemic hypothermia group), and at 24, 48, and 72 hours after reperfusion. The rat brains were perfusion fixed at 72 hours after ischemia, and infarct volumes and brain edema were determined. Both intraischemic and postischemic cooling to 32 degrees C led to similar significant reductions in cortical infarct volume (by 89% and 88%, respectively) and total infarct volume (by 54% and 69%, respectively), whereas postischemic cooling to 27 degrees C produced lesser reductions (64% and 49%, respectively), which were not statistically significant. All three hypothermic regimens significantly lessened hemispheric swelling and improved the neurological score at 24 hours. The authors' data confirm that a high degree of histological neuroprotection is conferred by postischemic cooling to 32 degrees C, which is virtually equivalent to that observed with intraischemic cooling to the same level.

CONCLUSIONS

These results may be relevant to the design of future clinical trials of therapeutic hypothermia for acute ischemic stroke.

Effects of systemic hypothermia and selective brain cooling on ischemic brain damage and swelling

The present study investigates the neuroprotective effects of temporary mild systemic hypothermia and selective brain cooling against focal cerebral infarction in the rat and the changes of cortical blood flow, and compares these two treatment modalities. In permanent middle cerebral artery (MCA) model, the treatments were induced 15 min following the artery occlusion. The animals were kept at the desired rectal or brain temperature (about 32 degrees C) for 30 min; (each, n = 6) and for 1 hr (each, n = 6), and then allowed to rewarm spontaneously, whereas control animals were kept at normothermia throughout the experiment. The volumes of brain infarction and edema were assessed 24 hr post-occlusion. The blood flow of the dorsolateral cortex was monitored by Laser-Doppler flowmetry (LDF) in the other experiments. Hemispheric infarct volume was attenuated only in the animals treated for 1 hr with systemic hypothermia (49.2%, P < 0.001) and selective brain cooling (26.7%, P < 0.01). The volume of brain swelling was diminished only in the animals treated with systemic hypothermia for 1 hr (23.6%, P < 0.05). LDF examination revealed a sharp drop in blood flow upon MCA occlusion and maintaining in low blood flow throughout the experiment in the control and systemic hypothermia. However, in the selective brain cooling, the reduced blood flow increased from 40% to 70% of baseline value while the brain was rewarmed. The present study indicates that mild systemic hypothermia has much stronger protective effects against focal cerebral infarction and edema than selective brain cooling. The lack of protective effects of selective brain cooling may be caused by post-cooling cerebral hyperemia in the ischemia area.

Mild hypothermia--a revived countermeasure against ischemic neuronal damages

Although hypothermia as a means of cerebral protection against and resuscitation from ischemic damage has a history of approximately six decades, extensive studies, both in basic and clinical fields, on the mechanisms, effects and methods of mild hypothermia at temperatures no less than 31 degrees C have started only in the last decade. In experiments on rodents, hypothermia in the postischemic period that is introduced up to several hours after reperfusion and is maintained for one day followed by a slow rewarming, significantly protects hippocampal neurons against damage. The mode of action of hypothermia is apparently non-specific and multi-focal in widely progressing cascade reactions in ischemic cells; namely, suppressing: (1) glutamate surge followed by; (2) intraneuronal calcium mobilization; (3) sustained activation of glutamate receptors; (4) dysfunction of blood brain barrier; (5) proliferation of microglial cells; and (6) production of superoxide anions and nitric oxide. In addition, mild hypothermia modulates processes in ischemic condition at the level of cell nucleus, such as the binding of transcription factor AP-1 to DNA, and ameliorates the depression of protein synthesis. This non-specific and widely affecting manner might explain why hypothermia is superior to any medicine developed. Recent clinical trials of mild hypothermia in various individual institutions have revealed significantly beneficial outcomes in some cases, along with an accumulation of practical knowledge of techniques and treatments. Large scale randomized studies involving multiple institutions as well as exchange of informations and ideas are needed for further development of hypothermia treatment.

Relationship between magnitude of hypothermia during ischemia and preventive effect against post-ischemic DNA fragmentation in the gerbil hippocampus

Protective effect of hypothermia against DNA fragmentation in hippocampal CA1 field after transient forebrain ischemia in gerbils was evaluated by changing the magnitude of hypothermia. Inhibition of DNA fragmentation was proportional to the magnitude of hypothermia. The result indicates that, in terms of susceptibility to ischemia, hippocampal CA1 neurons are sensitive to a relatively small decrement of temperature, with temperatures </=35 degreesC being critical for the prevention of apoptotic process following transient forebrain ischemia.

Mild hypothermia can attenuate nitroglycerin-induced vasodilation of pial arterioles in the cat

The purpose of the present study was to investigate the effect of mild hypothermia on nitroglycerin-induced vasodilation of cerebral vessels. The cranial window technique, combined with microscopic video recording, was used in an experiment involving 26 cats anesthetized with isoflurane. Animals were randomly assigned to either a normothermic or a mildly hypothermic group (33 degrees C). We administered three different concentrations of nitroglycerin (10[-6], 10[-5], 10[-4] M) under the window and measured the diameter of small (< 100 microm) and large (100-200 microm) pial arterioles. In the normothermic group (n = 13), nitroglycerin produced a significant dilation of both small and large arterioles in a dose-dependent manner. In the hypothermic group (n = 13), a significant dilation of arterioles was observed only after topical application of nitroglycerin at a concentration of 10(-4) M. The percent increase in diameter of small and large arterioles was less in the hypothermic group than the normothermic group. Our in vivo study demonstrates that topically applied nitroglycerin produces a dose-dependent dilation of pial arterioles in normothermic cats anesthetized with isoflurane, but the reduction of temperature to 33 degrees C significantly attenuates nitroglycerin-induced vasodilation of pial arterioles.

IMPLICATIONS

Although nitroglycerin may be used in hypothermic patients, the effect of mild hypothermia on nitroglycerin-induced vasodilation of cerebral vessels is unknown. In this study, we investigated the effects of nitroglycerin on pial arteriolar diameter in normothermic and hyperthermic cats. Hypothermia was found to attenuate nitroglycerin-induced vasodilation.

Hypothermia attenuates the activation of protein kinase C in focal ischemic rat brain: dual autoradiographic study of [3H]phorbol 12,13-dibutyrate and iodo[14C]antipyrine

Using phorbol 12,13-dibutyrate (PDBu) autoradiography, we investigated the effect of hypothermia or protein kinase C (PKC) activation in rat brain 2 h after focal ischemia. In normothermia, a significant increase of PDBu binding was observed in ischemic brain. Hypothermia suppressed the increase of PDBu binding in degree and extent. These observations suggest that intraischemic hypothermia attenuates the activation of PKC, and this may in part be participate in the protective effect of hypothermia.

Mild protective and resuscitative hypothermia for asphyxial cardiac arrest in rats

It has been shown in dogs that mild hypothermia (34 degrees C) during or immediately after ventricular fibrillation cardiac arrest can improve cerebral outcome. The effect of mild hypothermia on outcome after 8 minutes of asphyxiation (5 minutes' cardiac arrest) was studied for the first time in rats. Restoration of spontaneous circulation was with external cardiopulmonary resuscitation and observation to 72 hours. Three groups of 10 rats each were studied. At 72 hours postarrest, compared with the normothermic control group 1, final overall performance categories (OPC) and neurological deficit scores (NDS) were numerically better in the resuscitative (post-arrest) hypothermia group 2 and significantly better in the protective (pre-intra-arrest) hypothermia group 3 (P < .05). Total brain histopathological damage scores (HDS) were 17 +/- 5 in group 1, 14 +/- 6 in group 2 (NS), and 6 +/- 2 in group 3 (P < .001 versus group 1). HDS correlated with OPC (r = .6, P < .05) and NDS (r = .7, P < .05). Mild hypothermia improved cerebral outcome after asphyxial cardiac arrest in rats, more when induced before than after arrest. The model's insult is within the therapeutic window, which makes it also suitable for screening other cerebral resuscitation potentials.

Induction of hypercontractility in human cerebral arteries by rewarming following hypothermia: a possible role for tyrosine kinase

Induction of hypothermia is used routinely in neurosurgical and cardiovascular operations to protect the brain from ischemic insult. However, despite a plethora of experimental evidence supporting the use of hypothermia to protect the brain from ischemia, clinical experience using deliberate hypothermia in humans has not shown a convincing benefit. The authors tested the hypothesis that hypothermia and rewarming alter tone in human cerebral vessels and may interfere with cerebral perfusion in the setting of deliberate hypothermia. They examined human cerebral arteries during hypothermia (32 degrees C and 17 degrees C) and during rewarming to delineate the direct effects of cooling and rewarming on cerebrovascular tone. Artery segments obtained from autopsy material and from specimens excised at elective temporal lobectomies were tested in tissue baths using isometric tension measurements. Temperature-induced changes in vascular tone were measured and quantified with respect to contractile responses to serotonin (5-HT; 10(-6) M). Cooling induced mild relaxation in cerebral vessels (-38 +/- 12% 5-HT response in 50 vessels from autopsy specimens, -69 +/- 10% 5-HT response in 51 vessels from lobectomy specimens). On rewarming, vessels contracted significantly beyond their baseline tone (108 +/- 18% 5-HT response in 50 vessels from autopsy specimens, 42 +/- 12% 5-HT response in 51 vessels from lobectomy specimens). Rewarming-induced hypercontractility was inhibited by the tyrosine kinase inhibitor genistein (-5 +/- 7% vs. 70 +/- 23% 5-HT response, genistein vs. control, 14 segments, p < 0.05) and enhanced by the tyrosine phosphatase inhibitor sodium orthovanadate (339 +/- 54% vs. 104 +/- 20% 5-HT response, sodium orthovanadate vs. control, five segments, p < 0.05), indicating a possible role for tyrosine kinase activation in the rewarming-induced contraction.

Brief post-hypoxic-ischemic hypothermia markedly delays neonatal brain injury

The influence of post-insult temperature modulation on ischemic injury in immature brain was studied in 7-day-old rats that underwent a unilateral carotid artery ligation followed by exposure to hypoxia in 8% oxygen at an ambient temperature of 36.5 degrees C. After the hypoxic exposure, the animals were separated into three groups and placed for 3 h in temperature-controlled incubators set at 32 degrees C, 35 degrees C, and 38 degrees C. In Study 1, the influence of post-insult temperature modulation was assessed after graded cerebral hypoxic-ischemic injury. Brain damage was assessed 1 week after the insult by comparison of wet weights in the cerebral hemispheres ipsilateral and contralateral to the carotid artery ligation. Rectal temperatures of the animals significantly correlated with extent of brain injury after 60 min (Spearman correlation coefficient, p = 0.44, P = 0.005) and 90 min (p = 0.46, P = 0.004) but not 120 min of hypoxia (p = 0.18, P = 0.46). In Study 2, animals were exposed to 75 min hypoxia, and injury was assessed morphometrically and histologically at 1 and 4 weeks after the injury. Rectal temperatures significantly correlated with the extent of ischemic injury in the cerebral cortex (p = 0.3, P = 0.046) and striatum (p = 0.3, P = 0.048) at 1 week, but not 4 weeks, after the insult. The findings indicate that post-insult hypothermia delayed the expression of mild to moderate brain damage by more than a week, after which the damage was as severe as in normothermic animals. The results indicate that the events that determine the final expression of a neonatal hypoxic-ischemic insult can be extended over a long interval by post-insult hypothermia.

Brain cooling during transient focal ischemia provides complete neuroprotection

A review of the effects of reducing brain temperature on ischemic brain injury is presented together with original data describing the systematic evaluation of the effects of brain cooling on brain injury produced by transient focal ischemia. Male spontaneously hypertensive rate were subjected to transient middle cerebral artery occlusion (TMCAO; 80, 120 or 160 min) followed by 24 h of reperfusion. During TMCAO, the exposed skull was bathed with isotonic saline at various temperatures to control skull and deeper brain temperatures. Rectal temperature was always constant at 37 degrees C. Initial studies indicated that skull temperature was decreased significantly (i.e. to 32-33 degrees C) just as a consequence of surgical exposure of the artery. Subsequent studies indicated that maintaining skull temperature at 37 degrees C compared to 32 degrees C significantly (p < 0.05) increased the infarct size following 120 or 160 min TMCAO. In other studies, 80 min TMCAO was held constant, but deeper brain temperature could be varied by regulating skull temperature at different levels. At 36-38 degrees C brain temperature, infarct volumes of 102 +/- 10 to 91 +/- 9 mm3 occurred following TMCAO. However, at a brain temperature of 34 degrees C, a significantly (p < 0.05) reduced infarct volume of 37 +/- 10 mm3 was observed. Absolutely no brain infarction was observed if the brain was cooled to 29 degrees C during TMCAO. Middle cerebral artery exposure and maintaining brain temperature at 37 degrees C without artery occlusion did not produce any cerebral injury. These data indicated the importance of controlling brain temperature in cerebral ischemia and that reducing brain temperature during ischemia produces a brain temperature-related decrease in focal ischemic damage. Brain cooling of 3 degrees C and 8 degrees C can provide dramatic and complete, respectively, neuroprotection from transient focal ischemia. Multiple mechanisms for reduced brain temperature-induced neuroprotection have been identified and include reduced metabolic rate and energy depletion, decreased excitatory transmitter release, reduced alterations in ion flux, and reduced vascular permeability, edema, and blood-brain barrier disruption. Cerebral hypothermia is clearly the most potent therapeutic approach to reducing experimental ischemic brain injury identified to date, and this is emphasized by the present data which demonstrate complete neuroprotection in transient focal stroke. Certainly all available information warrants the evaluation of brain cooling for potential implementation in the treatment of human stroke.