Mild hypothermia fails to protect late hippocampal neuronal loss following forebrain cerebral ischaemia in rats

Summary Anaesthetized male rats (n=86) from both Long-Evans strain (LES) (n=43) and Wistar strain (WS) (n=43) were utilized for the experiments. While three animals from each strain were used as control, 40 rats from each strain underwent up to 10 minutes forebrain ischaemia by bilateral common carotid artery (CCA) occlusion combined with systemic hypotension [Mean Arterial Blood Pressure (MABP)=50 mm/Hg]. The animals from each strain were divided into four (n=10) groups. In both strains, groups (n=10) 1 and 2, temporalis muscle (TM) and body temperatures of the animals were kept at 36–37 °C during the experiments. The groups 1 and 2 were killed in 3 and 7 days after the ischaemic insult, respectively. The groups 3 and 4 were also killed 3 and 7 days after the ischaemic insult, but the forebrain ischaemia was carried out under mild cerebral hypothermia (TM temperature = 33 °C). Pyramidal neurons of the hippocampal CA1 region from each group was evaluated semiquantitatively. In WS, groups 1 and 2 showed moderate and severe neuronal loss in the CA1 region, respectively. However, in LES while the group 1 (3 days survival) did not show any neuronal loss, group 2 showed moderate neuronal loss of the CA1 region. While in group 3 (3 days survival, hypothermia) WS and LES, hypothermia protected the CA1 region, group 4 of LES showed mild neuronal loss. However WS, group 4 (7 days survival, hypothermia) showed severe neuronal loss of the CA1 region.It was concluded that mild hypothermia during ischaemic insults did not prevent the delayed postischaemic neuronal damage of the hippocampal CA1 region of both strains, and following 10 minutes forebrain ischaemia, male LES rats were found more resistant than male WS rats to neuronal loss of the CA1 region.     

Hypothermie et hypertension intracrânienne

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

Heart energy metabolism after intestinal ischaemia and reperfusion

Background/purpose

Multiple organ failure subsequent to intestinal ischaemia and reperfusion (I/R) includes cardiac failure, but little is known about heart energy metabolism in this setting. This study investigates the effects of intestinal I/R on heart energy metabolism and evaluates the effects of moderate hypothermia.

Methods

Adult rats underwent intestinal ischaemia for 60 minutes followed by 120 minutes of reperfusion. Animals were maintained at either normothermia (36° to 38°C) or moderate hypothermia (30° to 32°C). In experiment A, 2 groups were studied: (1) sham at normothermia; (2) I/R at normothermia. After death, the heart was removed. Cardiac phosphoenergetics were assessed by ³¹P magnetic resonance spectroscopy; data are expressed as micromoles per gram. In experiment B, 4 groups were studied: (1) sham at normothermia, (2) I/R at normothermia, (3) sham at hypothermia, (4) I/R at hypothermia. At the end of the experiment, the heart was harvested. The activity of carnitine palmitoyl transferase I (CPT I), an important enzyme in the control of fatty acid oxidation, was measured; data are expressed as nanomoles per minute per unit citrate synthase. Results are expressed as mean ± SEM.

Results

In experiment A, there were no differences between the 2 study groups in cardiac phosphocreatine, inorganic phosphate, adenosine triphosphate (ATP), or in the ratio of inorganic phosphate to ATP. In experiment B, CPT I activity was decreased significantly after I/R at normothermia compared with normothermic sham, but this enzyme inhibition was prevented by hypothermia (3.9 ± 0.2; v I/R).

Conclusions

These results suggest that although cardiac ATP supply was maintained during intestinal I/R at normothermia, the balance of substrate utilisation was shifted from fatty acid oxidation to carbohydrate utilisation. However, moderate hypothermia modified these changes. The beneficial effect of moderate hypothermia on cardiac metabolism during intestinal I/R has potential clinical application in various surgical conditions.     

Prolonged mild hypothermia after experimental hypothermic circulatory arrest in a chronic porcine model

Objectives: We sought to evaluate the potential efficacy of prolonged mild hypothermia after hypothermic circulatory arrest. Methods: Twenty pigs, after a 75-minute period of hypothermic circulatory arrest, were randomly assigned to be rewarmed to 37°C (normothermia group) or to 32°C and kept at that temperature for 14 hours from the start of rewarming (hypothermia group). Results: The 7-day survival was 30% in the hypothermia group and 70% in the normothermia group (P = .08). The hypothermia group had poorer postoperative behavioral scores than the normothermia group. Prolonged hypothermia was associated with lower oxygen extraction and consumption rates and higher mixed venous oxygen saturation levels during the first hours after hypothermic circulatory arrest. Decreased cardiac index, lower pH, and higher partial pressure of carbon dioxide were observed in the hypothermia group. There was a trend for beneficial effect of prolonged hypothermia in terms of lower brain lactate levels until the 4-hour interval and of intracranial pressure until the 10-hour interval. Postoperatively, total leukocyte and neutrophil counts were lower, and creatine kinase BB was significantly increased in the hypothermia group. At extubation, the hypothermia group had higher oxygen extraction rates and lower brain tissue oxygen tension. Conclusions: A 14-hour period of mild hypothermia after 75-minute hypothermic circulatory arrest seems to be associated with poor outcome. However, the results of this study suggest that mild hypothermia may preserve its efficacy when it is used for no longer than 4 hours, but the potentials of a shorter period of postoperative mild hypothermia still require further investigation.     

Effects of anaesthesia on thermoregulation

Hypothermia is a common and serious complication during anaesthesia and surgery. It mainly results from anaesthetic-induced inhibition of thermoregulatory control and exposure to cold operating room environment. Perioperative hypothermia develops in three distinct phases: (1) anaesthetic-induced vasodilation during induction of anaesthesia results in core-to-peripheral redistribution of body heat and decreases core temperature 1–1.5°C during the first hour of general anaesthesia; (2) subsequently core temperature decreases linearly as heat loss to the environment exceeds metabolic heat production; (3) after 3–5 h of anaesthesia, core temperature often stops decreasing. This core temperature plateau results from reactivation of thermoregulatory vasoconstriction which decreases cutaneous heat loss and constrains metabolic heat to the core thermal compartment. Perioperative hypothermia is associated with numerous complications such as myocardial ischaemia, increased risk of wound infection and coagulopathy. On the other hand temperatures only 1–3°C below normal provide substantial protection against cerebral ischaemia and hypoxaemia in numerous animal species. Consequently, most anaesthesiologists believe mild hypothermia is indicated during operations likely to cause cerebral ischaemia such as carotid endarterectomy and neurosurgery or cardiac procedures. Thermal perturbations, therefore, deserve the same risk/benefit analysis as other medical interventions. Fortunately, effective methods of cooling and warming surgical patients are now available.     

Therapeutic mild hypothermia improves early outcomes in rabbits subjected to traumatic uncontrolled hemorrhagic shock

BackgroundSurvival benefits of mild hypothermia in animals suffering from uncontrolled hemorrhagic shock (HS) may be influenced by trauma severity. We hypothesized that mild hypothermia would improve early outcomes based on our rabbit model of severe traumatic HS.Materials and methodsFifty male New Zealand rabbits weighing between 1.6 and 2.2 kg were randomized into one of the five groups: group 1 (sham), group 2 (37°C/80 mm Hg), group 3 (37°C/40 mm Hg), group 4 (34°C/80 mm Hg), and group 5 (34°C/40 mm Hg). Under urethane anesthesia, animals that suffered fractures and uncontrolled HS received prehospital fluid resuscitation (aggressive or limited) with temperature controlled at normothermia or mild hypothermia, hemostasis, and hospital resuscitation followed by observation.ResultsMild hypothermia significantly improved cardiac systolic function and decreased lung wet-to-dry weight ratios and total injury score compared with normothermia. Group 5 manifested the best results in lung injury. The decreased base excess and pH and increased lactate levels during HS and limited fluid resuscitation were not exacerbated by mild hypothermia. Electrolytes including potassium and calcium and blood glucose levels as well as coagulation were not significantly influenced after mild hypothermia treatment. Seven-hour survival in the hypothermic groups was higher than that in the normothermic groups, although there was no significant difference in survival between groups 5 and 3.ConclusionsTherapeutic mild hypothermia improves early outcomes through improving lung and cardiac performance without causing evident homeostasis disturbances in the rabbit model of traumatic uncontrolled HS. Animals may benefit most under the combination treatment with mild hypothermia and limited fluid resuscitation.     

Myocardial contractile function in survived neonatal piglets after cardiopulmonary bypass

Background  

Hemodynamic function may be depressed in the early postoperative stages after cardiac surgery. The aim of this study was the analysis of the myocardial contractility in neonates after cardiopulmonary bypass (CPB) and mild hypothermia.     

The Effect of Mild Hypothermia, Mannitol and Insulin-Induced Hypoglycaemia on Ischaemic Infarct Volume in the Early Period After Permanent Middle Cerebral Artery Occlusion in the Rat

Summary  We investigated the effect of mild hypothermia (32–34 °C), mannitol and insulin – induced hypoglycaemia on the ischaemic infarct volume on permanent middle cerebral artery occlusion with bilateral carotid artery ligation in rats. Temporalis muscle temperature as an indicator of brain temperature was monitored throughout the experiment in all rats, which were randomly divided into seven groups. During ischaemia, control rats received intravenous saline in a normothermic condition; treated rats had hypothermia and intravenous saline, hypothermia and mannitol, normothermia and mannitol, normothermia and insulin, normothermia, insulin and glucose, and hypothermia and insulin applied. After the experiment, the animals were killed, and brain sections were stained with haematoxylin and eosin. Images of infarct areas were determined using a camera attached to the microscope, and analysed by image analysis software. The total volume of infarcted tissue, right hemispheric volume, and the percentage of infarction were determined at the end of the image analysis investigation.  The infarct volume on the control group was found to be 128.16±6.67 mm³. Infarct volumes in hypothermic groups were significantly smaller than those of the control group (p<0.05). There were no significant differences between infarct volumes in the hypothermic groups. However, we found that hypothermia plus mannitol have the greatest neuro-protective effect. In normothermic rats, the infarct volume decreased proportionally but not statistically (p>0.05) whether mannitol or insulin was given. Our results also demonstrate that pre-, and post-ischaemic serum glucose concentrations influence the volume of infarction. Rats that had had pre-ischaemic high serum glucose concentrations had a higher volume of infarct than the hypothermic rats (p<0.05), while rats with post-ischaemic low serum glucose concentrations had a lower volume of infarct than the control rats.     

Association between intraoperative body temperature and postoperative delirium: A retrospective observational study

Study objectiveThe effect of perioperative body temperature derangement on postoperative delirium remains unclear. This study aimed to evaluate the association between intraoperative body temperature and postoperative delirium in patients having noncardiac surgery.DesignSingle-center retrospective observational study.SettingTertiary university hospital.PatientAdult patients who had major noncardiac surgery under general anesthesia for at least two hours between 2019 and 2021.InterventionsPatients were classified into three groups according to their intraoperative time-weighted average body temperature: severe hypothermia (<35.0 °C), mild hypothermia (35.0 °C–36.0 °C), and normothermia (≥36.0 °C) groups.MeasurementsThe primary outcome was the risk of delirium occurring within seven days after surgery, which was compared using logistic regression analysis. A multivariable procedure was performed adjusting for potential confounders including demographics, history of hypertension, diabetes, atrial fibrillation or flutter, myocardial infarction, congestive heart failure, and stroke or transient ischemic attack, preoperative use of antidepressants and statins, preoperative sodium imbalance, high-risk surgery, emergency surgery, duration of surgery, and red blood cell transfusion. Cox regression analysis was also performed using the same covariates.Main resultsAmong 27,674 patients analyzed, 5.5% experienced postoperative delirium. The incidence rates of delirium were 6.2% (63/388) in the severe hypothermia group, 6.4% (756/11779) in the mild hypothermia group, and 4.6% (712/15507) in the normothermia group. Compared with the normothermia group, the risk of delirium was significantly higher in the severe hypothermia (adjusted odds ratio, 1.43; 95% confidence interval, 1.04–1.97) and mild hypothermia (1.15; 1.02–1.28) groups. The mild hypothermia group also had a significantly increased risk of cumulative development of delirium than the normothermia group (adjusted hazard ratio 1.14; 95% confidence interval, 1.03–1.26).ConclusionsIntraoperative hypothermia (even mild hypothermia) was significantly associated with an increased risk of postoperative delirium.     

Mild hypothermia reduces expression of intercellular adhesion molecule-1 (ICAM-1) and the accumulation of neutrophils after acid-induced lung injury in the rat

BACKGROUND: The pathophysiology of the acute phase of acid-induced lung injury (AILI) has been elucidated. However, once acute respiratory distress syndrome (ARDS) develops, the mortality rate remains high and there is, as yet, no effective therapy. There are reports that application of mild hypothermia is an effective treatment for ARDS. In this study, we hypothesize that mild hypothermia inhibits activation of neutrophils and expression of intercellular adhesion molecule-1 (ICAM-1) in an injured lung. We studied the effects of mild hypothermia on the expression of ICAM-1 and the accumulation of neutrophils after AILI in the rat. METHODS: Male Sprague-Dawley rats were randomly allocated to one of the four groups: control normothermic group, induced mild hypothermia group, acid-instilled normothermic group, and acid-instilled group with mild hypothermia. At 6 h after instillation of acid, lungs were removed to measure neutrophil activity and to detect the expression of ICAM-1 in each group. RESULTS: Oxygenation in acid-instilled rats was significantly impaired as compared to that in non-instilled groups, but induction of mild hypothermia gradually improved oxygenation. Expression of ICAM-1 was enhanced in the acid-instilled normothermic group. By contrast, no overexpression of ICAM-1 and its mRNA was detected in the acid-instilled hypothermic group. In addition, accumulation of neutrophils was markedly inhibited after exposure to mild hypothermia irrespective of the instillation of acid. CONCLUSION: Our data suggest mild hypothermia can inhibit the adhesion, activation, and accumulation of neutrophils during the acute phase of AILI in the rat and may have the potential to reduce ongoing inflammation of ALI or ARDS.     

No additional neuroprotection provided by barbiturate-induced burst suppression under mild hypothermic conditions in rats subjected to reversible focal ischemia

OBJECT: Mild-to-moderate hypothermia is increasingly used for neuroprotection in humans. However, it is unknown whether administration of barbiturate medications in burst-suppressive doses-the gold standard of neuroprotection during neurovascular procedures-provides an additional protective effect under hypothermic conditions. The authors conducted the present study to answer this question. METHODS: Thirty-two Sprague-Dawley rats were subjected to 90 minutes of middle cerebral artery occlusion and randomly assigned to one of four treatment groups: 1) normothermic controls; 2) methohexital treatment (burst suppression); 3) induction of mild hypothermia (33 degrees C); and 4) induction of mild hypothermia plus methohexital treatment (burst suppression). Local cerebral blood flow was continuously monitored using bilateral laser Doppler flowmetry and electroencephalography. Functional deficits were quantified and recorded during daily neurological examinations. Infarct volumes were assessed histologically after 7 days. Methohexital treatment, mild hypothermia, and mild hypothermia plus methohexital treatment reduced infarct volumes by 32%, 71%, and 66%, respectively, compared with normothermic controls. Furthermore, mild hypothermia therapy provided the best functional outcome, which was not improved by additional barbiturate therapy. CONCLUSIONS: The results of this study indicate that barbiturate-induced burst suppression is not required to achieve maximum neuroprotection under mild hypothermic conditions. The magnitude of protection afforded by barbiturates alone appears to be modest compared with that provided by mild hypothermia.     

Consequences of hypothermia

Virtually all anaesthetics render patients poikilothermic and body temperature invariably decreases during surgery. For selected surgical procedures, hypothermia can protect vital organs from ischaemic injury. Hypothermia, however, is not without consequences as hypothermia-related complications are well known. As little as 2°C of core hypothermia impairs coagulation and predisposes to bleeding. Hypothermia slows emergence from general anaesthesia by both pharmacokinetic and pharmacodynamic mechanisms. Thermal discomfort is another commonly recognized perioperative problem. In the postoperative setting, even mild hypothermia exacerbates the stress response by activation of the sympathetic nervous system resulting in increased catecholamines. By this mechanism, hypothermia can precipitate myocardial ischaemia and cardiac morbidity in awake patients. In surgical patients, body temperature should be carefully monitored and controlled with the same level of attention that is given to the other vital signs. By controlling body temperature in the perioperative period, improved outcomes can be achieved.     

Mild induced hypothermia and coagulation and platelet function in patients with septic shock: Secondary outcome of a randomized trial

Coagulation abnormalities and microthrombi contribute to septic shock, but the impact of body temperature regulation on coagulation in patients with sepsis is unknown. We tested the hypothesis that mild induced hypothermia reduces coagulation and platelet aggregation in patients with septic shock. Secondary analysis of randomized controlled trial. Adult patients with septic shock who required mechanical ventilation from eight intensive care units in Denmark were randomly assigned to mild induced hypothermia for 24 h or routine thermal management. Viscoelastography and platelet aggregation were assessed at trial inclusion, after 12 h of thermal management, and 24 h after inclusion. A total of 326 patients were randomized to mild induced hypothermia (n = 163) or routine thermal management (n = 163). Mild induced hypothermia slightly prolonged activated partial thromboplastin time and thrombus initiation time (R time 8.0 min [interquartile range, IQR 6.6–11.1] vs. 7.2 min [IQR 5.8–9.2]; p = .004) and marginally inhibited thrombus propagation (angle 68° [IQR 59–73] vs. 71° [IQR 63–75]; p = .014). The effect was also present after 24 h. Clot strength remained unaffected (MA 71 mm [IQR 66–76] with mild induced hypothermia vs. 72 mm (65–77) with routine thermal management, p = .9). The proportion of patients with hyperfibrinolysis was not affected (0.7% vs. 3.3%; p = .19), but the proportion of patients with no fibrinolysis was high in the mild hypothermia group (8.8% vs. 40.4%; p < .001). The mild induced hypothermia group had lower platelet aggregation: ASPI 85U (IQR 50–113) versus 109U (IQR 74–148, p < .001), ADP 61U (IQR 40–83) versus 79 U (IQR 54–101, p < .001), TRAP 108 (IQR 83–154) versus 119 (IQR 94–146, p = .042) and COL 50U (IQR 34–66) versus 67U (IQR 46–92, p < .001). In patients with septic shock, mild induced hypothermia slightly impaired clot initiation, but did not change clot strength. Platelet aggregation was slightly impaired. The effect of mild induced hypothermia on viscoelastography and platelet aggregation was however not in a range that would have clinical implications. We did observe a substantial reduction in fibrinolysis.     

Mild hypothermia ameliorates lung ischemia reperfusion injury in an ex vivo rat lung model

BACKGROUND: Ischemia reperfusion (I-R) injury of the lung frequently occurs after cardiopulmonary bypass, pulmonary thromboendarterectomy, lung transplantation, and major pulmonary resection with vascular reconstruction. Mild hypothermia ameliorates ischemia reperfusion injury of the brain and the liver. However, the effect of mild hypothermia on I-R injury of the lung has not been investigated. METHODS: The lungs of Lewis rats underwent 80 min of ischemia followed by 60 min of reperfusion in an ex vivo perfusion model. The ambient temperature was maintained at either normothermia (38 degrees C, n=6) or mild hypothermia (35 degrees C, n=6) during the ischemia and reperfusion. RESULTS: Pulmonary shunt fraction, peak inspiratory pressure, mean pulmonary arterial pressure during reperfusion, and the wet/dry weight ratio of the lung tissue at the end of reperfusion in the mild hypothermia group were significantly (p<0.05) lower than those in the normothermia group. Total adenine nucleotide, adenosine triphosphate, adenosine diphosphate, and adenosine monophosphate after reperfusion in the mild hypothermia group were significantly (p<0.05) higher than those in the normothermia group. CONCLUSION: Mild hypothermia attenuates I-R injury of the lung with maintained levels of intrapulmonary high-energy phosphate compounds after reperfusion, suggesting its beneficial effect on warm lung I-R in clinical settings.     

Study of lymphocyte and NK cell activity during mild hypothermia therapy

Infectious disease is a common complication of mild hypothermia therapy. However, very little has been reported about immune response during hypothermia. In the present study, the number and subset of peripheral lymphocytes and mitogen response to phytohaemagglutinin (PHA) and concanavalin A (Con-A) were examined in 14 patients who received mild hypothermia therapy. NK cell ratio and activity were also examined in the same patients. Six out of 14 patients had complicated infectious diseases during mild hypothermia therapy. Five of them had pneumonia and the remaining one had thrombophlebitis. The number of peripheral lymphocytes decreased in patients whose rectal temperature was less than 34.5 degrees C, whereas mitogen response of lymphocytes to PHA and Con-A remained unchanged in patients whose rectal temperature was above 34.0 degrees C. NK cell ratio and cytotoxicity decreased in patients whose rectal temperature was less than 34.5 degrees C, including infectious cases. These results suggested that, under hypothermia therapy, immune responses of the patients whose rectal temperature was less than 34.5 degrees C were disturbed because of the reduced number of peripheral lymphocytes and depression of NK cell activity.     

The antiinflammatory effects of propofol in endotoxemic rats during moderate and mild hypothermia

Purpose We previously found that propofol attenuated the mortality rate and inflammatory responses during endotoxemia in rats; however, whether propofol retains its antiinflammatory effects during hypothermia has not been determined. We investigated the effects of propofol on endotoxemic rats subjected to moderate or mild hypothermia. Methods Male Wistar rats (n = 88) were anesthetized intraperitoneally with pentobarbital sodium and assigned to one of two protocols: one representing moderate hypothermia (30°–32°C) and the other representing mild hypothermia (33°–35°C). Each protocol included four equal-sized groups: group A, Escherichia coli endotoxin (15 mg·kg⁻¹, i.v.) and normothermia; group B, propofol (10 mg·kg⁻¹·h⁻¹, i.v.) and normothermia after endotoxin injection; group C, endotoxin (15 mg·kg⁻¹, i.v.) and hypothermia; and group D, propofol (10 mg·kg⁻¹·h⁻¹, i.v.) and hypothermia after endotoxin injection. Rats then were warmed or cooled to maintain rectal temperatures as above for 6 h. The mortality rate was assessed up to 6 h after endotoxin injection. In addition, we assessed hemodynamics, acid–base status, and plasma cytokine concentrations. Results Endotoxemic rats developed hypotension and metabolic acidosis as well as increased plasma cytokine concentrations. Mortality rates 6 h after endotoxin injection were 70%, 40%, 10%, and 0% for groups A–D, respectively, at moderate hypothermia. Propofol administration to endotoxemic rats with hypothermia, whether moderate or mild, also attenuated the high mortality rate, metabolic acidosis, and elevation of cytokines, but these effects were not superior to those of hypothermia alone. Conclusion During hypothermia, propofol administration does not have additive beneficial antiinflammatory effects.     

Continuous warm blood cardioplegia: a new technique for myocardial protection

Hypothermia is used to prolong the safe period of ischemic arrest by reducing the heart's oxygen demands. Due to this effect, hypothermia has been the fundamental component of most methods of myocardial protection during cardiac surgery. However, hypothermia has a number of unwanted side effects, such as detrimental effects on enzyme function, energy generation, and cell membranes. Since electromechanical arrest accounts for 90% of myocardial oxygen consumption, arresting the heart with chemical cardioplegia will reduce O2 consumption dramatically. Therefore, if the resting (arrested) heart is continuously perfused with oxygenated blood cardioplegia, one can easily provide the remaining 10% of O2 that it requires. Under these conditions, the need for hypothermia becomes questionable. In this paper, we describe the perfusionist's experience using the antegrade and retrograde technique of continuous warm blood cardioplegia.     

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.     

Hypothermia inhibits Fas-mediated apoptosis of primary mouse hepatocytes in culture

Apoptosis occurs during the isolation and even short-term storage and culture of hepatocytes, and in the pathogenesis of liver diseases, such as hepatic failure and hepatitis. Therapeutic hypothermia has beneficial effects in experimental models of fulminant hepatic failure. The mechanisms underlying the potential benefits of mild hypothermia on the liver have not been well investigated. We examined the effects of temperature on soluble Fas ligand-induced apoptosis in freshly isolated mouse hepatocytes. Decreasing the culture temperature from 37 degrees C to 32 degrees C produced significant suppression of Fas-mediated apoptosis in cultured hepatocytes over a 12-h period. This observation was supported by cell morphology, flow cytometry analysis of cellular DNA content, and Annexin V-FITC staining of membrane phosphatidylserine translocation. In hypothermic conditions, Fas-mediated cytochrome c release from mitochondria of hepatocytes and the proximate downstream activation of caspase-9 were suppressed under mild hypothermic conditions. Effector caspase-7 activity was also inhibited at 32 degrees C. In contrast, the activation of initiator caspase-8 and cleavage of Bid were not affected after Fas-ligand stimulation. These findings suggest that mild hypothermia suppresses Fas-mediated apoptosis of liver cells by the partial inhibition of signaling events including mitochondrial damage, cytochrome c release, and subsequent apoptosome formation and effector caspase activation.     

The effects of a temperature below 15°C on the myocardial calcium and ultrastructure in donor heart preservation in a canine model

The effects of 6-h hypothermic cardioplegic arrest on myocardial biochemical, morphologic, and functional recovery were investigated in two groups of dogs. Group 1 (n = 6) was subjected to hypothermia of 15°C and group 2 (n = 6) was subjected hypothermia of 5°C. Although the myocardial calcium (Ca) concentration was significantly higher at the end of reperfusion in group 2 compared to group 1, the MB-fraction of creatine kinase, mitochondrial aspartate aminotransferase, recovery of left ventricular systolic function, and mitochondrial morphologic integrity were better in group 2 than in group 1. These findings suggest that hypothermia of 5°C in 6-h cardioplegia is not necessarily coupled with interference in myocardial contractility, despite the Ca overload that occurs during reperfusion.