Heatstroke Model Research Articles

Activated protein C therapy in a rat heat stroke model*

To evaluate the therapeutic effects of activated protein C in an animal model of heat stroke. Laboratory investigation. Chi-Mei Medical Center research laboratory. Male Sprague-Dawley rats weighing 252-304 g. Anesthetized animals were subjected to heat stress (40 degrees C) to induce heat stroke. A bolus injection of normal saline or recombinant human activated protein C (drotrecogin alfa, activated) was conducted via femoral catheters immediately after the onset of heat stroke. Blood sampling was done before initiation of heat stress and 0 and 40 mins after the onset of heat stroke. When the vehicle-treated rats underwent heat exposure, their survival time values were found to be 56-64 mins (n = 16). Resuscitation with activated protein C significantly and dose-dependently improved survival during heat stroke (108-246 mins for doses of 0.5-20 mg of activated protein C per kilogram of body weight) (n = 32). All heat-stressed animals displayed systemic inflammation and activated coagulation, evidenced by increased tumor necrosis factor-alpha, prothrombin time, activated partial thromboplastin time, and D-dimer and decreased platelet count and protein C. Biochemical markers evidenced by cellular ischemia and injury/dysfunction included increased plasma levels of blood urea nitrogen, creatinine, glutamic oxaloacetic transaminase, glutamic pyruvic transaminase, and alkaline phosphatase; increased striatal levels of glutamate, glycerol, and lactate/pyruvate ratio; and decreased striatal levels of partial pressure of oxygen and local cerebral blood flow, which were all observed during heat stroke. These heat stroke reactions were all significantly suppressed by resuscitation with activated protein C but not vehicle solution. The results indicate that systemic delivery of human recombinant activated protein C at the time point of onset of heat stroke may improve survival by ameliorating systemic inflammation, hypercoagulable state, and tissue ischemia and injury in multiple organs.

Heat shock protein 72 overexpression protects against hyperthermia, circulatory shock, and cerebral ischemia during heatstroke

This study extends our earlier studies in rats by applying our heatstroke model to a new species. Additionally, transgenic mice are used to examine the role of heat shock protein (HSP) 72 in experimental heatstroke. Transgenic mice that were heterozygous for a porcine HSP70i gene ([+]HSP72), transgene-negative littermate controls ([-]HSP72), and normal Institute of Cancer Research strain mice (ICR) under pentobarbital sodium anesthesia were subjected to heat stress (40 degrees C) to induce heatstroke. In [-]HSP72 or ICR, the values for mean arterial pressure, the striatal blood flow, and the striatal PO2 after the onset of heatstroke were significantly lower than those in preheat controls. The core and brain temperatures, the extracellular concentrations of ischemic and injury markers in the striatum, and the striatal neuronal damage scores were significantly greater than those in the preheat controls. In [-]HSP72 or ICR, the body temperatures, cell ischemia content, and injury marker in the striatum were significantly higher, and the mean arterial pressure, striatal blood flow, and striatal PO2 concentration were significantly lower during heatstroke than in [+]HSP72. Accordingly, the latency and the survival times for [+]HSP72 significantly exceeded those of [-]HSP72 or ICR. These results demonstrate that the overexpression of HSP72 in multiple organs improves survival during heatstroke by reducing hyperthermia, circulatory shock, and cerebral ischemia and damage in mice.

The Roles of Exercise-Induced Immune System Disturbances in the Pathology of Heat Stroke

Heat stroke is a life-threatening condition that can be fatal if not appropriately managed. Although heat stroke has been recognised as a medical condition for centuries, a universally accepted definition of heat stroke is lacking and the pathology of heat stroke is not fully understood. Information derived from autopsy reports and the clinical presentation of patients with heat stroke indicates that hyperthermia, septicaemia, central nervous system impairment and cardiovascular failure play important roles in the pathology of heat stroke. The current models of heat stroke advocate that heat stroke is triggered by hyperthermia but is driven by endotoxaemia. Endotoxaemia triggers the systemic inflammatory response, which can lead to systemic coagulation and haemorrhage, necrosis, cell death and multi-organ failure. However, the current heat stroke models cannot fully explain the discrepancies in high core temperature (Tc) as a trigger of heat stroke within and between individuals. Research on the concept of critical Tc as a limitation to endurance exercise implies that a high Tc may function as a signal to trigger the protective mechanisms against heat stroke. Athletes undergoing a period of intense training are subjected to a variety of immune and gastrointestinal (GI) disturbances. The immune disturbances include the suppression of immune cells and their functions, suppression of cell-mediated immunity, translocation of lipopolysaccharide (LPS), suppression of anti-LPS antibodies, increased macrophage activity due to muscle tissue damage, and increased concentration of circulating inflammatory and pyrogenic cytokines. Common symptoms of exercise-induced GI disturbances include diarrhoea, vomiting, gastrointestinal bleeding, and cramps, which may increase gut-related LPS translocation. This article discusses the current evidence that supports the argument that these exercise-induced immune and GI disturbances may contribute to the development of endotoxaemia and heat stroke. When endotoxaemia can be tolerated or prevented, continuing exercise and heat exposure will elevate Tc to a higher level (>42 degrees C), where heat stroke may occur through the direct thermal effects of heat on organ tissues and cells. We also discuss the evidence suggesting that heat stroke may occur through endotoxaemia (heat sepsis), the primary pathway of heat stroke, or hyperthermia, the secondary pathway of heat stroke. The existence of these two pathways of heat stroke and the contribution of exercise-induced immune and GI disturbances in the primary pathway of heat stroke are illustrated in the dual pathway model of heat stroke. This model of heat stroke suggests that prolonged intense exercise suppresses anti-LPS mechanisms, and promotes inflammatory and pyrogenic activities in the pathway of heat stroke.

PLATONIN, A CYANINE PHOTOSENSITIZING DYE, CAUSES ATTENUATION OF CIRCULATORY SHOCK, HYPERCOAGULABLE STATE, AND TISSUE ISCHEMIA DURING HEAT STROKE

The aim of this study was to investigate the therapeutic effect of platonin, a cyanine photosensitizing dye as well as an inhibitor of proinflammatory cytokines, in an animal model of heat stroke. Anesthetized rats, immediately after the onset of heat stroke, were divided into two major groups and given the following: normal saline (1 mL per kg body weight) intravenously, or platonin (12.5-50 microg/mL per kg body weight) intravenously. They were exposed to ambient temperature of 43 degrees C to induce heat stroke. Another group of rats was exposed to room temperature (26 degrees C) and used as normothermic controls. Their physiologic and biochemical parameters were continuously monitored. When the vehicle-treated rats underwent heat exposure, their survival time values were found to be 18 to 22 min. Resuscitation with intravenous doses of platonin, but not normal saline, immediately at the onset of heat stroke, significantly improved survival during heat stroke (41-147 min). All heat-stressed animals displayed systemic inflammation and activated coagulation, evidenced by increased tumor necrosis factor-alpha, prothrombin time, activated partial thromboplastin time, fibrinogen degradation products, and D-dimer, and decreased platelet count and protein C. Biochemical markers evidenced cellular ischemia and injury/dysfunction: plasma levels of blood urea nitrogen, creatinine, glutamic oxaloacetic transaminase, glutamic pyruvic transaminase, and alkaline phosphatase, and striatal levels of partial pressure of oxygen, local cerebral blood flow, glycerol, glutamate, and lactate/pyruvate were all elevated during heat stroke. The systemic inflammation, hypercoagulable state, and cerebral ischemia and injury during heat stroke were all significantly suppressed by platonin. The data demonstrate that platonin therapy may resuscitate heat stroke victims by reducing circulatory shock, systemic inflammation, hypercoagulable state, and tissue ischemia and injury.

Vascular Endothelium Is Severely Perturbed and Undergoes Apoptosis in Experimental Heatstroke in Primates.

Introduction: Ultrastructural evidence of endothelial cell (EC) injury has been associated with diffuse microvascular thrombosis in human heatstroke (HS). In vitro studies have also shown that heat stress accelerates apoptotic cell death. Using a recently described baboon model of heatstroke, we sought to examine pathological changes in the vascular endothelium and whether apoptosis is a mechanism of endothelial cell death.Hypothesis: Major structural vascular endothelium alterations occur in HS and apoptosis is a mechanism of endothelial cell death in HS.Methods: Anesthetized baboons (Papio hamadyras) were heat-stressed in a neonatal incubator maintained at 44 1.5 °C, until rectal temperature attained 42.5°C (moderate heatstroke; n =4) or systolic blood pressure fell to < 90 mm Hg (severe heatstroke n =4). Animals were resuscitated with normal saline and allowed to cool at room temperature. Four sham-heated animals served as control group. Spleen, liver, heart, kidney, gut, lung and adrenal tissue were obtained either by immediate autopsy in non-survivors or after euthanasia at 72-h for survivors. Vascular endothelium ultrastructure was evaluated by transmission electron microscopy (TEM) of ultra-thin tissue sections. Biological activity of EC was determined by light microscopy (LM) using a polyclonal antibody targeting von Willebrand Factor (vWF). Apoptosis was assessed, also in tissue sections, by deoxyuridine triphosphate nick end-labeling (TUNEL) procedure.Results: In heatstroke animals, there were marked EC changes in lungs, spleen, jejunum, kidneys and liver, demonstrated by TEM, as increased cytoplasmic membrane convolutions that included formation of villi projecting into the vessel lumina, and increase in the width of the gaps between ECs. Migration of neutrophils, platelets and erythrocytes through these widened gaps was noted. Weibel-Palade bodies were increased both in size and number in EC of jejunum, lungs and kidneys. This increase correlated with increased endothelial expression of immunologically detectable vWF. TEM also showed that there was increased apoptosis manifested by nuclear chromatin condensation and karyorrhexis and formation of cytoplasmic myelin whorls. Increased EC apoptosis was also observed by TUNEL in the jejunum, lungs, liver and spleen. All these changes were greater in animals with severe HS than in animals with moderate HS, whereas sham heated control animals showed no significant changes.Conclusion: Widespread EC injury with apoptotic cell death is consistent with the hypothesis that the endothelium may play a pathogenic role in heatstroke.

Analysis of the Inflammatory and Hemostatic Changes and Their Relation to Outcomes in an Experimental Model of Heatstroke in Baboons.

The mortality and neurologic morbidity in heatstroke have been attributed to the host inflammatory and hemostatic responses to heat stress, with the corollary that immunomodulating them may improve outcome. We aimed to examine whether an experimental baboon model of heatstroke will reproduce these responses and clinical outcomes similar to that in humans, thereby providing a model for testing new therapeutic strategies. Eight anesthetized juvenile baboons (Papio hamadryas) were subjected to heat stress in an incubator where the environmental temperature was maintained at 44.2±1.5°C until rectal temperature attained 42.5°C (moderate heatstroke; n=4) or systolic arterial pressure fell to < 90 mmHg, (severe heatstroke; n=4). Animals were then allowed to recover at room temperature. Four sham-heated baboons served as a control group. Plasma IL-6, global coagulation tests and molecular markers of coagulation and fibrinolysis as well as endothelial cell activation/injury were determined at baseline, end of heat exposure or onset of heatstroke (T+0), then at T+1, +2, +3, +12, and +36 hours. The rectal temperature at the end of heat stress was 42.5±0.0 and 43.3±0.2°C for moderate and severe heatstroke respectively. All heat stressed animals had systemic inflammation, and activation of coagulation at onset of heatstroke, indicated by increased plasma IL-6 (345±67 vs 280±130 vs 24±15pg/ml, mean ± SD for severe, moderate and control group respectively, ANOVA-repeated measurements; p<0.001) and thrombin-antithrombin complexes (TAT) levels, 48 ±21 vs 13± 5 vs 6.6±0.9 ng/ml; p<0.01). Tissue factor pathway inhibitor (TFPI) levels were significantly elevated and protein C was decreased but this was not statistically significant. Early activation of fibrinolysis was evidenced by significantly increased D-dimer and tissue plasminogen activator (tPA) levels. Plasminogen activator inhibitor (PAI) was undetectable at onset of heatstroke and at anytime thereafter. No evidence of early endothelial cell injury/activation, as assessed by soluble thrombomodulin (TM), was noted. No baboons with severe heatstroke survived. Neurological morbidity, but no mortality was observed in animals with moderate heatstroke. During passive cooling at room temperature (T+0 to T+3hr), non-surviving animals displayed a different pattern, with significantly greater coagulopathy (peak or nadir levels of TAT, TFPI, and Protein C were 493±66 vs 18±8.6 ng/ml, 64±44 vs 7.2±3.2 ng/ml and 23±11 vs 62±8%), inflammatory activity (IL6, 1770±466 vs 320±76 pg/ml) and endothelial injury (TM, 46±16 vs 9.2±3.2ng/ml) than survivors. Sham heated animals had an uneventful course. These data show that heat stress elicits different patterns of inflammatory and hemostatic responses that were associated with outcome. Additionally, the data suggests that experimental baboon model of heatstroke is suitable for testing whether immunomodulation of the host responses can improve outcome.

The role of the gut in the pathogenesis of death due to hyperthermia.

Pathologic data from the gastrointestinal tract in heat-stroke victims, although documented, are confusing. The object of this study was to document the gastrointestinal changes observed during induced total body hyperthermia (42 degrees C) followed by cooling. An established heat-stroke model was used in a university animal laboratory. Group A underwent immersion hyperthermia for 1 hour, followed by cooling to normothermia. Group B underwent hyperthermia to cardiac arrest, followed by resuscitation plus cooling to normothermia. The postmortem findings in the gastrointestinal tract were evaluated. In group A, several hours after return to normothermia and stable vital signs, delayed secondary deterioration with massive gastrointestinal bleeding occurred. The postmortem findings revealed bleeding into the whole intestine and serosanguineous fluid in the peritoneal cavity. In group B, an adynamic gut was observed after 165 +/- 21 minutes (range 125-174) of heating when mean arterial pressure (MAP) decreased to 38 +/- 21 mm Hg (range 30-70). Cardiac arrest occurred at 178 +/- 26 minutes (range 140-208) of immersion. Eight monkeys could be resuscitated to spontaneous circulation with return of normal gut motility, then they rearrested at 158 +/- 68 minutes (range 45-228). The postmortem findings resembled those in group A. The Postmortem findings in the four monkeys in which restoration of spontaneous circulation failed, revealed only some intestinal wall edema and occasional petechial hemorrhages. It is concluded that after a hyperthermic event, tissue injury continues to develop. The pathologic findings are related to the time lapse between hyperthermia, cooling, and death. The similarity to the descriptions of septic shock, multiple organ failure, and the gut reperfusion syndrome is striking. An immunologic response as a mechanism for all these syndromes is discussed.

Hydrodynamic model of heat stroke.

This work presents an hydrodynamical model of heat stroke, which is a physiopathological state of stress, due to an exposure of animals to an ambient temperature of approximatively 40 degrees C during two hours. The evolution of body temperature during this stress process is characterised by three phases. A first phase of increase is followed by a plateau which occurs before a second phase of increase which can be lethal. The model is based on the analogy of a boat progressively caught in a whirlpool. The evolution of the degree of freedom lost by the boat is mathematically analysed and this study leads to the same three phases. The theoretical curves calculated during this study are well in agreement with the experimental curves obtained with animals. This analogy is compared to a previous one which has been made during another experiment with animals constrained by chemical intoxications. It seems that stress can be considered as a vital vorticity and that hydrodynamic models are powerful tools in understanding this physiopathological state.

A miniswine model of heatstroke

We developed a miniswine model of passive heatstroke, in part, to explain the variable hyper-, normo- and hypokalemia seen in heatstroke victims. After a baseline period ( T amb=26–27°C), anesthetized and instrumented miniswine ( n=13, mass=44.6 kg) were ramped to 41–43°C, 60% RH; 13 controls were treated identically, but T re was maintained at 38°C. T re of the experimental miniswine rose nearly linearly to 45–46°C until death (approx. 4 h). The response patterns of mean arterial pressure, heart rate, plasma K +, LPS, Ca 2+, inorganic phosphate, lactate and a variety of other clinical chemical and physiological variables were determined. An explanation for the variability of plasma K + in heatstroke victims was proposed. This model may be useful in characterizing the multisystemic pathology of severe heat injury and be useful for assessing innovative therapeutic regimens.

Explosive increase in Na+ entry to acidified cells at elevated temperature. Evidence for the energy depletion model of heat stroke?

Explosive increase in Na+ entry to acidified cells at elevated temperature. Evidence for the energy depletion model of heat stroke?

Evaporation versus iced peritoneal lavage treatment of heatstroke: Comparative efficacy in a canine model

The authors compared the speed of cooling and treatment efficacy for evaporative cooling versus iced peritoneal lavage in a canine heatstroke model. Nine random-source, mongrel dogs were anesthetized, shaved, and internally heated until the core temperature reached 43.0°C. The animals were then randomly assigned to be cooled to 37°C either by sterile normal saline (6°C) continuous peritoneal lavage at 250 mL/min (n = 4), or by spraying with tap water (15°C, 12 L/min) before a large fan blowing room temperature air (23°C) across the dog at 0.5 m/sec from a height of 50 cm (n = 5). Temperatures were monitored by thermocouples in both tympanic membranes. Electrocardiogram, blood pressure, and pulse were continuously monitored. Evaporative cooling was as rapid as iced peritoneal lavage (0.18 ± .03 versus 0.17 ± .07°C/min/m 2, P = NS). All animals survived, although one animal in each treatment group demonstrated a moderate neurologic deficit when measured 48 hours following resuscitation. A simple nonivasive evaporative cooling technique, readlly available in the emergency department, appears to be as rapid and effective as aggressive peritoneal lavage for cooling and treating heatstroke in the dog.

Effect of hyperthermia on somatosensory evoked potentials in the anaesthetized rat

In the rat heat stroke model, established by heating to a climatic chamber temperature of 42°C, the brain temperature was found to be consistently lower than the rectal temperature, suggesting efficient brain cooling mechanisms in the rat. In response to heating, with increasing brain temperature, the latencies of the somatosensory evoked potentials (SEPs) showed an initial decrease followed by an increase (inflection point). Studies were done on rats heated up to, before, or after the inflection point and then cooled. Reversibility with cooling of functional and structural changes induced by heat was evaluated by analysis of SEPs, survival time, brain blood perfusion and histopathology. The evidence from these studies demonstrated that the brain temperature at which the inflection in wave P 2 latency occurred was critical, beyond which hyperthermia produced irreversible changes in the SEP, shorter survival time, relative reduction in brain blood perfusion and evidence of brain histopathological damage. The suggestion that endorphins may mediate brain dysfunction in hyperthermia was investigated. In rats heated and then cooled after wave P 2 latency inflection naloxone, the endorphin antagonist, was injected (10 mg/kg, intravenously) just prior to the inflection. It produced reversibility of SEP changes as well as longer survival time ( P<0.001) compared to saline-treated rats.

Investigation on the hemodynamic alterations and their mechanism during heat stroke under hot environment.

Heat stroke model was made under artificial hot environment in 26 mongrel dogs, and hemodynamic alterations and their mechanism were investigated. When the rectal temperature rose above 41°C the stroke volume (SV) and cardiac output (CO) significantly decreased. The central venous pressure (CVP) declined at first, but increased later and rose above the control value at 43°C. The mean arterial pressure (MAP) began to decrease at 41°C, and decreased by 25.7% (P < 0.01) at 42.5°C comparison with the control value. MAP dropped dramatically (68.4 %) at 43°C. Besides, the serum concentration of potassium increased by 6.1±1.6 mg (P < 0.01) in comparison with the control value. Naloxone (0.1–0.3 mg/kg) was injected i.v. at 40. 5°C of rectal temperature. SV and CO increased markedly at 41°C and 42 °C. CVP declined significantly, and MAP showed elevation at 41°C. These findings suggested that the circulatory failure during heat stroke might be related to the increased β-endorphin release from the anterior pituitary gland and the increase of serum potassium concentration.

Iced gastric lavage for treatment of heatstroke: Efficacy in a canine model

This study was undertaken to evaluate a central cooling technique for heatstroke, gastric lavage with iced tap water, for speed of cooling and efficacy of treatment in an anesthetized canine heatstroke model. Eleven mongrel dogs were anesthetized, shaved, and externally heated until core temperature reached 43.0 C (109.4 F). Control animals (n = 6) were cooled passively in room air. Treatment animals (n = 5) were cooled with the addition of gastric lavage with iced tap water through a large-bore orogastric tube. Temperatures were monitored using thermocouples in the brain, pulmonary artery, rectum, and subcutaneous tissue of the chest wall. Serial enzymatic and hematologic laboratory studies and hemodynamic parameters were measured prior to heating, at the end of heating, and at 0, 0.5, 1, 2, and 12 hours after heating. Animals were sacrificed 12 hours after heatstroke induction and gross and microscopic pathologic examination was performed. Central cooling rates achieved in the lavage group were five to six times faster than in the control group, (P less than .01) and are comparable with those reported in the literature for external cooling techniques. No significant complications were induced by tap water lavage. Rapid cooling in the lavage group resulted in a statistically significant improvement in serum creatinine and LDH after heatstroke induction (P less than .05). Gastric lavage with iced tap water appears to be an effective alternative or adjunctive cooling technique for heatstroke in the dog.

Diagnostic significance of selected serum enzymes in a rat heatstroke model

A total of 171 untrained, unacclimatized, and unanesthetized rats were either exercised to exhaustion at one of four ambient temperatures (5, 20, 26, or 30 degrees C), or were restrained and heated at an ambient temperature of 41.5 degrees C until their core temperatures reached a preselected end point between 41.0 and 43.3 degrees C. The serum levels of creatine phosphokinase (CRK) and two transaminases (SGOT and SGPT) were determined at 30 min, 24, 48, 72, and 96 h posttreatment. Peak enzyme activity for CPK was noted primarily at the 30-min sampling period and at 24 h for the transaminases. The data indicated that under these conditions a) the transaminase SGOT was elevated in the serum as a consequence of the extent and duration of prior hyperthermia, b) the transaminase SGOT was released in moderate amounts after exhaustive exercise but reached its greatest activity levels following hyperthermia, and c) the activity of CPK was increased by the duration of exhaustive exercise and was less sensitive than either transaminase to prior hyperthermia. As a result, each of the three experimental conditions: a) exercise without hyperthermia, b) exercise with hyperthermia, and c) sedentary hyperthermia, produced a unique pattern of serum enzyme activity that would appear useful in diagnosing a variety of heat- and/or work-induced disorders.

An anesthetized dog heatstroke model.

This study was undertaken to develop an anesthetized dog heatstroke model. Forty-six animals were anesthetized with pentobarbital sodium (25 mg/kg) intravenously, and maintained at an ambient temperature of (42-46 degrees C) with a water-heated blanket over 2.5-3.0 h until rectal temperatures rose to 43.0-44.5 degrees C. Animals then cooled passively until death occurred or until 18 h elapsed, and were prepared for autopsy. Liver, kidney, and brain temperature, mean weighted skin temperature, mean weighted surface heat loss, and metabolic rates were obtained. There were no significant differences between liver, kidney, brain, and rectal temperatures during the heating and cooling periods. Cardiac output rose to 127% of initial value, and dropped rapidly to zero at 43.4 degrees C rectal temperature. The rapid decline was accompanied by a doubling of heart rate and a rapid drop in blood pressure and respiratory rate. Cheyne-Stokes respiration and apnea preceded bradycardia followed by asystole or ventricular fibrillation. Certain serum constituents demonstrated modest elevations suggestive of widespread tissue damage. Autopsy did not reveal a clear pattern of heat injury, with the exception of consistent congestion of the major organs and karyorrhexis of lymphocytes. These data are in agreement with similar data from human heatstroke victims and other heatstroke modeling in dogs, and support the concept that the anesthetized dog can in many respects provide an adequate model for human heatstroke.