Clinical characteristics, prognostic factors, and outcomes of heat-related illness (Heatstroke Study 2017-2018)

Heat stress-induced platelet dysfunction is associated with loss of fibrinogen and is improved by fibrinogen supplementation

INTRODUCTION

Heatstroke is a critical heat-related condition characterized by coagulopathy and multiple organ dysfunction. One of the most severe complications of heatstroke is disseminated intravascular coagulation. This condition manifests as excessive clot formation and bleeding that are primarily due to platelet depletion and dysfunction. Fibrinogen plays a crucial role in hemostasis because it links integrin αIIbβ3 on adjacent platelets, thereby promoting the platelet activation and aggregation necessary for clot formation. However, reduced fibrinogen levels may impair the formation of the initial platelet plug and increase the risk of bleeding. The current study explored the effect of fibrinogen on platelet dysfunction in a heatstroke model.

MATERIALS AND METHODS

Male Wistar rats were subjected to heat stress, and subsequent changes in hemodynamic, biochemical, and coagulation parameters were analyzed. Platelet viability, aggregation, adhesion, spreading and fibrin clot retraction were assessed.

RESULTS

The rats with heatstroke exhibited a variety of clinical symptoms, including hypotension, tachycardia, multiple organ dysfunction, and coagulopathy. Platelet viability in the heatstroke group was comparable to that in the healthy control group. However, the heatstroke group exhibited significant reductions in plasma fibrinogen levels and platelet aggregation, adhesion, spreading, and fibrin clot retraction. Notably, fibrinogen supplementation markedly augmented the aggregation responses of platelets in the heatstroke group. The impairment of platelet adhesion, spreading, and fibrin clot retraction in the rats with heatstroke was partially ameliorated by fibrinogen supplementation.

CONCLUSIONS

An early use of fibrinogen replacement may serve as a therapeutic intervention to alleviate platelet hyporeactivity and prevent the complications in patients with heatstroke.

Prediction of in-hospital mortality in patients with exertional heatstroke: a 13-year retrospective study

Hyperactivity of coagulation is common in exertional heatstroke (EHS). Disseminated intravascular coagulation (DIC) is the most severe form of coagulation dysfunction and associated with poor outcome. DIC, temperature and Glasgow coma scale score were identified as independent risk factors for in-hospital mortality by multivariate logistic regression analysis, and we developed a nomogram for predicting in-hospital mortality in a 13-year EHS patient cohort. The nomogram was assessed by calibration curves and bootstrap with 1,000 resamples. The receiver operating characteristic curve was constructed, and the area under the curve (AUC) was compared. Two hundred and ten patients were included. The in-hospital mortality was 9.0%, and the incidence of DIC was 17.6%. The AUC of the nomogram was 0.897 (95% CI 0.848-0.935, p < .0001) and was non-inferior to SOFA and APACHE II scores but superior to SIRS score, which were widely-used score systems of disease severity. The nomogram contributed to the adverse outcome prediction of EHS.

Thromboinflammation and microcirculation damage in heatstroke

Rising temperatures associated with climate change have significantly increased the risk of heatstroke. Unfortunately, the trend is anticipated to persist and increasingly threaten vulnerable populations, particularly older adults. According to Japan's environment ministry, over 1000 people died from heatstroke in 2021, and 86% of deaths occurred in those above 65. Since the precise mechanism of heatstroke is not fully understood, we examined the pathophysiology by focusing on the microcirculatory derangement. Online search of published medical literature through MEDLINE and Web of Science using the term "heatstroke," "heat-related illness," "inflammation," "thrombosis," "coagulation," "fibrinolysis," "endothelial cell," and "circulation." Articles were chosen for inclusion based on their relevance to heatstroke, inflammation, and thrombosis. Reference lists were reviewed to identify additional relevant articles. Other than preexisting conditions (genetic background, age, etc.), factors such as hydration status, acclimatization, dysregulated coagulation, and inflammation are the additional major factors that promote tissue malcirculation in heatstroke. The fundamental pathophysiologic mechanisms significantly overlap with those seen in the systemic inflammatory response to sepsis, and as a result, coagulation-predominant coagulopathy develops during heat stress. Although a bleeding tendency is not common, bleeding frequently occurs in the microcirculation, causing additional injury. Sterile inflammation is mediated by proinflammatory cytokines, chemokines, and other humoral mediators in concert with cellular factors, including monocytes, neutrophils, platelets, and endothelial cells. Excess inflammation results in inflammatory cell death, including pyroptosis and necroptosis, and the release of danger signals that further propagate systemic inflammation and coagulopathy. Consequently, thromboinflammation is the critical factor that induces microcirculatory disturbance in heatstroke.

Lactate combined with SOFA score for improving the predictive efficacy of SOFA score in patients with severe heatstroke

BACKGROUND

The relationship between lactate levels and multiple organ dysfunction in patients with severe heatstroke remains unclear. In this study, we aimed to elucidate the clinical significance of lactate in severe heatstroke prognosis and assess whether incorporating lactate in the SOFA score improves its predictive efficacy.

METHODS

This study was a multicenter retrospective cohort investigation included 275 patients. Logistic regression analysis was performed to examine the relationship between lactate levels and patient outcomes and complications, including acute kidney injury (AKI), disseminated intravascular coagulation (DIC), and myocardial injury. Further, receiver operating characteristic (ROC) curves and clinical decision curve analysis (DCA) were used to evaluate the predictive power of lactate and SOFA scores in severe heatstroke-associated death. Lastly, the Kaplan-Meier survival curve was employed to differentiate the survival rates among the various patient groups.

RESULTS

After adjusting for confounding factors, lactate was demonstrated as an independent risk factor for death (OR = 1.353, 95% CI [1.170, 1.569]), AKI (OR = 1.158, 95% CI [1.007, 1.332]), DIC (OR = 1.426, 95% CI [1.225, 1.659]), and myocardial injury (OR = 2.039, 95% CI [1.553, 2.679]). The area under the curve (AUC) of lactate for predicting death from severe heatstroke was 0.7540, with a cutoff of 3.35. The Kaplan-Meier survival curve analysis showed that patients with elevated lactate levels had higher mortality rates. Additionally, the ROC curves demonstrated that combining lactate with the SOFA score provided better predictive efficacy than the SOFA score alone in patients with severe heatstroke (AUC: 0.9025 vs. 0.8773, DeLong test, P < 0.001). Finally, the DCA curve revealed a higher net clinical benefit rate for lactate combined with the SOFA score.

CONCLUSIONS

Lactate is an independent risk factor for severe heatstroke-related death as well as a risk factor for AKI, DIC, and myocardial injury associated with severe heatstroke. Thus, combining lactate with the SOFA score can significantly improve its predictive efficacy in patients with severe heatstroke.

A newly proposed heatstroke-induced coagulopathy score in patients with heat illness: A multicenter retrospective study in China

PURPOSE

In patients with heatstroke, disseminated intravascular coagulation (DIC) is associated with greater risk of in-hospital mortality. However, time-consuming assays or a complex diagnostic system may delay immediate treatment. Therefore, the present study proposes a new heatstroke-induced coagulopathy (HIC) score in patients with heat illness as an early warning indicator for DIC.

METHODS

This retrospective study enrolled patients with heat illness in 24 Chinese hospitals from March 2021 to May 2022. Patients under 18 years old, with a congenital clotting disorder or liver disease, or using anticoagulants were excluded. Data were collected on demographic characteristics, routine blood tests, conventional coagulation assays and biochemical indexes. The risk factors related to coagulation function in heatstroke were identified by regression analysis, and used to construct a scoring system for HIC. The data of patients who met the diagnostic criteria for HIC and International Society on Thrombosis and Haemostasis defined-DIC were analyzed. All statistical analyses were performed using SPSS 26.0.

RESULTS

The final analysis included 302 patients with heat illness, of whom 131 (43.4%) suffered from heatstroke, including 7 death (5.3%). Core temperature (OR = 1.681, 95% CI 1.291 - 2.189, p < 0.001), prothrombin time (OR = 1.427, 95% CI 1.175 - 1.733, p < 0.001) and D-dimer (OR = 1.242, 95% CI 1.049 - 1.471, p = 0.012) were independent risk factors for heatstroke, and therefore used to construct an HIC scoring system because of their close relation with abnormal coagulation. A total score ≥ 3 indicated HIC, and HIC scores correlated with the score for International Society of Thrombosis and Hemostasis -DIC (r = 0.8848, p < 0.001). The incidence of HIC (27.5%) was higher than that of DIC (11.2%) in all of 131 heatstroke patients. Meanwhile, the mortality rate of HIC (19.4%) was lower than that of DIC (46.7%). When HIC developed into DIC, parameters of coagulation dysfunction changed significantly: platelet count decreased, D-dimer level rose, and prothrombin time and activated partial thromboplastin time prolonged (p < 0.05).

CONCLUSIONS

The newly proposed HIC score may provide a valuable tool for early detection of HIC and prompt initiation of treatment.

Wilderness Medical Society Clinical Practice Guidelines for the Prevention and Treatment of Heat Illness: 2024 Update

The Wilderness Medical Society (WMS) convened an expert panel in 2011 to develop a set of evidence-based guidelines for the recognition, prevention, and treatment of heat illness. The current panel retained 5 original members and welcomed 2 new members, all of whom collaborated remotely to provide an updated review of the classifications, pathophysiology, evidence-based guidelines for planning and preventive measures, and recommendations for field- and hospital-based therapeutic management of heat illness. These recommendations are graded based on the quality of supporting evidence and the balance between the benefits and risks or burdens for each modality. This is an updated version of the WMS clinical practice guidelines for the prevention and treatment of heat illness published in Wilderness & Environmental Medicine. 2019;30(4):S33-S46.

Construction and Validation of the Nomogram Based on von Willebrand Factor Predicting Mortality in Patients with Heatstroke

Heatstroke (HS), a severe condition, can develop multiple organ dysfunction syndrome and death. However, at present, no early reliable index exists for risk stratification and prognosis. von Willebrand factor (vWF), a marker of vascular endothelial injury, is a key regulatory target of inflammation and coagulation, which is closely associated with the pathogenesis of HS. vWF was reported as a prognostic marker in several infectious and noninfectious severe illness such as COVID-19, sepsis, and trauma. Although early increased level of vWF is seen in HS, the relationship between vWF and mortality is to be elucidated. Clinical data of patients with HS in a tertiary hospital were recorded and analyzed. It was shown that plasma vWF concentrations at admission were significantly increased in the nonsurvivors (351% ± 105%) compared with survivors (278% ± 104%, p = 0.021). After multivariate logistic regression analysis it was shown that vWF (odds ratio [OR] = 1.010; 95% confidence interval [CI], 1.002-1.18; p = 0.017), hemoglobin (Hb) (OR = 0.954; 95% CI, 0.931-0.979; p < 0.001), and hematocrit (HCT) in blood (OR = 0.859; 95% CI, 0.790-0.934; p < 0.001) were independent factors of in-hospital mortality in HS. The nomogram based on vWF and Hb was constructed in patients with HS. The area under curve under the receiver operating characteristic of this prediction model was 0.860 (95% CI, 0.773-0.923) and cutoff was 0.15, with Youden index 0.5840, which were not significantly different to sequential organ failure assessment (p = 0.0644), Acute Physiology and Chronic Health Evaluation II (APACHE II) (p = 0.7976), and systemic inflammatory response syndrome (SIRS) scores (p = 0.3274). The prediction model that integrated vWF and Hb showed a better predicting efficiency than single variable, and a higher specificity (81.48%) than APACHE II (72.84%) and SIRS (72.84%) scores. In summary, vWF, as an independent risk factor for in-hospital mortality, combined with Hb, could effectively prognosis the mortality in HS patients at early stage.

Image Improved Intravoxel Incoherent Motion MRI With Optimized Trigger Delays Based on Strain Curve Analysis to Evaluate Myocardial Microvascular Dysfunction of Exertional Heat Illness

BACKGROUND

Intravoxel incoherent motion (IVIM) MRI has not been widely used and its role in evaluating exertional heat illness (EHI)-related myocardial involvement remains unknown.

PURPOSE

To investigate the feasibility of strain curve-derived trigger delay (TD) IVIM-MRI and its role in assessing myocardial diffusion and microvascular perfusion of EHI patients.

STUDY TYPE

Prospective.

SUBJECTS

A total of 42 male EHI patients (median age: 21 years) and 22 age- and sex-matched healthy controls (HC).

FIELD STRENGTH/SEQUENCE

A 3-T, diffusion-weighted spin-echo echo-planar-imaging sequence.

ASSESSMENT

IVIM-MRI was acquired by conventional TD method (group A) or strain curve-based TD method (group B) in random order. IVIM image quality was evaluated on a 3-point Likert scale (1, nondiagnostic; 2, moderate; 3, good). Technical success was defined as image quality score = 3. IVIM-MRI-derived parameters (pseudo diffusion in the capillaries [D*], perfusion fraction [f], and slow apparent diffusion coefficient [D]) were compared between EHI and HC.

STATISTICAL TESTS

Student's t-tests, chi-square tests, one-way analysis of variance, receiver operating characteristic (ROC) curve analysis, Pearson's correlation coefficient (r). The statistical significance level was set at P < 0.05.

RESULTS

IVIM-MRI image quality score (median [interquartile range]: 3 [2, 3] vs. 2 [1-3]) and technical success rate (61.9%[13/21] vs. 28.6%[6/21]) were significantly improved in group B. EHI patients showed significantly decreased D* (118.1 ± 23.3 × 10-3  mm2 /sec vs. 142.7 ± 42.6 × 10-3  mm2 /sec) and f values (0.42 ± 0.12 vs. 0.51 ± 0.11) and significantly higher D values (3.0 ± 0.9 × 10-3  mm2 /sec vs. 2.5 ± 0.6 × 10-3  mm2 /sec) compared to HC. Relative to D and D*, f showed the most robust efficacy for detecting EHI-related myocardial injury with the highest area under the ROC curve (0.906: 95% confidence interval, 0.799, 0.967) and sensitivity of 88.5% and specificity of 85.6%.

CONCLUSION

The strain curve-based TD method significantly improved image quality and technical success rate of IVIM-MRI, and f value may be an effective biomarker to assess myocardial microcirculation abnormalities of EHI patients.

EVIDENCE LEVEL

2.

TECHNICAL EFFICACY

Stage 3.

Development and validation of a prognostic model of survival for classic heatstroke patients: a multicenter study

Classic heatstroke (CHS) is a life-threatening illness characterized by extreme hyperthermia, dysfunction of the central nervous system and multiorgan failure. Accurate predictive models are useful in the treatment decision-making process and risk stratification. This study was to develop and externally validate a prediction model of survival for hospitalized patients with CHS. In this retrospective study, we enrolled patients with CHS who were hospitalized from June 2022 to September 2022 at 3 hospitals in Southwest Sichuan (training cohort) and 1 hospital in Central Sichuan (external validation cohort). Prognostic factors were identified utilizing least absolute shrinkage and selection operator (LASSO) regression analysis and multivariate Cox regression analysis in the training cohort. A predictive model was developed based on identified prognostic factors, and a nomogram was built for visualization. The areas under the receiver operator characteristic (ROC) curves (AUCs) and the calibration curve were utilized to assess the prognostic performance of the model in both the training and external validation cohorts. The Kaplan‒Meier method was used to calculate survival rates. A total of 225 patients (median age, 74 [68-80] years) were included. Social isolation, self-care ability, comorbidities, body temperature, heart rate, Glasgow Coma Scale (GCS), procalcitonin (PCT), aspartate aminotransferase (AST) and diarrhea were found to have a significant or near-significant association with worse prognosis among hospitalized CHS patients. The AUCs of the model in the training and validation cohorts were 0.994 (95% [CI], 0.975-0.999) and 0.901 (95% [CI], 0.769-0.968), respectively. The model's prediction and actual observation demonstrated strong concordance on the calibration curve regarding 7-day survival probability. According to K‒M survival plots, there were significant differences in survival between the low-risk and high-risk groups in the training and external validation cohorts. We designed and externally validated a prognostic prediction model for CHS. This model has promising predictive performance and could be applied in clinical practice for managing patients with CHS.

Holistic view of heat acclimation alleviated intestinal lesion in mice with heat stroke based on microbiome-metabolomics analysis

The severity of heat stroke (HS) is associated with intestinal injury, which is generally considered an essential issue for HS. Heat acclimation (HA) is considered the best strategy to protect against HS. In addition, HA has a protective effect on intestinal injuries caused by HS. Considering the essential role of gut microbes in intestinal structure and function, we decided to investigate the potential protective mechanism of HA in reducing intestinal injury caused by HS. HA model was established by male C57BL/6J mice (5-6 weeks old, 17-19 g) were exposed at (34 ± 0.7)°C for 4 weeks to establish an animal HA model. The protective effect of HA on intestinal barrier injury in HS was investigated by 16S rRNA gene sequencing and nontargeted liquid chromatography-mass spectrometry (LC-MS) metabolomics. According to the experimental results, HA can change the composition of the gut microbiota, which increases the proportion of lactobacilli, faecal bacteria, and urinobacteria but decreases the proportion of deoxycholic acid. Moreover, HA can reduce liver and kidney injury and systemic inflammation caused by HS and reduce intestinal injury by enhancing the integrity of the intestinal barrier. In addition, HA regulates inflammation by inhibiting NF-κB signalling and increasing tight junction protein expression in HS mice. HA induces changes in the gut microbiota, which may enhance tight junction protein expression, thereby reducing intestinal inflammation, promoting bile acid metabolism, and ultimately maintaining the integrity of the intestinal barrier. In conclusion, HA induced changes in the gut microbiota. Among the gut microbiota, lactobacilli may play a key role in the potential protective mechanism of HA.

The role of platelets in heat-related illness and heat-induced coagulopathy

Heat-related illness is becoming more problematic due to ongoing global warming. Heat-related injury causes systemic inflammation and coagulopathy, due to leukocyte, platelet, and vascular endothelial cell activation and injury. Hyperthermia directly modulates platelet function and can induce cellular damage. Meanwhile, heat also affects platelet function via activated coagulation, excess inflammation, production of cytokines, and heat shock proteins. Aberrant hyperthermia-induced interactions between leukocytes and endothelial cells are also involved in platelet regulation. Heat-induced coagulopathy commonly progresses to disseminated intravascular coagulation (DIC), leading to multiple organ failure and in some cases enhanced bleeding. Consequently, platelet count, prothrombin time, and DIC score are useful for evaluating the severity of heat-related illness in addition to other organ damage markers such as Glasgow Coma Scale, creatinine, and bilirubin. Despite the increasing risk, therapeutic modalities targeting platelets are limited and no established therapy exists. In this review, we summarize the current knowledge about the role of platelets in the pathogenesis, diagnosis, and management of heat-related illness.

Inflammation, coagulation, and cellular injury in heat-induced shock

BACKGROUND

The number of heatstroke victims hit record numbers in 2022 as global warming continues. In heat-induced injuries, circulatory shock is the most severe and deadly complication. This review aims to examine the mechanisms and potential approaches to heat-induced shock and the life-threatening complications of heatstroke.

METHODS

A computer-based online search was performed using the PubMed database and Web of Science database for published articles concerning heatstroke, shock, inflammation, coagulopathy, endothelial cell, cell death, and heat shock proteins.

RESULTS

Dehydration and heat-induced cardiomyopathy were reported as the major causes of heat-induced shock, although other heat-induced injuries are also involved in the pathogenesis of circulatory shock. In addition to dehydration, the blood volume decreases considerably due to the increased vascular permeability as a consequence of endothelial damage. Systemic inflammation is induced by factors that include elevated cytokine and chemokine levels, dysregulated coagulation/fibrinolytic responses, and the release of damage-associated molecular patterns (DAMPs) from necrotic cell death that cause distributive shock. The cytoprotective heat shock proteins can also facilitate circulatory disturbance under excess heat stress.

CONCLUSIONS

Multiple mechanisms are involved in the pathogenesis of heat-induced shock. In addition to dehydration, heat stress-induced cardiomyopathy due to the thermal damage of mitochondria, upregulated inflammation via damage-associated molecular patterns released from oncotic cells, unbalanced coagulation/fibrinolysis, and endothelial damage are the major factors that are related to circulatory shock.

Current status of active cooling, deep body temperature measurement, and face mask wearing in heat stroke and heat exhaustion patients in Japan: a nationwide observational study based on the Heatstroke STUDY 2020 and 2021

Aim

The study aimed to determine the current status of face mask use, deep body temperature measurement, and active cooling in patients suffering from heat stroke and heat exhaustion in Japan.

Methods

This was a prospective, observational, multicenter study using data from the Heatstroke STUDY 2020-2021, a nationwide periodical registry of heat stroke and heat exhaustion patients. Based on the Bouchama heatstroke criteria, we classified the patients into two groups: severe and mild-to-moderate. We compared the outcomes between the two groups and reclassified them into two subgroups according to the severity of the illness, deep body temperature measurements, and face mask use. Cramer's V was used to determine the effect sizes for a comparison between groups.

Results

Almost all patients in this study were categorized as having degree III based on the Japanese Association for Acute Medicine heatstroke criteria (JAAM-HS). However, the severe group was significantly worse than the mild-to-moderate group in outcomes like in-hospital death and modified Rankin Scale scores, when discharged. Heat strokes had significantly higher rates of active cooling and lower mortality rates than heat stroke-like illnesses. Patients using face masks often use them during labor, sports, and other exertions, had less severe conditions, and were less likely to be young male individuals.

Conclusions

It is suggested that severe cases require a more detailed classification of degree III in the JAAM-HS criteria, and not measuring deep body temperature could have been a factor in the nonperformance of active cooling and worse outcomes.

Estimation of the number of heat illness patients in eight metropolitan prefectures of Japan: Correlation with ambient temperature and computed thermophysiological responses

The number of patients with heat illness transported by ambulance has been gradually increasing due to global warming. In intense heat waves, it is crucial to accurately estimate the number of cases with heat illness for management of medical resources. Ambient temperature is an essential factor with respect to the number of patients with heat illness, although thermophysiological response is a more relevant factor with respect to causing symptoms. In this study, we computed daily maximum core temperature increase and daily total amount of sweating in a test subject using a large-scale, integrated computational method considering the time course of actual ambient conditions as input. The correlation between the number of transported people and their thermophysiological temperature is evaluated in addition to conventional ambient temperature. With the exception of one prefecture, which features a different Köppen climate classification, the number of transported people in the remaining prefectures, with a Köppen climate classification of Cfa, are well estimated using either ambient temperature or computed core temperature increase and daily amount of sweating. For estimation using ambient temperature, an additional two parameters were needed to obtain comparable accuracy. Even using ambient temperature, the number of transported people can be estimated if the parameters are carefully chosen. This finding is practically useful for the management of ambulance allocation on hot days as well as public enlightenment.

Disseminated Intravascular Coagulation: The Past, Present, and Future Considerations

Disseminated intravascular coagulation (DIC) has been understood as a consumptive coagulopathy. However, impaired hemostasis is a component of DIC that occurs in a progressive manner. The critical concept of DIC is systemic activation of coagulation with vascular endothelial damage. DIC is the dynamic coagulation/fibrinolysis disorder that can proceed from compensated to decompensated phases, and is not simply impaired hemostasis, a misunderstanding that continues to evoke confusion among clinicians. DIC is a critical step of disease progression that is important to monitor over time. Impaired microcirculation and subsequent organ failure due to pathologic microthrombi formation are the pathophysiologies in sepsis-associated DIC. Impaired hemostasis due to coagulation factor depletion from hemodilution, shock, and hyperfibrinolysis occurs in trauma-associated DIC. Overt-DIC diagnostic criteria have been used clinically for more than 20 years but may not be adequate to detect the compensated phase of DIC, and due to different underlying causes, there is no "one-size-fits-all criteria." Individualized criteria for heterogeneous conditions continue to be proposed to facilitate the diagnosis. We believe that future research will provide therapeutics using new diagnostic criteria. Finally, DIC is also classified as either acute or chronic, and acute DIC results from progressive coagulation activation over a short time and requires urgent management. In this review, we examine the advances in research for DIC.

Morphological Changes in Blood Cells in a Rat Model of Heatstroke: A Pilot Study

Despite the increasing threat of heatstroke with global warming, pathophysiologic injury continues to be defined. In addition, morphological changes of the peripheral blood cells in heatstroke have not been well characterized. We evaluated pathophysiologic changes in bone marrow and blood cells in a rat heatstroke model using a 39.5 °C climate chamber. After three hours of incubation, blood and bone marrow samples were collected for morphology, and the direct effects of heat on leukocytes in vitro were evaluated using time-lapse observation. The blood cell count and peripheral/bone marrow smear were examined either in a lethal model (core body temperature exceeded 42.5 °C) or in a sublethal model (<41.5 °C). Significant decreases in platelet and white blood counts occurred in the lethal model (>35% and >20% decreases, respectively) and changes were less in the sublethal model. Platelet clumping with the appearance of large platelets was observed. The neutrophils often demonstrated hyper-segmented nuclei, and lymphocytes showed reactive or blast-like changes. Further, the direct effect of heat on leukocytes noted apoptotic cell death at 41.5 °C, but subsequent necrosis at 43 °C. In summary, our rodent model showed that heatstroke causes platelet aggregation, leukocyte injury, and aponecrotic cell death. Such changes were milder and reversible in sublethal heatstroke. The appearance of immature cells may result from damage to the bone marrow microenvironment. These findings may provide useful information for potential diagnostic and therapeutic considerations.

Outer membrane vesicles derived from heatstroke-associated intestinal microbiota promote multiple organ injury in mice

Multiple organ injury is a common issue in heatstroke (HS); however, the underlying pathogenesis remains unclear. As an early event in HS, intestinal injury is an active participant that drives organ injury. Outer membrane vesicles (OMVs), a group of vesicles shed by unbalanced intestinal microbiota as "danger signals," mediate different functional cargo transport in cells and modulate varying biological events in distant target cells. However, the role of OMVs in HS-mediated organ damage remains unclear. Therefore, this study examined OMV production in HS and explored the effect of regulating multiple organ injury. To construct a mouse model, animals were exposed to hyperthermia. OMVs from the intestinal microbiota of HS and control mice were extracted by standardized differential ultracentrifugation. Thereafter, OMVs were characterized and infused into recipient mice via the tail vein. Cl-amidine (a pan-peptidylarginine deiminase inhibitor and OMV production inhibitor) was injected intraperitoneally (2 mg/kg) 2 h before HS treatment, and the absorption of HS OMVs by different organs was tracked. The effect of OMVs on inducing organ pathological changes, inflammatory infiltration, inflammatory cytokine expression, and serum organ injury biomarkers was demonstrated. HS increased OMV production by intestinal microbiota; OMVs were absorbed by different organs in vivo, and were especially enriched in the liver and lung. Compared to control OMVs, infusion with HS OMVs induced significant organ pathological changes, elevated inflammatory cell (macrophages and neutrophil) infiltration, inflammatory cytokine (TNF-ɑ, IL-1β, IL-6) expression, as well as serum biomarkers of organ injury. Similarly, inhibition of endogenous OMVs alleviated these organ injury indicators induced by HS. To our knowledge, the present study is the first to illustrate that OMVs induce acute organ impairment during severe HS, offering a foundation for subsequent studies and providing novel therapeutic targets.

Early biomarkers for kidney injury in heat-related illness patients: a prospective observational study at Japanese Self-Defense Force Fuji Hospital

BACKGROUND

Since heatstroke-induced acute kidney injury (AKI) can progress to chronic kidney disease, it would be useful to detect heatstroke-induced AKI and severe heat-related illness in the early phase. We studied the epidemiology of heat-related illness among patients in the Japanese Ground Self-Defense Force and evaluated the relationship between heat-related illness severity and early urinary biomarkers for AKI.

METHODS

We enrolled patients who were diagnosed with heat-related illness at the Self-Defense Force Fuji Hospital from 1 May to 30 September 2020. We compared the urinary kidney injury molecule-1 (KIM-1), neutrophil gelatinase-associated lipocalin (NGAL), liver fatty acid-binding protein (L-FABP), N-acetyl-β-D-glucosaminidase (NAG) and β2-microglobulin levels according to the severity of heat-related illness as defined by positive scores for the Japanese Association of Acute Medicine Heatstroke Working Group (JAAM-HS-WG) criteria (0, mild; 1, moderate; ≥2, severe).

RESULTS

Of the 44 patients, kidney injury, defined as serum creatinine (sCr) ≥1.2 mg/dL, was seen in 9 (20.5%) patients. Urinary NAG, NGAL and L-FABP levels were significantly higher in the ≥2 JAAM-HS-WG criteria group than in the 0 group. Furthermore, urinary L-FABP levels were positively correlated with sCr levels. In contrast, the urinary KIM-1 levels showed the best correlation with serum cystatin C (sCysC) among these biomarkers.

CONCLUSIONS

We conclude even mild to moderate heatstroke could lead to AKI. Urinary L-FABP is useful for detecting heatstroke-induced AKI and patients with severe heat-related illness requiring immediate treatment. Urinary KIM-1 may detect heatstroke-induced AKI in terms of sCysC, although it was not related to the severity of heat-related illness.

Heatstroke-induced coagulopathy: Biomarkers, mechanistic insights, and patient management

Heatstroke is increasingly becoming a significant concern due to global warming. Systemic inflammation and coagulopathy are the two major factors that provoke life-threatening organ dysfunction in heatstroke. Dysregulated thermo-control induces cellular injury, damage-associated molecular patterns release, hyperinflammation, and hypercoagulation with suppressed fibrinolysis to produce heatstroke-induced coagulopathy (HSIC). HSIC can progress to disseminated intravascular coagulation and multiorgan failure if severe enough. Platelet count, D-dimer, soluble thrombomodulin, and inflammation biomarkers such as interleukin-6 and histone H3 are promising markers for HSIC. In exertional heatstroke, the measurement of myoglobin is helpful to anticipate renal dysfunction. However, the optimal cutoff for each biomarker has not been determined. Except for initial cooling and hydration, effective therapy continues to be explored, and the use of antiinflammatory and anticoagulant therapies is under investigation. Despite the rapidly increasing risk, our knowledge is limited, and further study is warranted. In this review, we examine current information and what future efforts are needed to better understand and manage HSIC.

Wearing a face mask during controlled-intensity exercise is not a risk factor for exertional heatstroke: A pilot study

Aim

This study aimed to measure the influence of wearing face masks on individuals' physical status in a hot and humid environment.

Methods

Each participant experienced different physical situations: (i) not wearing a mask (control), (ii) wearing a surgical mask, (iii) wearing a sport mask. An ingestible capsule thermometer was used to measure internal core body temperature during different exercises (standing, walking, and running, each for 20 min) in an artificial weather room with the internal wet-bulb globe temperature set at 28°C. The change in the participants' physical status and urinary liver fatty acid-binding protein (L-FABP) were measured.

Results

Six healthy male volunteers were enrolled in the study. In each participant, significant changes were observed in the heart rate and internal core temperatures after increased exercise intensity; however, no significant differences were observed between these parameters and urinary L-FABP among the three intervention groups.

Conclusion

Mask wearing is not a risk factor for heatstroke during increased exercise intensity.

CCAAT/Enhancer-Binding Protein Homologous Protein (CHOP) Deficiency Attenuates Heatstroke-Induced Intestinal Injury

The intestine is one of the main target organs involved in the pathological process of heatstroke. CCAAT/enhancer-binding protein homologous protein (CHOP) is involved in endoplasmic reticulum (ER) stress-induced apoptosis. This study aimed to explore the role of CHOP in heatstroke-induced intestinal injury and potential therapy. An in vitro heat stress (HS) model using Caco-2 cells was employed. We observed the role of CHOP in apoptosis-mediated intestinal epithelial cell injury secondary to HS by evaluating cell viability, lactate dehydrogenase release, apoptosis levels, and GRP78, PERK, ATF4, CHOP, Bcl-2, and BAX mRNA and protein expression. To further study the role of CHOP in HS-induced intestinal barrier dysfunction, we assessed transepithelial electrical resistance, paracellular tracer flux, ultrastructure of tight junctions, and protein expression of ZO-1 and occludin. Male wild-type mice and CHOP knockout mice were used for in vivo experiments. We evaluated serum d-lactate and diamine oxidase levels, histopathological changes, intestinal ultrastructure, and ZO-1 and occludin protein expression. HS activated the PERK-CHOP pathway and promoted apoptosis by upregulating BAX and downregulating Bcl-2; these effects were prevented by CHOP silencing. Intestinal epithelial barrier function was disrupted by HS in vitro and in vivo. CHOP silencing prevented intestinal barrier dysfunction in Caco-2 cells, whereas CHOP knockout mice exhibited decreased intestinal mucosal injury. The ER stress inhibitor 4-phenylbutyrate (4-PBA) prevented HS-induced intestinal injury in vitro and in vivo. This study indicated that CHOP deficiency attenuates heatstroke-induced intestinal injury and may contribute to the identification of a novel therapy against heatstroke associated with the ER stress pathway.

Association between active cooling and lower mortality among patients with heat stroke and heat exhaustion

Body cooling is recommended for patients with heat stroke and heat exhaustion. However, differences in the outcomes of patients who do or do not receive active cooling therapy have not been determined. The best available evidence supporting active cooling is based on a case series without comparison groups; thus, the effectiveness of this method in improving patient prognoses cannot be appropriately quantified. Therefore, we compared the outcomes of heat stroke patients receiving active cooling with those of patients receiving rehydration-only therapy. This prospective observational multicenter registry-based study of heat stroke and heat exhaustion patients was conducted in Japan from 2010 to 2019. The patients were stratified into the "severe" group or the "mild-to-moderate" group, per clinical findings on admission. After conducting multivariate logistic regression analyses, we compared the prognoses between patients who received "active cooling + rehydration" and patients who received "rehydration only," with in-hospital death as the endpoint. Sex, age, onset situation (i.e., exertional or non-exertional), core body temperature, liver damage, renal dysfunction, and disseminated intravascular coagulation were considered potential covariates. Among those who received active cooling and rehydration-only therapy, the in-hospital mortality rates were 21.5% and 35.5%, respectively, for severe patients (n = 231) and 3.9% and 5.7%, respectively, for mild-to-moderate patients (n = 578). Rehydration-only therapy was associated with a higher in-hospital mortality in patients with severe heat illness (adjusted odds ratio [aOR], 3.29; 95% confidence interval [CI], 1.21-8.90), whereas the cooling methods were not associated with lower in-hospital mortality in patients with mild-to-moderate heat illness (aOR, 2.22; 95% CI, 0.92-5.84). Active cooling was associated with lower in-hospital mortality only in the severe group. Our results indicated that active cooling should be recommended as an adjunct to rehydration-only therapy for patients with severe heat illness.

Machine learning-based mortality prediction model for heat-related illness

In this study, we aimed to develop and validate a machine learning-based mortality prediction model for hospitalized heat-related illness patients. After 2393 hospitalized patients were extracted from a multicentered heat-related illness registry in Japan, subjects were divided into the training set for development (n = 1516, data from 2014, 2017–2019) and the test set (n = 877, data from 2020) for validation. Twenty-four variables including characteristics of patients, vital signs, and laboratory test data at hospital arrival were trained as predictor features for machine learning. The outcome was death during hospital stay. In validation, the developed machine learning models (logistic regression, support vector machine, random forest, XGBoost) demonstrated favorable performance for outcome prediction with significantly increased values of the area under the precision-recall curve (AUPR) of 0.415 [95% confidence interval (CI) 0.336–0.494], 0.395 [CI 0.318–0.472], 0.426 [CI 0.346–0.506], and 0.528 [CI 0.442–0.614], respectively, compared to that of the conventional acute physiology and chronic health evaluation (APACHE)-II score of 0.287 [CI 0.222–0.351] as a reference standard. The area under the receiver operating characteristic curve (AUROC) values were also high over 0.92 in all models, although there were no statistical differences compared to APACHE-II. This is the first demonstration of the potential of machine learning-based mortality prediction models for heat-related illnesses.

A novel mouse model of heatstroke accounting for ambient temperature and relative humidity

Background

Heatstroke is associated with exposure to high ambient temperature (AT) and relative humidity (RH), and an increased risk of organ damage or death. Previously proposed animal models of heatstroke disregard the impact of RH. Therefore, we aimed to establish and validate an animal model of heatstroke considering RH. To validate our model, we also examined the effect of hydration and investigated gene expression of cotransporter proteins in the intestinal membranes after heat exposure.

Methods

Mildly dehydrated adult male C57/BL6J mice were subjected to three AT conditions (37 °C, 41 °C, or 43 °C) at RH > 99% and monitored with WetBulb globe temperature (WBGT) for 1 h. The survival rate, body weight, core body temperature, blood parameters, and histologically confirmed tissue damage were evaluated to establish a mouse heatstroke model. Then, the mice received no treatment, water, or oral rehydration solution (ORS) before and after heat exposure; subsequent organ damage was compared using our model. Thereafter, we investigated cotransporter protein gene expressions in the intestinal membranes of mice that received no treatment, water, or ORS.

Results

The survival rates of mice exposed to ATs of 37 °C, 41 °C, and 43 °C were 100%, 83.3%, and 0%, respectively. From this result, we excluded AT43. Mice in the AT 41 °C group appeared to be more dehydrated than those in the AT 37 °C group. WBGT in the AT 41 °C group was > 44 °C; core body temperature in this group reached 41.3 ± 0.08 °C during heat exposure and decreased to 34.0 ± 0.18 °C, returning to baseline after 8 h which showed a biphasic thermal dysregulation response. The AT 41 °C group presented with greater hepatic, renal, and musculoskeletal damage than did the other groups. The impact of ORS on recovery was greater than that of water or no treatment. The administration of ORS with heat exposure increased cotransporter gene expression in the intestines and reduced heatstroke-related damage.

Conclusions

We developed a novel mouse heatstroke model that considered AT and RH. We found that ORS administration improved inadequate circulation and reduced tissue injury by increasing cotransporter gene expression in the intestines.

Safety and efficacy of cold-water immersion in the treatment of older patients with heat stroke: a case series

Background

Heat stroke treatment focuses on rapid cooling because symptom severity correlates with the duration of hyperthermia (i.e., time during which the core body temperature is sustained above the critical threshold). Several reports have revealed that cold‐water immersion is a safe and appropriate therapy for exertional heat stroke in young, otherwise healthy patients. However, few reports have assessed cold‐water immersion in older patients. We document three cases of cold‐water immersion in older heat stroke patients and evaluate its safety and efficacy.

Case presentation

Three older patients with severe heat stroke were treated with cold‐water immersion. Core body temperatures decreased rapidly, and no complications occurred during the treatment.

Conclusion

Cold‐water immersion can achieve rapid cooling and is effective in treating heat stroke. With special precautions, it can be performed safely for older patients. Further investigation is warranted to establish appropriate cooling methods in older adults.

Indicators of Acute Kidney Injury as Biomarkers to Differentiate Heatstroke from Coronavirus Disease 2019: A Retrospective Multicenter Analysis

BACKGROUND

Coronavirus disease 2019 (COVID-19) and heat-related illness are systemic febrile diseases. These illnesses must be differentiated during a COVID-19 pandemic in summer. However, no studies have compared and distinguished heat-related illness and COVID-19. We compared data from patients with early heat-related illness and those with COVID-19.

METHODS

This retrospective observational study included 90 patients with early heat-related illness selected from the Heatstroke STUDY 2017-2019 (nationwide registries of heat-related illness in Japan) and 86 patients with laboratory-confirmed COVID-19 who had fever or fatigue and were admitted to one of two hospitals in Tokyo, Japan.

RESULTS

Among vital signs, systolic blood pressure (119 vs. 125 mm Hg, p = 0.02), oxygen saturation (98% vs. 97%, p < 0.001), and body temperature (36.6°C vs. 37.6°C, p<0.001) showed significant between-group differences in the heatstroke and COVID-19 groups, respectively. The numerous intergroup differences in laboratory findings included disparities in white blood cell count (10.8 × 10³/μL vs. 5.2 × 10³/μL, p<0.001), creatinine (2.2 vs. 0.85 mg/dL, p<0.001), and C-reactive protein (0.2 vs. 2.8 mg/dL, p<0.001), although a logistic regression model achieved an area under the curve (AUC) of 0.966 using these three factors. A Random Forest machine learning model achieved an accuracy, precision, recall, and AUC of 0.908, 0.976, 0.842, and 0.978, respectively. Creatinine was the most important feature of this model.

CONCLUSIONS

Acute kidney injury was associated with heat-related illness, which could be essential in distinguishing or evaluating patients with fever in the summer during a COVID-19 pandemic.