Exertional heat stroke: An evidence based approach to clinical assessment and management

Molecular Investigation and Preliminary Validation of Candidate Genes Associated with Neurological Damage in Heat Stroke

Heat stroke (HS) is a severe medical condition characterized by a systemic inflammatory response that may precipitate multi-organ dysfunction, with a particular predilection for inducing profound central nervous system impairments. We aim to employ bioinformatics techniques for the retrieval and analysis of genes associated with heat stroke-induced neurological damage. We performed a comprehensive analysis of the GSE64778 dataset from the Sequence Read Archive, resulting in the identification of 1178 significantly differentially expressed genes (DEGs). We retrieved 2914 genes associated with heat stroke from the GeneCards database and 2377 genes associated with heat stroke from the Comparative Toxicogenomics Database (CTD). The intersection of the top 300 DEGs in the GSE64778 dataset intersected with the search results of GeneCards and CTD, yielding 25 final candidates for DEGs associated with heat stroke. Gene Ontology functional annotation results indicated that the target genes were mainly involved in apoptosis, stress response, and negative regulation of cellular processes and function in processes such as protein dimerization and protein binding. The Kyoto Encyclopedia of Genes and Genomes pathway enrichment analysis revealed a predominant enrichment of candidate target genes within the PI3K-AKT signaling pathway. Subsequent protein-protein interaction network analysis highlighted HSP90aa1 as a central gene, indicating its pivotal role by possessing the highest number of edges among the genes enriched in the PI3K-AKT signaling pathway. Quantitative reverse transcription-polymerase chain reaction analysis performed on blood samples from patients validated the expression of Hsp90aa1 in individuals exhibiting early neurological damage in HS, consistent with the findings from the mRNA bioinformatics analysis. Additionally, the bioinformatics analysis of the upstream microRNAs (miRNAs) regulating HSP90aa1 and the target miRNAs associated with candidate long non-coding RNAs (lncRNAs) identified three lncRNAs, eight miRNAs, and one mRNA in the regulatory network. The DIANA Tools database and algorithms were employed for pathway enrichment and correlation analysis, revealing a significant association between LOC102547734 and MIR-206-3p, with the latter being identified as a target binding site Moreover, the analysis unveiled a correlation between MIR-206-3p and HSP90aa1, implicating the latter as a potential target binding site within the regulatory network.

Cardiovascular magnetic resonance feature tracking derived strain analysis can predict return to training following exertional heatstroke

BACKGROUND

Exertional heatstroke (EHS) is increasingly common in young trained soldiers. However, prognostic markers in EHS patients remain unclear. The objective of this study was to evaluate cardiovascular magnetic resonance (CMR) feature tracking derived left ventricle (LV) strain as a biomarker for return to training (RTT) in trained soldiers with EHS.

METHODS

Trained soldiers (participants) with EHS underwent CMR cine sequences between June 2020 and August 2023. Two-dimensional (2D) LV strain parameters were derived. At 3 months after index CMR, the participants with persistent cardiac symptoms including chest pain, dyspnea, palpitations, syncope, and recurrent heat-related illness were defined as non-RTT. Multivariable logistic regression analysis was used to develop a predictive RTT model. The performance of different models was compared using the area under curve (AUC).

RESULTS

A total of 80 participants (median age, 21 years; interquartile range (IQR), 20-23 years) and 27 health controls (median age, 21 years; IQR, 20-22 years) were prospectively included. Of the 77 participants, 32 had persistent cardiac symptoms and were not able to RTT at 3 months follow-up after experiencing EHS. The 2D global longitudinal strain (GLS) was significantly impaired in EHS participants compared to the healthy control group (-15.8 ± 1.7% vs -16.9 ± 1.2%, P = 0.001), which also showed significant statistical differences between participants with RTT and non-RTT (-15.0 ± 3.5% vs -16.5 ± 1.4%, P < 0.001). 2D-GLS (≤ -15.0%) (odds ratio, 1.53; 95% confidence interval: 1.08, 2.17; P = 0.016) was an independent predictor for RTT even after adjusting known risk factors. 2D-GLS provided incremental prognostic value over the clinical model and conventional CMR parameters model (AUCs: 0.72 vs 0.88, P = 0.013; 0.79 vs 0.88, P = 0.023; respectively).

CONCLUSION

Two-dimensional global longitudinal strain (≤ -15.0%) is an incremental prognostic CMR biomarker to predict RTT in soldiers suffering from EHS.

Suspected exertional heat stroke; A case study of worker cooling in a hot and humid field environment

BACKGROUND

In the event of a severe occupational heat-related illness, paramedic assistance may not be immediately available. A worker's survival may depend on their co-workers access to efficacious field-based cooling modalities. One cooling method that has been claimed to be practical in field-based settings is the ice towel method.

OBJECTIVES

This case study assessed the practicality of the ice towel method in an industrial setting, where criteria for use include cost effectiveness, portability, scalability, and implementation by a single worker under the stress of an emergency.

METHODS

This case study describes the emergency application of the ice towel method while awaiting paramedics, for a worker suffering suspected exertional heat stroke on a remote job site.

RESULTS

Ice towels were able to be transported to a remote field site and applied successfully by a single worker under the stress of a potentially life-threatening emergency.

CONCLUSIONS

The ice towel method was cost effective, scalable, transportable, and rapidly applied in a field-based emergency. This case study demonstrates the importance of organizations assessing their heat-related risks, and determining controls based upon their efficacy and practicality for their unique setting.

Every Picture Tells A Story: Managing Exertional Heatstroke with Rotating Ice Water Towels

ABSTRACT

A 23-year-old woman completing her first marathon collapsed near the finish line at 4 hours 6 min with a rectal temperature of 41.8°C. She was in good health before the race with no recent illness, had completed a full training program, and was taking no medications or supplements. On the initial exam, she was unconscious with a response to painful stimulus, spontaneous breathing, rapid pulse, eyes closed, fully dilated pupils, poor muscle tone, and pale skin that was warm to touch. The medical team initiated whole-body cooling using rapidly rotating ice water towels and ice packs placed in the neck, axilla, and groin. She developed echolalia during active cooling. About 20 minutes into the cooling procedure, she "woke up," was able to answer questions coherently, and her pupils were normal size and reactive. She was discharged home with instructions to follow-up in 2 d for evaluation and blood chemistry testing.

Heat stroke burden and validity of wearable-derived core temperature estimation during elite military training

OBJECTIVES

Exertional heat stroke (EHS) remains a significant health concern while training in hot environments, prompting the development of noninvasive wearable technology for monitoring estimated core temperature (ECT). The objective of this study was to review the effectiveness of an ECT device during elite military training by providing a case series of individuals who developed EHS.

METHODS

This is a retrospective study of EHS cases during high-intensity training while wearing the ECT device, Zephyr BioharnessTM. Data was collected from January 2021 through September 2021 at the Air Force Special Warfare Training pipeline in San Antonio, TX. Rectal temperatures of EHS diagnoses, defined by central nervous system (CNS) dysfunction and rectal temperature approaching or >40°C (104°F), were compared to ECT reading via Zephyr BioharnessTM. Incidence rates and psychometric properties were calculated using R package.

RESULTS

A total of 47,058 daily peak ECT measurements were collected among 1,364 trainees. A total of 499 trainees flagged as potential EHS by Zephyr BioharnessTM reading >39.7°C (103.5°F). The incidence of confirmed EHS was 0.8/1000 person-months. Of the 10 confirmed EHS cases (9 males, 1 female; age = 23.4 ± 2.7 yrs; BMI = 25 ± 2; body fat = 13 ± 5%), 8 trainees had a peak ECT reading below 39.7°C which resulted in a sensitivity of 20%, specificity of 98.9%, positive likelihood ratio (LR) of 18.93 (95% Confidence Interval [CI] 5.5-65.6), and a negative LR of 0.81 (95% CI 0.6-1.1).

CONCLUSION

ECT had substantial false positive and negative rates. Further studies are needed to validate this technology in other populations, and the algorithm used in this device needs to be refined to better capture the environmental and physical requirements in the special operations population.

Recommended water immersion duration for the field treatment of exertional heat stroke when rectal temperature is unavailable

INTRODUCTION

The recommended treatment for exertional heat stroke is immediate, whole-body immersion in < 10 °C water until rectal temperature (Tre) reaches ≤ 38.6 °C. However, real-time Tre assessment is not always feasible or available in field settings or emergency situations. We defined and validated immersion durations for water temperatures of 2-26 °C for treating exertional heat stroke.

METHODS

We compiled data for 54 men and 18 women from 7 previous laboratory studies and derived immersion durations for reaching 38.6 °C Tre. The resulting immersion durations were validated against the durations of cold-water immersion used to treat 162 (98 men; 64 women) exertional heat stroke cases at the Falmouth Road Race between 1984 and 2011.

RESULTS

Age, height, weight, body surface area, body fat, fat mass, lean body mass, and peak oxygen uptake were weakly associated with the cooling time to a safe Tre of 38.6 °C during immersions to 2-26 °C water (R2 range: 0.00-0.16). Using a specificity criterion of 0.9, receiver operating characteristics curve analysis showed that exertional heat stroke patients must be immersed for 11-12 min when water temperature is ≤ 9 °C, and for 18-19 min when water temperature is 10-26 °C (Cohen's Kappa: 0.32-0.75, p < 0.001; diagnostic odds ratio: 8.63-103.27).

CONCLUSION

The reported immersion durations are effective for > 90% of exertional heat stroke patients with pre-immersion Tre of 39.5-42.8 °C. When available, real-time Tre monitoring is the standard of care to accurately diagnose and treat exertional heat stroke, avoiding adverse health outcomes associated with under- or over-cooling, and for implementing cool-first transport second exertional heat stroke policies.

Hot water immersion: Maintaining core body temperature above 38.5°C mitigates muscle fatigue

PURPOSE

Hot water immersion (HWI) has gained popularity to promote muscle recovery, despite limited data on the optimal heat dose. The purpose of this study was to compare the responses of two exogenous heat strains on core body temperature, hemodynamic adjustments, and key functional markers of muscle recovery following exercise-induced muscle damage (EIMD).

METHODS

Twenty-eight physically active males completed an individually tailored EIMD protocol immediately followed by one of the following recovery interventions: HWI (40°C, HWI40 ), HWI (41°C, HWI41 ) or warm water immersion (36°C, CON36 ). Gastrointestinal temperature (Tgi ), hemodynamic adjustments (cardiac output [CO], mean arterial pressure [MAP], and systemic vascular resistance [SVR]), pre-frontal cortex deoxyhemoglobin (HHb), ECG-derived respiratory frequency, and subjective perceptual measures were tracked throughout immersion. In addition, functional markers of muscle fatigue (maximal concentric peak torque [Tpeak ]) and muscle damage (late-phase rate of force development [RFD100-200 ]) were measured prior to EIMD (pre-), 24 h (post-24 h), and 48 h (post-48 h) post-EIMD.

RESULTS

By the end of immersion, HWI41 led to significantly higher Tgi values than HWI40 (38.8 ± 0.1 vs. 38.0°C ± 0.6°C, p < 0.001). While MAP was well maintained throughout immersion, only HWI41 led to increased (HHb) (+4.2 ± 1.47 μM; p = 0.005) and respiratory frequency (+4.0 ± 1.21 breath.min-1 ; p = 0.032). Only HWI41 mitigated the decline in RFD100-200 at post-24 h (-7.1 ± 31.8%; p = 0.63) and Tpeak at post-48 h (-3.1 ± 4.3%, p = 1).

CONCLUSION

In physically active males, maintaining a core body temperature of ~25 min within the range of 38.5°C-39°C has been found to be effective in improving muscle recovery, while minimizing the risk of excessive physiological heat strain.

A century of exercise physiology: concepts that ignited the study of human thermoregulation. Part 3: Heat and cold tolerance during exercise

In this third installment of our four-part historical series, we evaluate contributions that shaped our understanding of heat and cold stress during occupational and athletic pursuits. Our first topic concerns how we tolerate, and sometimes fail to tolerate, exercise-heat stress. By 1900, physical activity with clothing- and climate-induced evaporative impediments led to an extraordinarily high incidence of heat stroke within the military. Fortunately, deep-body temperatures > 40 °C were not always fatal. Thirty years later, water immersion and patient treatments mimicking sweat evaporation were found to be effective, with the adage of cool first, transport later being adopted. We gradually acquired an understanding of thermoeffector function during heat storage, and learned about challenges to other regulatory mechanisms. In our second topic, we explore cold tolerance and intolerance. By the 1930s, hypothermia was known to reduce cutaneous circulation, particularly at the extremities, conserving body heat. Cold-induced vasodilatation hindered heat conservation, but it was protective. Increased metabolic heat production followed, driven by shivering and non-shivering thermogenesis, even during exercise and work. Physical endurance and shivering could both be compromised by hypoglycaemia. Later, treatments for hypothermia and cold injuries were refined, and the thermal after-drop was explained. In our final topic, we critique the numerous indices developed in attempts to numerically rate hot and cold stresses. The criteria for an effective thermal stress index were established by the 1930s. However, few indices satisfied those requirements, either then or now, and the surviving indices, including the unvalidated Wet-Bulb Globe-Thermometer index, do not fully predict thermal strain.

Exertional Heat Stroke and Rhabdomyolysis: A Medical Record Review and Patient Perspective on Management and Long-Term Symptoms

INTRODUCTION

Exertional heat stroke (EHS) is a medical emergency, occurring when the body generates more heat than it can dissipate, and frequently associated with exertional rhabdomyolysis (ERM). In the present study we aimed to (I) identify clinical features and risk factors, (II) describe current prehospital management, (III) investigate long-term outcomes including the impact on mental health, and review the guidance received during restarting activities. We hope that our approach will improve individual and organizational heat illness preparedness, and improve follow-up care.

METHODS

We performed a prospective online survey and retrospective medical record review among athletes and military personnel with an episode of EHS/ERM in the Netherlands between 2010 and 2020. We evaluated prehospital management, risk factors, clinical features and long-term outcomes at 6 and 12 months after the event, including mental health symptoms. Furthermore, we investigated what guidance participants received during follow-up, and assessed the patients' perspective on these outcomes.

RESULTS

Sixty participants were included, 42 male (70%) and 18 female (30%), of which 47 presented with EHS (78%) and 13 with ERM (22%). Prehospital management was inconsistent and in the majority of participants not conducted according to available guidelines. Self-reported risk factors included not feeling well-acclimatized to environmental heat (55%) and peer pressure (28%). Self-reported long-term symptoms included muscle symptoms at rest (26%) or during exercise (28%), and neurological sequelae (11%). Validated questionnaires (CIS, HADS and SF-36) were indicative of severe fatigue (30%) or mood/anxiety disorders (11%). Moreover, 90% expressed a lack of follow-up care and that a more frequent and intensive follow-up would have been beneficial for their recovery process.

CONCLUSION

Our findings indicate major inconsistencies in the management of patients with EHS/ERM, emphasizing the compelling need for implementing standardized protocols. Based on the results of long-term outcome measures, we recommend to counsel and evaluate every patient not only immediately after the event, but also in the long-term.

Neuromotor deficits and altered physiological responses to repeated exertional heat stroke exposures in mice

Exertional heat stroke (EHS) is a life-threatening illness that can lead to negative health outcomes. Using a "severe" preclinical mouse model of EHS, we tested the hypotheses that one EHS exposure results in altered susceptibility to a subsequent EHS and reduced neuromotor performance. Female C57BL/6 mice underwent two protocols, 2 wk apart, either an EHS trial (EHS) or a sham exercise control trial (EXC). For EHS, mice ran in a forced running wheel at 37.5°C/40% relative humidity until loss of consciousness, followed by a slow cooling protocol (2 h recovery at 37.5°C). EXC mice exercised equally but in ∼22°C. Mice were randomized into three groups: 1) EXC-EXC (two consecutive EXC, n = 6, 2) EHS-EXC (EHS followed by EXC, n = 5), and 3) EHS-EHS (repeated EHS, n = 9). Mice underwent noninvasive neuromotor and behavioral tests during recovery and isolated soleus force measurements at the end of recovery. At the first EHS, mice reached average peak core temperatures (Tc,max) of 42.4°C, (46% mortality). On the second EHS, average Tc,max was reduced by ∼0.7°C (P < 0.05; mortality 18%). After the first EHS, both EHS-EX and EHS-EHS showed significant reductions in maximum strength (24 h and 1 wk post). After the second EHS, strength, horizontal rotation, hindlimb tone, suspended hindlimb splay, trunk curl, and provoked biting continued to decline in the EHS-EHS group. In conclusion, exposure to a second EHS after 2 wk leads to increased exercise times in the heat, symptom limitation at a lower Tc,max, and greater deficits in neuromotor and behavioral function during recovery.

Exertional heat stroke in sport and the military: epidemiology and mitigation

NEW FINDINGS

What is the topic of this review? Exertional heat stroke epidemiology in sport and military settings, along with common risk factors and strategies and policies designed to mitigate its occurrence. What advances does it highlight? Individual susceptibility to exertional heat stroke risk is dependent on the interaction of intrinsic and extrinsic factors. Heat policies in sport should assess environmental conditions, as well as the characteristics of the athlete, clothing/equipment worn and activity level of the sport. Exertional heat stroke risk reduction in the military should account for factors specific to training and personnel.

ABSTRACT

Exertional heat illness occurs along a continuum, developing from the relatively mild condition of muscle cramps, to heat exhaustion, and in some cases to the life-threatening condition of heat stroke. The development of exertional heat stroke (EHS) is associated with an increase in core temperature stemming from inadequate heat dissipation to offset the rate of metabolically generated heat. Susceptibility to EHS is linked to the interaction of several factors including environmental conditions, individual characteristics, health conditions, medication and drug use, behavioural responses, and sport/organisational requirements. Two settings in which EHS is commonly observed are competitive sport and the military. In sport, the exact prevalence of EHS is unclear due to inconsistent exertional heat illness terminology, diagnostic criteria and data reporting. In contrast, exertional heat illness surveillance in the military is facilitated by standardised case definitions, a requirement to report all heat illness cases and a centralised medical record repository. To mitigate EHS risk, several strategies can be implemented by athletes and military personnel, including heat acclimation, ensuring adequate hydration, cold-water immersion and mandated work-to-rest ratios. Organisations may also consider developing sport or military task-specific heat stress policies that account for the evaporative heat loss requirement of participants, relative to the evaporative capacity of the environment. This review examines the epidemiology of EHS along with the strategies and policies designed to reduce its occurrence in sport and military settings. We highlight the nuances of identifying individuals at risk of EHS and summarise the benefits and shortcomings of various mitigation strategies.

Exertional heat stroke: pathophysiology and risk factors

Exertional heat stroke, the third leading cause of mortality in athletes during physical activity, is the most severe manifestation of exertional heat illnesses. Exertional heat stroke is characterised by central nervous system dysfunction in people with hyperthermia during physical activity and can be influenced by environmental factors such as heatwaves, which extend the incidence of exertional heat stroke beyond athletics only. Epidemiological data indicate mortality rates of about 27%, and survivors display long term negative health consequences ranging from neurological to cardiovascular dysfunction. The pathophysiology of exertional heat stroke involves thermoregulatory and cardiovascular overload, resulting in severe hyperthermia and subsequent multiorgan injury due to a systemic inflammatory response syndrome and coagulopathy. Research about risk factors for exertional heat stroke remains limited, but dehydration, sex differences, ageing, body composition, and previous illness are thought to increase risk. Immediate cooling remains the most effective treatment strategy. In this review, we provide an overview of the current literature emphasising the pathophysiology and risk factors of exertional heat stroke, highlighting gaps in knowledge with the objective to stimulate future research.