Relation of body surface area-to-mass ratio to risk of exertional heat stroke in healthy men and women

Ambient temperature and wet bulb globe temperature outperform heat index in predicting hydration status and heat perception in a semi-arid environment

BACKGROUND

Climate change is increasing temperatures, frequency of heatwaves, and erratic rainfall, which threatens human biology and health, particularly in already extreme environments. Therefore, it is important to understand how environmental heat stress measures are tied to human water needs and thermoregulation under increasingly hot conditions.

AIM

To test how ambient temperature, heat index, and wet bulb globe temperature (WBGT) relate to hydration status and thermal heat perception in a hot, semi-arid environment.

SUBJECTS AND METHODS

Urine samples, perceived heat stress, and anthropometrics were collected among Daasanach semi-nomadic pastoralists (n = 187 children, n = 231 adults) in northern Kenya. Environmental heat stress measures were recorded at sample collection; samples' urine specific gravity (USG) was measured.

RESULTS

Multiple linear and logistic regressions indicate that all environmental heat stress measures were associated with USG, odds of dehydration, and heat perception. Ambient temperature performed marginally better than WBGT, and both performed better than heat index. These associations were stronger among children than adults.

CONCLUSION

In a hot, semi-arid climate, ambient temperature and WBGT accurately predict human water needs and heat stress, with children more vulnerable to dehydration. To mitigate consequences of extreme heat, local bioculturally-appropriate hydration (e.g. tea) and cooling (e.g. shade) strategies should be encouraged.

Exertional heat illness: international military-oriented lessons learned and best practices for prevention and management

Climate change has resulted in more frequent and intense heat waves, leading to elevated global temperatures and posing a significant health threat to individuals working in hot environments such as military personnel. Ensuring both safety and performance, alongside the increasing risk of exertional heat illnesses (EHI) due to rising temperatures, is hence even more crucial. Extensive research conducted over many years has aimed to understand the causes and impacts of EHI and develop prevention and treatment strategies. This review summarizes the research on the impacts of heat on health and performance in military settings, consolidates evidence-based strategies for EHI prevention and pre-hospital management, summarizes sex differences in heat tolerance, and discusses best practices for recovery and return to duty post-EHI. The aim is to share the knowledge and practices derived from military research to protect the health and performance of individuals in various populations exposed to heat.

Risk for exertional heat illness among US army enlistees: climate indexes, intrinsic factors and their interactions

OBJECTIVES

To characterise intrinsic and extrinsic (climatic) risks for mild and severe exertional heat illness (EHI) among first-year army enlistees.

METHODS

We examined 337 786 soldiers who enlisted between 2012 and 2019. Survival models were used to predict incident EHI from intrinsic factors (demographics, healthcare utilisation, chronic conditions, body mass index (BMI), Army Physical Fitness Test (APFT), upper/lower respiratory tract infections (URTI and LRTI), skin and soft-tissue infections (SSTI), extrinsic factors (geographical region, daily mean Universal Thermal Climate Index (UTCI), wet bulb globe temperature (WBGT)) and interactions.

RESULTS

There were 1390 cases of mild and 359 cases of severe EHI. Females had a higher risk for mild (adjusted OR (aOR) 1.78; 95% CI 1.57 to 2.02) but a lower risk for severe (aOR 0.61; 95% CI 0.38 to 0.87) EHI. Obesity was associated with severe EHI (aOR: 1.76; 95% CI 1.09 to 2.84) but not mild EHI (aOR: 1.03; 95% CI 0.76 to 1.39). URTI was associated with severe (aOR: 2.44; 95% CI 1.12 to 5.30) and mild (aOR 3.72, 95% CI 2.84 to 4.87) EHI, as were LRTI (severe, aOR: 11.40; 95% CI 6.09 to 21.32; mild, aOR 2.06; 95% CI 1.22 to 3.46), but not SSTI. UTCI outperformed WBGT in predicting EHI. Outside the Southern USA, EHI risk was elevated at lower UTCI. Associations varied over climate conditions and generally did not increase with climatic heat stress.

CONCLUSIONS

Respiratory infections were associated with the highest risk for EHI in soldiers. Risk mitigation strategies may include monitoring prevention and recovery from respiratory infections. Female sex and obesity may have different associated risks over climate conditions.

Regulation of body temperature and blood pressure in women: Mechanisms and implications for heat illness risk

Increasing global temperatures due to ongoing climate change phenomena have resulted in increased risk of exertional heat illness in otherwise healthy, young individuals who work or play in the heat. With increasing participation of women in athletic, military and industrial activities that involve exertion in the heat, there is a growing need to study female physiology in this context. Mechanisms controlling blood pressure and body temperature have substantial overlap in humans, largely due to autonomic mechanisms which contribute to both. Similarly, illnesses that result from excessive heat exposure can often be traced back to imbalances in one or more of these autonomic mechanisms. In recent years, there has been increased recognition of the importance of sex as a biological variable for basic and applied research in these areas. The goal of this paper is to present an update on the integrative physiology and pathophysiology of responses to heat stress in women (thermoregulation and blood pressure regulation). In this context, it is often the case that differences between sexes are presented as 'advantages' and 'disadvantages' of one sex over the other. In our opinion, this is an over-simplification of the physiology which ignores the nuances and complexities of the integrative physiology of responses to heat exposure and exercise, and their relevance for practical outcomes.

The Significance of Body Surface Area to Mass Ratio for Thermal Responses to a Standardized Exercise-Heat Stress Test

PURPOSE

To evaluate the significance of body surface area-to-mass ratio (BSA/mass) on the heat-tolerance test (HTT) results. We hypothesized that individuals defined as heat tolerant (HT) would have on average higher BSA/mass compared with heat intolerant (HI) individuals.

METHODS

A retrospective reanalysis of the HTT results of 517 soldiers (age, 18-38 yr; M/F, 96%:4%), who were tested by the Israel Defense Forces HTT protocol. The criterion for heat tolerance in the current analysis was a rectal temperature (T re ) plateau during the second hour of the test. A logistic regression analysis to evaluate the predictive power of BSA/mass for heat intolerance was performed; the spline model was applied to show the odds for heat intolerance across BSA/mass.

RESULTS

In men BSA/mass of HI individuals was lower than HT individuals (248 ± 19 vs 262 ± 18 cm 2 ·kg -1 , P < 0.01, d = 0.76). In women a similar trend was noted but with no statistical significance between HT and HI groups. The odd ratio for heat intolerance for every unit increase in BSA/mass was 0.97 (95% confidence interval, 0.95-0.99). The spline model plateaued above BSA/mass of 270 cm 2 ·kg -1 .

CONCLUSIONS

The results imply that body-core temperature responses to a standard exercise-heat stress (fixed external work rate and climatic conditions) are influenced by BSA/mass. More specifically, lack of a steady state in T re (indicating heat intolerance) was more likely to occur with every unit decrease in BSA/mass. These findings contribute to a better understanding of the role of body anthropometry in the response to a standard exercise-heat task that might have an implication on clinical decision making about return to duty/play of soldiers, athletes, and others who deemed to be identified as HI.

Sex differences in biomarkers of end-organ damage following exertional heat stroke in humans

Women are participating in military and athletic activities in the heat in increasing numbers, but potential sex differences in sequelae from exertional heat illness remain poorly understood. We tested the hypothesis that women suffering from exertional heat stroke (EHS) would have similar severity of organ damage biomarkers compared with men, as measured in a hospital setting. We studied women and men presenting with EHS to the emergency department at Fort Moore, GA. We measured creatinine (CR), creatine kinase (CK), alanine-transaminase (ALT), aspartate aminotransferase (AST), and estimated glomerular filtration rate (eGFR). Core temperature was also assessed by medical personnel. Biomarker data were obtained for 62 EHS cases (11 women). Men were significantly taller, and heavier, and had larger body mass index (BMI) and body surface area (P < 0.05 for all). The highest recorded body core temperature was not different between groups [women: 41.11°C (40.06, 41.67); men: 41.11°C (40.28, 41.72), P = 0.57]. Women had significantly lower peak CR [women: 1.39 (1.2, 1.48) m·dL-1; men: 1.75 (1.53, 2.16) mg·dL-1, P < 0.01] and peak CK [women: 584 (268, 2,412) U·L-1; men: 2,183 (724, 5,856) U·L-1, P = 0.02]. Peak ALT and AST were not different between groups; during recovery time points, ALT and AST were either similar or lower in women. Women spent approximately half as much time in the hospital following admittance compared with men. Our findings suggest that women may be less susceptible to organ injury resulting from EHS. Further research is necessary to understand the pathophysiology underlying these differences and how biomarkers of end-organ damage severity can differ between women and men following EHS.NEW & NOTEWORTHY We studied otherwise healthy women and men after exertional heat stroke in a military training environment. Peak values for biomarkers of kidney and muscle damage were lower in women compared with men despite similar (highest recorded) body core temperatures. During recovery, organ damage markers were similar or lower in women. These sex differences may indicate differences in the pathophysiology of responses, but more work is needed to clarify specific mechanisms.