Revisiting the evaluation of central versus peripheral thermoregulatory control in humans

Shifting focus: Time to look beyond the classic physiological adaptations associated with human heat acclimation

Planet Earth is warming at an unprecedented rate and our future is now assured to be shaped by the consequences of more frequent hot days and extreme heat. Humans will need to adapt both behaviorally and physiologically to thrive in a hotter climate. From a physiological perspective, countless studies have shown that human heat acclimation increases thermoeffector output (i.e., sweating and skin blood flow) and lowers cardiovascular strain (i.e., heart rate) during heat stress. However, the mechanisms mediating these adaptations remain understudied. Furthermore, several possible benefits of heat acclimation for other systems and functions involved in maintaining health and performance during heat stress remain to be elucidated. This review summarizes recent advances in human heat acclimation, with emphasis on recent studies that (1) advanced our understanding of the mechanisms mediating improved thermoeffector output and (2) investigated adaptations that go beyond those classically associated with heat acclimation. We highlight that these studies have contributed to a better understanding of the integrated physiological responses underlying human heat acclimation while leaving key unanswered questions that will need to be addressed in the future.

Association of hypermagnesemia at birth and admission hypothermia in pre-term infants: A secondary analysis of a prospective cohort study

BACKGROUND

Hypothermia on admission is associated with increased mortality in preterm infants. Drugs administered to pregnant women is implicated in its occurrence. Since magnesium sulfate has a myorelaxant effect, we aimed evaluating the association of hypermagnesemia at birth and admission hypothermia (axillary temperature <36.5°C) in preterm infants.

METHODS

We performed a secondary analysis of a prospective cohort study database including inborn infants <34 weeks, without congenital malformations. Hypermagnesemia was considered if the umbilical magnesium level > 2.5 mEq/L. Maternal and neonatal variables were used to adjust the model, submitted to the multivariate hierarchical modelling process.

RESULTS

We evaluated 249 newborns with median birth weight and gestational age of 1375 (IQR 1020-1375) g and 31 (IQR 28-32) weeks, respectively. Hypermagnesemia occurred in 28.5% and admission hypothermia occurred in 28.9%. In the univariate analysis, the following variables were identified as being associated with admission hypothermia: hypermagnesemia (OR 3.71; CI 2.06-6.68), resuscitation (OR 2.39; CI 1.37-4.19), small to gestational age (OR 1.91; CI1.03-3.53), general anesthesia (OR 3.34; CI 1.37-8.13), birth weight (OR 0.998; CI 0.998-0.999) and gestational age (OR 0.806; CI 0.725-0.895). In the hierarchical regression model, hypermagnesemia remained independent associated with admission hypothermia (OR 3.20; CI 1.66-6.15), as well as birth weight (OR 0.999; CI 0.998-0.999) and tracheal intubation (3.83; CI 1.88-7.80).

CONCLUSION

Hypermagnesemia was associated with an increased risk of admission hypothermia, as did tracheal intubation and lower birth weight.

Comparing thermoregulatory responses between short and long moderate intensity intermittent exercise protocols with the same duty cycle

To date, the thermoregulatory response between continuous and intermittent exercises has been investigated whilst limited studies are available to examine the thermoregulatory responses between different modes of intermittent exercises. We sought to determine the effect of two patterns of short duration intermittent exercises (180:180 (3-min) and 30:30 s (30-s) work: rest) on thermoregulatory responses in a temperate environment (25 °C, 50% RH, vapor pressure: 1.6 kPa) with low airflow (0.2 m/s). Twelve male participants (Age:24.0(5.0) year; VO2max: 53(8) mL.kg-1.min-1; BSA:1.7(0.1) m2) cycled at 50% VO2max for 60 min in 3-min and 30-s intervals to result in the same 30-min net exercise duration. Core and skin temperatures, the percent increase of skin blood flow (forearm and chest) from baseline and local sweat rate (forearm and chest) were not different between 3-min and 30-s (all P > 0.35) from the onset of exercise to the end of the exercise. Similarly, the mean body temperature onsets of skin blood flow (forearm and chest) and local sweat rates (forearm and chest) were not different between different mode of intermittent exercises (all P > 0.1). Furthermore, thermal sensitivities of skin blood flow (forearm and chest) and local sweat rate (forearm and chest) with increasing mean body temperature were not different between different mode of intermittent exercises (all P > 0.1). We conclude that intermittent exercises with different work periods at moderate exercise intensity did not alter core temperature and thermoeffector responses in a temperate environment. (241/250).

Sex differences in thermal sensitivity and perception: Implications for behavioral and autonomic thermoregulation

Temperature sensitive receptors in the skin and deep body enable the detection of the external and internal environment, including the perception of thermal stimuli. Changes in heat balance require autonomic (e.g., sweating) and behavioral (e.g., seeking shade) thermoeffector initiation to maintain thermal homeostasis. Sex differences in body morphology can largely, but not entirely, account for divergent responses in thermoeffector and perceptual responses to environmental stress between men and women. Thus, it has been suggested that innate differences in thermosensation may exist between men and women. Our goal in this review is to summarize the existing literature that investigates localized and whole-body cold and heat exposure pertaining to sex differences in thermal sensitivity and perception, and the interplay between autonomic and behavioral thermoeffector responses. Overall, it appears that local differences in thermal sensitivity and perception are minimized, yet still apparent, when morphological characteristics are well-controlled. Sex differences in the early vasomotor response to environmental stress and subsequent changes in blood flow likely contribute to the heightened thermal awareness observed in women. However, the contribution of thermoreceptors to observed sex differences in thermal perception and thermoeffector function is unclear, as human studies investigating these questions have not been performed.

Influence of topical capsaicin cream on thermoregulation and perception during acute exercise in the heat

PURPOSE

Determine if topical capsaicin, a transient receptor potential vanilloid heat thermoreceptor activator, alters thermoregulation and perception when applied topically prior to thermal exercise.

METHODS

Twelve subjects completed 2 treatments. Subjects walked (1.6 m s^-1, 5% grade) for 30 min in the heat (38 °C, 60% relative humidity) with either a capsaicin (0.025% capsaicin) or control cream applied to the upper (shoulder to wrist) and lower (mid-thigh to ankle) limbs covering ∼50% body surface area. Skin blood flow (SkBF), sweat (rate, composition), heart rate, temperature (skin, core), and perceived thermal sensation were measured prior to and during exercise.

RESULTS

The relative change in SkBF was not different between treatments at any time point (p = 0.284). There were no differences in sweat rate between the capsaicin (1.23 ± 0.37 L h^-1) and control (1.43 ± 0.43 L h^-1, p = 0.122). There were no differences in heart rate between the capsaicin (122 ± 38 beats·min^-1) and control (125 ± 39 beats·min^-1, p = 0.431). There were also no differences in weighted surface (p = 0.976) or body temperatures (p = 0.855) between the capsaicin (36.0 ± 1.7 °C, 37.0 ± 0.8 °C, respectively) and control (36.0 ± 1.6 °C, 36.9 ± 0.8 °C, respectively). The capsaicin treatment was not perceived as hotter than the control treatment until minute 30 of exercise (2.8 ± 0.4, 2.5 ± 0.5, respectively, p = 0.038) CONCLUSIONS: Topical capsaicin application does not alter whole-body thermoregulation during acute exercise in the heat despite perceiving the treatment as hotter late in exercise.

Effects of heat exposure on the thermoregulatory responses of young children

The purpose of this study was to determine whether young children's thermoregulation during heat exposure varies with age and body size. A total of 34 young children (aged 6 months-8 years)-18 boys and 16 girls-participated in the study. They were divided into five groups according to age (<1 year, 1 year, 2-3 years, 4-5 years, and 8 years). The participants sat for 30 min in a 27°C, 50% rh room, then moved to a 35°C, 70% rh room and remained seated for at least 30 min. They then returned to the first 27°C room and remained stationary for 30 min. Rectal temperature (Tre) and skin temperature (Tsk) were continuously recorded, and the amount of whole-body sweat rate (SR) was measured. Local SR of the back and upper arm were collected with filter paper to calculate local sweating volume, and Na + concentration was measured later. The smaller the age, the significantly greater the increase in Tre. There was no significant difference in the amount of whole-body SR and the increase in Tsk during the heating among the five groups. Furthermore, there was no significant difference in whole-body SR per increase in Tre during heating between the five groups, but a significant difference was found in back local SR per increase in Tre with age. Difference in local SR between upper arm and back was observed at age 2 years and above, and difference in Na⁺ concentrations in sweat was observed at age 8 years. The development of thermoregulatory responses with growth was observed. The results indicate that the thermoregulatory response is disadvantaged by immature mechanisms and small body size in younger children.

Induction and decay of seasonal acclimatization on whole body heat loss responses during exercise in a hot humid environment with different air velocities

Whether whole body heat loss and thermoregulatory function (local sweat rate and skin blood flow) are different between summer and autumn and between autumn and winter seasons during exercise with different air flow in humid heat remain unknown. We therefore tested the hypotheses that whole body sweat rate (WBSR), evaporated sweat rate, and thermoregulatory function during cycling exercise in autumn would be higher than in winter but would be lower than in summer under hot-humid environment (32 C, 75% RH). We also tested the hypothesis that the increase of air velocity would enhance evaporated sweat rate and sweating efficiency across winter, summer, and autumn seasons. Eight males cycled for 1 h at 40% V̇o_2max in winter, summer, and autumn seasons. Using an electric fan, air velocity increased from 0.2 m/s to 1.1 m/s during the final 20 min of cycling. The autumn season resulted in a lower WBSR, unevaporated sweat rate, and a higher sweating efficiency compared with summer (all P ≤ 0.05) but WBSR and unevaporated sweat rate in autumn were higher than in winter and thus sweating efficiency was lower when compared with winter only at the air velocity of 0.2 m/s (All P ≤ 0.05). Furthermore, evaporated sweat rate and core temperature (T_core) were not different among winter, summer, and autumn seasons (All P > 0.19). In conclusion, changes in WBSR across different seasons do not alter T_core during exercise in a hot humid environment. Furthermore, increasing air velocity enhances evaporated sweat rate and sweating efficiency across all seasons.

Human temperature regulation under heat stress in health, disease, and injury

The human body constantly exchanges heat with the environment. Temperature regulation is a homeostatic feedback control system that ensures deep body temperature is maintained within narrow limits despite wide variations in environmental conditions and activity-related elevations in metabolic heat production. Extensive research has been performed to study the physiological regulation of deep body temperature. This review focuses on healthy and disordered human temperature regulation during heat stress. Central to this discussion is the notion that various morphological features, intrinsic factors, diseases, and injuries independently and interactively influence deep body temperature during exercise and/or exposure to hot ambient temperatures. The first sections review fundamental aspects of the human heat stress response, including the biophysical principles governing heat balance and the autonomic control of heat loss thermoeffectors. Next, we discuss the effects of different intrinsic factors (morphology, heat adaptation, biological sex, and age), diseases (neurological, cardiovascular, metabolic, and genetic), and injuries (spinal cord injury, deep burns, and heat stroke), with emphasis on the mechanisms by which these factors enhance or disturb the regulation of deep body temperature during heat stress. We conclude with key unanswered questions in this field of research.