Exertional Hyperthermia Research Articles

Post-exercise management of exertional hyperthermia in dogs participating in dog sport (canicross) events in the UK

Exercise is a common trigger of heat-related illness (HRI) events in dogs, accounting for 74% of canine HRI cases treated under primary veterinary care in the United Kingdom. However, few empirical studies have evaluated the effectiveness of differing cooling methods for dogs with exertional hyperthermia or HRI. This study aimed to prospectively evaluate effects of ambient conditions and post-exercise management practices (cooling methods and vehicular confinement) on the post-exercise temperature change of dogs participating in UK canicross events. Canine temperature was recorded at three intervals post-exercise: as close as possible to 0- (immediately post-exercise), 5-, and 15-min post-exercise. Ambient conditions and post-exercise management were recorded for 115 cooling profiles from 52 dogs. In 28/115 (24.4%) profiles, the dog's temperature increased during the first 5-min post-exercise. Overall, 68/115 (59.1%) profiles included passive cooling (stood or walked outside), 35 (30.4%) active cooling (cold-water immersion or application of a cooling coat), and 12 (10.4%) involved no cooling and were immediately housed in vehicles. No dogs developed hypothermia during the study and no adverse effects were observed from any cooling method. In hyperthermic dogs, overall post-exercise body temperature change was significantly negatively associated (i.e. the dogs cooled more) with 0-min post-exercise body temperature (β = −0.93, p < 0.001), and not being housed in a vehicle (β = −0.43, p = 0.013). This study provides evidence cold-water immersion (in water at 0.1–15.0 °C) can be used to effectively and safely cool dogs with exertional hyperthermia. Progressive temperature increases in many dogs - even after exercise has terminated - supports the message to “cool first, transport second” when managing dogs with HRI. When transporting dogs post-exercise or with HRI even after active cooling, care should be taken to cool the vehicle before entry and promote air movement around the dog during transport to facilitate ongoing cooling and prevent worsening of hyperthermia during travel.

Lower limb hyperthermia augments functional hyperaemia during small muscle mass exercise similarly in trained elderly and young humans.

What is the central question of the study? Ageing is postulated to lead to underperfusion of human limb tissues during passive and exertional hyperthermia, but findings to date have been equivocal. Thus, does age have an independent adverse effect on local haemodynamics during passive single-leg hyperthermia, single-leg knee-extensor exercise and their combination? What is the main finding and its importance? Local hyperthermia increased leg blood flow over three-fold and had an additive effect during knee-extensor exercise with no absolute differences in leg perfusion between the healthy, exercise-trained elderly and the young groups. Our findings indicate that age per se does not compromise lower limb hyperaemia during local hyperthermia and/or small muscle mass exercise. Heat and exercise therapies are recommended to improve vascular health across the lifespan. However, the haemodynamic effects of hyperthermia, exercise and their combination are inconsistent in young and elderly people. Here we investigated the acute effects of local-limb hyperthermia and exercise on limb haemodynamics in nine healthy, trained elderly (69±5 years) and 10 young (26±7 years) adults, hypothesising that the combination of local hyperthermia and exercise interact to increase leg perfusion, albeit to a lesser extent in the elderly. Participants underwent 90min of single whole-leg heating, with the contralateral leg remaining as control, followed by 10min of low-intensity incremental single-leg knee-extensor exercise with both the heated and control legs. Temperature profiles and leg haemodynamics at the femoral and popliteal arteries were measured. In both groups, heating increased whole-leg skin temperature and blood flow by 9.5±1.2°C and 0.7±0.2Lmin-1 (>3-fold), respectively (P<0.0001). Blood flow in the heated leg remained 0.7±0.6 and 1.0±0.8Lmin-1 higher during exercise at 6 and 12W, respectively (P<0.0001). However, there were no differences in limb haemodynamics between cohorts, other than the elderly group exhibiting a 16±6% larger arterial diameter and a 51±6% lower blood velocity following heating (P<0.0001). In conclusion, local hyperthermia-induced limb hyperperfusion and/or small muscle mass exercise hyperaemia are preserved in trained older people despite evident age-related structural and functional alterations in their leg conduit arteries.

Exertional, But Not Passive, Hyperthermia Increases Gastrointestinal Permeability

Exertional hyperthermia reduces splanchnic blood flow which leads to gastrointestinal ischemia. This is associated with increased gastrointestinal perturbations and endotoxemia which are potential precursors of exertional heat stroke. However, whether hyperthermia alone can result in significant disruption to gastrointestinal integrity remains unclear. PURPOSE: To determine whether exertional or passive hyperthermia contributes to greater changes in gastrointestinal permeability METHODS: 16 physically active males (mean ± SD; age: 26 ± 3, VO2max: 65 ± 6 ml/kg/min) completed three randomised trials. Blood samples were obtained at baseline and after (a) 70% VO2max run (RUN; Tdb: 30.0 ± 0.1 °C, RH: 71 ± 1%), (b) 50% VO2max walk (WALK; Tdb: 30.0 ± 0.0 °C, RH: 71 ± 0%) and (c) passive heating via warm water immersion at 42 °C (PaH). Serum concentrations of I-FABP, LPS and IL-6 were determined using ELISA. Gastrointestinal temperature (Tgi), heart rate (HR), physiological strain index (PSI) and changes in blood biomarkers were assessed using 1-way ANOVA or paired t-test with p < 0.05 as statistically significant. RESULTS: Participants observed similar baseline Tgi (37.0 ± 0.2 °C, p = 0.204) and HR (63 ± 7 bpm; p = 0.118). End Tgi was similar for RUN (39.4 ± 0.2 °C) and PaH (39.3 ± 0.3 °C, p = 0.289), but lower in WALK (37.9 ± 0.3 °C, p < 0.001). Mean PSI was greatest in RUN (7.4 ± 0.5), followed by PaH (5.2 ± 0.6) and WALK (3.1 ± 0.7, p < 0.001). Serum concentrations of I-FABP, LPS and IL-6 were similar at baseline (p > 0.05). Serum I-FABP and IL-6 concentrations increased from baseline to post-trial in RUN (I-FABP: 797 ± 467 pg/ml vs 1801 ± 1432 pg/ml, p = 0.007; IL-6: 4.3 ± 3.4 pg/ml vs 9.3 ± 5.4 pg/ml, p < 0.001) and WALK (I-FABP: 772 ± 460 pg/ml vs 1121 ± 966 pg/ml, p = 0.044; IL-6: 4.5 ± 4.1 pg/ml vs 9.1 ± 8.5 pg/ml, p = 0.004) but not in PaH (I-FABP: 888 ± 572 pg/ml vs 893 ± 550 pg/ml, p = 0.935; IL-6: 5.3 ± 5.4 pg/ml vs 5.5 ± 5.2 pg/ml, p = 0.285). No differences in post-trial serum LPS concentrations were observed (p = 0.337). CONCLUSION: Hyperthermia alone may be insufficient to trigger changes in gastrointestinal permeability. Disruptions to gastrointestinal integrity during exertional hyperthermia may instead be exercise-induced, stemming from mechanical and metabolic stress arising from physical exertion. Supported by NUS Internal Research Funding

Influence of sex and biological maturation on the sudomotor response to exercise-heat stress: are girls disadvantaged?

Both adult females and children have been reported to have a lower sweating capacity and thus reduced evaporative heat loss potential that may increase their susceptibility to exertional hyperthermia in the heat. Compared with males, females have a lower maximal sweat rate and thus a theoretically lower maximum skin wettedness due to a lower sweat output per gland. Similarly, children have been suggested to be disadvantaged in high ambient temperatures due to a lower sweat production and therefore reduced evaporative capacity, despite modifications of heat transfer due to physical attributes and possible evaporative efficiency. The reported reductions in the sudomotor activity of females and children suggest a lower sweating capacity in girls. However, because of the complexities of isolating sex and maturation from the confounding effects of morphological differences (e.g., body surface area-to-mass ratio) and metabolic heat production, limited evidence exists supporting whether children, and, more specifically, girls are at a thermoregulatory disadvantage. Furthermore, a limited number of child-adult comparison studies involve females and very few studies have directly compared regional and whole body sudomotor activity between boys and girls. This minireview highlights the exercise-induced sudomotor response of females and children, summarizes previous research investigating the sudomotor response to exercise in girls, and suggests important areas for further research.

The Management of Hyperthermia and Exercise-Associated Hyponatremia in Low-Resource and Prehospital Settings

Between 2000 and 2016, the number of people exposed to heat waves worldwide increased by around 125 million. As global warming increases, there is growing concern regarding the effect of heat stress on health outcomes, particularly in low- and middle-income tropical countries. The likelihood of developing heat-related injury depends on three factors: an individual, their environment, and their workload. Both exertional hyperthermia and exercise-induced hyponatremia are potentially life-threatening conditions that may develop in environments with increased heat stress. These can present with vague and overlapping symptoms such as confusion, headache, vomiting, and in severe cases, coma. The management of these two conditions is extremely different, and making a correct diagnosis can be challenging, particularly for health-care professionals who are working in low-resource or prehospital settings.

2020 American Heart Association and American Red Cross Focused Update for First Aid.

2020 American Heart Association and American Red Cross Focused Update for First Aid.

2020 International Consensus on First Aid Science With Treatment Recommendations

This is the summary publication of the International Liaison Committee on Resuscitation’s 2020 International Consensus on First Aid Science With Treatment Recommendations. It addresses the most recent published evidence reviewed by the First Aid Task Force science experts. This summary addresses the topics of first aid methods of glucose administration for hypoglycemia; techniques for cooling of exertional hyperthermia and heatstroke; recognition of acute stroke; the use of supplementary oxygen in acute stroke; early or first aid use of aspirin for chest pain; control of life- threatening bleeding through the use of tourniquets, haemostatic dressings, direct pressure, or pressure devices; the use of a compression wrap for closed extremity joint injuries; and temporary storage of an avulsed tooth. Additional summaries of scoping reviews are presented for the use of a recovery position, recognition of a concussion, and 6 other first aid topics. The First Aid Task Force has assessed, discussed, and debated the certainty of evidence on the basis of Grading of Recommendations, Assessment, Development, and Evaluation criteria and present their consensus treatment recommendations with evidence-to-decision highlights and identified priority knowledge gaps for future research.The 2020 International Consensus on Cardiopulmonary Resuscitation (CPR) and Emergency Cardiovascular Care (ECC) Science With Treatment Recommendations (CoSTR) is the fourth in a series of annual summary publications from the International Liaison Committee on Resuscitation (ILCOR). This 2020 CoSTR for first aid includes new topics addressed by systematic reviews performed within the past 12 months. It also includes updates of the first aid treatment recommendations published from 2010 through 2019 that are based on additional evidence evaluations and updates. As a result, this 2020 CoSTR for first aid represents the most comprehensive update since 2010.

2020 International Consensus on First Aid Science With Treatment Recommendations

This is the summary publication of the International Liaison Committee on Resuscitation's 2020 International Consensus on First Aid Science With Treatment Recommendations. It addresses the most recent published evidence reviewed by the First Aid Task Force science experts. This summary addresses the topics of first aid methods of glucose administration for hypoglycemia; techniques for cooling of exertional hyperthermia and heatstroke; recognition of acute stroke; the use of supplementary oxygen in acute stroke; early or first aid use of aspirin for chest pain; control of life-threatening bleeding through the use of tourniquets, hemostatic dressings, direct pressure, or pressure devices; the use of a compression wrap for closed extremity joint injuries; and temporary storage of an avulsed tooth. Additional summaries of scoping reviews are presented for the use of a recovery position, recognition of a concussion, and 6 other first aid topics. The First Aid Task Force has assessed, discussed, and debated the certainty of evidence on the basis of Grading of Recommendations, Assessment, Development, and Evaluation criteria and present their consensus treatment recommendations with evidence-to-decision highlights and identified priority knowledge gaps for future research.

Combined Effects of Hypohydration, Muscle Damage, and Exertional Hyperthermia on Biomarkers of Acute Kidney Injury

Dehydration, exertional hyperthermia, and muscle damage commonly occur in athletic, military, and occupational settings, yet, their combined effects on biomarkers of acute kidney injury are not well understood. PURPOSE: Investigate the combination of dehydration, muscle damage, and exercise in the heat on biomarkers of renal stress. METHODS: Six male participants (age 24±5 y, body mass 74.9±6.3 kg, body fat 14.5±4.1%) completed two trials, one euhydrated (EU; fluid replacement ≤2% body mass loss) and one hypohydrated (HY; fluid restriction 24-h prior to and throughout exercise), separated by ≥28 days. Trials consisted of muscle damaging unilateral eccentric knee flexion, 60 minutes of treadmill running (~60% VO2max) in the heat (33 °C, 54% RH), and 30 minutes of passive recovery. Participants were provided a rehydration protocol in both trials and returned 24-h later for a follow-up visit. RESULTS: Urine osmolality when HY was greater pre- (HY 1045±102, EU 612±142 mOsm/kg; P <0.01) and post-trial (HY 1007±105, EU 503±205 mOsm/kg; P <0.01), but not 24-h post (HY 543±310, EU 545±404 mOsm/kg; P = 0.98). Serum osmolality was also different pre-trial (HY 301±5, EU 290±5 mOsm/kg; P = 0.02), but was similar 24-h post (HY 295±4, EU 293±3 mOsm/kg; P = 0.25). Isometric strength was reduced regardless of condition immediately after eccentric exercise (grand mean Δ = -33.6±27.9 N·m, P = 0.03). Rectal temperature increased to a greater degree when HY (2.11±0.60 °C) compared to EU (1.65±0.44 °C; P = 0.01). Plasma neutrophil gelatinase-associated lipocalin (pNGAL) increased independent of condition (grand means: pre- 59.9±7.3, post-exercise 77.6±12.0 ng/mL, P <0.01), but was not different between trials (P = 0.84). However, percent changes from baseline in pNGAL were greater, regardless of time, when HY (19.1±7.5%) compared to EU (6.1±11.2%, P < 0.01). CONCLUSION: Exercise in the heat with muscle damage increased renal strain when HY and resulted in greater changes in pNGAL, a biomarker of acute kidney injury. These preliminary findings suggest that improper fluid consumption prior to and during exercise may augment renal stress, yet the long-term consequences of these detriments require further investigation. *Supported by funding from Central States ACSM Student Grant Award

Defining Acceptable Cold‐Water Immersion Times for the Treatment of Exertional Hyperthermia When Rectal Temperature Measurements are not Available

Exertional hyperthermia is most effectively treated with immediate whole‐body cold‐water immersion to rapidly cool the body to the safe rectal temperature (Tre) of 38.6°C, which is the cooling limit recommended to negate any risk associated with overcooling hyperthermic individuals. However, the safe use of cold‐water immersion is reliant on real‐time measurement of Tre to identify the appropriate immersion time, which is often unavailable in emergency situations. The purpose of this study was to determine appropriate immersion times for the immediate treatment of exertional hyperthermia for a range of water temperatures (2°C to 26°C) and following a treatment delay (20 min and 40 min) to guide emergency management in the absence of continuous Tre measurements. Data were extracted from seven published studies (143 experimental sessions) involving 79 participants (65 males, 14 females; age: 26 ± 6 years; height: 177 ± 9 cm; weight, 76.9 ± 13.0 kg; body fat: 17.3 ± 6.9%; peak oxygen consumption: 56.9 ± 7.9 mL/kg/min). In each study, individuals rendered hyperthermic (Tre 39.5–40.0°C) during exercise in the heat were treated immediately with immersion in water at 2°C, 8°C, 14°C, or 26°C. Separate study conditions assessed cold‐water immersion (2°C) following a 20 and 40 min treatment delay. From these data, receiver operating characteristics curves were used to define acceptable cooling times (time for Tre to reach 38.6°C) for each condition (all p&lt;0.05). Our analysis revealed that 9 min would be the recommended cooling time for immersion in water temperatures ranging from 2°C to 8°C (average sensitivity at 2°C: 0.84, average sensitivity at 8°C: 0.82; average specificity at 2°C: 0.82, average specificity at 8°C: 0.96). For immersion in water temperatures between 14°C and 20°C, the suggested cooling time was 10 min (average sensitivity: 0.80; average specificity: 0.76), whereas it was 14 min at 26°C (average sensitivity: 0.73; average specificity: 0.79). When the gold standard water temperature of 2°C is available, a cooling time of 11 min is recommended if immersion is delayed between 20 and 40 min (average sensitivity: 0.84; average specificity: 0.94). Our results provide the first clinical guidance to health practitioners for the emergency treatment of exertional hyperthermia using water immersion at a range of water temperatures and following a treatment delay when Tre measurements are unavailable.Support or Funding InformationNatural Sciences and Engineering Research Council of Canada (grant held by Dr. Glen P. Kenny).This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Physical characteristics cannot be used to predict cooling time using cold-water immersion as a treatment for exertional hyperthermia.

We examined if physical characteristics could be used to predict cooling time during cold water immersion (CWI, 2 °C) following exertional hyperthermia (rectal temperature ≥39.5 °C) in a physically heterogeneous group of men and women (n = 62). Lean body mass was the only significant predictor of cooling time following CWI (R2 = 0.137; P < 0.001); however, that prediction did not provide the precision (mean residual square error: 3.18 ± 2.28 min) required to act as a safe alternative to rectal temperature measurements.

Effectiveness of Ice-Sheet Cooling Following Exertional Hyperthermia.

The procedure of wrapping a heat casualty in ice-water soaked bed sheets to reduce core temperature has received little investigation, despite the practice and recommendation for its use in some military settings. Thus, the purpose of this study was to investigate the cooling efficacy of ice-sheet cooling (ISC) following exertional hyperthermia. 13 (11 males, 2 females) participants (age = 23 ± 3 years, height = 176.5 ± 10.3 cm, mass = 78.6 ± 15.3 kg, body fat = 19.6 ± 8.6%, and body surface area = 1.95 ± 0.22 m2) volunteered to complete 2 randomized, crossover design trials on an outdoor recreation field (34.4 ± 1.4°C, 54.4 ± 4.1% relative humidity). Each trial consisted of exercise (self-paced 400-m warm-up, 1,609-m run, and 100-m sprints) followed by 15 minutes of either lying supine in the shade with no treatment (control [CON]) or being treated with ice-water soaked sheets wrapped around their body (ISC). Physiological (rectal temperature [Tre], heart rate, mean-weighted skin temperature) and perceptual measures (thermal sensation, rating of perceived exertion) were assessed after each exercise protocol, every 3 minutes during treatment, and every 5 minutes during recovery. By design, there were no differences during exercise between ISC and CON for Tre (p = 0.16), skin temperature (p = 0.52), heart rate (p = 0.62), thermal sensation (p = 0.89), or rating of perceived exertion (p = 0.99). There were greater decreases in Tre at 3 (ISC 0.33 ± 0.26°C vs. CON 0.03 ± 0.30°C, p = 0.01) and 6 minutes (ISC 0.47 ± 0.27°C vs. CON 0.30 ± 0.19°C, p = 0.05) of treatment; however, the overall rate of cooling was not different between trials (CON 0.05 ± 0.02°C/min vs. ISC 0.06 ± 0.02°C/min, p = 0.72). Skin temperature (Tsk) was significantly reduced from 3 minutes (ISC 34.4 ± 1.7°C vs. CON 36.6 ± 0.5°C, p = 0.007) through 15 minutes (ISC 32.4 ± 1.5 vs. CON 36.1 ± 0.4°C, p < 0.001) of treatment. There was a trend for lower heart rate with ISC (p = 0.051). Thermal sensation was reduced from 3 minutes of treatment (ISC 3.5 ± 0.9 vs. CON 4.5 ± 0.6, p = 0.002) through 15 minutes (ISC 2.8 ± 1.0 vs. CON 3.9 ± 0.4, p = 0.005). ISC does not provide effective reduction in Tre following exertional hyperthermia compared to no treatment. However, perceptual benefits may warrant the use of ISC in settings where rapid reductions in core temperature are not a concern (i.e., recovery from exercise). Thus, clinicians should continue to utilize validated techniques (i.e., cold-water immersion) for the treatment of exertional heat illnesses.

Curcumin Improves Systemic Responses to Exertional Hyperthermia but Doesn’t Alter Protein Expression in Circulating Leukocytes

Exertional heat stress increases gastrointestinal barrier permeability and risk of exertional heatstroke (EHS) via a TLR4-mediated inflammatory pathway. Curcumin has been shown to inhibit TLR4 signaling in vitro but has not been examined in a human exertional heat stress model. PURPOSE: This work investigated the effect of 3d of 500mg/d dietary curcumin supplementation on the cellular and systemic responses to exertional heat stress in non-heat acclimated humans. METHODS: Subjects (N=6) ran (65%VO2max) for 60min inside an environmental chamber (37°C/26%RH) two times (CURCUMIN/PLACEBO). Core temperature (Tc), heart rate (HR), and physiological strain index (PSI) were measured throughout exercise. Peripheral blood mononuclear cells (PBMC) were isolated from blood samples that were taken before (PRE), after (POST), 1hr (1-POST), and 4hrs after (4-POST) exercise. The protein content of markers along the TLR4 signaling pathway (TLR4, MyD88, pNFKB, NFKB) and indicators of cellular energy status (SIRT1 & p-AMPK) were determined with Western Blot. Group differences were determined with 2-Way (Condition x Time) RM ANOVAs. RESULTS: Under CURCUMIN, Tc rose less (0.23±0.15°C; p<0.01) and both HR and PSI were lower from 45-60min of exercise (HR: 9±2 bpm, PSI: 12±1%; p< 0.05). In PBMC, the ratio of pNFKB to NFKB at 1-POST was increased by 64% in PLACEBO and 51% in CURCUMIN (p=0.05). Intriguingly, TLR4 was reduced at 1POST in both conditions (PLACEBO: -28%, CURCUMIN: -17%; p=0.05), as were pAMPK (PLACEBO: -62%, CURCUMIN: -57%; p<0.01) and SIRT1 (PLACEBO: -61%, CURCUMIN: -48%; p=0.02). CONCLUSIONS: Despite robust improvements in systemic responses to exertional heat stress under CURCUMIN, there was no difference in the protein expression profile of PBMC that were collected under CURCUMIN and PLACEBO conditions. However, in both conditions we did note a significant elevation in pNFKB:NFKB at 1-POST, which (ironically) coincided with a significant reduction in TLR4 (but not MyD88). At present, our working hypothesis is that pAMPK and SIRT1 were downregulated in an effort to maintain the pro-inflammatory capacity of PBMC during this “open window”, an effect that has not previously been described in PBMC collected under exertional heat stress conditions.

Differences in tolerance to exertional hyperthermia between male and female mice

Surveillance reports suggest possible reductions in heat stroke susceptibility in female vs. male active component members of the US Armed Forces, but whether these differences reflect behavior or underlying biology is unknown. Previous studies in mice have shown that females exhibit markedly better resistance to moderate, passive heat. However, whether heat tolerance translates to acute settings or to exertional heat stroke (EHS) has not been tested. In this study, we compared responses in male and female mice to an established model of EHS. Exercise trained mice (3 wks) were maintained at 37.5°C (35% RH) and ran using a preprogrammed incremental protocol on a forced running wheel. The EHS end point was defined as loss of consciousness. Female mice on average ran longer than males (177 vs. 124 min; p=0.0001), and were exposed to greater heat loads (241 vs. 160 °C • min; p=0.0001). Male and female mice ran to nearly identical average peak core temperatures, both 42.2°C (n.s.). There were no differences in the minimum temperature during post EHS hypothermia 32°C (n.s.) or the time to reach the minimum temperature. However, females lost a greater % body weight (9.2% vs 7.5% p &lt; 0.001), demonstrated significantly higher levels of circulating corticosterone (286 vs 183 ng/ml, p = 0.001, 3 h) and higher levels of resistin polypeptide (8891 vs. 3781 pg/ml, p = 0.004, 3 h). These results demonstrate that female mice have greater resistance to EHS during exercise in hyperthermia. Possible mechanisms include greater body surface to mass ratio in females vs. males (3.3 vs. 3.2 m2/kg; p=0.0001), greater aerobic conditioning in females (characteristic of mice), or a hormonally or genetically induced resistance to hyperthermia. Though controversial, marked elevations in circulating corticosterone and resistin in females have the capacity to contribute to improved heat tolerance. We conclude that female mice are significantly more resistant to EHS than male mice. Inherent thermal tolerance in female mice may provide an evolutionary advantage because metabolic rate and heat production have been shown to double during pregnancy and lactation. Author views not official US Army or DoD policy. W81XWH‐15‐2‐0038

Cooling Effectiveness of a Modified Cold-Water Immersion Method After Exercise-Induced Hyperthermia.

Recommended treatment for exertional heat stroke includes whole-body cold-water immersion (CWI). However, remote locations or monetary or spatial restrictions can challenge the feasibility of CWI. Thus, the development of a modified, portable CWI method would allow for optimal treatment of exertional heat stroke in the presence of these challenges. To determine the cooling rate of modified CWI (tarp-assisted cooling with oscillation [TACO]) after exertional hyperthermia. Randomized, crossover controlled trial. Environmental chamber (temperature = 33.4°C ± 0.8°C, relative humidity = 55.7% ± 1.9%). Sixteen volunteers (9 men, 7 women; age = 26 ± 4.7 years, height = 1.76 ± 0.09 m, mass = 72.5 ± 9.0 kg, body fat = 20.7% ± 7.1%) with no history of compromised thermoregulation. Participants completed volitional exercise (cycling or treadmill) until they demonstrated a rectal temperature (Tre) ≥39.0°C. After exercise, participants transitioned to a semirecumbent position on a tarp until either Tre reached 38.1°C or 15 minutes had elapsed during the control (no immersion [CON]) or TACO (immersion in 151 L of 2.1°C ± 0.8°C water) treatment. The Tre, heart rate, and blood pressure (reported as mean arterial pressure) were assessed precooling and postcooling. Statistical analyses included repeated-measures analysis of variance with appropriate post hoc t tests and Bonferroni correction. Before cooling, the Tre was not different between conditions (CON: 39.27°C ± 0.26°C, TACO: 39.30°C ± 0.39°C; P = .62; effect size = -0.09; 95% confidence interval [CI] = -0.2, 0.1). At postcooling, the Tre was decreased in the TACO (38.10°C ± 0.16°C) compared with the CON condition (38.74°C ± 0.38°C; P < .001; effect size = 2.27; 95% CI = 0.4, 0.9). The rate of cooling was greater during the TACO (0.14 ± 0.06°C/min) than the CON treatment (0.04°C/min ± 0.02°C/min; t15 = -8.84; P < .001; effect size = 2.21; 95% CI = -0.13, -0.08). These differences occurred despite an insignificant increase in fluid consumption during exercise preceding CON (0.26 ± 0.29 L) versus TACO (0.19 ± 0.26 L; t12 = 1.73; P = .11; effect size = 0.48; 95% CI = -0.02, 0.14) treatment. Decreases in heart rate did not differ between the TACO and CON conditions (t15 = -1.81; P = .09; effect size = 0.45; 95% CI = -22, 2). Mean arterial pressure was greater at postcooling with TACO (84.2 ± 6.6 mm Hg) than with CON (67.0 ± 9.0 mm Hg; P < .001; effect size = 2.25; 95% CI = 13, 21). The TACO treatment provided faster cooling than did the CON treatment. When location, monetary, or spatial restrictions are present, TACO represents an effective alternative to traditional CWI in the emergency treatment of patients with exertional hyperthermia.

Effects of mild hypohydration on cooling during cold-water immersion following exertional hyperthermia.

We investigated the effects of mild hypohydration compared to euhydration on the cooling efficacy of cold-water immersion (CWI). Fourteen participants (eight male, six female; age 26 ± 5 years; ht 1.77 ± 0.08 m; wt 72.2 ± 8.8 kg; 20.6 ± 7.4 % body fat) completed one euhydrated (EU) trial followed by one hypohydrated trial (HY; via 24 h fluid restriction) in an environmental chamber (33.6 ± 0.9 °C, 55.8 ± 1.7 % RH). Volitional exercise was performed in a manner that matched end-exercise rectal temperature (T re) through repeating exercise mode and intensity. Participants were then immersed in ice water (2.0 ± 0.8 °C) until T re reached 38.1 °C or for a maximum of 15 min. T re, heart rate (HR), skin blood flux (SBF) and mean skin temperature (T sk) were monitored continuously during cooling. Pre-cooling body mass was decreased in the HY trial (-2.66 ± 1.23 % body mass) and maintained in the EU trial (-0.66 ± 0.44 %) compared to baseline mass (P < 0.001). Cooling rates were faster when EU (0.14 ± 0.05 °C/min) compared to HY (0.11 ± 0.05 °C/min, P = 0.046). HR, SBF, and T sk were not different between EU and HY trials (P > 0.05), however, all variables significantly decreased with immersion independent of hydration status (P < 0.001). The primary finding was that hypohydration modestly attenuates the rate of cooling in exertionally hyperthermic individuals. Regardless of hydration status, the cooling efficacy of CWI was preserved and should continue to be utilized in the treatment of exertional hyperthermia.

The Influence of Physical Characteristics in the Treatment of Exertional Hyperthermia by Cold Water Immersion

Cold water immersion is the gold standard treatment for the immediate cooling of exertional heat stroke victims. However, current cooling guidelines for the use of cold water immersion do not consider physical characteristics, which are known to affect core temperature cooling rates when normothermic individuals are immersed in cold water. PURPOSE: To examine the relative influence of physical characteristics on rectal temperature cooling rates during cold water immersion following exercise-induced hyperthermia (as defined by rectal temperature of ≥39.5°C). METHODS: Data from 71 participants (59 males, 12 females) from 6 previously published studies performed in our laboratory were retrospectively analyzed using stepwise multiple regression to assess the relative influence of physical characteristics (height, body mass lean body mass, percent body fat, body surface area, body surface area to mass ratio and body surface area to lean body mass ratio) on rectal temperature cooling rate during cold water immersion performed on individuals rendered hyperthermic (39.5-40.0°C) during exercise in the heat. Whole-body cooling began within 5 min of end-exercise in a circulated cold water bath (2-8°C) and was continued until rectal temperature returned to 37.5°C. RESULTS: Participants physical characteristics were as follows: height; 177 ± 8 cm, body mass; 77.3 ± 13.0 kg, body surface area; 1.94 ± 0.19 m2, body surface area to mass ratio; 253.9 ± 19.2 cm2/kg and body surface area to lean body mass ratio; 310.3 ± 25.9 cm2/kg. Participants were immersed for 13.6 ± 6.2 min and cold water immersion provided a mean rectal temperature cooling rate of 0.21 ± 0.10°C/min. Lean body mass was the only physical characteristic included in the model that significantly explained some (7.4%) of the variance in rectal temperature cooling rate (Adjusted R2 = 0.074, P = 0.013). CONCLUSIONS: Within a broad range of individuals, lean body mass was the only predictor of rectal temperature cooling rate, however it only explained a small proportion of the variance. These data suggest that cold water immersion may be effective in negating the potential influence of physical characteristics on rectal temperature cooling rate during the treatment of exertional heat stroke. SUPPORT: Natural Sciences and Engineering Research Council of Canada (RGPIN-298159-2009, RGPIN-06313-2014).

Incidence and predictors of exertional hyperthermia after a 15-km road race in cool environmental conditions

Current knowledge about the incidence and risk factors for exertional hyperthermia (core body temperature ≥40°C) is predominantly based on military populations or small-sized studies in athletes. We assessed the incidence of exertional hyperthermia in 227 participants of a 15-km running race, and identified predictors for exertional hyperthermia. Observational study. We measured intestinal core body temperature before and immediately after the race. To identify predictive factors of maximum core body temperature, we entered sex, age, BMI, post-finish dehydration, number of training weeks, fluid intake before and during the race, finish time, and core body temperature change during warming-up into a backward linear regression analysis. Additionally, two subgroups of hyperthermic and non-hyperthermic participants were compared. In a WBGT of 11°C, core body temperature increased from 37.6±0.4°C at baseline to 37.8±0.4°C after warming-up, and 39.2±0.7°C at the finish. A total of 15% of all participants had exertional hyperthermia at the finish. Age, BMI, fluid intake before the race and the core body temperature change during warming-up significantly predicted maximal core body temperature (p<0.001). Participants with hyperthermia at the finish line had a significantly greater core body temperature rise (p<0.01) during the warming-up compared to non-hyperthermic peers, but similar race times (p=0.46). 15% of the recreational runners developed exertional hyperthermia, whilst core body temperature change during the warming-up was identified as strongest predictor for core body temperature at the finish. This study emphasizes that exertional hyperthermia is a common phenomenon in recreational athletes, and can be partially predicted.

L-Arginine Supplementation Prevents Increases in Intestinal Permeability and Bacterial Translocation in Male Swiss Mice Subjected to Physical Exercise under Environmental Heat Stress1–3

Dietary supplementation with L-arginine has been shown to improve the intestinal barrier in many experimental models. This study investigated the effects of arginine supplementation on the intestinal permeability and bacterial translocation (BT) induced by prolonged physical exercise under heat stress. Under anesthesia, male Swiss mice (5-wk-old) were implanted with an abdominal sensor to record their core body temperature (TCORE). After recovering from surgery, the mice were divided into 3 groups: a non-supplemented group that was fed the standard diet formulated by the American Institute of Nutrition (AIN-93G; control), a non-supplemented group that was fed the AIN-93G diet and subjected to exertional hyperthermia (H-NS), and a group supplemented with L-arginine at 2% and subjected to exertional hyperthermia (H-Arg). After 7 d of treatment, the H-NS and H-Arg mice were forced to run on a treadmill (60 min, 8 m/min) in a warm environment (34°C). The control mice remained at 24°C. Thirty min before the exercise or control trials, the mice received a diethylenetriamine pentaacetic acid (DTPA) solution labeled with technetium-99m (99mTc-DTPA) or 99mTc-Escherichia coli by gavage to assess intestinal permeability and BT, respectively. The H-NS mice terminated the exercise with Tcore values of ∼40°C, and, 4 h later, presented a 12-fold increase in the blood uptake of 99mTc-DTPA and higher bacterial contents in the blood and liver than the control mice. Although supplementation with arginine did not change the exercise-induced increase in Tcore, it prevented the increases in intestinal permeability and BT caused by exertional hyperthermia. Our results indicate that dietary L-arginine supplementation preserves the integrity of the intestinal epithelium during exercise under heat stress, acting through mechanisms that are independent of Tcore regulation.

Neck cooling and cognitive performance following exercise-induced hyperthermia

To assess the efficacy of neck cooling on cognitive performance following exertional hyperthermia. Twelve healthy men completed two experimental trials [control (CON) and neck cooling collar (NCC)] in a counter-balanced design. They ran on a treadmill at 70% VO2peak under warm and humid conditions (dry bulb temperature: 30.2 ± 0.3 °C, relative humidity: 71 ± 2 %) for 75 min or until volitional exhaustion. Gastrointestinal, neck and skin temperatures, heart rate and subjective ratings were assessed. Serum brain-derived neurotrophic factor (BDNF) levels were measured before and after each run. Cognitive performance comprising symbol digit matching, search and memory, digit span, choice reaction time and psychomotor vigilance test (PVT) were assessed before and after exercise. Mean gastrointestinal temperature was similar after exercise between trials (CON: 39.5 ± 0.4 °C vs. NCC: 39.6 ± 0.3 °C; p = 0.15). Mean neck temperature was lowered in NCC compared to CON after the run (36.4 ± 1.6 °C vs. NCC: 26.0 ± 0.3 °C; p < 0.001). Exercise-induced hyperthermia improved mean reaction time in the symbol digit matching test (-134 ± 154 ms; p < 0.05) and the PVT (-18 ± 30 ms; p < 0.05). Maximum span was increased in the digit span test (1 ± 2; p < 0.05). Application of NCC reduced the number of search errors made in level 3 of the search and memory test (p < 0.05). Mean serum BDNF levels were increased following exercise-induced hyperthermia in both trials (p < 0.05). Exercise-induced hyperthermia improves working memory and alertness. Neck cooling may only enhance performance in tasks of higher complexity.