Heat Tolerance Test Research Articles

Repeatability of a running heat tolerance test

At present there is no standardised heat tolerance test (HTT) procedure adopting a running mode of exercise. Current HTTs may misdiagnose a runner's susceptibility to a hyperthermic state due to differences in exercise intensity. The current study aimed to establish the repeatability of a practical running test to evaluate individual's ability to tolerate exercise heat stress. Sixteen (8M, 8F) participants performed the running HTT (RHTT) (30min, 9kmh−1, 2% elevation) on two separate occasions in a hot environment (40°C and 40% relative humidity). There were no differences in peak rectal temperature (RHTT1: 38.82±0.47°C, RHTT2: 38.86±0.49°C, Intra-class correlation coefficient (ICC)=0.93, typical error of measure (TEM)=0.13°C), peak skin temperature (RHTT1: 38.12±0.45, RHTT2: 38.11±0.45°C, ICC=0.79, TEM=0.30°C), peak heart rate (RHTT1: 182±15 beatsmin−1, RHTT2: 183±15 beatsmin−1, ICC=0.99, TEM=2 beatsmin−1), nor sweat rate (1721±675gh−1, 1716±745gh−1, ICC=0.95, TEM=162gh−1) between RHTT1 and RHTT2 (p>0.05). Results demonstrate good agreement, strong correlations and small differences between repeated trials, and the TEM values suggest low within-participant variability. The RHTT was effective in differentiating between individuals physiological responses; supporting a heat tolerance continuum. The findings suggest the RHTT is a repeatable measure of physiological strain in the heat and may be used to assess the effectiveness of acute and chronic heat alleviating procedures.

Heat tolerance testing: association between heat intolerance and anthropometric and fitness measurements.

This study investigated associations between heat intolerance, as determined by performance on a heat tolerance test (HTT), and anthropometric measurements (body surface-to-mass ratio, percent body fat, body mass index, and waist circumference) and cardiorespiratory fitness (maximal oxygen uptake [VO2max]). Relationships between predictive variables and specific physiological measurements recorded during the HTT were examined. A total of 34 male and 12 female participants, recruited from the military community, underwent anthropometric measurements, a maximal aerobic exercise test, and a standardized HTT, which consisted of walking on a treadmill at 5 km/h at 2% grade for 120 minutes at 40°C and 40% relative humidity. VO2max negatively correlated with maximum core temperature (r = -0.30, p < 0.05) and heart rate (HR) (r = -0.48, p < 0.01) although percent body fat showed a positive correlation with maximum HR (r = 0.36, p < 0.05). VO2max was the only independent attribute that significantly influenced both the maximum HR and core temperature attained during HTT. Logistic regression analyses indicated that VO2max was the only independent parameter (OR = 0.89, p = 0.026) that significantly contributed to overall HTT performance. Low cardiorespiratory fitness was associated with heat intolerance, as defined by HTT performance, and can be addressed as a preventative measure for exertional heat illness. This study provides further evidence that the HTT can be an effective tool for assessment of thermoregulatory patterns.

Cyp11A1 Canola plants under short time heat stress conditions

In order to investigate the high temperature tolerance of spring canola plants (Brassica napus L.) constitutively expressing cyp11A1 gene which encodes bovine cytochrome P450(scc) the growth features were analyzed under short time heat stress (42 degrees C) in growth chamber. Earlier it was documented that results of the heat tolerance test positively correlated with improvement of high temperature resistance in field trial. Higher relative water content (by 13%) and superoxide dismutase (SOD) activity, lower electrolyte leakage (up 1.4-fold) and smaller increase in chlorophyll a and carotenoid contents in cyp11A1 canola leaves in comparison with wild-type plants under stress allowed to conclude cyp11A1 plants are more tolerant to high temperature than the control ones. We suppose that SOD activity increase which revealed in our transgenic canola in normal condition plays the defining role in the biochemical alterations in plant metabolism for the thermotolerance improvement. SOD activity increment could be caused by heterologous cytochrome P450(scc) activity which resulted in the superoxide radical formation. Cyp11A1 canola plants might be resistant to the other stress conditions of different origin.

Changes in heart rate variability during the induction and decay of heat acclimation.

We evaluated the changes in core temperature, heart rate, and heart rate variability (HRV) during the induction and decay of heat acclimation. Ten males (23 ± 3 years; 79.5 ± 3.5 kg; 15.2 ± 4.5 percent body fat; 51.13 ± 4.61 mLO(2)∙kg(-1)∙min(-1) peak oxygen uptake) underwent a 14-day heat acclimation protocol comprising of 90-min cycling at ~50 % peak oxygen uptake at 40 °C and ~20 % relative humidity. Core temperature, heart rate, and 102 HRV measures were recorded during a heat tolerance test conducted at baseline (day 0) and at the end of the induction (day 14) and decay (day 28) phases. Heat acclimation resulted in significantly reduced core temperature [rectal (χ (2) = 1298.14, p < 0.001); esophageal (χ (2) = 1069.88, p < 0.001)] and heart rate (χ (2) = 1230.17, p < 0.001). Following the decay phase, 26, 40, and 60 % of the heat acclimation-induced reductions in rectal temperature, esophageal temperature, and heart rate, respectively, were lost. Heat acclimation was accompanied by profound and broad changes in HRV: at the end of the induction phase, 75 of the 102 variability measures computed were significantly different (p < 0.001), compared to only 47 of the 102 at the end of the decay phase. Heat acclimation is accompanied by reduced core temperature, significant bradycardia, and marked alterations in HRV, which we interpret as being related to vagal dominance. The observed changes in core temperature persist for at least 2 weeks of non-exposure to heat, while the changes in heart rate and HRV decay faster and are only partly evident after 2 weeks of non-exposure to heat.

Refining the distinction between heat tolerant and intolerant individuals during a Heat tolerance test

BackgroundThe heat tolerance test (HTT) is a standardized physiological test that constitutes one of the considerations in the Israel Defence Forces (IDF) for return to duty after a heat injury. The HTT consists of a 2h controlled exercise-heat stress with a threshold of maximal rectal temperature (Tc) and heart rate (HR) values above which subjects are referred to as heat intolerant; the dynamics of the HR and Tc during the test, which tend to plateau during the 2nd hour of the test, is also considered. Since “tendency to plateau” is a subjective measure, this study aimed to quantify the tendency to plateau during a HTT. Material and methodsThe physiological results of 102 HTT subjects (83 normal and 19 heat-intolerant) served as the database for analysis. The first 28 subjects, who were considered heat tolerant (HT) by an experienced examiner, served to evaluate a “normal” dynamic of Tc and HR during a HTT. Then, we applied the results on seven heat tolerant and seven heat intolerant (HI) subjects in order to determine which of the two variables (dTc or dHR) in different time intervals (t120–0, t60–0, t120–60, and t120–100) may best distinguish heat tolerant from heat intolerant subjects. During the 3rd stage of the study 60 random test results (post factum: 48 heat tolerant and 12 heat intolerant subjects) were evaluated. Post-hoc results of heat tolerance determined by dTc were compared to the expert diagnosis of the test. ResultsA rise of less than 0.45°C in Tc during the 2nd hour of the HTT was found acceptable to define a tendency to plateau of the Tc with a 100% sensitivity and specificity. We did not find a numerical value of HR increase over time during the HTT acceptable to define a tendency of the HR to plateau. ConclusionDuring the last hour of the heat tolerance test, there is a tendency to plateau in Tc in heat tolerant individuals. It is concluded that a rise in Tc of less than 0.45°C during this period (Tc120–60) can be used as a supporting measure to distinguish between heat tolerant and heat intolerant individuals.

Specific Exercise Heat Stress Protocol for a Triathlete’s Return from Exertional Heat Stroke

A triathlete collapsed with exertional heatstroke (EHS) during 2 races over 3 months. The American College of Sports Medicine recommends a heat tolerance test (HTT) following EHS if there is a concern with return to play. The classical walking HTT may not be the best test to evaluate elite triathletes' heat tolerance due to race intensity, nor is it suited to evaluate acclimation ability, which may play a role in risk of heat illness. Is the athlete capable of returning to racing or should he retire from sport due to heat intolerance? Up to 90 min of cycling (70% of V˙O2max; 36°C, 50% relative humidity) was followed by 9 d of exercise heat acclimation and a final identical exercise heat stress test. After acclimation, exercise duration before reaching a gastrointestinal temperature (Tgi) of 39.5°C increased 25 min, sweat rate increased 0.5 L·h, initial Tgi decreased 0.85°C, and rate of Tgi rise decreased 0.6°C·h. Adaptations were deemed acceptable, and the athlete was allowed to return to competition. The athlete has since raced in hot environments without incident.

Acute Care for Exercise-Induced Hyperthermia to Avoid Adverse Outcome from Exertional Heat Stroke

Exertional heat stroke (EHS) is a major concern in military trainees performing intense physical exercise, with substantial morbidity rates. Prehospital diagnosis of EHS is essentially clinical. Thus, soldiers, command personnel, and medical staff are taught to recognize this injury and immediately begin aggressive treatment to prevent further deterioration. During 2007, 5 otherwise healthy Israeli Defense Forces (IDF) soldiers were diagnosed with EHS while performing strenuous exercise. They were treated vigorously according to the IDF EHS-treatment protocol and were referred to the emergency department. On arrival at the emergency department, physical examination including rectal temperature was unremarkable in all soldiers. Blood and urine workup showed near-normal values. No other medical conditions that could have explained the clinical presentation were found. All soldiers were discharged shortly afterward, with no further consequences. A heat-tolerance test was performed several weeks after the event and was interpreted as normal. All soldiers returned to active service. Because the initial clinical findings were very suggestive of EHS and because no other condition could have explained the prehospital transient hyperthermia, we suggest that these soldiers were correctly diagnosed with EHS, and we propose that rapid vigorous cooling prevented further deterioration and complications. We suggest calling this condition aborted heat stroke.

Heat tolerance of Nelore, Senepol x Nelore and Angus x Nelore heifers in the southeast region of Brazil

The Brazilian beef industry has experienced an increase in the utilization of adapted and non-adapted Bos taurus breeds in crossbreeding systems. In spite of this, little is known about the adaptability of these groups and of their crossbred progeny when raised in a tropical climate. The aim of this study was to evaluate the physiological responses related to adaptability of Nelore (NE), crossbred Angus x Nelore (TA) and Senepol x Nelore (SN) cattle submitted to a heat tolerance trial. The study was conducted in the Southeast – Embrapa Cattle (CPPSE), Sao Carlos, Brazil. During the summer of 2008, 45 heifers, 15 of each breed type, were evaluated over three days: at 7:00 a.m. (resting measure), at 13:00 (after six hours in the sun with no access to water and shade) and at 15:00 (after a further two hours in the sun with access to shade). Rectal temperature and sweating rate were measured and the data were analyzed using restricted maximum likelihood. The effect of breed type was significant for rectal temperature only at 15:00 and for the sweating rate at all three test periods. Based on these results, Senepol x Nelore heifers showed a better adaptation in the heat tolerance test than the other breed types.

The Heat Intolerant Phenotype. Physiological Responses And Lymphocyte Transcriptome

INTRODUCTION: During physical training in the heat, extreme metabolic heat storage manifests (pending on the individual features) as a rise in body core temperature (Tc). Individuals, previously exposed to heat exhaustion/stroke and demonstrating an earlier/faster Tc and heart rate rise within a cohort study, are defined as "heat intolerant" (HI). Whether HI is a transient phenotypic or genotypic response is yet unknown. PURPOSE: To investigate whether HI and heat tolerant (HT) subjects differ in their lymphocyte gene profile. METHODS: 68 healthy young male volunteers, 1-3 months post-exertion heatstroke, underwent a heat tolerance test (HTT), consisting of walking on a treadmill for 2h at a speed of 5 km·h-1 and 2% grade in a climatic chamber (40°C, 40% relative humidity). Tc, Tskin (3 sites), and heart rate were continuously monitored and recorded. Blood samples were drawn before, at the end of the HTT and after 1h of recovery in an air-conditioned room (23°C). Global gene profile of 6 HT and 6 HI lymphocytes RNA samples (2 pools per group) was detected and analyzed using two-color gene chips and bioinformatic tools. RESULTS: According to the HTT, 10 subjects were diagnosed as HI and 58 subjects as HT. Tc at the end of HTT was elevated by 1.4 and 0.77°C in the HI and the HT subjects, respectively. Tskin vs. Tc, indicative of skin blood flow, implied a HI sluggish response (vs. HT) (P<0.01). Two different gene chip experimental paradigms indicated differences in gene profiles of the HI vs. the HT group, prior to and following recovery from HTT. Basal differences in biological functional categories among the groups included genes associated with immune responses, stress (e.g., apoptotic pathways), transport, molecular processes (DNA and RNA regulation and organization) and metabolism. CONCLUSIONS:: This study (as well as our previous reports) characterizes a distinct HI physiological phenotype. This characterization of the HI phenotype demonstrates a differential genomic profile found even prior to heat stress tests, adding a novel category to the defined reasons for potential congenital HI.

The Heat Tolerance Test: An Efficient Screening Tool for Evaluating Susceptibility to Heat

Individuals in the population who are not able to sustain heat and whose body temperature will start rising earlier and at a higher rate than that of others, under the same conditions, are defined as "heat intolerant." The applicability of the heat tolerance test (HTT) in identifying individuals' tolerance/intolerance to heat is presented. HTT is performed according to the following protocol: 120 minutes exposure to 40 degrees C and 40% relative humidity in a climatic chamber while walking on a treadmill, dressed in shorts and T-shirt, at a pace of 5 km/h and 2% elevation. Rectal temperature and heart rate are continuously monitored, and sweat rate is calculated. The HTT that is based on controlled exposure to an exercise-heat stress is an applicable and an efficient tool in differentiating between a temporary and permanent state of heat susceptibility.

Guidelines for Return to Duty (Play) after Heat Illness: A Military Perspective

Since Biblical times, heat injuries have been a major focus of military medical personnel. Heat illness accounts for considerable morbidity during recruit training and remains a common cause of preventable nontraumatic exertional death in the United States military. This brief report describes current regulations used by Army, Air Force, and Navy medical personnel to return active duty warfighters who are affected by a heat illness back to full duty. In addition, a description of the profile system used in evaluating the different body systems, and how it relates to military return to duty, are detailed. Current guidelines require clinical resolution, as well as a profile that that protects a soldier through repeated heat cycles, prior to returning to full duty. The Israeli Defense Force, in contrast, incorporates a heat tolerance test to return to duty those soldiers afflicted by heat stroke, which is briefly described. Future directions for U.S. military medicine are discussed.

Effect of Chronic Heat Stress on Broiler Performance in Jordan

2 Abstract: An experiment was conducted in the poultry farm of the faculty of agriculture in the campus of Jordan University to evaluate the effect of heat stress on the performance and some physiological parameters of 4-8 week old Isa Vedette broilers. Environmental temperatures in controlled environmental rooms were constant 25 (control), 30 and 35±2 C, in addition to a variable-natural temperature, 24 - 28 ± 2 C o o in the open-sided poultry house. At the end of the experiment, a representative sample was randomly selected from each treatment and exposed to heat tolerance test at 40±2 C in the environmental physiology o lab. The results showed that, growth rate, body gain, feed consumption and feed efficiency were significantly (p<0.05) reduced of 4-8 week old broilers reared in hot-closed environment (30 and 35±2 C) and variable- o natural temperature in open-sided environment. The heat tolerance test showed that, broilers reared at high environmental temperatures of 30 and 35±2 C had lower mortality rate and RITr than those reared at o

Rapid IV versus Oral Rehydration

This study sought to determine the effect of rapid intravenous (IV) versus oral (ORAL) rehydration immediately after dehydration, on cardiovascular, thermoregulatory, and perceptual responses during subsequent exercise in the heat. Eight males (21.4 +/- 0.7 yr; 176.2 +/- 1.6 cm; 75.2 +/- 3.7 kg; 63.7 +/- 3.6 mL.kg.min VO2max, 9.0 +/- 1.7% fat) participated in three randomized trials. Each trial consisted of a 75-min dehydration phase (36 degrees C; 42.5% rh, 47 +/- 0.9% VO2max) where subjects lost 1.7 L (IV and no-fluid (NF) trials) to 1.8 L of fluid (ORAL trial). In the heat, fluid lost was matched with 0.45% saline in 20 min by either IV or ORAL rehydration; no fluid was given in the NF trial. Subjects then performed a heat-tolerance test (HTT; 37.0 degrees C, 45% rh, treadmill speed of 2.4 m.s, 2.3% grade) for 75 min or until exhaustion (Tre of 39.5 degrees C). During the HTT, thermal and thirst sensations, RPE, rectal temperature (Tre), heart rate (HR), and mean weighted skin temperature (Tsk) were measured. Plasma volume in the IV treatment was greater (P < 0.05) after rehydration compared with ORAL and NF. However, during the HTT there were no overall differences (P > 0.05) in HR, Tre, Tsk, RPE, thermal sensations, or HTT time (ORAL, 71 +/- 8 min; IV, 73 +/- 5 min; NF, 39 +/- 29 min) between the ORAL and IV treatments. Sensations of thirst were lower (P < 0.05) in ORAL compared with IV and NF, likely because of oropharyngeal stimuli. Despite a more rapid restoration of plasma volume, IV rehydration was not advantageous over ORAL rehydration in regards to physiological strain, heat tolerance, RPE, or thermal sensations.

Hsp70 Induction and hsp70 Gene polymorphisms as Indicators of acclimatization under hyperthermic conditions

The possibility of using Hsp70 and hsp70 gene polymorphisms as markers of acclimatization was investigated. Volunteers (22) were subjected to an acclimatization regimen and blood analysed for Hsp70 (Hsp72) and hsp70 polymorphisms before and after a heat tolerance test. Physiological parameters denoting acclimatization, or not, were correlated to levels of Hsp70 and combination of hsp70 genes. Only individuals that acclimatized had decreased basal Hsp70 levels and increased ability to induce Hsp70 together with a specific hsp70 genotype combination. We propose that Hsp70 levels (basal vs. induced) with the genotype combination have the potential to be used as markers of acclimatization.

Effect of Chronic Heat Stress and Long-Term Feed Restriction on Broiler Performance

An experiment was conducted to investigate the effects of long-term feed restriction on alleviating the effects of chronic heat stress on the performance of 4-8-wk old ISA Vedette broiler birds. Environmental temperatures were constant 25, and 35±2 C in controlled environmental rooms and variable-natural o temperature ranged between 21 and 30±2 C in an open-sided poultry house. At 4 weeks of age, Daily feed o consumption in all experimental rooms was restricted to 75% and 50% of the daily ad libitum consumption recommended in the management guide for this strain. At the end of experiment, a representative sample of 8-wk-old broilers from each treatment was selected and placed randomly in controlled-environmental chambers set at 40±1 C for heat tolerance test. The results showed that, rearing temperature of 35±2 C in o o closed environment and variable-natural temperature of 21-30 ± 2 C in the open-sided environment had a o significant (p<0.05) negatively effects on broiler performance. Long-term feed restriction to 75% or 50% reduced significantly (p<0.05) the growth rate and body gain of 4-8-wk old broilers. The body weight gain of birds reared at 35 C and fed 50% of ad libitum was higher than their counterparts reared at constant 25 C o o or variable-natural temperature (21-30 C). Feed conversion ratio and survivability were significantly (p< 0.05) o improved with increasing feed restriction at high ambient temperature of 35± C. However, feed restriction o at lower ambient temperature of 25 C had negative effects on feed conversion ratio. On the other hand, heat o tolerance test showed that, feed restriction improved heat tolerance and increased the heat resistance of 8wk old broilers when exposed to 40 C. Lower mortality rate and lower rate of increase of rectal temperature o of feed restricted birds indicated this. From the results of this experiment, it could be concluded that, ambient temperature and long-term feed restriction significantly affect broiler performance. Moreover, long-term feed restriction at high ambient temperature increase heat resistance and improve the heat tolerance of growing broilers, when exposed to heat waves in summer season.

Heat intolerance: does gene transcription contribute?

During exertion in the heat, heat-intolerant (HI) subjects have a physiological disability in metabolic heat dissipation. The HI state is either permanent or temporary, depending on whether it stems from transient predisposing factors or inherent thermoregulatory dysfunction. In this investigation, we studied protein levels of heat shock protein (HSP) 70 and HSP72, HSP90, bcl-2xL, glutathione S-transferase-p, heat shock factor-1, TATA-binding protein-associated factor, and NF-kappaB transcripts using Western blot and quantitative RT-PCR, respectively, in lymphocytes of HI and tolerant (T) male volunteers of similar anthropometric features. Measurements were made from blood drawn before, during the heat tolerance test (3.5 mph, 40 degrees C, 40% relative humidity, 2 h), and 1 h after recovery at 24 degrees C. Rectal and skin temperatures, as well as heart rate, were continuously recorded. Of 58 subjects, 7 were identified as HI, with a significantly higher physiological strain index than in the T group (6.3 +/- 0.9 vs. 3.8 +/- 0.6, P < 0.001). The responsiveness of the vasculature to thermal stimuli was decreased in the HI group, as indicated by rectal temperature minus skin temperature. The HSP72 level in the HI group dropped during the recovery session (P < 0.01), whereas that of the T group continued to rise. A significantly increased expression of the transcription factors in the T subjects and significantly decreased expression in the HI group (P < 0.009, 0.013, and 0.005 for heat shock factor-1, NF-kappaB, and TATA-binding protein-associated factor, respectively) points to impaired transcriptional processes in the HI group. Our data suggest that transcriptional malfunction and sluggishness of the vasculature to thermal stimuli are predisposing factors in the HI group.

Rehydration after Exercise Dehydration in Heat: Effects of Caffeine Intake

Context:Dieticians, physiologists, athletic trainers, and physicians have recommended refraining from caffeine intake when exercising because of possible fluid-electrolyte imbalances and dehydration.Objective:To assess how 16-hour rehydration is affected by caffeine ingestion.Design:Dose–response.Setting:Environmental chamber.Participants:59 college-age men.Intervention:Subjects consumed a chronic caffeine dose of 0 (placebo), 3, or 6 mg · kg−1 · day−1 and performed an exercise heat-tolerance test (EHT) consisting of 90 minutes of walking on a treadmill (5.6 km/h) in the heat (37.7 °C).Outcome Measures:Fluid-electrolyte measures.Results:There were no between-group differences immediately after and 16 hours after EHT in total plasma protein, hematocrit, urine osmolality, specific gravity, color, and volume. Body weights after EHT and the following day (16 hours) were not different between groups (P &gt; .05).Conclusion:Hydration status 16 hours after EHT did not change with chronic caffeine ingestion.

Influence of Caffeine Ingestion on Fluid-Electrolyte, Psychological and Physiological Responses During an Exercise Heat-Tolerance Test

Influence of Caffeine Ingestion on Fluid-Electrolyte, Psychological and Physiological Responses During an Exercise Heat-Tolerance Test

Influence of Caffeine Ingestion on Fluid-Electrolyte, Psychological and Physiological Responses During an Exercise Heat-Tolerance Test

Influence of Caffeine Ingestion on Fluid-Electrolyte, Psychological and Physiological Responses During an Exercise Heat-Tolerance Test

Heat intolerance in patients with chronic schizophrenia maintained with antipsychotic drugs.

Schizophrenia may be associated with hyperthermic syndromes such as febrile catatonia, neuroleptic malignant syndrome, and heatstroke. The authors hypothesized that an exercise-heat tolerance test would disclose abnormal thermoregulation in schizophrenic patients. Seven male schizophrenic outpatients in remission maintained on depot antipsychotic treatment and eight healthy comparison subjects completed a heat tolerance test that consisted of two 50-minute bouts of walking a motor-driven treadmill at 40xC (relative humidity=40%). A significantly higher rise in rectal and skin temperatures was observed in the patient group. No differences in heart rate, blood pressure, or perspiration were detected. Schizophrenic patients maintained on antipsychotic drugs exhibit impaired heat tolerance. Possible explanations are a reduced ability to convey heat from the body's core to the periphery with or without excessive heat production. The hyperthermic response to the heat tolerance test may reflect a dysfunction associated with schizophrenia, a neuroleptic-induced side effect, or both.