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Exertional Heat Illness: The Role of Heat Tolerance Testing

Kazman, Josh Ben MS1; Heled, Yuval PhD, FACSM2; Lisman, Peter J. PhD, ATC1; Druyan, Amit MD2; Deuster, Patricia Anne PhD, MPH, FACSM1; O’Connor, Francis G. MD, MPH, FACSM1

doi: 10.1249/JSR.0b013e3182874d27
Environmental Conditions: Case Reports

Exertional heat stroke (EHS) is a common clinical problem for both athletes and warriors; however, evidence-based guidance for return-to-play/duty (RTP/RTD) decisions is limited. Heat tolerance testing (HTT) has been proposed as a potential tool that, when combined with appropriate clinical information, may assist in RTP/RTD decisions. However, currently, no standard of care is available for performing HTT. The Israeli Defense Forces (IDF) HTT protocol, which was developed over decades of careful research, has proven useful for IDF warriors and is utilized by other militaries to assist in RTD decisions. The present case studies are used to discuss the efficacy of the IDF HTT in determining RTD for two warriors who experienced EHS. Strengths and limitations of the IDF HTT, along with current and potential roles in clinical decision-making and in future thermoregulation research, are discussed.

1Department of Military and Emergency Medicine, Uniformed Services University, Bethesda, MD; and 2The Institute of Military Physiology, Heller Institute of Medical Research, Sheba Medical Center, Tel HaShomer, Israel

Address for correspondence: Josh Ben Kazman, MS, Department of Military and Emergency Medicine, Uniformed Services University, 4301 Jones Bridge Rd., Building 53, Ground Floor, Room 26, Bethesda, MD 20814; E-mail:

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Exertional heat stroke (EHS) is the leading preventable (8–11,15), nontraumatic cause of exertional sudden death in both athletes (30,37) and warriors (1,4,7,36). EHS is characterized by a rise in core body temperature over 40°C and accompanied by central nervous system dysfunction (e.g., delirium, convulsions, and coma) and multiple organ system failure (5). In contrast to classic heat stroke, EHS is brought on by physical exertion (5) and most commonly strikes young, highly motivated individuals (35). EHS is the most severe type of exertional heat illness (EHI), which also includes exertional heat cramps and exhaustion (5). Key preventive measures include altering physical activity on hot and humid days (12), ensuring sufficient heat acclimation at the start of training cycles (9), adhering to hydration schedules, implementing institutional restrictions, and providing regular educational updates for all athletic staff or key personnel.

Return-to-play/duty (RTP/RTD) guidelines following EHS remain controversial (31,32). The American College of Sports Medicine (ACSM) currently has several consensus-based recommendations for RTP, including the consideration of heat tolerance testing (HTT) for athletes having difficulty (2) (Table). However, HTT is not commonly performed in the United States, and few health care providers have knowledge of or access to this testing. Various HTT protocols have been used (3,6) among EHS experts in the United States, and a consensus for how to standardize the test or interpret its results has not been reached (31). This manuscript focuses on the role of HTT within the RTP/RTD decision-making process. Two clinical cases illustrating the application of the HTT to RTD decisions in military athletes are described. In addition, relevant HTT background information, as well as performance and interpretation of the HTT, and current controversies are discussed.

Table AC

Table AC

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The Israeli HTT protocol

Currently, no standard of care exists for HTT in warriors or athletes. The U.S. military does not routinely perform HTT in warriors who have sustained an EHS event; RTD is guided by clinical judgment and graduated acclimatization (32). In the Israeli Defense Forces (IDF), however, all warriors who sustain an EHS event are cleared with HTT prior to being returned to duty. The Israeli HTT consists of walking for 2 h at 5 km·h−1 (3.1 mph) with a 2% incline in an environmental chamber set at 40°C (104°F) and 40% relative humidity (15,25). The test also controls for modifiable EHS risk factors: participants wear light (shorts and no shirt for men) clothing; are asked to avoid medications, alcohol, tobacco and caffeine; and are advised to arrive well-rested and hydrated. For practical reasons, the test does not control for time-of-year, patient acclimatization, or baseline hydration status, although these factors may influence EHS risk or heat tolerance.

Key outcome measures to assess heat tolerance are core body temperature (measured with a rectal thermistor) and heart rate. Those with a core body temperature >38.5°C and/or heart rate >150 bpm are classified as heat intolerant (15,25). It is additionally important that patients’ core body temperature and heart rate demonstrate a tendency to plateau, which suggests compensation to a given heat load. Measurements of core body temperature and heart rate can be combined also into the physiological strain index (PSI), which is calculated by weighted changes in core body temperature and heart rate relative to starting the test (23,29). PSI scores range from 0 to 10 and are classified as minimal (PSI = 0 to 2), low (3 to 4), moderate (5 to 6), high (7 to 8), and very high (9 to 10). Although the PSI is not an outcome criterion, it may assist when comparing results across tests by combining two variables while controlling for baseline measures. Additionally, sweat rate, calculated using change in nude weight after adjusting for fluid input/output, is expected to be 0.5 to 1 L·h−1 (25).

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Case presentation 1

OA, an 18-yr-old previously healthy, moderately fit IDF army recruit (weight: 94.9 kg, height: 181 cm), who had experienced EHS during a selection process to an elite unit, presented to the Israeli Institute for Military Physiology at Tel HaShomer Israel. During a 2-km run under low heat load (<29°C and discomfort index <22 (16,38)) in January, he felt weakness and dizziness and fell to the ground. Rectal temperature, measured immediately after his collapse, was 40.2°C. OA was cooled by spraying copious amounts of water on him; his rectal temperature was 38.7°C about 20 min later upon arrival to the emergency room.

Two months after his EHS, OA underwent HTT in February, which was abnormal: his rectal temperature reached 38.5°C, and his heart rate was 178 bpm; neither tended to plateau (left panel, Fig. 1). The test was stopped after 115 min due to his complaints. PSI was calculated to be 7.8 and sweat rate was 0.803 L·h−1. Based on the HTT results, OA was given a temporary medical profile, and because he wanted to join a combat unit, his enrollment in the Army was postponed. Three months later, in July, OA completed another HTT, which was normal: rectal temperature and heart rate at the end of the test were within the normal range (37.8°C and 149 bpm, respectively), and tended to a plateau (right panel, Fig. 1), PSI was 5.0 and sweat rate was 0.910 L·h−1. OA’s medical profile has been updated, and he joined a combat unit, without any known recurrent heat illness episodes at 1-year follow-up.

Figure 1

Figure 1

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Case presentation 2

RK, a 24-year-old male American warrior (weight: 85.5 kg, height: 181 cm), presented to the Uniformed Services University Human Performance Laboratory following two episodes of EHI. For the initial EHI event, RK manifested general weakness and dizziness and collapsed during the final quarter mile of a 4-mile run in September while wearing a heavy backpack; his rectal temperature was noted to be 41.6°C. The ambient air temperature was 18.0°C with 94% relative humidity in that area. RK was immediately transported to the first-aid station where standardized cooling procedures were instituted, including the application of ice bags to his groin, armpits, and neck; intravenous fluid replacement; and fanning his body with air. His core body temperature decreased, and he was subsequently transported to the hospital for additional testing. Laboratory results revealed rhabdomyolysis with no apparent associated renal or liver dysfunction. He was released from the hospital the same day, placed on medical profile for 30 d, and returned to full duty approximately 60 d following the EHI episode. Just under 12 months after the first event, the warrior sustained a second EHI event (also in September). He manifested syncope during the final half mile of a 5-mile timed run. On this day, the ambient air temperature was 21.7°C, and the relative humidity was 96%. He had a rectal temperature of 39.7°C and was immediately transported to the hospital; while in route, ice bags were applied to his groin, armpits, and neck. He was released from the hospital the same day and placed on medical profile for another 30 d. In light of the second EHI event being within a 1-year period, and unique operational requirements for this individual warrior, the treating clinician referred him for HTT.

During the initial HTT, which was in the last week of October, his maximal rectal temperature was 38.2°C; maximal heart rate (HR), 153 bpm; PSI, 5.9; and sweat rate, 0.93 L·h−1. According to these results, (HR >150), the participant was identified as a borderline case, profiled for heat and activity restrictions and scheduled for a follow-up HTT in 3 months. Throughout the time between tests, the warrior’s physical activity level was limited and excluded work in the heat.

During the follow-up HTT, which was in the last week of January, maximal rectal temperature was 38.2°C; maximal HR, 140 bpm; PSI, 5.9; and sweat rate, 0.687 L·h−1. Based on these test findings, RK was recognized as heat tolerant and permitted to return to duty with initial training limitations. The warrior returned to full duty without further complications following an acclimatization protocol as described by Casa et al. (8), where heat and activity levels were controlled; no recurrent heat illness episodes were noted at 1-year follow-up.

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The IDF has developed a model of heat intolerance to both conceptualize EHS (14,17) and aid in clinical decisions following EHS (15,25). Exercise in the heat elicits a thermoregulatory response that is expected to prevent a dangerous rise in core body temperature, mostly by an increase in cardiac output, peripheral vasodilation, and sweating (14,17). The ability to tolerate a given heat load with an appropriate demonstration of exercise-induced hyperthermia, and maintenance of core body temperature, is termed “compensable.” If core body temperature continues to rise, however, exercise-heat stress becomes uncompensable, and if not removed from the heat, EHI is likely. An EHS may lead to systemic inflammatory responses, which can cause multiple organ failure and/or death (5).

The Israeli HTT consists of a mild aerobic exercise challenge (25). However, since it is performed in conjunction with a significant external heat load, the HTT allows researchers and clinicians to quantify the individual’s ability to physiologically respond to a given exercise-heat stress with an expected and appropriate thermoregulatory response. Individuals who do not adequately adapt to exercise in the heat are defined as heat intolerant, given that they were not able to sufficiently physiologically adjust to compensable heat strain in this environment. Heat-intolerant individuals are subsequently given a temporary medical profile and then retested after 3 months. If still heat intolerant after 3 months, they are given a permanent profile, which minimizes their strenuous activity in a thermally challenged environment. Warriors identified as heat tolerant are cleared for RTD, with the caveat of beginning an appropriately graduated exercise and heat acclimatization protocol prior to returning to full duty.

The Israeli HTT was developed over multiple experiments wherein ambient temperature, exercise type, and test duration were manipulated to optimize discrimination of heat tolerant/intolerant individuals (18,26,34). Over the course of development, the HTT was used also to further basic research into EHS, by investigating the role of other variables, such as body type (18,34), fitness level (18,34), hydration (27), role of protective gear (19,26), gender (13,28), and gene transcription (24) in thermoregulation. Using a standardized HTT has allowed comparisons of results across these many studies, and the HTT has proved clinically useful for the IDF as well (15,25).

However, use of the HTT is still controversial among some sports medicine clinicians and scientists and is far from a widely accepted standard of care. During a 2008 ACSM Roundtable convened at Uniformed Services University in Bethesda, MD, a number of questions about the Israeli HTT were raised, such as its ability to predict future EHS, the causal relations between heat intolerance and EHS, and the recovery process for heat-intolerant individuals (31). Although the test has been utilized successfully for more than 30 years, to date, no published literature documents the diagnostic or prognostic validity of the HTT. That being stated, a premature return to duty with strenuous exercise-heat exposure would likely be problematic; in 2011, 362 incidents of EHS were reported for U.S. service members, and in that year alone, 68 of the cases suffered more than one EHS episode (1).

Many specific facets of heat tolerance warrant further discussion, but the Israeli HTT represents a touchstone for further research. The role of women in the IDF and U.S. military has increased over the past few decades, and women may have a higher HTT failure rate than men (13). These gender differences may justify new criteria for women. Some researchers have experimented with HTTs more suited toward particular groups by varying the workload according to aerobic capacity or by simulating high-altitude conditions (20). However, warriors are often required to perform group tasks regardless of their aerobic capacity, and most cases of EHS occur during military marches and timed runs (33). A constant workload represents the majority of high-risk EHS conditions and allows for test standardization. The decision to control for modifiable risk factors is another trade-off, which may limit the applicability of the HTT to specific groups. For instance, some warriors are routinely exposed to known risk factors, such as wearing chemical suits, being sleep deprived, or taking dietary supplements with stimulants, and other substances. By eliminating these risk factors, the HTT may be less applicable to their everyday performance, but once again, it allows for standardization. The Israeli HTT represents a trade-off between these multiple known and unknown factors and has been improved upon over decades of iterative experiments and clinical experience. Despite some weaknesses, it remains the most widely used HTT.

Many of the questions surrounding the Israeli HTT can be answered through future research. Determining the predictive value of the HTT will be difficult due to both the rarity of EHS and the IDF practice of removing heat-intolerant patients from active duty. However, test validity can be indirectly studied by analyzing known EHS risk factors and their correlates (21). It is also important to analyze changes in HTT performance over time, both following fitness or acclimation/acclimatization protocols and without any structured intervention. The HTT may be useful also in identifying future preventive interventions, such as certain nutrients and natural products, identifying new biologic markers for EHS susceptibility, elucidating conditions that are associated with EHS, such as exertional rhabdomyolysis or sickle cell trait, and even determining conditions that may predispose to heat intolerance, such as traumatic brain injury.

Determining when an athlete or warrior should RTP/RTD following EHS is a clinical challenge (1,22,31,32). The IDF, as demonstrated in case 1, utilizes the HTT in all cases of EHS prior to RTD. As previously noted, the HTT is not routinely used by the U.S. military and is not the standard of care for RTP in the athletic community. However, the authors believe that the HTT can be an efficacious option for selected individuals. A potential clinical algorithm illustrating how to make RTP/RTD decisions following EHS and the role of HTT is presented in Figure 2.

Figure 2

Figure 2

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A sports clinician needs to be able to use the best information available when he or she is confronted with an EHS patient and has to make a clinical decision. Potential clinical risk is higher in the military where limited medical resources and extreme operational requirements exist. Whereas inappropriately returning a warrior back to duty could endanger the lives of others, incorrectly pulling a warrior from duty could cut short a valuable military career. The latest data from the Medical Surveillance Monthly Report suggest that military physicians may lack sufficient information to make good RTD decisions for EHS patients (1), and civilian epidemiologic studies reflect increasing morbidity and mortality from EHS (30,37). Thus, the HTT represents a potential tool and should be considered when combined with other requisite clinical information, and the sound judgment of a clinician, to assist in RTP/RTD decisions.

The project was funded by the Office of Naval Research, grant no. N0001411MP20023.

The authors declare no conflicts of interest and do not have any financial disclosures

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