Current Clinical Concepts: Heat Tolerance Testing

With appropriate and timely recognition and treatment of EHS, most athletes, warfighters, and laborers fully recovered without complications² ; however, when this was not the case, HTT could have been used as a functional tool to objectively facilitate the RTA process.¹  The concept of HTT was initially developed in the first half of the 20th century to identify laborers who could tolerate the hot working conditions in the South African gold mining industry.¹⁸  Heat tolerance testing as we know it was created by the Israeli Defense Forces in the 1980s as a means to test a soldier's ability to return to duty after experiencing EHS.^1,19–21  Currently, the Israeli Defense Forces uses HTT as part of its clinical decision-making tool to determine if soldiers can safely RTA after EHS,^20,22  and the US military uses various protocols for treatment and testing post-EHS that are service and clinician dependent. The components of the traditional Israeli Defense Forces model are described in Table 2.

Although the Israeli Defense Forces model is the most widely used and accepted method for HTT, it does have some limitations. These include but are not restricted to EHS case specificity, individual factors, characteristics of heat intolerance, and timing of the HTT. We discuss how each of these factors relates to heat tolerance and why it may be important to use and adapt HTT protocols based on individual circumstances. When deciding on the need to use HTT, clinicians are strongly advised to first consider the most common HTT protocols described in this article. However, based on individual circumstances, providers may decide to use a less common protocol or, rarely, to adapt their own. Consulting experts who routinely perform and interpret HTT (see “Resources” section) is highly recommended when in doubt about which protocol to use.

Exertional heat illness is an umbrella term that may include exercise-associated muscle cramps; heat syncope; heat exhaustion; exertional hyponatremia; and the most severe and sometimes fatal condition, EHS.²  During HTT, core body temperature is monitored to determine susceptibility to the heat.²⁰  Yet other heat-related conditions, such as heat exhaustion and heat cramps, have little association with body temperature, indicating that HTT may not be relevant for an RTA protocol in those situations.²  Therefore, with EHS, it is important to rule out other conditions and to distinguish between EHS and other exertional heat illnesses that would result in different treatment approaches, recoveries, and RTA plans.²  Best practice for the diagnosis of EHS is observed cognitive impairment in addition to a core body temperature >40.5°C (105°F) measured rectally,^2,24  although the diagnostic criterion of 40.5°C versus 40.0°C remains controversial.¹³ 

After an appropriate diagnosis of EHS, the modality and timing of treatment are important for the survival, recovery, and RTA of the individual. Numerous cooling methods have been cited as treatments for EHS,²⁵  although cold-water immersion was considered the criterion standard treatment by the American College of Sports Medicine²⁶  and National Athletic Trainers' Association²  due to unsurpassable cooling rates. Delayed cooling or suboptimal cooling rates increased the risk for adverse outcomes, such as organ damage or failure, resulting in an increased duration of hospital stay,²⁷  and consequently may have affected heat tolerance. As such, whether an individual received rapid cooling could ultimately influence recovery in addition to the time frame of initiating HTT.^1,27 SOR for adapting HTT protocol: C.

An important and frequently overlooked factor to consider with HTT is the fitness level of the person. Most researchers^6,14,23  assessed HTT only in well-trained athletes (including tactical athletes), although individuals with lower levels of physical fitness were at the greatest risk for EHS. In addition, general health and comorbidities may have affected the recovery from EHS^11,28  and also inhibited performance on HTT. In contrast, the literature^6,12  supported the premise that some HTT may be insufficient to challenge individuals participating in athletic activities of high intensity. Just as when determining the appropriateness of return to sport for athletes after a musculoskeletal injury, the specificity of training, intensity, and the environment should be considered to help determine when to initiate HTT and an appropriate HTT protocol. SOR for adapting HTT protocol: A.

Heat acclimation occurs after repeated heat exposures and results in physiological, perceptual, and performance adaptations.²⁹  These adaptations allow an individual to better tolerate exercise in the heat, which could be reflected by improved performance on HTT.³⁰  Furthermore, given that an HTT protocol requires exercising in the heat, it could also be beneficial as a heat-acclimation strategy. Although the benefits of these strategies occur as early as 1 to 2 weeks after initiation, heart rate and core temperature adaptations are lost at approximately 2.5% per day when sufficient exposures are no longer achieved.³¹  Therefore, increased time between HTT trials or time spent in recovery from an EHS may result in the loss of any potential heat acclimation or training adaptations already achieved. Additionally, most research on HTT involved men,^6,14,23  yet emerging evidence suggested sex-dependent factors influence thermoregulation in general and heat acclimation adaptations specifically. As such, future HTT interpretations should take into account possible sex and menstrual cycle phase differences.^32,33  The most commonly used protocol, the Israeli Defense Forces model, does not control for heat acclimation status,²³  thereby limiting its interpretation and translation. As such, it is important to consider fluctuating the heat acclimation status for each patient with EHS and HTT. SOR for adapting HTT protocol: B.

Heat intolerance occurs during exercise when the body is unable to maintain thermal balance, resulting in an extreme increase in body temperature. The mechanisms of heat intolerance are not fully understood but have been suggested to originate from 2 characteristics, state and trait, that can operate simultaneously. Heat intolerance may be a state of the human body when it occurs after an EHS episode.³⁴  Unlike trait, heat intolerance due to state would allow individuals to improve tolerance through training and recovery. This is supported by cases in which athletes who had experienced EHS were able to eventually pass the HTT.⁷  Additionally, heat intolerance may be a trait of the human body when genetic predispositions limit one's ability to tolerate heat during exercise. Genetic mutations cause altered sensitivity to heat and exercise stress. Even though these specific mutations are not fully understood, this mechanism was supported by the literature, showing that people who experienced a previous EHS were at greater risk for experiencing another EHS.⁷  Given that growing evidence supports the attribution of both state and trait to heat intolerance,³³  we can conclude that heat intolerance is multifactorial and is highly dependent on the individual. SOR for adapting HTT protocol: C.

In the Israeli Defense Forces model, HTT was traditionally performed after 4 to 6 weeks of rest post-EHS treatment. It is most common to allow only 1 attempt; still, a second test was allowed in some cases after 1 month.²²  Schermann et al¹⁴  found no difference in HTT outcomes between those who were tested <6 weeks after an EHS event and those who were tested at >6 weeks. However, they did not account for the variables of each EHS case and individual factors. As discussed, HTT results depend on a myriad of situational and individual characteristics that indicate the timeline should not be standardized across cases. Appropriate diagnosis and fast, aggressive treatment of EHS benefit recovery, whereas delayed treatment or treatment with unfavorable cooling rates³⁵  may result in complications or prolonged recovery. Also, the fitness level and heat acclimation status may change during recovery or between HTT trials. Additionally, individuals who are intolerant to heat because of state would benefit from appropriate training before HTT more than a person who is intolerant because of trait. SOR for adapting HTT protocol: B.

The clinical algorithm to guide RTA decisions after an EHS, originally constructed by Kazman et al,²³  was adapted to incorporate these considerations (Table 3).

Although the Israeli Defense Forces model is the only validated HTT and the most used in laboratories and clinics, it is important to explore modified versions that have been created throughout the world in military, occupational medicine, athletics, and research laboratories to test the thermoregulatory response to exercise in the heat under different conditions (Table 4). For example, the US Naval Health Research Center (NHRC), located in San Diego, California, conducts a modified version of the Israeli Defense Forces HTT primarily on US Navy and Marine Corps special warfare members who have experienced EHS and have not been allowed to resume normal duties.³⁷  Because of the high metabolic workloads of these warfighters, this protocol requires the participant to walk slightly faster (5.3 versus 5.0 km/h) on a greater incline (4% versus 2%).⁴⁴  However, to date, no validity studies have ascertained the effectiveness or utility of the NHRC protocol.

To our knowledge, no other HTT protocols have been replicated or used outside the context of their published studies (Table 4). The examples outlined in Table 4 may provide opportunities for the future direction of the HTT and to adapt the test to the individual's needs. Although outside the scope of this review, characterizing an individual's metabolic heat production may be an alternative or additional approach to determining and investigating impairment to heat tolerance post-EHS.⁴⁵ 

Despite the lack of a current standard HTT, most look to the Israeli Defense Forces protocol, as all warriors who sustain an EHS in the Israeli Defense Forces are required to be cleared via the Forces' version of the HTT. Consequently, over the last several decades, a large pool of data has become available as a result of using this method. Under this protocol, if a participant demonstrates an abnormal thermoregulatory response (rectal temperature >38.5°C, heart rate >150 beats per minute [bpm]) or if neither rectal temperature nor heart rate reaches a plateau,²⁰  the soldier will be scheduled for a second test 1 to 3 months later. The appropriate time between a failed attempt and a second attempt is based on test results and previous EHS severity. After a second failed attempt, the individual is deemed heat intolerant and will not continue service in a combat military unit.

Similarly, with the NHRC protocol, if participants have failed HTT (ending core temperature >38.5°C and heart rate >150 bpm), their chain of command or clinician may recommend additional attempts. As of 2020, the NHRC had explored using a slightly higher temperature (38.8°C) as a final core temperature cutoff.⁴⁶ 

As discussed in earlier sections, heart rate, core temperature, and the plateauing of either are the traditional methods of interpreting HTT. Additional metrics, such as dynamic physiological variables, may be considered when performing and interpreting HTT. The thermal-circulatory ratio index, which is the ratio between rectal temperature and heart rate, directly affects thermoregulatory processes.⁴⁷  Suggested cutoffs for heat intolerance are a maximum value of ≤0.279°C/bpm at the end of the 120-minute test or ≤0.320°C/bpm at the 60-minute mark. However, the data are currently limited regarding the latter, as it may shorten HTT.⁴⁸  In general, the thermal-circulatory ratio index is intended to prevent misinterpretation of heat intolerance.

Another dynamic variable is the magnitude of increase of rectal temperature during HTT, which has been defined as >0.45°C during the second hour of the test and may help identify those who are heat intolerant. Finally, a more recent variable that has been explored on a continuous scale is the probability of heat tolerance, which was designed to be used with borderline HTT results.³⁶  The probability of heat tolerance is defined using an algorithm calculator available via open access (probability of heat tolerance; https://phtheller.shinyapps.io/HTTest/). The calculator requires the time, rectal temperature, and heart rate and then provides a probability of heat tolerance value and cut-off scores. Although still relatively new, this variable may provide a more standardized interpretation, especially in the event of borderline results. Nonetheless, as addressed in previous sections, the decision to perform HTT and the interpretation thereof is multifactorial and should be individualized. SOR: C.

To date, the US military services have different consensus recommendations regarding RTA post-EHS, and they do not routinely use HTT but instead rely on clinical judgment and heat acclimation variables.²²  They primarily use HTT for individuals with abnormal recoveries or multiple EHSs; in contrast, the Israeli Defense Forces uses HTTs for all warfighters who have sustained an EHS. Similarly, the RTA decision for athletes or laborers post-EHS is typically at the discretion of the physician or medical team to whom they are assigned. However, HTT can supply more objective, functional measures of the person's thermoregulatory state. Guidance for the RTA of warfighters, athletes, and laborers is offered in Table 3. SOR: C.

Unfortunately, no evidence-based, consensus guidelines or tools currently exist to direct clinicians, coaches, or athletes regarding HTT.⁴⁸  As of this writing, general recommendations for overseeing the care, recovery, and reintroduction into the heat of a person post-EHS included the synthesis of the individual's medical history; clinical biomarkers; subjective reports; and, if available or required, HTT.

Controversy exists regarding current HTT models, as many research gaps need to be clarified. For example, Schermann et al³⁶  used a mathematical model to obtain a quantitative estimate of the probability of heat tolerance, demonstrating sensitivity, specificity, and accuracy of 100%, 90%, and 92.06%, respectively. However, these psychometric properties have not been validated in other studies. In addition, current HTT models do not account or adjust for sex differences, age, varying workloads by aerobic capacity, high altitude, simulated real-world conditions (wearing heavy gear or uniforms, being sleep deprived, using dietary supplements with stimulants, etc), heat acclimation or training status, or body fat.^12,13  To more accurately measure an individual's ability to thermoregulate after an EHS, further investigation is warranted to better understand how these variables may influence the assessment of heat tolerance.³⁵  Therefore, the future of HTT may include activities more specific to the individual's needs, whether in an athletic event, at work, or in a military setting. For health care professionals, clinical judgment and individualized considerations are key for using, adapting, and interpreting HTT.

We provide information on the steps for performing HTT, yet caution is needed when attempting to conduct such testing without proper training, equipment, manpower, and expertise. As such, we recommend that individuals who request to perform HTT post-EHS should consult with a laboratory that specializes in heat physiology. Although not all-inclusive, the following are examples of locations within the United States that perform HTT.

• Naval Health Research Center Warfighter Performance Laboratory (San Diego, CA)

  Population: primarily US Navy and Marine warfighters

• Korey Stringer Institute, University of Connecticut (Storrs, CT)

  https://ksi.uconn.edu/services/athlete-testing/ Population: athletes, warfighters, laborers

• Uniformed Services University (Bethesda, MD)

  https://champ.usuhs.edu/for-the-provider Population: military warfighters

We thank Drs Francis O'Connor and Doug Jones for their time and expertise in answering questions and clarifying how their respective organizations perform and interpret heat tolerance testing for US military warfighters.