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the role of an HTT as a clinical tool to help determine the RTD heat tolerant (9), 4 were heat intolerant, and 3 were border-
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timeline is a topic of robust debate. While the utility of HTT line. All but 1 of the 20 SS males were heat tolerant, with the
in determining risk of recurrent EHI is not well understood, 1 SS being borderline. Of the seven women, three were RC and
there are risk factors for EHI recurrence that are measurable. heat tolerant, and four were SS (with two intolerant and two
For example, environmental conditions during the initial and heat tolerant). The participant characteristics and HTT time-
recurrent EHI, acute or chronic medical conditions, and an- lines for all participants are provided in Table 1 and Figure 1,
thropometric measurement should all be considered in com- respectively. While CHAMP classifies Servicemembers as heat
bination with HTT results when determining EHI recurrence tolerant, heat intolerant, or borderline and makes appropriate
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risks. Our project evaluated the use of those variables in recommendations, the final return to duty decisions and accli-
combination with HTT long-term follow-up data to establish matization strategies are made by the referring providers.
the predictive validity of HTT for recurrent EHI.
The medical record was used to review for recurrent EHI in
the two years following the HTT, with two Servicemembers
Methods
being diagnosed with another EHI. The first was a 23-year-
The study protocol (FAM-81-3173, 407231) was reviewed old woman who had been classified as SS and heat intolerant.
and approved by the Uniformed Services University (USU) At the initial evaluation, her Vo max (44.0mL·kg ·min ) was
-1
-1
2
Institutional Review Board. Informed consent was obtained similar to the other women, but her percentage of body fat
from all participants. The study was performed by the was slightly higher (35.4%). Records indicate she had EHS in
Consortium for Health and Military Performance (CHAMP) August 2018, was evaluated in June 2019, and had a recur-
research staff in the Human Performance Laboratory at USU. rent EHS in August 2019. The second was a 29-year-old man
Participants (n=43) with prior EHS were recruited for the classified as RC and borderline. At the initial evaluation, his
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study by a physician’s referral to CHAMP for an HTT. Exclu- Vo max (43.6mL·kg ·min ) and other anthropometric mea-
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sion criteria included heart disease, high blood pressure, his- surements were similar to those of the other men. Records in-
tory of malignant hyperthermia, anemia, use of psychotropic dicate a self-reported EHI in June 2014, a documented EHS in
medications, use of diabetes medications, use of glucocorti- June 2019, an evaluation in September 2019, and a recurrent
coids, use of beta blockers, and pregnancy. Participants (Ser- EHS in October 2019. These two recurrent EHIs are depicted
vicemembers with prior EHS) completed demographic (age, by darker lines in Figure 1.
sex) and anthropometric screening (% body fat, body mass
index, body surface area, body surface area to mass ratio), The predictive utility of the HTT depended on how borderline
maximal oxygen consumption (Vo max), and an HTT (2h cases were classified. When borderline cases were classified as
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walk with 2% incline at 5km/h pace in 40°C heat and 40% heat intolerant, the HTT had a high NPV, indicating a high
relative humidity). We estimated percentage of body fat using probability that those who were heat tolerant did not have a
a two-point bioelectrical impedance analysis (RJL Quantum recurrent EHI (NPV: 100%; 95%CI 89, 100) and low PPV of
II, RJL Systems, Clinton Township, MI; https://rjlsystems.com) 20% (95% CI 3, 56) (Table 2, first column). Specificity (81%;
and then applied estimation equations derived from the Na- 95% CI 65, 91) was moderate, and sensitivity (100%; 95%
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tional Health and Nutrition Examination Survey III. Maxi- CI 16, 100) was high. When borderline cases were classified
mal aerobic capacity via Vo max was assessed with a maximal as heat tolerant, NPV (97%; 95% CI 86, 100), PPV (17%;
2
graded exercise test on the treadmill using open-circuit spi- 95% CI 0, 64), and especially sensitivity (50%; 95% CI 1, 99)
rometry (Oxycon Mobile portable system, Viasys Healthcare all decreased, and specificity (88%; 95% CI 74, 96) slightly
Inc, Yorba Linda, CA; product/manufacturer discontinued). 12 increased (Table 2, second column). In contrast, when we used
Participants were classified as heat tolerant (rectal tempera- EHI history (i.e., RC, SS), there were much lower values for
ture <38.5°C and heart rate <150beats/min [BPM]), heat in- specificity (56%; 95% CI 40, 72) and PPV (5%; 95% CI 0, 26)
tolerant (consistently rising rectal temperature and heart rate (Table 2, third column).
>150BPM), or borderline (equivocal thermoregulatory ability)
following the HTT. We then categorized Servicemembers as Discussion
single/simple prior EHI (SS) or recurrent/complex EHI (RC)
by conducting a retrospective review of the specific conditions Our project was carried out to establish the predictive validity
or factors in the HTT record, such as time of year, ambient of the HTT for recurrent EHI in our cohort of Servicemem-
temperature, and activity being conducted during the EHI ep- bers who had long-term follow-up data. We examined medical
isode. Lastly, the electronic medical records of all participants records up to two years after an initial EHI evaluation, and
were followed for two years following the HTT for a recurrent our results indicate that heat-tolerant Servicemembers with a
EHI. Using these data, we calculated positive predictive value history of EHI have a low risk of EHI recurrence during those
(PPV; proportion of heat intolerant subjects who had recur- two years. Importantly, we demonstrated that the HTT had an
rent events), and negative predictive value (NPV; proportion NPV of 100% for a future EHI over two years of follow-up
of heat tolerant subjects who did not have a recurrent event). for Servicemembers with a history of recurrent heat injury and
The 95% CIs were calculated using the meddecide module in a negative HTT. Thus, it has clinical utility.
jamovi (version 2.3, www. jamovi.org, the jamovi group). 13
As in our study, the Israelis demonstrated considerable clinical
value in HTT’s accuracy to predict recurrent EHI through their
Results
use of defined heat intolerant and heat tolerant descriptive test
The retrospective review of HTT records was used to catego- parameters. In their study of 145 patients (110 heat tolerant,
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rize 44% of Servicemembers as RC. Most were classified as 35 heat intolerant) over about 8 years, 6 (4.1%) experienced re-
heat tolerant (77%), and the rest as heat intolerant (14%) or current EHI events. Four were considered heat intolerant, and
borderline (9%). Among the 16 RC males, more than half were two were heat tolerant by their test parameters. Among those

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