Page 50 - JSOM Spring 2024

Heat Tolerance Testing and the Return to Duty Decision

A Two-Year Case Cohort Analysis

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Rachel M. Kester, MD ; Preetha A. Abraham, MA, DOL ;
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Jeffrey C. Leggit, MD ; Jacob B. Harp, MS ; Josh B. Kazman, MS ;
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Patricia A. Deuster, PhD, MPH, FACSM ; Francis O’Connor, MD, MPH *
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ABSTRACT
Background: Among individuals with prior exertional heat ill- “exertional.” Classic heat stroke (CHS) is typically observed in
ness (EHI), heat tolerance testing (HTT) may inform risk and patients with preexisting conditions. In constrast, exertional
return to duty/activity. However, little is known about HTT’s heat stroke (EHS) occurs primarily in healthy people who gen-
predictive validity, particularly for EHI recurrence. Our proj- erate increased metabolic heat from physical work with con-
ect sought to demonstrate the predictive validity of HTT in current exposure to high ambient temperatures. Regardless of
EHI recurrence and HTT’s utility as a diagnostic tool in ex- the origin, heat stroke is a medical emergency requiring rapid
ertional heat stroke (EHS). Methods: Participants with prior recognition and treatment to prevent permanent complica-
EHS were recruited for the study by a physician’s referral and tions and death; in the absence of prompt treatment, mortality
were classified as heat tolerant or intolerant after completing approaches 80% from CHS and 33% from EHS. 1,3
demographics and an HTT. Participants were further catego-
rized as single/simple (SS) EHI or recurrent/complex (RC) EHI EHI is a direct threat to military operational readiness. In
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by conducting a retrospective record review of the following 2020, there were 475 incident cases of heat stroke and 1,667
two years. We calculated the positive (PPV) and negative pre- incident cases of heat exhaustion. Despite the risk of recurrent
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dictive values (NPV) of HTT. Results: The retrospective review EHI as a threat to readiness, currently no universal standards
of HTT records was used to categorize 44% of Servicemem- inform return to duty (RTD) timelines. In the U.S. military,
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bers as RC, with 77% classified as heat tolerant, 14% as heat RTD decisions are largely based on successful demonstration
intolerant, and 9% as borderline. When borderline cases were of a graduated response to both exercise and environmental ac-
classified as heat intolerant, HTT had a high NPV, indicat- climatization. Warfighters who fail to demonstrate a favorable
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ing a high probability that heat-tolerant individuals did not response to re-acclimatization or sustain a second EHI event
have recurrent EHI. When borderline cases were classified as are generally considered candidates for medical discharge. Pro-
heat tolerant, NPV and sensitivity decreased while specificity longed loss of duty and training time from EHI, in addition
increased. Conclusion: We demonstrated that the HTT had a to losses from medical discharge, pose a significant threat to
100% NPV for future EHI over two years of follow-up for operational readiness. Currently, the U.S. military has no spe-
Servicemembers with a history of recurrent heat injury and cific clinical tool or biomarker required or routinely utilized to
negative HTT results. An HTT can provide critical data points assess a warfighter’s future risk for a recurrent EHI event.
to inform return to duty decisions and timelines by predicting
the risk of EHI recurrence. The Israel Defense Force has reduced the number of recurrent
EHIs by requiring all military personnel with a medical history
Keywords: exertional heat stroke; heat stroke; heat toler- of EHI to undergo a heat tolerance test (HTT) prior to re-
ance testing; return to duty; heat tolerance; exertional heat turning to duty. Because of the Israel Defense Force’s success
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illness; recurrent heat injury with the HTT, the U.S. military began conducting research on
the HTT as an adjunctive tool to assist with clinical decision
making on high-risk EHS cases. The U.S. Navy and Army
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Special Operations Forces (SOF) currently use the HTT as
Introduction
both a diagnostic and informative tool, particularly for com-
Exertional heat illness (EHI) is a clinical spectrum of disorders plex and borderline EHI cases. It has been used successfully
ranging from heat exhaustion to life-threatening heat stroke. to return 74 Naval Special Warfare personnel to duty, with 19
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The estimated overall incidence of EHI in the military was successfully becoming U.S. Navy SEALs. 7
reported to be 0.2–10.5 per 1,000 years across 14 studies.
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One form of EHI is heat stroke. Heat stroke, characterized Although the HTT has been used in the U.S. military for over
by multisystem involvement, is subcategorized as “classic” or 15 years, a significant knowledge gap remains. In particular,
*Correspondence francis.oconnor@usuhs.edu
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1 CPT Rachel M. Kester is an emergency medicine resident at Madigan Army Medical Center. Dr. Preetha A. Abraham is the Director of Research
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and Scholarship. COL Jeffrey C. Leggit is a professor at the Department of Family Medicine, F. Edward Hébert School of Medicine, Uniformed
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Services University, Bethesda, MD. Jacob B. Harp is a research assistant, and Josh B. Kazman is a biostatistician at the Consortium for Health
and Military Performance, Department of Military and Emergency Medicine, F. Edward Hébert School of Medicine, Uniformed Services Univer-
sity, Bethesda, MD, and with the Henry M. Jackson Foundation for the Advancement of Military Medicine, Inc., Bethesda, MD. Dr. Patricia A.
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Deuster is a professor and the Acting Executive Director, and COL (Ret) Francis G. O’Connor is a professor and the Chief Medical Officer at
the Consortium for Health and Military Performance, Department of Military and Emergency Medicine, F. Edward Hébert School of Medicine,
Uniformed Services University, Bethesda, MD.
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