Page 115 - JSOM Summer 2019

TABLE 3 Guidelines for Return to Activity for Individuals cardiovascular strain. Voluntary water consumption increases.
Experiencing Exertional Heat Stroke 27 Many of these adaptations occur over the first week of heat
Refrain from exercise for at least 7 days following release exposure and are largely completed within 7–14 days, requir-
1
from medical care ing only 2–4 hours of exposure per day. 15,77,78
Follow up in about 1 week for physical exam and repeat
2 lab testing or diagnostic imaging of affected organs that Individuals should be gradually acclimatized to heat over 7–14
may be indicated, based on physician’s evaluation days to obtain favorable physiological adaptations mentioned
When cleared for activity, begin exercise in a cool earlier. Acclimation involves gradually increasing the fre-
77
environment and gradually increase the duration,
3 quency, intensity, and duration of physical activity in the heat.
intensity, and heat exposure for 2 weeks to acclimatize
and demonstrate heat tolerance. A recent meta-analysis on the loss of heat acclimation indi-
If return to activity is difficult, consider a cated that for every day without heat exposure about 2.5% of
4 laboratory–heat tolerance test about one month post adaptations in heart rate and core temperature were lost, but
incidence. 49,74,105,106 data on loss of sweat adaptations are not clear. 79
Clear the individual for full competition if heat tolerance
5
exists after 2–4 weeks of training. Thermal Environment
Besides acclimation, prevention of EHS involves an under-
plateau during the exercise in heat-tolerant individuals. 51,74 In standing of the thermal environment, control of physical ac-
the IDF, a second test is repeated 2–3 months later if the SM tivity, and judicious water consumption. With regard to the
is found to be heat intolerant and a second positive test will thermal environment, if the ambient temperature is higher
remove the SM from combat service. The HTT has been shown than skin temperature the body will absorb heat. Wind assists
to have a sensitivity (ability to correctly identify those with heat in the evaporation of sweat to cool the body, but exposure to
intolerance) of 67% and a specificity (ability to detect those sunlight increases body heat by absorbing solar radiation. The
75
who are heat tolerant) of 78%. More recently, a thermal-cir- wet blub globe temperature (WBGT) is an index that takes
culatory ratio (TCR) was developed for use in conjunction into account these and other factors related to the thermal en-
with the HTT. This is simply calculated as the ratio of rectal vironment and human heat stress. The WBGT incorporates air
temperature to heart rate. A TCR of ≤0.279°C per beats/min temperature, relative humidity, wind speed, and solar radia-
during the last 5 minutes of the HTT test was found to have a tion into a single number. It is calculated from three thermom-
100% sensitivity and 89% specificity for heat tolerance/intol- eters that include a dry blub, a wet blub, and a black globe.
erance when tested on an independent sample of Soldiers with The wet blub responds to the level of water evaporation (influ-
76
prior exertional heat injury. Limitations to the HTT include enced by wind and humidity) thus giving an index of cooling.
the fact that it does not account for sex, training level, accli- The dry bulb is shaded and provides the air temperature. The
matization status, and body fat, all of which may affect test black globe absorbs solar radiation and heats above the tem-
results. Nonetheless, the HTT may be useful as one item (in perature of the dry blub. The WBGT is a weighted average of
addition to the other guidelines listed in Table 3) to assist in these thermometers calculated as:
determining an individual’s ability to return to normal duty. 73
WBGT = 0.7 · T + 0.2 · T + 0.1 · T ,
wb gt db
Prevention
where T is the temperature of the wet blub, T is the tem-
gt
wb
Key to prevention of EHS and other heat-related injuries is en- perature of the black globe, and T is the temperature of the
db
suring heat acclimation, adequate work/rest cycles, and proper dry bulb. Simple criteria for purchase of an appropriate in-
80
hydration during activity. Also, certain dietary supplements strument are available, but WBGT is typically provided by
81
(DSs) may have effects on energy expenditure, gastrointestinal messaging, e-mail, or other means to trainers, Operators, and
function, and thermoregulation that should be considered. healthcare personnel at military facilities during times of high
temperature.
Acclimation
When exposed to a hot environment over several days, adap- Work/Rest Cycles and Fluid Intake
tive mechanisms allow individuals to work and/or exercise Because of the likelihood of EHS increases in higher tempera-
in conditions that were previously intolerable or dangerous. tures, rest and rehydration should be emphasized during train-
These adaptations are collectively called heat acclimatization. ing in higher temperatures. Individuals should begin physical
In an acclimatized individual, total body water increases by activity in the heat under hydrated conditions. Training should
2–3L, distributed to extracellular (plasma and interstitial adhere to the recommendations compiled to integrate the level
fluid) and intracellular areas. Plasma volume is expanded of physical activity with the level of environmental heat stress.
and cardiac output is more effectively distributed to both Adequate work/rest cycles should be planned to eliminate
the skin and muscles, resulting in greater stability in blood overheating. Work/rest and proper rehydration guidelines
pressure during physical activity. Vasodilation starts earlier have been developed for the military and are included in Table
82
and larger total amounts of blood are shunted to the skin, so 4. Note that there is a limit to the volume of recommended
heat is more effectively dissipated from the body core. The fluids fluid consumption aimed at avoiding hyponatremia and
temperature threshold for the onset of sweating is lower so overdrinking, which had been a problem in previous military
evaporative cooling can begin earlier and the total amount of operations. 83–85
sweat increases to maximize the potential evaporative cooling.
The concentration of salt in the sweat becomes lower as the Dietary Supplements
86
sweat glands reabsorb salt so that electrolytes are preserved in About 50% of Americans use DSs, but use among SMs is
the extracellular fluid. Rectal temperature and heart rate at a considerably higher, with 70% reporting DS use. 87–89 Among
given activity intensity decrease, reducing thermal stress and elite military personnel (Army Special Forces, Army Rangers,

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