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Year : 2021  |  Volume : 19  |  Issue : 3  |  Page : 165-170

The management of hyperthermia and exercise-associated hyponatremia in low-resource and prehospital settings

Portfolio GP, Event Medic and Expedition Doctor, South Central Ambulance Service NHS Foundation Trust, United Kingdom

Date of Submission02-Apr-2021
Date of Decision04-Apr-2021
Date of Acceptance05-Apr-2021
Date of Web Publication05-Jul-2021

Correspondence Address:
Dr. Daniel Grace
Portfolio GP, Event Medic and Expedition Doctor, South Central Ambulance Service NHS Foundation Trust
United Kingdom
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/cmi.cmi_15_21

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Between 2000 and 2016, the number of people exposed to heat waves worldwide increased by around 125 million. As global warming increases, there is growing concern regarding the effect of heat stress on health outcomes, particularly in low- and middle-income tropical countries. The likelihood of developing heat-related injury depends on three factors: an individual, their environment, and their workload. Both exertional hyperthermia and exercise-induced hyponatremia are potentially life-threatening conditions that may develop in environments with increased heat stress. These can present with vague and overlapping symptoms such as confusion, headache, vomiting, and in severe cases, coma. The management of these two conditions is extremely different, and making a correct diagnosis can be challenging, particularly for health-care professionals who are working in low-resource or prehospital settings.

Keywords: Hyperthermia, hyponatremia, low resource, prehospital setting, temperature

How to cite this article:
Grace D. The management of hyperthermia and exercise-associated hyponatremia in low-resource and prehospital settings. Curr Med Issues 2021;19:165-70

How to cite this URL:
Grace D. The management of hyperthermia and exercise-associated hyponatremia in low-resource and prehospital settings. Curr Med Issues [serial online] 2021 [cited 2023 Jun 6];19:165-70. Available from: https://www.cmijournal.org/text.asp?2021/19/3/165/320640

  Introduction Top

Between 2000 and 2016, the number of people exposed to heat waves worldwide increased by around 125 million.[1] Within most high-income countries, public perception, and indeed the majority of academic researches, has focused upon the associated morbidity and mortality that affects the elderly, the very young, and the chronically ill during seasonal heat waves.[2] As global warming increases, there is growing concern regarding the effect of heat stress on health outcomes, particularly in low- and middle-income tropical countries (LMICs).[3] Here, heat stress is a significant problem, especially among occupational populations, where a combination of ambient heat exposure and internal heat, generated from heavy physical work, is associated with an increased incidence of heat-related illness and death from exertional heat stroke.[4] Information regarding mortality in LMICs is unfortunately limited, however, data from the US Bureau of Labor showed that between 1992 and 2008, 487 worker deaths occurred due to exposure to environmental heat and it is likely that this figure is an underestimate.[2]

Countries in economic transition will often experience rapid urbanization, with workers performing heavy labor for long periods of time, often under hot and humid conditions, especially if they come from lower socioeconomic backgrounds.[3] These populations are usually much younger than the patient cohorts that are affected by seasonal heat waves. As well as direct morbidity from heat stress, which can have long-term deleterious effects on the kidneys, there is also an increased risk of indirect injury: individuals report reduced comfort, strength, endurance, vision, coordination, concentration, and judgment which can in turn lead to fainting, confusion, and potential traumatic injury.[2],[4]

There are also groups within the sporting and tourism industries that have an increased risk of developing heat-related illnesses, most notably amateur athletes and charity trek participants. Over the last two decades, the number of ultra-marathon races hosted worldwide has grown considerably, and prior to the COVID-19 pandemic, these events have occurred, often annually, all over the world, from sub-Saharan Africa to Nepal and Patagonia.[5],[6] Pre-COVID-19, the charity fundraising market was also thriving, with increasing numbers of participants taking part in treks or similar challenges at a more advanced age than they would have done previously. These “silver trekkers” are likely to have a higher incidence of comorbidities, which may in turn predispose them to developing a heat-related illness if they are exposed to a high heat stress environment.

This review shall briefly examine hyperthermia and “exercise-associated hyponatremia” (EAH) and explore how these conditions can be diagnosed and managed in low-resource and prehospital settings. Both of these conditions can occur in environments that have increased heat stress and they may present with similar, and often quite vague, symptoms. Crucially, the management of these is very different. Making the wrong clinical diagnosis and subsequently starting treatment can have disastrous consequences, leading to increased morbidity and mortality.

  Heat-Related Injury: The Environment Top

The likelihood of developing heat-related injury (HRI) depends on three factors: the environment, the individual, and their workload.[7] The environment plays a significant role in determining perceived heat stress, and this is influenced by air temperature, radiant heat load, absolute humidity, and wind speed,[7] as illustrated in [Figure 1].
Figure 1: Heat loss and gain mechanisms.

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A wet-bulb globe temperature (WBGT) reading can provide an environmental heat stress index, and devices can be purchased for this purpose. Importantly, WBGT does not represent human heat strain, as it does not account for metabolic heat production or clothing.[8] It also underestimates heat stress in high humidity or conditions with reduced air movement which will restrict sweat evaporation.[8] Despite these limitations, it can be used to calculate how much physical activity should be undertaken in specific conditions, and may be used by employers, sporting organizations, and the military to ascertain a suitable workload strategy for a particular environment, as illustrated in [Table 1].
Table 1: Recommended maximum workloads in different WBGT conditions

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  Heat-Related Injury: The Individual Top

Multiple individual factors affect thermoregulatory ability, including a lack of sleep, missing meals, and getting sunburnt.[9] Medical conditions that reduce cardiac output, such as heart failure, can predispose to HRI, as can fever and infection, which augment the body's normal hyperthermic response.[9] Certain medications also impair thermoregulation, as illustrated in [Table 2]. These include prescribed and over-the-counter medications. Drinking alcohol inhibits vasomotor reflexes that aid heat loss, and illicit drugs, most notably ecstasy (C1115NO2), also increase the risk of HRI.[9]
Table 2: Prescribed and over-the-counter medications can increase the risk of heart rate

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Advancing age reduces the protective thermoregulatory responses that occur during exercise and increases the probability of HRI.[10] Clothing choice can also play a role: synthetic base-layer garments are more effective than cotton at actively reducing moisture retention, while maintaining desired skin and rectal temperatures.[10],[11],[12],[13] Vapor barrier coveralls, such as those worn by construction workers, can raise the effective WGBT by up to 11°C.[4] In contrast, light-colored single-layer clothing can be protective against heat stress.[2] Ensuring adequate shelter is fundamental, in order to avoid the hottest time of day with the greatest potential for radiant heat gain. Health promotion to at-risk occupational groups may be an effective strategy, however, this is only likely to be effective if their workload is also taken into account.

  Heat-Related Injury: The Workload Top

Data from the American Conference of Governmental Industrial Hygienists show that going from rest to a “moderate work activity” increases an individual's metabolic rate by nearly 200%.[2] Groups at particular risk of this include farmers, construction workers, firefighters, miners, soldiers, and manufacturing workers.[11] Reviewing and adjusting work-rest cycles, and allowing time for adequate physiological recovery, is vital in order to compensate for the increased heat stress that can occur in these roles.[2] In 2015, Costa Rica introduced legislation requiring employers of agricultural workers who labor outdoors to provide shade, water, rest breaks, and protective clothing, in response to a growing epidemic of chronic kidney disease linked to occupational heat stress and chronic dehydration among young men.[14]

Many jobs, particularly within agriculture, can make it hard for employees to maintain a healthy balance of heat gain and loss: much of the work is physically demanding, during warm months, and pay can be output based for self-paced tasks, which often leads to high levels of exertion.[2] Construction workers are also a high-risk group, especially in the southern United States, the Middle East, Asia, Latin America, and Africa, where workers are regularly exposed to extremely high temperatures with long working hours and limited or no access to shade or water.[14] Due to the dangerous nature of building sites, the construction industry consistently has higher fatality rates as a result of heat stress compared to other industries.[14]

Educating workers and their supervisors about the risks posed by heat stress is important. In Gujarat India, educational pamphlets have been distributed, alongside “high temperature warnings,” to raise public awareness of heat stress prevention strategies, however, the efficacy of such public education campaigns to prevent or reduce HRI among construction workers is unknown.[12]

  Prevention of Heat-Related Injury Top

Acclimatization is an important prophylactic strategy when addressing HRI. It requires an individual to exercise in the heat for a minimum of 60 to 90 min every day for 1 to 2 weeks.[13] This improves sweating, increases skin blood flow, lowers core body temperature, reduces cardiovascular strain, improves fluid balance, and alters an individual's metabolism.[15]

Heat acclimatization can also lead to protective adaptations on a cellular level. The intracellular heat shock protein 72 (HSP 72) is involved in the maintenance of cellular protein conformation and homeostasis during hyperthermia, inflammation, and injury.[16] After a 10-day program of heat and exercise acclimation, increased levels of HSP 72 have been found in peripheral blood mononuclear cells.[16] These physiological changes allow an individual to work at a given output level with a significantly lower increase in core temperature.[16] It takes 10–14 days for these to occur[7] and various acclimatization protocols exist depending on the proposed activity and an individual's aerobic fitness level. This is important for expedition and sporting companies to be aware of when organizing and running events and is also an important consideration for organizations that employ large numbers of seasonal migrant workers.

Avoiding dehydration is one method to reduce the likelihood of HRI, however, it is important to educate individuals regarding fluid intake as misconceptions are widespread. Until the 1960s, it was thought that drinking during exercise would impair athletic performance[17] and this resulted in cases of hypernatremia and dehydration.[18] Subsequently, athletes were advised to consume as much fluids as possible during exercise.[18] This came from the US military during the 1980s, where water was presented as an important “tactical weapon.”[19] Military scientists advocated drinking up to 1.8 liters of fluid per hour and combining this with alternating periods of activity and rest, stating that this would give soldiers a tactical advantage in warm environments.[19] As these scientists left the military and became advisors for sporting bodies, these ideas spread.[19] Hyperhydration has become engrained in everyday health advice,[19] resulting in cases of EAH among both trekkers and climbers.[19]

Advising people to “drink to thirst” is a strategy that may reduce the chances of developing both HRI and EAH, particularly in the high-risk groups that are the focus of this article. The sensation of thirst is a behavioral urge, driven largely by physiologic mediators that are activated when total body water content is low and antidiuresis is maximal.[20] Concerns also exist with this approach, especially in individuals working hard in hot environments or those drinking lots of caffeine who may be at risk of chronic dehydration.[7] Surrogate markers such as urine color, quantity, and frequency can be helpful in assessing fluid status alongside monitoring for any weight loss,[7] and these simple low-tech measures should be highlighted to individuals at risk of HRI.[10],[20]

  Assessment and Treatment of Exertional Hyperthermia and Exercise-Associated Hyponatremia Top

Exertional hyperthermia (EH), also known as exertional heat stroke, and EAH, are two potentially fatal conditions that can present with nonspecific, overlapping symptoms in the context of a high heat stress environment. The most commonly used definition of EH is the presence of a core body temperature that rises above 40°C, accompanied by hot dry skin and central nervous system abnormalities, such as delirium, convulsions, or coma.[21] An alternative definition has been produced by the Japanese Association for Acute Medicine.[22] This definition includes patients who have been exposed to high environmental temperatures with one of more of the following: A Glasgow Coma Scale score of ≤14, creatinine or total bilirubin levels of ≥1.2 mg/dL, or a JAAM DIC score of ≥4. As this article focuses upon the prehospital or low-resource diagnosis and management of EH and EAH, the clinical definition made by Bouchama et al. is felt to be more appropriate in this context.[20]

EAH is defined as: hyponatremia that occurs during, or up to 24 h after, prolonged exertion.[16] Making a correct diagnosis is key as the subsequent management of each of these conditions is very different. Making a wrong diagnosis can be fatal, however, without laboratory facilities, it may be impossible to tell clinically whether a collapse is due to EH or EAH. Point-of-care sodium testing is therefore a valuable resource in a prehospital or low-resource setting.

Preceding events may point toward a diagnosis: If a patient has been drinking excess fluids, for example, they are more likely to have developed EAH. Female sex, a low bodyweight, and taking nonsteroidal anti-inflammatory drugs can also increase the risk of EAH.[23] A patient's temperature may be diagnostic, and if this is raised over 40°C, the patient has HRI. It is important to appreciate that EH and EAH can coexist and this can make management decisions hazardous.[24] The challenges of making a correct clinical diagnosis are exemplified by a study looking at marathon participants with and without hyponatremia who presented with collapse. Vomiting was the only symptom differentiating hyponatremia from other conditions that can also cause collapse.[25]

  Treatment of Exertional Hyperthermia Top

As with all medical emergencies, it is important to undertake a primary survey and resuscitate the patient appropriately. Prompt recognition of EH is essential, and rapidly, reducing the core body temperature to below 38.5–39°C i<30–60 min is key to survival.[26] Resting the casualty in the shade can help to reduce the external ambient temperature, and placing them on an insulating barrier can decrease heat conduction from the ground.[27] Stripping them down to their underwear will help optimize convective heat exchange.[27] Lying them in a string hammock can have a similar effect due to increased airflow.[7]

Cold water immersion (CWI) is the optimal field treatment to achieve rapid temperature reduction, as the thermal conductivity of water is 24 times greater than air.[27] Aggressive stirring or continuous water motion will replace warmed water at the skin with cold water and wrapping a cold wet towel around the top of the head will enhance cooling further.[28] [Figure 2] shows the relative differences in cooling rates for different treatments.
Figure 2: Mean cooling rates for different treatment options.

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It is important to balance these treatment benefits against the potential risks of hazardous currents, aspiration, and drowning. If immersion is not feasible, evaporative cooling techniques should be used. Spraying or dousing the victim with water and fanning are suitable techniques, and using cold water mist can achieve cooling rates from 0.04°C/min to 0.08°C/min.[26] Wrapping the patient in wet towels or sheets may be beneficial, although it is less effective than CWI, especially in humid conditions.[7] Rehydration is an important consideration, and both oral and intravenous hydration are equally effective in replenishing water deficiencies due to heat stress.[27] As discussed, EAH and EH can present similarly and there is a substantial risk of further harm if fluids are administered incorrectly to a patient with EAH.

  Treatment of Exercise Induced Hyponatremia Top

As with EH, an awareness of EAH as a potential diagnosis is important. Treatment protocols for EAH are based on serum sodium estimations which are may be unavailable in a prehospital or low-resource setting.[27] Machines such as the “i-STAT” analyzer are suitable for point-of-care testing of electrolytes in critically ill patients and results can be available within 130–200 s [Figure 3]. However, a unit costs £5191 the equivalent of 518761.46 rupees, and each test cartridge costs £12–20 (1199–1998 rupees).[27],[28],[29],[30],[31] They also require appropriate calibration, and hot environments may affect the reliability of cartridges which are primarily designed for inhospital use.
Figure 3: I-STAT handheld analyzer.

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If EAH is suspected clinically, but serum sodium testing is unavailable, intravenous fluids can only be justified to treat frank hypovolemia.[27] If EAH is confirmed biochemically and there are signs or symptoms of hyponatremic encephalopathy, a 100-mL bolus of 3% NaCl should be given.[27] This may be repeated up to two more times at 10-min intervals if there is no clinical improvement.[27] Mild and nonspecific symptoms may resolve spontaneously with appropriate fluid restriction, as illustrated in [Figure 4]. Recent studies suggest that oral hypertonic saline solutions may improve symptoms and provided patients do not have significant neurological symptoms, they can safely be given at a dose of 100 mL of 3% saline orally.[29],[32]
Figure 4: Management of biochemically confirmed exercise-associated hyponatremia.

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  Take Home Points Top

  • EH and EAH are both medical emergencies
  • Management of each of these conditions is very different and relies upon checking the patient's core temperature and serum sodium level
  • In a prehospital or low-resource setting, with no ability to check serum sodium, fluids should only be given if a patient is hypotensive
  • Following appropriate resuscitation and stabilization, it is important to arrange prompt medical evacuation to definitive care, so that casualties can have appropriate investigation and ongoing management. This may involve intensive care unit level care.

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Conflicts of interest

There are no conflicts of interest.

  References Top

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Hew-Butler T, Loi V, Pani A, Rosner MH. Exercise-Associated Hyponatremia: 2017 Update. Front Med (Lausanne). 2017;4:21. Published 2017 Mar 3. doi:10.3389/fmed.2017.00021.  Back to cited text no. 32


  [Figure 1], [Figure 2], [Figure 3], [Figure 4]

  [Table 1], [Table 2]


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