Heat exhaustion

Heat exhaustion can affect anyone, though children, older adults, and individuals with comorbid conditions are more susceptible. It is caused by elevated body temperatures, typically due to environmental factors or overexertion, which impairs thermoregulation. This can lead to significant fluid and electrolyte loss, such as sodium and potassium. If untreated, heat exhaustion can progress to more severe and potentially life threatening heat-related illnesses.

Signs and symptoms

Heat exhaustion versus heat stroke,
U.S. National Weather Service resource

Common signs and symptoms

• Elevated heart rate
• Lowered blood pressure
• Elevated core body temperature (not exceeding 40°C or 104°F)
• Excessive sweating
• Dehydration
• Loss of electrolytes
• Weakness and fatigue
• Persistent muscle cramps
• Skin tingling
• Nausea and vomiting
• Dizziness and light-headedness
• Irritability
• Headache

Less common signs and symptoms

• Palor
• Hot and dry skin
• Core body temperature exceeding 40°C or 104°F
• Syncope
• Central nervous system dysfunction (e.g., altered mental status, loss of spatial awareness, loss of bodily movement control, seizures, etc.)

Comparison with other heat-related illnesses

Common signs and symptoms of heat exhaustion can also be observed in other heat-related illnesses such as heat cramps, heat syncope, and heat stroke. Heat cramps, a mild form of heat-related illness, is characterized by persistent abdominal, quadricipital, and calf muscle contractions. Heat syncope, also referred to as exercise-associated collapse, is a moderate form of heat-related illness. It is characterized by a temporary loss of consciousness. Unlike heat exhaustion, heat cramps and heat syncope do not have systemic effects. Heat exhaustion is a precursor to heat stroke, a severe heat-related illness. Heat stroke is more likely than heat exhaustion to cause palor, hot and dry skin, syncope, and dysfunction of the central nervous system (e.g., altered mental status, loss of spatial awareness, loss of bodily movement control, seizures, etc.). Central nervous system dysfunction and a core body temperature exceeding 40°C or 104°F are the primary differentiators between heat exhaustion and heat stroke. One of the earliest indicators of heat stroke is altered mental status, which can manifest as delirium, confusion, reduced alertness, loss of consciousness, etc. Prompt recognition and treatment are crucial to prevent multi-organ failure and death.

Physiology

The human body maintains a core body temperature around 37°C or 98.6°F through mechanisms controlled by the thermoregulatory center within the hypothalamus. When the body is exposed to high ambient temperatures, intense physical exertion, or both, the thermoregulatory center will initiate several processes to dissipate more heat:

• Blood vessels near the skin surface dilate, increasing blood flow to the skin to facilitate heat loss through radiation and convection
• Heart rate increases to support elevated blood flow to the skin
• Eccrine sweat glands in the skin produce sweat, which evaporates from the skin surface

As the body's core temperature rises and thermoregulatory mechanisms are unable to maintain it, different heat illnesses can develop. The muscle contractions in heat cramps occur due to an imbalance of electrolytes, including sodium, potassium, and calcium, resulting from excessive sweating during intense physical exertion in high temperatures. The impairment in blood flow to the brain in heat syncope occurs because of excessive dilation of blood vessels near the skin surface, leading to a drop in blood pressure and therefore blood flow to the brain.

Causes

As global temperatures continue to rise, there is growing evidence linking higher temperatures to a range of diseases, disorders, and increased mortality and morbidity rates. The Intergovernmental Panel on Climate Change (IPCC) forecasts an upward trend in temperatures, rising by up to 1.5°C in the future as a result of ongoing greenhouse gas emissions. Extreme temperatures, exacerbated by climate change, are leading to more intense and frequent heat waves, putting populations with greater susceptibility to heat exhaustion, such as children, older adults, and individuals with chronic diseases, more at risk.

Common causes of heat exhaustion and other heat-related illnesses include:

• Hot, sunny, or humid weather
• Prolonged heat exposure
• Physical exertion through work, exercise, or sports, particularly in high temperatures
• Tight or non-breathable clothing that traps heat
• Dehydration due to insufficient fluid intake or as a side effect of certain medications
• Overhydration due to depletion of electrolytes and bodily fluids
• Medications affecting thermoregulation

Risk factors

Risk factors for heat exhaustion include:

• Wearing dark, padded, or insulated clothing; hats; and/or helmets (for example, football pads, turnout gear, etc.)
• Having a higher percentage of body fat
• Fever
• Due to impaired thermoregulation, children younger than four years old and adults older than 65 are at a higher risk of serious heat illness, even at rest, if the weather outside is hot and humid, and they are not getting enough cool air.
• Limited access to water or air conditioning
• Some medications, such as diuretics, antihistamines, beta-blockers, antipsychotics, MDMA ('Ecstasy', 'Molly'), and other amphetamines, can cause an increased risk of heat exhaustion.

Medication

The body naturally responds to extreme temperatures by sweating to maintain thermoregulation and lower the body temperature. However, medications taken for chronic conditions, such as diuretics, antihypertensives, anticholinergics, and antidepressants, can impact these thermoregulation processes. These medications can cause electrolyte imbalances, drug-induced hypohidrosis, or drug-induced hyperhydrosis, which can disrupt an individual's ability to regulate body temperature by affecting sweat production. This can lead to heat intolerance, where an individual may feel more sensitive to higher temperatures, increasing the risks of heat exhaustion.

Anticholinergic medications can affect cognitive functions, autonomic mechanisms and impair sweat responses, resulting in common adverse effects consisting of dry mouth, which leads to increased thirst and dehydration. Similarly, other medications containing anticholinergic properties, such as certain antidepressants and antihistamines, can also induce comparable side effects such as dry mouth and dehydration. For patients at risk of or experiencing heat exacerbation, taking medications with anticholinergic properties can further worsen their condition.

Certain antidepressants, including those in the tricyclic or selective serotonin reuptake inhibitor (SSRI) classes, as well as opiods that stimulate histamine release, can cause hyperhidrosis, indicated by increased sweat production. This leads to greater fluid loss and imbalance of electrolytes due to the loss of sodium.

Though the mechanisms are still unclear and not fully understood, antihypertensives such as ACE inhibitors, beta-blockers, and diuretics have shown to decrease heat tolerance and increase one's sensitivity to heat. ACE inhibitors and diuretics can also cause electrolyte imbalances, increasing dehydration and thirst. Beta-blockers limits the body's ability to redirect hyperthermic blood away from the body's core and towards the skin for cooling.

If dehydration and electrolyte imbalances are left untreated, they can be fatal and potentially lead to death.

Special Populations

Pediatrics

Children (under the age of 18 years old) have a higher sensitivity to temperature change compared to adults due to their decreased homeostatic regulatory systems, increased metabolic rates, and decreased cardiac output.

Prevention

Ways to prevent and lower risk of heat exhaustion include:

• Public widespread announcements of heat waves or rapid increases in temperature
• Staying up to date on daily weather reports
• Heat shelters throughout communities
• Wearing loose fitting and lighter fabric clothing
• Try to stay well hydrated unless fluid intake is limited
• For those who are doing lots of extraneous activities or work, find shady cool areas to rest
• Avoid prolonged exposure to hot environments, such as tropical sunshine in the middle of the day, or a boiler room
• Drink adequate fluids
• Avoid exertion and exercise in hot weather
• Avoid medications that can be detrimental to the regulation of body heat

Diagnosis

A diagnosis of heat exhaustion most commonly is diagnosed by medical professionals with various physical examinations. Through examination a person would have their temperature checked and questioned about their recent activity. If the medical professionals suspect a person's heat exhaustion has progressed into heat stroke they may then lead with these varying tests to verify;

• Blood test, medical professionals when conducting a blood test look for low blood sugar or potassium. They may also look for the presence of unwanted gases in a person's blood.
• Urinalysis, an urinalysis or urine test is a test to measure color, clarity, pH levels, glucose concentration, and protein levels. The test additionally can check a person's kidney function, which is common to be affected by classic heat stroke.
• Muscle function tests, medical professionals use muscle function tests to check for rhabdomyolysis. Which is severe damage to a persons skeletal muscle tissue.

Treatment

First aid

First aid for heat exhaustion or heat stroke includes:

• Moving the person to a shaded, fanned, or air-conditioned place
• Removing any excess or tight clothing to facilitate cooling
• Applying wet towels or ice packs wrapped in cloth to the forehead, neck, armpits, and groin, and using a fan to cool the person down
• Lying the person down on their back and elevating their feet above head level to improve blood circulation
• Having the person drink cool water or sports drinks, also referred to as electrolyte drinks, provided they are conscious, alert, and not vomiting (Only applies to heat exhaustion)
• Turning the person on their side if they are vomiting to prevent choking
• Monitoring the person's vital signs, which includes their heart rate, blood pressure, breathing rate, and body temperature
• Monitoring the person's mental status (i.e., confusion, delirium, reduced alertness etc.)
• Contacting emergency medical services
• Transporting or having emergency medical services transport the person to an emergency department if their situation does not improve quickly

Emergency medical treatment

If an individual with heat exhaustion receives medical treatment, Emergency Medical Technicians (EMTs), doctors, and/or nurses may also:

• Provide supplemental oxygen
• Administer intravenous fluids and electrolytes if they are too confused to drink and/or are vomiting

Do Not

If an individual is experiencing heat exhaustion or any other heat related illness DO NOT:

• Administer fever medications such as aspirin or Tylenol as they can be harmful for the individual
• Administer salt tablets as they can worsen dehydration
• Use alcohol or caffeine containing products as they can make it harder for the individual to control their body temperature
• Give anything by mouth if the person is vomiting or unconscious

Heat warning resources

With high temperatures becoming more frequent, there are resources available to stay up to date on sudden changes in the weather. In the United States, OSHA in collaboration with the NIOSH have a Heat Safety Tool app that notifies their users with real time data on weather forecasts in a certain location, common side effects of heat related illnesses, and how the temperature feels like outside allowing individuals to safely plan out their day based on the weather. Additional resources include monitoring weather in your area of the United States based on zip code using weather.gov, being aware of cooling centers in your area, knowing how to save and use less energy within your household, and being well informed of certain populations who are more vulnerable to heat related illnesses than others. Apart from these resources, there are radio stations and news weather forecasts that continue to provide information on changes in the weather and temeprature both globally and within your area.

Prognosis

When heat exhaustion is left untreated, the most common disease progression is heat stroke. According to the CDC, a typical trait indicating a person is having a heat stroke is when their body temperature reaches 106°F or higher in a span of 10 to 15 minutes. In addition to a high body temperature, they will also experience central nervous system dysfunctions such as alteration in their mental status and slurred speech. Another possible illness that heat stroke can lead to is rhabdomyolysis or rapid injury to skeletal muscle especially when heat stroke is caused by physical exertion. When an individual experiences rhabdomyolysis, that damaged skeletal tissue releases toxic muscle components such as myoglobin into the bloodstream and can cause issues such as coca cola colored urine, myalgia, and kidney damage due to the blocked tubules to name a few. If a person is experiencing a heat stroke and is not properly treated, that can further progress to metabolic abnormalities, irreversible damage to multiple organs in the body, and death as a result.

See also

• Occupational heat stress