Heat-related illnesses

Heat-related illnesses are pathological conditions caused by hyperthermia, an elevated body temperature caused by an impaired ability to maintain temperature at the normal hypothalamic set point. Underlying causes of hyperthermia include endogenous, environmental, and behavioral factors which can be present concurrently, increasing the risk of heat-related illnesses. Minor heat-related illnesses include heat syncope, heat rash, heat edema, and heat cramps. Heat exhaustion, a moderate heat-related illness, is characterized by dehydration, normal or slightly elevated body temperature (typically ≤ 40°C), and normal mental status. Clinical features include symptoms of dehydration, nausea, headache, weakness, and ataxia. Heat exhaustion is typically treated with rehydration, passive cooling, and, if needed, evaporative and convective cooling (e.g., mist and fan). Aggressive active cooling is usually unnecessary and complications are uncommon. Heatstroke is a life-threatening heat-related illness characterized by severe hyperthermia (with a body temperature typically > 40°C) and severe CNS dysfunction. The cornerstone of treatment is aggressive active cooling to reduce core body temperature to less than 40°C. Methods include conductive cooling (e.g., ice water immersion) or evaporative and convective cooling combined with adjunct cooling measures (e.g., cooled IV fluids or ice packs). Management of complications (e.g., acute kidney injury and rhabdomyolysis) and ICU admission are often required.

Hyperthermia

• Elevated body temperature caused by an inability to maintain temperature at the normal hypothalamic setpoint, i.e., thermoregulatory dysfunction or failure
• Causes include endogenous, environmental, and behavioral factors and many disease states (see “Causes of hyperthermia”).
• Can be a manifestation of an underlying disease or the cause of a heat-related illness
• Treated primarily with cooling (see “Management of hyperthermia”)

Elevated body temperature alone is insufficient to diagnose hyperthermia or heat-related illness in athletes, as it is a common physiological response to strenuous exertion. ^[1][2]

Overview of heat-related illnesses

Heat-related illnesses are pathological conditions caused by hyperthermia.

Heat-related illnesses exist on a spectrum, and manifestations of heat exhaustion and heatstroke can overlap. When in doubt, treat any heat-related illness with severe features as heatstroke.

Causes of hyperthermia ^[3][4][5]

Multiple concurrent risk factors for hyperthermia increase the likelihood of developing a heat-related illness.

Increased heat production

• Behavioral factors
  • Strenuous exercise
  • Stimulant intoxication
• Conditions
  • Sepsis
  • Status epilepticus
  • Thyroid storm
  • Neuroleptic malignant syndrome
  • Malignant hyperthermia
  • Serotonin syndrome

Decreased heat dissipation

• Environmental factors
  • High ambient temperature and/or humidity
  • Heavy clothing and/or equipment
• Conditions
  • Skin disease, e.g., erythroderma, anhidrosis, ichthyoses, scleroderma
  • Burns
  • Reduced cutaneous perfusion, e.g., due to heart failure
• Substance-related factors
  • Medications that impair thermoregulation, e.g., anticholinergics, antihistamines, beta blockers, calcium channel blockers
  • Alcohol use

Limitations in adaptive behavior

Environmental, cognitive, and/or physical factors may limit appropriate responses to hot environments, e.g.:

• Inability to increase water intake
• Inability to remove oneself from the hot environment
• See “Classification of heatstroke” for detailed risk factors.

• The normal physiological response to elevated body temperature includes:
  • Hypothalamus-mediated peripheral vasodilation (increased heat convection and radiation)
  • Sweating (increased evaporation)
• In hyperthermia, the hypothalamic thermoregulatory response is inadequate to maintain a normal temperature.
• High body temperature → protein denaturation, phospholipid and lipoprotein damage, and membrane lipid liquefaction → cell damage and loss of function (including myocardiocytes and neurons) → cardiovascular collapse → multi-organ failure and possibly death

Hyperthermia vs. fever

Sepsis with hyperpyrexia can be mistaken for a heat-related illness. ^[4]

The differential diagnoses listed here are not exhaustive.

The following cooling methods can be used to treat all types of hyperthermia. The choice of a specific method depends on the cause of hyperthermia, local resources, and the patient's baseline condition. See “Heat exhaustion” and “Heatstroke” for specific treatment approaches.

Approach ^[1][2][5]

Rapid reduction of core body temperature is critical to reducing mortality.

• Provide advanced cardiac life support and/or airway management if indicated.
• Begin passive cooling: Remove heat stressors, e.g., stop exercise, move to a cooler environment.
• Provide fluid resuscitation and immediate hemodynamic support.
• Initiate active cooling promptly for heatstroke and/or persistent hyperthermia.
• Continue active cooling until the core temperature is < 39°C (102°F).
• Consider adjunctive cooling measures (e.g., cold IV fluids) alongside other active cooling methods, if indicated.

Manage hyperthermia with active cooling if there is any doubt about the severity of the illness.

Passive cooling ^[2]

• Definition: cooling methods that separate a patient from external heat sources and/or barriers to heat dissipation
• Indications
  • Minor or moderate heat-related illness
  • Severe heat-related illness only if active cooling is not immediately available
• Methods
  • Move the patient to a cooler environment.
  • Remove equipment and heavy clothing, e.g., helmet, pads, jersey, overalls.

Passive cooling is insufficient for treating heatstroke; initiate active cooling as soon as possible.

Active cooling

• Definition: cooling methods that involve the application of a medium to facilitate heat transfer from the patient's body
• Methods
  • Conductive cooling (most rapid and effective), e.g., ice water immersion, skin lavage or wrap
  • Evaporative and convective cooling (usually most practical), e.g., mist and fan
  • Adjunctive methods (limited use), e.g., cold IV fluids, ice packs, cooling blankets, internal lavage

Conductive cooling ^[1][2]

• Conductive cooling involves direct heat transfer from the body to another material (typically water).
• It is the most effective method for rapid temperature reduction. ^[1][2]

Immersion has the fastest cooling rate (∼ 0.2°C/min) compared to skin wrap and evaporative and convective cooling methods (≤ 0.1°C/min). ^[1][2][3]

Immersion

• Indications
  • Typically the first choice to treat severe hyperthermia
  • Preferred method for exertional heatstroke; consider for nonexertional heatstroke.
• Technique: Place the patient in a tub of cold water (∼ 20°C) or ice water (∼ 2°C). ^[1]

Immersion may interfere with other necessary lifesaving measures (e.g., airway management) in unstable patients with heatstroke.

Skin lavage or wrap

• Indications: alternative if immersion is unavailable, e.g., in the field
• Techniques
  • Rotation of ice water-soaked towels or ice packs over the entire body
  • Application of crushed ice slurry over a patient lying between plastic sheets ^[2]

Evaporative and convective cooling ^[2][5][6]

• Evaporative cooling involves the transfer of heat from the body to surrounding air and water molecules.
• Cooling is usually supported through convection, i.e., increasing the movement of air around the body.

Although less effective than conductive cooling, evaporative and convective cooling methods are easier to implement and may be less stressful for frail patients. ^[7]

Indications

• Immersion unavailable
• Frailty and/or older age
• Simultaneous treatment of many patients, e.g., severe urban heat wave, marathon runners
• Associated conditions that make immersion difficult, e.g., airway compromise, active vomiting

“Mist and fan” technique

• Remove clothing.
• Spray or douse skin with cold water.
• Fan or blow air across wet skin to facilitate evaporation.

Adjunctive active cooling measures ^[3][4][5]

Avoid the following as primary cooling methods; consider only in patients with severe hyperthermia if immersion is impractical or unavailable:

• Infusion of cold IV fluids
• Ice packs to the neck, groin, axillae
• Bladder and/or gastric lavage with cold fluids
• Commercial cooling blankets or vests

Definition ^[2][3][4]

Heat exhaustion is a heat-related illness characterized by all of the following:

• Decrease in blood volume and/or total body water caused by heat stress
• Normal or slightly elevated core body temperature (typically defined as ≤ 40°C)
• Normal mental status ^[4][5]

In contrast with heatstroke, the homeostatic thermoregulatory system retains partial function, and heat continues to actively dissipate.

Clinical features ^[3][4][8]

• Nausea, vomiting
• Diaphoresis
• Dehydration, tachycardia, orthostatic hypotension
• Headache, weakness, fatigue
• Dizziness, vertigo, ataxia

Diagnostics ^[5]

• Diagnostics for dehydration: BMP, CBC, UA, urine electrolytes
• CPK: to assess for rhabdomyolysis

Treatment ^[2][3][8]

• Discontinue exercise.
• Initiate passive cooling
• Treat dehydration, typically with oral rehydration therapy.
• Severe heat exhaustion
  • Consider IV fluid resuscitation.
  • Initiate evaporative and convective cooling.
  • Begin conductive cooling if refractory, i.e., manage as heatstroke.

If heatstroke cannot be excluded clinically, begin intensive active cooling without delay. ^[5]

Disposition ^[4][5]

• Discharge after rehydration: Consider for young, healthy patients without major laboratory abnormalities.
• Hospital admission: Consider for patients with coexisting disease, advanced age, and/or significant laboratory abnormalities.

Definition ^[2][3][7]

Heatstroke is a life-threatening heat-related illness characterized by all of the following:

• Significantly elevated core body temperature; typically defined as > 40°C (104°F)
• Severe CNS dysfunction, e.g., encephalopathy, seizures, coma
• Recent intense physical exertion and/or exposure to extreme heat

Classification

Clinical features ^[1][2][7]

• Body temperature usually > 40°C (104°F)
• Severe CNS dysfunction
  • Persistently altered mental status (AMS)
  • Delirium, hallucinations
  • Loss of consciousness, coma
  • Seizures
  • Ataxia and tremors
• Hot skin (wet or dry)
• Flaccid or persistently rigid muscles
• Tachypnea
• Tachycardia
• Hypotension
• Nausea, vomiting

Differential diagnosis ^[2][3]

• Causes of fever: e.g., infection/sepsis
• Other causes of hyperthermia, e.g., stimulant intoxication
• Ischemic stroke
• High-altitude cerebral edema
• Electrolyte abnormality
  • Hypoglycemia
  • Exercise-associated hyponatremia
  • Hypernatremia

If there is diagnostic uncertainty about heat exhaustion vs. heatstroke, immediately begin empiric treatment for heatstroke.

Diagnostics ^[1][3][5]

Heatstroke is a clinical diagnosis. Diagnostic studies can support the diagnosis, help rule out alternative causes, and assess for complications (e.g., AKI).

Laboratory studies ^[4][9]

• CBC
  • ↑ WBC (stress leukocytosis)
  • ↓ Platelet count if there is DIC
• CMP
  • Findings of AKI (↑ BUN, ↑ creatinine) in up to 30% of patients ^[5][10]
  • Hypernatremia or hyponatremia
  • Hyperkalemia or hypokalemia
  • Hypoglycemia
  • ↑ Transaminases in hepatic injury
• ABG
  • ↓ PaO₂
  • Metabolic acidosis or respiratory alkalosis
  • ↑ Lactate
• CPK, myoglobin: to assess for rhabdomyolysis
• Coagulation panel: to assess for DIC
• Troponin: to assess for cardiomyopathy

Additional diagnostics

• ECG: Nonspecific ST segment abnormalities and ischemic ECG changes may indicate stress-induced cardiomyopathy. ^[11]
• CXR: Consider in patients with signs or symptoms of respiratory distress.
• Other diagnostics for AMS: e.g., CT head

Management ^[1][2][3]

Initial management

• Begin advanced cardiac life support if indicated, and perform an ABCDE survey.
• Initiate rapid active cooling. ^[1][2]
  • Goal: core body temperature < 39°C (102.2°F) within 30 minutes
  • Conductive cooling with immersion in ice water is the preferred method in exertional heatstroke.
  • Evaporative and convective cooling may be considered in nonexertional heatstroke, but the cooling rate is slower. ^[2]
  • Start other active cooling methods immediately if immersion is not readily available.
• Begin IV fluid resuscitation while avoiding overhydration.

Active cooling is the highest priority for patients with heatstroke and should be considered even before initiating transport. ^[12]

Supportive care and monitoring

• Initiate continuous temperature monitoring.
• Replete electrolytes guided by serial laboratory studies.
• Consider benzodiazepines for shivering and seizures ^[3][4][5]
• Monitor for and treat delayed complications (e.g., management of rhabdomyolysis).
• Continue cooling until:
  • Normal body temperature
  • Clinically stable with normal CNS function

Antipyretics interrupt the change in the hypothalamic set point caused by pyrogens and are ineffective in heatstroke. They may even be harmful because of the risk of bleeding. ^[2][7]

Disposition ^[4][5][8]

• All patients require hospital admission.
• Most patients require ICU admission to monitor for and manage organ failure.

Complications ^[7]

• Multisystem organ failure
• Acute respiratory distress syndrome
• Rhabdomyolysis and acute kidney injury
• Acute hepatic dysfunction
• Disseminated intravascular coagulation

All patients with moderate to severe heat-related illness

• Measure core temperature and vital signs.
• Assess mental status.
• Begin passive cooling.
• Consider evaporative and convective cooling (e.g., mist and fan) if passive cooling is ineffective.
• Begin oral or IV rehydration.
• Obtain diagnostics for dehydration and CPK.
• Monitor temperature and reevaluate frequently.
• Escalate management of hyperthermia if there is clinical deterioration.
• Admit patients with older age and significant laboratory abnormalities and/or comorbidities.

Suspected heatstroke (AMS and/or temperature > 40°C)

• Begin aggressive active cooling to bring temperature < 39°C.
  • If feasible, start immersion cooling (e.g., ice bath).
  • If immersion cannot be performed, start evaporative and convective cooling or skin lavage.
  • Consider adjunctive active cooling methods, e.g., cold IV fluids, ice packs, cooling blanket.
• Begin continuous core temperature monitoring.
• Obtain CBC, CMP, CPK, ABG, troponin, and coagulation panel.
• Start IV fluid resuscitation.
• Treat shivering with benzodiazepines.
• Admit all patients; consider ICU admission in severe cases.
• Monitor for complications, e.g., rhabdomyolysis, ARDS, AKI, or multisystem organ failure.

• Ensure adequate hydration and take breaks from the heat.
• Minimize the use of substances and medications that impair thermoregulation.
• Identify and prepare vulnerable individuals for heat events.
• Athletes: Consider heat acclimatization.