Sections View Full Chapter Figures Tables Videos Annotate Full Chapter Figures Tables Videos Supplementary Content + Download Section PDF Listen ++ For further information, see CMDT Part 37-02: Disorders due to Heat + Key Features Download Section PDF Listen +++ +++ Essentials of Diagnosis ++ Spectrum of preventable heat-related illnesses: heat cramps, heat exhaustion, heat syncope, and heat stroke Heat stroke: the most serious disorder, heat stroke, is defined as hyperthermia with cerebral dysfunction in a patient with heat exposure Best outcome: early recognition, initiation of rapid cooling, and avoidance of shivering during cooling; delays in cooling result in higher morbidity and mortality in heat stroke victims Best choice of cooling method: whichever can be instituted the fastest with the least compromise to the patient +++ General Considerations +++ Heat syncope ++ Defined as a transient loss of consciousness with spontaneous return to normal mentation Results from volume depletion and cutaneous vasodilation with subsequent systemic and cerebral hypotension May occur during or immediately following exercise +++ Heat cramps ++ Exercise-associated painful involuntary muscle contractions during or immediately after exercise Muscle cramps result from dilutional hyponatremia as sweat losses are replaced with water alone or sometimes from high utilization of aldosterone leading to potassium wasting +++ Heat exhaustion ++ Characterized by dehydration, sodium depletion, or isotonic fluid loss with accompanying cardiovascular changes Exhaustion results from prolonged strenuous activity in a hot environment with inadequate water or salt intake +++ Heat stroke ++ A life-threatening medical emergency Severe form of heat-related illness resulting in cerebral dysfunction with core body temperature over 40°C May present in one of two forms: classic or exertional Classic heat stroke occurs in patients with impaired thermoregulatory mechanisms or in extreme environmental conditions Exertional heat stroke occurs in healthy persons undergoing strenuous exertion in a hot or humid environment +++ Demographics ++ Persons at greatest risk for heat stroke Very young Elderly (age > 65 years) Chronically ill Patients taking medications that interfere with heat-dissipating mechanisms + Clinical Findings Download Section PDF Listen +++ +++ Symptoms and Signs +++ Heat syncope ++ Systolic blood pressure usually < 100 mm Hg; weak pulse Skin typically cool, moist +++ Heat cramps ++ Slow, painful skeletal muscle contractions Severe muscle spasms last 1–3 min Muscles tender, hard, and lumpy; may be twitching Skin moist, cool Victim alert, with stable vital signs, but may be agitated and complain of focal pain Body temperature may be normal or slightly increased +++ Heat exhaustion ++ Rectal temperature > 37.8°C (> 100°F), tachycardia, and moist skin Symptoms associated with heat syncope and heat cramps may be present Additional symptoms Nausea, vomiting Malaise Myalgias Hyperventilation Thirst Weakness Central nervous system symptoms Headache Dizziness Fatigue Anxiety Paresthesias Impaired judgment Psychosis (occasionally) Hyperventilation secondary to heat exhaustion can lead to respiratory alkalosis Lactic acidosis may also occur due to poor tissue perfusion +++ Heat stroke ++ Presenting symptoms include all those seen in heat exhaustion with additional symptoms of Dizziness Weakness Emotional lability Confusion Delirium Blurred vision Convulsions Collapse Unconsciousness Core body temperature usually > 40°C (> 104°F) Skin hot; initially covered with perspiration, later dries so sweating may not be present Pulse initially strong Blood pressure may be slightly elevated at first, but hypotension develops later Tachycardia Hyperventilation (with subsequent respiratory alkalosis) Exertional heat stroke may present with sudden collapse and loss of consciousness followed by irrational behavior 25% of patients have prodromal symptoms (dizziness, weakness, nausea, confusion, disorientation, drowsiness, irrational behavior) +++ Differential Diagnosis +++ Heat stroke ++ Neuroleptic malignant syndrome Malignant hyperthermia (anesthetic associated) Serotonin syndrome, caused by selective serotonin reuptake inhibitors (SSRIs) used with other drugs Anticholinergics Antihistamines Tricyclic antidepressants Monoamine oxidase inhibitors (MAOIs) Salicylates Amphetamines Cocaine Thyrotoxicosis Prolonged seizures + Diagnosis Download Section PDF Listen +++ ++ Using an internal rectal, Foley, or esophageal thermometer is important in assessing all types of heat-related illnesses because skin temperature may not reflect core temperature +++ Laboratory Tests ++ Muscle cramps Low serum sodium Hemoconcentration Elevated blood urea nitrogen (BUN) and serum creatinine Heat exhaustion Respiratory alkalosis secondary to hyperventilation Lactic acidosis resulting from poor tissue perfusion Heat stroke Dehydration Hemoconcentration Coagulopathy Elevated white blood cell count Elevated BUN Elevated serum uric acid, creatine kinase, troponin, and liver biochemical tests Acid-base abnormalities (eg, lactic acidosis, respiratory alkalosis) Decreased serum glucose, sodium, calcium, and phosphorus Urine may be concentrated, with proteinuria, hematuria, tubular casts, and myoglobinuria Potassium may be high or low +++ Diagnostic Procedures ++ In heat stroke, ECG findings may include ST–T changes consistent with myocardial ischemia PCO2 may be < 20 mm Hg + Treatment Download Section PDF Listen +++ +++ Medications +++ Heat stroke ++ Antipyretics (aspirin, acetaminophen) have no effect on environmentally induced hyperthermia and are contraindicated Oral or intravenous fluid administration to ensure an adequate urinary output +++ Therapeutic Procedures +++ Heat cramps ++ Move the patient to a shaded, cool environment Provide oral isotonic or hypertonic rehydration solution to replace both electrolytes and water Oral salt tablets are not recommended Advise the patient to rest for at least 2 days with continued dietary supplementation before returning to work or resuming strenuous activity in the heat +++ Heat exhaustion ++ Move the patient to a shaded, cool environment, provide adequate fluid and electrolyte replacement, and active cooling (eg, fans, ice packs) if necessary Physiologic saline or isotonic glucose solution should be administered intravenously when oral administration is not appropriate Intravenous 3% (hypertonic) saline may be necessary if sodium depletion is severe At least 24 h of rest and rehydration is suggested +++ Heat syncope ++ Rest and recumbency in a cool place, with electrolyte replacement orally (or intravenously if necessary) +++ Heat stroke ++ See Hyperthermia Aim to reduce the core body temperature rapidly (within 1 h) and control the secondary effects Immersion in ice water or cold water is the preferred method of cooling for exertional heat stroke in the field Ice packs are most effective when covering the whole body, as opposed to the traditional method of just the axilla and groin Continue treatment until core body temperature drops to 39°C Fluid output should be monitored by an indwelling urinary catheter Because shivering must be avoided to prevent an increase of internal heat production and to inhibit effectiveness of cooling, the following medications can be used to suppress shivering: Magnesium Quick-acting opioid analgesics Benzodiazepines Quick-acting anesthetic agents + Outcome Download Section PDF Listen +++ +++ Follow-Up ++ Because sensitivity to high environmental temperature may persist for prolonged periods following an episode of heat stroke, immediate reexposure should be avoided +++ Complications ++ Hypovolemic and cardiogenic shock Acute kidney injury from rhabdomyolysis, hypokalemia, cardiac arrhythmias, coagulopathy, and hepatic failure Multiorgan dysfunction Can be predicted by combination of elevated CK, metabolic acidosis, coagulopathy, and elevated liver enzymes May continue after temperature is normalized Usual cause of heat stroke–related death Hypokalemia may not appear until rehydration +++ Prognosis ++ Heat stroke is associated with high mortality +++ Prevention ++ Athletic competition is not recommended when the wet bulb globe temperature (WBGT) index exceeds 26–28°C (78.8-82.4°F) Workers and athletes need acclimatization before exertion in hot ambient temperatures Fluid consumption should include balanced electrolyte fluids and water Protective cooled suits have been used successfully in industry for prolonged work in environments up to 60°C (140°F) +++ When to Refer ++ Potential consultants include a surgeon for suspected compartment syndrome, nephrologist for acute kidney injury, and transplant surgeon for fulminant liver failure +++ When to Admit ++ All patients with suspected heat stroke must be admitted to the hospital for close monitoring + References Download Section PDF Listen +++ + +Epstein Y et al. Heatstroke. N Engl J Med. 2019 Jun 20;380(25):2449–59. [PubMed: 31216400] + +Gauer R et al. Heat-related illnesses. Am Fam Physician. 2019 Apr 15;99(8):482–9. [PubMed: 30990296] + +King MA et al. Influence of prior illness on exertional heat stroke presentation and outcome. PLoS One. 2019 Aug 20;14(8):e0221329. [PubMed: 31430332] + +Lipman GS et al. Wilderness Medical Society practice guidelines for the prevention and treatment of heat illness: 2019 update. Wilderness Environ Med. 2019 Dec;30(4S):S33–46. [PubMed: 31221601] + +Peiris AN et al. Heat stroke. JAMA patient page. 2017 Dec 26;318(24):2503. [PubMed: 29279936] + +Tustin AW et al. Evaluation of occupational exposure limits for heat stress in outdoor workers—United States, 2011–2016. MMWR Morb Mortal Wkly Rep. 2018 Jul 6;67(26):733–7. [PubMed: 29975679]