Heat emergencies are classified into three distinct categories, heat cramps, heat exhaustion, and heat stroke. Each develops from the inability to respond adequately to environmental conditions, inadequate correction of fluid and electrolyte deficiencies, and a malfunction of the system through exogenous and endogenous causes. Risk factors include infectious diseases, anticholingeric drugs, neuroleptics, diuretics, anesthetic agents, and cerebral vascular accidents. Individuals exercising or working in a hot, humid environment are at risk. This is especially true if they have not taken adequate fluids/electrolyte solutions, or have consumed moderate to large amounts of alcohol the night before.

Heat Injuries

Heat cramps are characterized by painful spasm of skeletal muscles including the muscles of the extremities and abdomen, related to sodium and potassium depletion. Clinically, body temperature is normal and there is rarely evidence of dehydration. This condition is benign and is easily treated with rest and oral/IV sodium replacement. If the condition does not resolve in a timely manner with the above treatment, the provider should consider checking a creatine phosphokinase (CPK) to rule out exertional rhabdomyolysis.

Heat exhaustion is characterized by volume depletion. Fluid and electrolyte losses due to sweating coupled with inadequate replacement result in hypovolemia and tissue hypoperfusion. Symptoms include fatigue, vertigo, headache, nausea, vomiting, and finally impaired judgement, hyperventilation, tachycardia, and hypotension. A headache, weakness, or transient confusion may be present but there should be no signs of significant global mental status changes. Core temperature is only moderately elevated (less than 40°C or 104°F). If left untreated, heat exhaustion may lead to heat stroke.

Treatment includes rest in a cool environment with volume and electrolyte replacement. Rapid administration of moderate amounts of normal saline (1 to 2 L) may be necessary if significanttissue hypoperfusion is evident. Elderly patients and patients with moderate to severe symptoms should receive a workup (see heat stroke) and should be considered for hospital admission.

Heat stroke is defined as hyperpyrexia (greater than 40.5°C or 105°F) and neurological impairment, usually severe CNS dysfunction. While in younger individuals the onset of heat stroke is most commonly associated with exertional activity; classic heat stroke is described as non-exertional onset in the elderly. It represents a failure of the body's thermoregulatory mechanisms. As temperatures climb above 105°F, the normal cellular systems for tasks such as aerobic metabolism fail. Lack of sweating is often present; however, patients with early heat stroke will typically demonstrate marked sweating. In contradistinction to heat cramps and heat exhaustion, heat stroke is a medical emergency which may result in multisystem tissue damage and organ dysfunction with a potentially high mortality rate.

The clinical onset of heat stroke begins with alteration of neurological function, often a sudden loss of consciousness with little or no prodrome. Irritability, bizarre behavior, combativeness, hallucinations, or coma may occur. Virtually any neurological abnormality may be present, and signs of trauma should be assessed.

On presentation, the provider should immediately measure core temperature. A conservative approach should be taken as the patient may present with a lower than peak core temperature if cooling measures have already been initiated. The primary goal in treatment is rapid reduction of body temperature. This can be achieved initially by removing the patient from external sources of heat, removing clothing, and applying cool or ice water to the entire skin surface, and fanning with humidified air/mist.

Additional immediate supporting measures should include oxygen therapy or endotracheal intubation, and IV fluid administration. It is important to note that dehydration and volume depletion may be limited in heat stroke (the majority of previously healthy individuals have lost approximately 2 liters), as opposed to heat exhaustion, and overzealous fluid administration may produce pulmonary edema (especially in the elderly). This is compounded by the predisposition of heat stroke patients to circulatory failure. A nasogastric (NG) tube and Foley catheter should be placed and urine output monitored. If there is inadequate urine output after continued IV fluid resuscitation, osmotic diuretics may be considered. Small doses of benzodiazepines may assist in controlling shivering and agitation. Antipyretic agents are not indicated.

Labs should be drawn for CBC, platelets, PT/PTT, electrolytes (including calcium, magnesium, and phosphate), glucose, BUN, creatinine, CPK, liver function tests, urinalysis, and urine for myoglobin. An ECG and chest x-ray should be performed. If an arterial blood gas (ABG) is deemed necessary, the lab should be informed of the patient's body temperature in order to make corrections in determining the results. As in any patient with altered mental status and fever, a toxicologic screen and lumbar puncture may be indicated if the etiology is unclear.

Additional methods of reducing the patient's body temperature include hypothermia blankets, iced water gastric lavage, enemas, bladder irrigation, and peritoneal lavage. These aggressive measures should be ceased when shivering begins or the temperature is less than 102° F. Submersing a patient in an iced bath may produce intense vasoconstriction and decrease heat transfer from the skin. All patients suffering from heat stroke should be hospitalized for potential complications, which can include the following: heart failure, cardiovascular collapse, hepatic failure, disseminated intravascular coagulation (DIC), rhabdomyolysis, cerebral edema, pulmonary edema, and myoglobinemia with renal failure.

Prevention of heat stroke should follow these basic tenets:

• Conditioning and acclimation to environmental heat.

• Adequate fluid intake (600-1200 cc/hr) with strenuous activity.

• Avoidance of strenuous activity during peak hours of hot, humid weather.

• Avoidance of strenuous activity with certain medications (thyroid drugs, sympathomimetics, antihistamines, sedatives, and phenothiazines).

1. Emergency Medicine: A Comprehensive Study Guide, 3rd Ed. Tintinalli et al. McGraw-Hill, 1996.

2. Emergency Medicine: Concepts and Clinical Practice, 3rd Ed. Rosen et al. Mosby, 1992.

3. NAVMED P-5052-5, Technical Information Manual for Medical Corps Officers, chapter 5, (1980) Prevention and Control of Heat Injuries. S/N 0510-LP-060-005.

4. NEHC-TM92-6, June 1992, Prevention and Treatment of Heat and Cold Injuries.