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Emerg Med Clin North Am. 2004;22:28 1.

Heat-Related Illness

Natalie Radford, MD

Key Points

• Always consider secondary causes of hyperthermia.

Heat exhaustion and heat stroke should be diagnoses

of exclusion.

• Do not fluid overload elderly patients while rehydrating

them in the emergency department (ED). Remember

that their fluid and electrolyte deficits developed over

INTRODUCTION

Heat exhaustion and heat stroke are on a continuum of disease severity. Heat exhaustion occurs when the body can no

longer dissipate heat adequately, resulting in hyperthermia.

Heat stroke is the result of complete thermoregulatory dysfunction. Classic heat injury occurs in the elderly or ill with

prolonged exposure to high environmental temperatures.

Physical exertion is not required. Elevated temperatures and

high humidity overwhelm the body's normal c ooling mechanisms. Ex:ertional heat injury occurs in physically fit individ -

uals who exert themselves during conditions with high heat

and humidity. Heat gain from the environment combined

with internal heat production overwhelms the body's normal

cooling mechanisms, creating hyperthermia.

There are about 400 deaths from heat-related illness in

the United States every year. Extremes in weather condi ­

tions can greatly affect these numbers. The Midwest heat

wave in July 1 995 caused 465 deaths in the city of Chicago

alone. The mortality rate in patients with heat stroke can

range between 10% and 70% and is affected by a patient's

physical ability to adapt to changes in the ambient tern ­

perature and medical comorbidities.

The body normally maintains its core temperature

between 36°C (96.8°F) and 38°C (100.4°F). In hyperthermia,

as opposed to fever, there is an elevated body temperature

without a resetting of the hypothalamic t emperature center.

days, and they do not need to be fully repleted while

in the ED.

• Begin cooling the severely hyperthermic patient as

soon as other life-threatening conditions and airway,

breathing, and circulation have been addressed. Delays

in treatment can increase morbidity and mortality.

The body reacts to a heat stress to decrease body temperature

via 3 main mechanisms: increased sweat production,

decreased internal heat production, and removal from the hot

environment. Any factors that impede these responses can

lead to heat exhaustion or heat stroke.

Evaporation of sweat is the main mechanism through

which the body dissipates heat. Evaporative mechanisms

are impaired by both environmental and physical factors.

High humidity, as seen with an elevated heat index,

impedes the body's ability to evaporate sweat and cool.

Elderly, infants, and those with chronic illness have

decreased ability to adapt to hot conditions. Certain medications including antipsychotics, anticholinergics, betablockers, and diuretics also interfere with sweat e vaporation

and cooling. Alcoholics, those with decreased mobility, and

some patients with chronic medical conditions including

obesity, poor cardiac function, and scleroderma have

impaired abilities to evaporate heat as well.

Radiation, conduction, and convection of heat also

allow the body to lose heat, but only when the ambient

temperature is lower than body temperature. Utilizing these

mechanisms can aid in cooling a hyperthermic patient.

CLINICAL PRESENTATION

In patients presenting with heat exhaustion, core body temperature is usually normal, but can be elevated to 40°C

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