Heat Illness

Published on 14/03/2015 by admin

Filed under Emergency Medicine

Last modified 22/04/2025

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Heat Illness

Definitions

The term heat illness encompasses a spectrum of syndromes ranging from muscle cramps to heatstroke, which is a life-threatening emergency. Predisposing factors include the following:

1. Environmental temperature exceeding 35° C (95° F) with humidity level greater than 80%

2. Dehydration (one indicator in the field is dark yellow urine)

4. Cardiovascular disease

6. Hyperactivity

c. Cocaine or amphetamine intoxication

7. Muscular exertion

8. Burns (including sunburn)

a. Anticholinergic agents (antihistamines, phenothiazines, antispasmodics)

b. β-Adrenergic blockers, angiotensin-converting enzyme (ACE) inhibitors, diuretics

d. α-Adrenergic agonists

10. Extremes of age

11. Fatigue or lack of sleep

13. Excessive clothing

Disorders

Signs and Symptoms

Peripheral edema develops during the first few days in a hot environment in unacclimatized travelers.

Treatment

The edema is usually self-limited and does not require medical therapy. Diuretics should be avoided because the dehydration that may ensue predisposes a person to more serious heat illness syndromes.

“Prickly Heat” (Miliaria Rubra)

Signs and Symptoms

1. Erythematous, papular, and pruritic rash

2. In dry climates, the rash is confined to skin sufficiently occluded by clothing to produce local sweating

Treatment

1. Cool and dry affected skin.

2. Administer antihistamines (diphenhydramine, adult dose 25 to 50 mg q4-6h) to relieve itching.

3. Desquamation of the affected epidermis and recovery of sweat gland function occurs in 7 to 10 days.

Heat Syncope

Signs and Symptoms

Syncope occurs after prolonged standing in a hot environment or after rapidly standing up from a lying or sitting position.

Treatment

1. Perform a full secondary assessment after the primary survey to assess for any trauma that may have occurred because of a fall.

2. Place the patient in the Trendelenburg position.

3. Cool the patient, and administer oral fluids when he or she is awake and alert. The body can absorb a carbohydrate-containing beverage, such as Gatorade, faster than plain water. The concentration of carbohydrates in such a beverage should not exceed 6%; otherwise, gastric emptying and fluid absorption by the intestines may be delayed. Responders should target an intake for the patient of 1 to 2 L (1.1 to 2.1 qt) over the first hour.

4. Patients with heat syncope usually recover rapidly with treatment. If the patient does not improve or worsens, he or she should be evaluated for heatstroke or other potential cause of syncope and transported to a hospital immediately.

Heat Cramps

Heat cramps result from fluid and electrolyte deficits and occur most often in persons who have not been fully acclimated to a combination of intense muscular activity and environmental heat. Individuals who are susceptible to heat cramps are often believed to be profuse sweaters who sustain large sweat sodium losses.

Signs and Symptoms

1. Painful, spasmodic muscle cramps that usually occur in heavily exercised muscles

2. Recurrent cramps that may be precipitated by manipulation of the muscle

3. Onset during or after exercise

Treatment

1. Administer an oral fluid containing sodium chloride (see later).

2. The affected muscles often respond to passive stretching to “work out” the cramp.

3. Allow the patient to rest in a cool environment.

Heat Exhaustion

Signs and Symptoms

1. Nonspecific symptoms (malaise, headache, weakness, nausea, anorexia)

2. Vomiting may occur

3. Orthostatic hypotension

5. Core body temperature is usually less than 38° to 40° C (100.4° to 104° F) and may be normal

6. Sweating is present

7. Normal mental status and normal findings on neurologic examination

Treatment

1. Stop all exertion, and move the patient to a cool and shaded environment.

2. Remove restrictive clothing.

3. Administer oral fluids (see Heat Cramps, earlier).

4. Cool the patient by placing ice or cold packs on the neck, chest wall, axillae, and groin. Do not place ice directly against skin to avoid frostbite injury. Fanning the patient, while spraying with tepid water, is also an effective cooling method.

5. Generally patients recover rapidly, and hospitalization is not necessary.

Heatstroke

Environmental heatstroke can be regarded as the end stage of heat exhaustion when compensatory mechanisms for dissipating heat have failed. The transition from heat exhaustion to heatstroke is often recognized when a patient begins to show abnormal mental status and neurologic function. Mental status changes in an individual who is performing exertion in the heat should be the defining characteristic of heatstroke. Sweating is still likely to be present in the early stages of heatstroke. Heatstroke is a true medical emergency; if not promptly and effectively treated, morbidity and mortality are high. This necessitates immediate cooling measures.

Signs and Symptoms

1. Elevated core body temperature, usually above 40.5° C (105° F)

2. Altered neurologic state (confusion, disorientation, bizarre behavior, ataxia, seizures, coma). Loss of coordination is one of the earliest manifestations.

6. Sweating may be present or absent.

Treatment

1. Cool the patient rapidly. Prognosis is a function of the magnitude and duration of hyperthermia. The faster cooling is accomplished, the lower the morbidity and mortality.

a. Place ice or cold packs on the neck, axillae, chest wall, and groin. Take care to avoid creating a frostbite injury.

b. Wet the patient with tepid (not cold) water, then fan rapidly to facilitate evaporative cooling.

c. Immerse in cool water if available.

2. Protect the airway, and do not give anything by mouth because of the risk for vomiting and aspiration.

3. Administer fluid intravenously (1 to 2 L of normal saline solution as an initial bolus in adults and 20 to 40 mL/kg in children).

4. Treat seizures and combative behavior with a benzodiazepine (diazepam 0.1 to 0.3 mg/kg IV or IM adult dose; midazolam 0.2 mg/kg IV or IM adult dose).

5. Suppress shivering by administering a benzodiazepine (diazepam 5 to 10 mg IV adult dose) or chlorpromazine (25 to 50 mg IM or IV adult dose).

6. Evacuate the patient immediately to the nearest medical facility. Continue to cool the patient during transport until his or her core body temperature has fallen to 38° to 39° C (100.4° to 102.2° F).

7. Recheck the temperature at least every 30 minutes.

Hyponatremia

Symptomatic hyponatremia is diagnosed when serum sodium level is less than 130 mEq/L and is generally caused by drinking large volumes of water or markedly hypo-osmotic fluids. It may be difficult to differentiate in the field between heat illness and hyponatremia from water intoxication because of considerable overlap of symptoms. One hint is that in heat illness core body temperature is greater than 39° C (102.2° F), whereas in hyponatremia, core temperature is usually normal or close to normal.

Signs and Symptoms

4. Muscle cramps

5. Altered neurologic state (lethargy, apathy, confusion, disorientation, agitation, psychosis, seizures, coma)

Treatment

1. If the patient is mentating normally and capable of safely consuming oral liquids, have him or her drink a full-strength sports beverage, such as Gatorade.

2. If available, administer 2 L of IV normal saline, initially at 500 to 1000 mL/hr.

Prevention

The best indicator of environmental heat stress is the wet bulb globe temperature (WBGT). Whereas a regular thermometer measures the dry-air temperature, a wet bulb thermometer (WBT) measures the effect of humidity as well as temperature. The standard dry bulb thermometer temperature by itself is a poor predictor of heat stress because humidity is such an important factor in heat dissipation accomplished by sweating. Because the WBGT is complex and 70% of the value is derived from the WBT, a simple alternative in the field is to use a sling psychrometer. This instrument has a thermometer with a wick surrounding the bulb attached to an aluminum frame with a hinged handle. After the wick is moistened, the psychrometer is slung over the head for approximately 2 minutes. Air passing over the wetted thermometer bulb cools the bulb in inverse proportion to the humidity. The WBGT can be used as a guide for recommended activity levels (Table 5-1).

Table 5-1

Wet Bulb Globe Temperature and Recommended Activity Levels

°C	°F	RECOMMENDATIONS
15.5	60	No precautions necessary
16.1-21.1	61-70	No precautions if adequate hydration maintained
21.7-23.8	71-75	Unacclimatized: curtail exercise
		Acclimatized: exercise with caution; rest periods and water breaks every 20 to 30 minutes
24.4-26.6	76-80	Unacclimatized: avoid hiking or sports or sun exposure
		Acclimatized: heavy to moderate work with caution
27.2-29.4	81-85	Limited brief activity for acclimatized, fit persons only
31	88	Avoid activity and sun exposure

The U.S. Army has developed fluid replacement and work pacing guidelines that incorporate work intensity, environment, work-to-rest cycles, and fluid intake. These guidelines use WBGT to mark levels of environmental heat stress and emphasize both the need for sufficient fluid replacement during heat stress and concern for the dangers of overhydration. These recommendations specify an upper limit for hourly and daily water intake, which safeguards against overdrinking and water intoxication. The guidelines do not account for individual variability.

The Institute of Medicine provides general guidance for composition of “sports beverages” for persons performing prolonged physical activity in hot weather. They recommend that fluid replacement beverages contain approximately 20 to 30 mEq/L sodium (chloride as the anion), approximately 2 to 5 mEq/L potassium, and approximately 6% carbohydrate. The sodium and potassium are used to help replace sweat electrolyte losses, while sodium also helps to stimulate thirst, and carbohydrate provides energy and facilitates intestinal absorption. These components can also be consumed using nonfluid sources such as gels, energy bars, and other foods. Drinks containing sodium, such as sports beverages, may be helpful, but many foods can supply the needed electrolytes. A little extra salt may be added to meals and recovery fluids when sweat sodium losses are high. Table 5-2 presents the electrolyte contents of common sport drinks, tablets, and powdered additives that can be used to help replace electrolytes lost during activity or exercise.

Table 5-2

Electrolyte Contents of Common Sport Drinks, Tablets, and Powdered Additives

Acclimatization

Physiologic acclimatization to a hot environment is an important adaptive response. It usually requires 8 to 10 days to reach maximum benefit and is facilitated by a minimum amount of daily exercise (1 to 2 hr/day). During initial exposure to a hot environment, workouts should be moderate in intensity and duration. A gradual increase in the time and intensity of physical exertion over 8 to 10 days should allow for optimal acclimatization. As with physical conditioning, there are limits to the degree of protection that acclimatization provides from heat stress. Given a sufficiently hot and humid environment, no one is immune to heat injury. It is important to note that heat acclimatization is specific to the climate and activity level. If individuals will be working in a hot, humid climate, heat acclimatization should be conducted under similar conditions.

Once heat acclimatization is achieved, skin vasodilation and sweating are initiated at a lower core temperature threshold, and higher sweat rates can be sustained without the sweat glands becoming “fatigued.” Whereas an unacclimatized individual will secrete sweat with a sodium concentration of approximately 60 mEq/L (or higher), the concentration of secreted sodium from the sweat glands of an acclimatized individual is significantly lower, at approximately 5 mEq/L. Provided that fluids are not restricted during physical activities, heat-acclimated individuals will be better able to maintain hydration during exercise. Thirst is a poor indicator of adequate hydration because it is not stimulated until plasma osmolarity rises 1% to 2% above normal.

Related

Field Guide to Wilderness Medicine