International travelers encounter extremes of climate to which they are not accustomed. Exposure to heat and cold can result in serious injury or death. Travelers should investigate the climate extremes they will face during their journey and prepare with proper clothing, equipment, and knowledge.
Many of the most popular travel destinations are tropical or desert areas. Travelers who sit on the beach or by the pool and do only short walking tours incur minimal risk of heat illness. Those participating in strenuous hiking, biking, or work in the heat are at risk, especially travelers coming from cool or temperate climates who are not in good physical condition and not acclimatized to the heat.
Unlike in the cold, where adaptive behaviors play a more important role in body heat conservation, tolerance to heat depends largely on physiologic factors. The major means of heat dissipation are radiation (while at rest) and evaporation of sweat (during exercise), both of which become minimal with air temperatures above 95°F (35°C) and high humidity.
Two major organs are involved in temperature regulation: the cardiovascular system, which must increase blood flow to shunt heat from the core to the surface, while meeting the metabolic demands of exercise; and the skin, where sweating and heat exchange take place. Cardiovascular status and conditioning are the major physiologic variables affecting the response to heat stress at all ages. Many chronic illnesses (in particular, those involving the cardiovascular system or the skin) limit tolerance to heat and predispose to heat illness; these include cardiovascular disease, diabetes, renal disease, and extensive skin disorders or scarring that limits sweating.
Apart from environmental conditions and intensity of exercise, dehydration is the most important predisposing factor in heat illness. Dehydration also reduces exercise performance, decreases time to exhaustion, and increases internal heat load. Temperature and heart rate increase in direct proportion to the level of dehydration. Sweat is a hypotonic fluid containing sodium and chloride. Sweat rates commonly reach 1 L per hour or more, resulting in substantial fluid and sodium loss.
Heat cramps are painful muscle contractions following exercise. They begin an hour or more after stopping exercise and most often involve heavily used muscles in the calves, thighs, and abdomen. Rest and passive stretching of the muscle, supplemented by commercial rehydration solutions or water and salt, rapidly relieve symptoms. Water with a salty snack is usually sufficient. Travelers can make a simple oral salt solution by adding one-fourth to one-half teaspoon of table salt (or two 1-g salt tablets) to 1 L of water. To improve taste, a few teaspoons of sugar or orange or lemon juice may be added to the mixture.
Heat syncope—sudden fainting caused by vasodilation—occurs in unacclimatized people standing in the heat or after 15–20 minutes of exercise. Consciousness rapidly returns when the patient is supine. Rest, relief from heat, and oral rehydration are adequate treatment.
Heat edema, another minor heat disorder, occurs more frequently in women than in men. Characterized by mild swelling of the hands and feet during the first few days of heat exposure, this condition typically resolves spontaneously. Travelers should not treat heat edema with diuretics, which can both delay heat acclimatization and cause dehydration.
Prickly heat (miliaria or heat rash) manifests as small, red, raised itchy bumps on the skin caused by obstruction of the sweat ducts. It resolves spontaneously, aided by relief from heat and avoiding continued sweating. Travelers can best prevent prickly heat by wearing light, loose clothing and avoiding heavy, continuous sweating.
Most people who experience acute collapse or other symptoms associated with exercise in the heat are suffering from heat exhaustion—the inability to continue exertion in the heat. The presumed cause of heat exhaustion is loss of fluid and electrolytes, but there are no objective markers to define the syndrome, which is a spectrum ranging from minor complaints to a vague boundary shared with heat stroke. Transient mental changes, such as irritability, confusion, or irrational behavior, may be present in heat exhaustion, but major neurologic signs such as seizures or coma indicate heat stroke or profound hyponatremia. Body temperature may be normal or mildly to moderately elevated.
Most cases of heat exhaustion can be treated with supine rest in the shade or other cool place and oral water or fluids containing glucose and salt; subsequently, spontaneous cooling occurs, and patients recover within hours. As previously described, travelers can prepare a simple oral salt solution by adding one-fourth to one-half teaspoon of table salt (or two 1-g salt tablets) to 1 L of water. Adding 4–6 teaspoons of sugar, one-quarter cup of orange juice, or 2 teaspoons of lemon juice can improve the taste. Commercial sports-electrolyte drinks are also effective. Plain water plus salty snacks may be more palatable and equally effective. Subacute heat exhaustion may develop over several days and is often misdiagnosed as “summer flu” because of findings of weakness, fatigue, headache, dizziness, anorexia, nausea, vomiting, and diarrhea. Treatment is as described for acute heat exhaustion.
Symptoms of heat exhaustion and early exercise-associated hyponatremia are similar. Hyponatremia can be distinguished from heat illnesses by persistent alteration of mental status without elevated body temperature, delayed onset of major neurologic symptoms (confusion, seizures, or coma), or deterioration up to 24 hours after cessation of exercise and removal from heat. Where medical care and clinical laboratory resources are available, measure serum sodium to diagnose hyponatremia and guide treatment.
Hyponatremia occurs in both endurance athletes and recreational hikers, due in some measure to physiologic mechanisms that result in failure of the kidneys to correct salt and fluid imbalances properly. Excess fluid retention occurs when antidiuretic hormone (secreted inappropriately) influences the kidneys to both retain water and excrete sodium. Sodium losses through sweat also contribute to hyponatremia. In the field setting, altered mental status in a patient with normal body temperature and a history of taking in large volumes of water suggests hyponatremia. The vague and nonspecific symptoms are the same as those described for hyponatremia in other settings, including anorexia, nausea, emesis, headache, muscle weakness, lethargy, confusion, and seizures.
The recommendation to force fluid intake during prolonged exercise and the attitude that “you can’t drink too much” are major contributors to exercise-associated hyponatremia. Prevention includes drinking only enough to relieve thirst. During prolonged exercise (>12 hours) or heat exposure, supplemental sodium should be taken. Most sports-electrolyte drinks do not contain sufficient amounts of sodium to prevent hyponatremia; on the other hand, salt tablets often cause nausea and vomiting. For hikers, food is the most efficient vehicle for salt replacement. Snacks should include not just sweets, but salty foods such as trail mix, crackers, and pretzels.
Restrict fluid if hyponatremia is suspected (neurologic symptoms in the absence of hyperthermia or other diagnoses). In conscious patients who can tolerate oral intake, give salty snacks with sips of water or a solution of concentrated broth (2–4 bouillon cubes in 1/2 cup of water). Obtunded patients may require hypertonic saline.
Heat stroke is an extreme medical emergency requiring aggressive cooling measures and hospitalization for support. Heat stroke is the only form of heat illness in which the mechanisms for thermal homeostasis have failed, and the body does not spontaneously restore the temperature to normal. Uncontrolled fever and circulatory collapse cause organ damage to the brain, liver, kidneys, and heart. Damage is related to duration as well as peak elevation of body temperature.
The onset of heat stroke may be acute or gradual. Acute (also known as exertional) heat stroke is characterized by collapse while exercising in the heat, usually with profuse sweating. It can affect healthy, physically fit people. By contrast, gradual or nonexertional (referred to sometimes as classic or epidemic) heat stroke occurs in chronically ill people experiencing passive exposure to heat. Sufferers of nonexertional heat stroke tend not to perspire. Victims of both exertional and nonexertional heat stroke demonstrate altered mental status and markedly elevated body temperature.
Early symptoms are similar to those of heat exhaustion, with confusion or change in personality, loss of coordination, dizziness, headache, and nausea that progress to more severe symptoms. A presumptive diagnosis of heat stroke is made in the field when people have elevation of body temperature (hyperpyrexia) and marked alteration of mental status, including delirium, convulsions, and coma. Body temperatures in excess of 106°F (41°C) can occur in heat stroke; even without a thermometer, people will feel hot to the touch. If a thermometer is available, a rectal temperature is the safest and most reliable way to check the temperature of someone with suspected heat stroke; an axillary temperature may give a reasonable estimation.
In the field, immediately institute cooling measures by these methods:
Heat stroke is life threatening, and many complications occur in the first 24–48 hours, including liver or kidney damage and abnormal bleeding. Most victims have significant dehydration and many require hospital intensive care management to replace fluid losses. If evacuation to a hospital is delayed, monitor closely for several hours for temperature swings.
Heat acclimatization is a process of physiologic adaptation that occurs in residents of and visitors to hot environments. Increased sweating with less salt content, and decreased energy expenditure with lower rise in body temperature for a given workload, is the result. Only partial adaptation occurs from passive exposure to heat. Full acclimatization, especially cardiovascular, requires 1–2 hours of exercise in the heat each day. With a suitable amount of daily exercise, most acclimatization changes occur within 10 days. Decay of acclimatization occurs within days to weeks if there is no heat exposure.
If possible, all travelers should acclimatize before departing for hot climates by exercising ≥1 hour daily in the heat. Physically fit travelers have improved exercise tolerance and capacity but still benefit from acclimatization. If this is not possible, advise travelers to limit exercise intensity and duration during their first week of travel. It is a good idea to conform to the local practice in most hot regions and avoid strenuous activity during the hottest part of the day.
Clothing should be lightweight, loose, and light colored to allow maximum air circulation for evaporation yet give protection from the sun (see Sun Exposure in this chapter). A wide-brimmed hat markedly reduces radiant heat exposure.
During exertion, fluid intake improves performance and decreases the likelihood of illness. Reliance on thirst alone is not sufficient to prevent mild dehydration, but forcing a person who is not thirsty to drink water creates the potential danger of hyponatremia. During mild to moderate exertion, electrolyte replacement offers no advantage over plain water. For those exercising many hours in the heat, however, salt replacement is recommended. Eating salty snacks or lightly salting mealtime food or fluids is the most efficient way to replace salt losses. Salt tablets swallowed whole may cause gastrointestinal irritation and vomiting; they may be better tolerated if dissolved in 1 L of water. Urine volume and color are a reasonable means to monitor fluid needs.
Travelers do not have to be in an arctic or high-elevation environment to encounter problems with cold. Humidity, rain, and wind can produce hypothermia with temperatures around 50°F (10°C). Even in temperate climates, people can rapidly become hypothermic in the water. Although reports of severe hypothermia in international travelers are rare, those planning trips to wilderness areas should be familiar with the major mechanisms of heat loss (convection, conduction, radiation) and how to mitigate them (by taking shelter from the wind, getting and staying dry, and keeping warm by building a fire).
Being caught without shelter in a wilderness environment represents a significant risk for accidental hypothermia. Many high-elevation travel destinations, however, are not wilderness areas. Local inhabitants and villages offer shelter and protection from extreme cold weather. In Nepal, for example, trekkers almost never experience hypothermia except in rare instances in which they get lost in a storm.
Hypothermia is defined as a core body temperature below 95°F (35°C). When people are faced with an environment in which they cannot keep warm, they first feel chilled. They then shiver, and eventually stop shivering as their metabolic reserves are exhausted. Body temperature continues to decrease, depending on ambient temperatures. As core body temperature falls, neurologic function decreases; almost all hypothermic people with a core temperature of 86°F (30°C) or lower are comatose. The record low core body temperature in an adult who survived is 56°F (13°C).
Travelers headed to cold climates should ask questions and research clothing and equipment. Modern clothing, gloves, and particularly footwear have greatly decreased the chances of suffering cold injury in extreme climates. Cold injuries occur more often after accidents, such as avalanches or unexpected nights outside, than during normal recreational activities.
Those engaging in recreational activities or working around cold water face a different sort of risk. Within 15 minutes, immersion hypothermia can render a person unable to swim or float. In these cases, a personal flotation device is critical, as is knowledge about self-rescue and righting a capsized boat.
Other medical conditions associated with cold affect mainly the skin and the extremities. These can be divided into nonfreezing cold injuries and freezing injuries (frostbite).
Nonfreezing cold injuries include trench foot (immersion foot), pernio (chilblains), and cold urticaria. Trench foot is caused by prolonged immersion of the feet in cold water (32°F–59°F; 0°C–15°C). The damage is mainly to nerves and blood vessels, and the result is pain aggravated by heat and a dependent position of the limb. Severe cases can take months to resolve. Unlike frostbite, avoid rapid rewarming of immersion foot, which can make the damage much worse.
Pernio are localized, inflammatory lesions occurring mainly on the hands after exposure to only moderately cold weather. The bluish-red lesions are thought to be caused by prolonged, cold-induced vasoconstriction. Rapid rewarming makes the pain worse; slow rewarming is preferred. Nifedipine may be an effective treatment.
Cold urticaria are localized or general wheals with itching. It is not the absolute temperature but the rate of change of temperature that induces this form of skin lesion.
Frostbite describes tissue damage caused by direct freezing of the skin. Once severe tissue damage occurs, little can be done. Fortunately, modern equipment and clothing are available to protect adventure tourists from frostbite. The condition now occurs mainly as the result of accidents, severe unexpected weather, or failure to plan appropriately.
Frostbite is usually graded like burns. First-degree frostbite involves reddening of the skin without deeper damage. The prognosis for complete healing is virtually 100%. Second-degree frostbite involves blister formation. Blisters filled with clear fluid have a better prognosis than blood-tinged blisters. Third-degree frostbite represents full-thickness injury to the skin and possibly the underlying tissues. No blisters form, the skin darkens over time and may turn black. If the tissue is completely devascularized, amputation will be necessary.
Severely frostbitten skin is numb and appears whitish or waxy. The generally accepted method for treating a frozen digit or limb is rapid rewarming in water heated to 104°F–108°F (40°C–42°C). Immerse the frozen area completely in the heated water. Use a thermometer to ensure the water is kept at the correct temperature. Rewarming can be associated with severe pain, so analgesics should be given if needed. Once rewarmed, protect frostbitten skin against freezing again. It is better to keep digits frozen a little longer and rapidly rewarm them than to allow them to thaw out slowly or to thaw and refreeze. A cycle of freeze-thaw-refreeze is devastating to tissue, often resulting in amputation.
Once the area has rewarmed, examine for blisters and note whether they extend to the end of the digit. Proximal blisters usually mean that the tissue distal to the blister has suffered full-thickness damage. For treatment, avoid further mechanical trauma to the area and prevent infection. In the field, wash the area thoroughly with a disinfectant such as povidone iodine, put dressings between the toes or fingers to prevent maceration, use fluffs (expanded gauze sponges) for padding, and cover with a roller gauze bandage. These dressings can be left on safely for up to 3 days at a time. By leaving the dressings on longer, travelers can preserve what may be limited supplies of bandages. Prophylactic antibiotics are not needed in most situations.
In the rare situation in which a foreign traveler suffers frostbite and can be evacuated to an advanced medical setting within 24–72 hours, there may be a role for thrombolytic agents, such as prostacyclin and recombinant tissue plasminogen activator. Clinicians managing a case of frostbite within the first 72 hours should carefully consider the risks and benefits of using these drugs; consultation with an expert is strongly recommended. Beyond 72 hours after thawing, these interventions are probably not beneficial.
Once a frostbite patient has reached a definitive medical setting, there should be no rush to do surgery. The usual time from injury to surgery is 4–5 weeks. Technetium (Tc)-99 scintigraphy and magnetic resonance imaging can be used to define the extent of the damage. Once the delineation between dead and viable tissue becomes clear, surgery that preserves the remaining digits can be planned.
Howard D. Backer, David R. Shlim