Injuries and illnesses due to heat

Burn injuries

See page 128.

Heat-related illness (hyperthermia)

Human core temperature is maintained at 98.6°F (37°C), with little variation from individual to individual. Heat is generated by all of the metabolic processes that contribute to life, from the blink of an eyelid to the running of a marathon and must be shed constantly to avoid a condition of overheating. The resting person generates enough heat (60 to 80 kilocalories per hour) to raise body temperature by 1.8°F (1°C) per hour. A person exposed to the sun can absorb 150 kilocalories of energy an hour. Vigorous exercise can increase endogenous heat production 10-fold. As outlined in the section on hypothermia (see page 321), heat is lost to the environment through conduction, convection, radiation, and evaporation:

• Conduction: Heat exchange between two surfaces in direct contact. Lying uninsulated on hot (or cold) ground can result in significant heat exchange. The same is true for immersion into hot or cold water.

• Convection: Heat transferred from a surface to a gas or liquid, commonly air or water. When air temperature exceeds skin temperature, heat is gained by the body. Loose-fitting clothing allows air movement and assists conductive heat loss.

• Radiation: Heat transfer between the body and the environment by electromagnetic waves. Clothing protects the body from radiant heat, and the skin radiates heat away from the body. Highly pigmented skin absorbs more heat than does nonpigmented skin.

• Evaporation: Consumption of heat energy as liquid is converted to a gas. Evaporation of sweat is an effective cooling mechanism.

In the normal situation, skin is the largest heat-wasting organ, and radiates approximately 65% of the daily heat loss. The skin is also largely responsible for evaporation (of sweat). Extreme humidity impedes evaporation and greatly diminishes human temperature control. The National Weather Service heat index ( Fig. 226 ) roughly correlates air temperature and relative humidity to derive an “apparent temperature.” At all temperatures, humidity makes the situation worse. For instance, at an air temperature of 85°F, [29°C] if the relative humidity is 80%, the apparent temperature is 97°F. [36°C]

To summarize these recommendations:

When maximally effective, the complete evaporation of 1 quart (liter) of sweat from the skin removes 600 kilocalories of heat (equivalent to the total heat produced with strenuous exercise in 1 hour). The scalp, face, and torso are most important in terms of sweating. Sweat that drips from the skin without evaporating does not contribute to the cooling process, but might contribute to dehydration. World-class distance runners who are acclimated to the heat can sweat in excess of 3 ½ quarts per hour. Since the maximum rate of gastric emptying (a surrogate for fluid absorption) is only 1.2 quarts per hour, it’s easy to see how a person can become dehydrated. Thus, a person should be able to tolerate a 1 quart per hour sweat rate and manage rehydration with oral fluids.

When heat-control mechanisms are overloaded, the body responds unfavorably. As opposed to hypothermia, in which moderate cooling might offer a protective effect, the syndromes of true hyperthermia (in which core body temperature is measurably elevated) can rapidly become life threatening as elevated temperature destroys vital organs and dismembers chemical systems essential to life. Fever in and of itself can set off a vicious cycle, because raising the body temperature by 1.8°F (1°C) can increase metabolism by approximately 13%, which hastens the generation of more heat. Dehydration might by itself raise body temperature. For all of these reasons, it’s crucial to be familiar with heat illness, and to be prepared to respond promptly and decisively.

Heat exhaustion and heat stroke

Heat exhaustion and heat stroke are part of the same continuum, but of differing severity. Heat exhaustion is illness caused by an elevation of body temperature that does not result in permanent damage or altered mental status. Heat stroke is life threatening and can permanently disable the victim.

The signs and symptoms of heat exhaustion are fatigue, a rapid weak pulse, dizziness, nausea, diarrhea, headache, and minor temperature elevation (up to 104°F [40°C]). It’s important to note that sweating might be present or absent, and that the skin of the victim might feel cool to the touch. It is the core temperature that is elevated and that must be measured (rectally).

The signs and symptoms of heat stroke are extreme confusion, weakness, dizziness, unconsciousness, low blood pressure or shock (see page 70), seizures, increased bleeding (bruising, vomiting blood, bloody urine), diarrhea, vomiting, shortness of breath, red skin rash (particularly over the chest, abdomen, and back), tea-colored to deeply darkened (“machine oil”) urine (caused by breakdown of muscle tissue that releases pigment into the bloodstream), and major core body temperature elevation (> than 104°F [40°C]) (up to 115.7°F [46.5°C] has been reported in a heat stroke survivor). Again, it’s important to note that sweating might be present or absent. At the time of collapse, most victims of heat stroke are still sweating copiously. It’s rare for someone to feel cool externally when their temperature exceeds 105°F (40.5°C), but it is not impossible.

The skin will usually be warm or hot to the touch when a victim suffers heat exhaustion or heat stroke, but, again, this is not constant. Carry a rectal thermometer so you can take a temperature reading. If no thermometer is available, and you’re fairly certain that the victim is suffering from heat exhaustion or heat stroke, proceed with therapy.

The most important aspect of therapy is to lower the temperature as quickly as possible. “Cool first and transport second.” The body might lose its ability to control its own temperature at 106°F (41.1°C), so from that point upward, temperature can skyrocket. Manage the airway (see page 18) and administer oxygen (see page 431) at a flow rate of 10 liters per minute by face mask. Don’t give liquids by mouth unless the victim is awake and capable of purposeful swallowing. Cooled liquids don’t assist the cooling process enough to risk choking the uncooperative or confused victim.