Summertime and Heat-Related Illness
Hot Temperatures, Humidity and Exertion can be a Deadly Combination
May 14, 2009
Humans are warm-blooded animals: Normal metabolism and physiologic activity lead to the production of heat. Additional heat can be acquired from one’s surroundings.
In order to maintain an internal environment that supports cellular health, some of the body’s heat must be transferred to the external environment.
When heat input and production exceed heat output, heat-related illness can develop.
Heat output occurs through three mechanisms: radiation (direct heat transfer to surrounding objects); convection (loss of heat due to the movement of air around the body); evaporation (cooling of the body surface via sweat).
Radiation is the predominant method of heat loss at room temperature, but rising ambient temperatures lessen its efficiency. As environmental temperatures approach body temperature, evaporation becomes the primary means of heat dissipation, providing nearly 100% of cooling at temperatures above 95º F (35º C).
However, if rising environmental temperatures are accompanied by increases in humidity, evaporation, too, becomes less effective. (The Merck Manual, 18th Edition. Heat illness. 2006:2606-09)
Heat disorders occur on a spectrum of severity. Some victims of heat-related illness may not recognize their progression from milder to more severe forms.
Dehydration
- Loss of plasma volume occurs with increased sweating or respiratory activity (water vapor is lost with exhalation).
- Electrolyte depletion due to sweating is mild.
- Body temperature is normal with uncomplicated dehydration, but heart rate and respiratory rate usually increase.
- Victims may feel lightheaded, especially upon standing.
- Thirst is a poor indicator of dehydration during exertion.
Heat Cramps
- Significant amounts of sodium are lost in sweat, causing disruption of normal muscular electrophysiology.
- Abnormal muscular contractions accompanied by severe cramping may occur during or following exertion.
- Body temperature is normal, but signs of dehydration are usually present.
Heat Exhaustion
- As heat exposure, dehydration, and electrolyte loss continue, weakness, nausea, fainting, headache, and dizziness ensue.
- Victims are usually sweating profusely; heart and respiratory rates are increased.
- Mental status is typically normal.
- Temperature is usually normal, but, if elevated, remains below 40º C (104º F).
Heatstroke
- Continued heat exposure—or intense exertion in a hot environment—leads to a loss of thermoregulatory mechanisms; core temperature increases inappropriately (>40º C).
- Mental status deterioration (confusion, delirium, seizures, coma, etc.) is the hallmark of heatstroke.
- Classically, the skin is hot and dry, but victims of exertional heatstroke may sweat profusely.
- Heart rate and respiratory rate are increased.
- Untreated heatstroke progresses to a massive inflammatory response with production of cytokines, activation of the coagulation cascade, multiple organ system failure, and death.
Treatment of Heat-Related Illness
- Dehydration can be addressed with adequate fluid intake. Cool water is usually sufficient during most activities, but heavy sweating may necessitate replacement of sodium. Adding a rounded teaspoon of table salt to five gallons of punch is sufficient. Sports drinks are usually not required.
- Heat cramps respond to passive stretching, although they may last up to several hours. Replacement of fluids and sodium is important; adding two teaspoons of table salt to a quart (1 liter) of water is sufficient for most cases.
- Heat exhaustion requires moving the victim to a cool environment and the provision of intravenous fluids (oral rehydration may not be sufficient). Normal saline is an appropriate IV solution; the volume and rate of administration vary with the patient’s age and medical condition; 1 to 2 liters at 500 cc/hr is usually adequate.
- Heatstroke necessitates hospitalization. In the field, rapid and aggressive external cooling is critical (continual wetting, immersion in cold water, spray from a hose, wet towels and fanning, etc.). IV rehydration is critical and should be started at the earliest opportunity.
(From Wexler R. Evaluation and treatment of heat-related illnesses. Am Fam Phys. 2002;65(11):2307-14)
