Hyperthermia, in its advanced state referred to as heat stroke or sunstroke, is an acute condition which occurs when the body produces or absorbs more heat than it can dissipate. It is usually caused by prolonged exposure to high temperatures. The heat-regulating mechanisms of the body eventually become overwhelmed and unable to effectively deal with the heat, causing the body temperature to climb uncontrollably. Hyperthermia is a medical emergency which requires immediate treatment.
Hyperthermia can be created artificially by drugs or medical devices. In these instances it may be used to treat cancer and other conditions. Malignant hyperthermia is a rare complication of some types of general anesthesia.
The opposite of hyperthermia is hypothermia, caused when an organism's temperature drops below that required for normal metabolism. Hypothermia is caused by prolonged exposure to low temperatures and is also a medical emergency requiring immediate treatment.
A fever occurs when the body sets the core temperature to a higher temperature, through the action of the pre-optic region of the anterior hypothalamus. For example, in response to a bacterial or viral infection, the body will raise its temperature to allow the immune system to work better and to deteriorate the condition of the invaders. In contrast, hyperthermia occurs when the body temperature is raised without the consent of the heat control centers.
Heat stroke may come on suddenly, but usually follows a less-threatening condition commonly referred to as heat exhaustion or heat prostration.
After effects may include sensitivity to heat.
One of the body's most important methods of temperature regulation is perspiration. This process draws heat from inside, allowing it to be carried off by radiation or convection. Evaporation of the sweat furthers cooling, since this endothermic process draws yet more heat from the body. When the body becomes sufficiently dehydrated to prevent the production of sweat this avenue of heat reduction is closed. When the body is no longer capable of sweating core temperature begins to rise swiftly.
Victims may become confused, may become hostile, often experience headache, and may seem intoxicated. Blood pressure may drop significantly from dehydration, leading to possible fainting or dizziness, especially if the victim stands suddenly. Heart rate and respiration rate will increase (tachycardia and tachypnea) as blood pressure drops and the heart attempts to supply enough oxygen to the body. The skin will become red as blood vessels dilate in an attempt to increase heat dissipation. The decrease in blood pressure will cause blood vessels to contract as heat stroke progresses, resulting in a pale or bluish skin colour. Complaints of feeling hot may be followed by chills and trembling, as is the case in fever. Some victims, especially young children, may suffer convulsions. Acute dehydration such as that accompanying heat stroke can produce nausea and vomiting; temporary blindness may also be observed. Eventually, as body organs begin to fail, unconsciousness and coma will result.
The body temperature must be lowered immediately. The victim should be moved to a cool area (indoors, or at least in the shade) and clothing removed to promote heat loss (passive cooling). Active cooling methods may be used: The person is bathed in cool water, a hyperthermia vest can be applied, however, wrapping the victim in wet towels or clothes can actually act as insulation and increase the body temperature. Cold compresses to the torso, head, neck, and groin will help cool the victim. A fan may be used to aid in evaporation of the water (evaporative method).
Immersing a victim into a bathtub of cool - but not cold - water (immersion method) is a recognized method of cooling. This method requires the effort of 4-5 persons and the victim should be monitored carefully during the treatment process. This should be avoided for an unconscious victim; if there is no alternative, the victim's head must be held above water. Be careful not to make the water too cold as Immersion in ice or very cold water is dangerous as this may cause vasoconstriction in the skin, preventing heat from escaping the body core.
Hydration is of paramount importance in cooling the victim. This is achieved by drinking water (Oral rehydration). Commercial isotonic drinks may be used as a substitute. Some authoritiesare opposed to giving any fluids, except by emergency personnel. Intravenous hydration (via a drip) is necessary if the victim is confused, unconscious, or unable to tolerate oral fluids.
Alcohol rubs will cause further dehydration and impairment of consciousness and should be avoided. The victim's condition should be reassessed and stabilized by trained medical personnel. The victim's heart rate and breathing should be monitored, and CPR may be necessary if the victim goes into cardiac arrest.
The risk of heatstroke can be reduced by observing precautions to avoid overheating and dehydration. Light, loose-fitting clothing will allow perspiration to evaporate. Wide-brimmed hats in bright colour keep the sun from warming the head and neck; vents on a hat will allow perspiration to cool the head. Strenuous exercise should be avoided during daylight hours in hot weather; so should remaining in enclosed spaces (such as automobiles). People who must be outside should be aware that humidity and the presence of direct sunlight may cause the heat index to be 10 °C (18 °F) hotter than the temperature indicated by a thermometer.
In hot weather people need to drink plenty of liquids to replace fluids lost from sweating. Thirst is not a reliable sign that a person needs fluids. A better indicator is the color of urine. A dark yellow color indicates dehydration. Water, not sports drinks, is the most effective in replacing lost fluids. However, drinking only water without ingesting any salts (for instance, in the form of dried banana chips) will lead to a condition known as hyponatremia, which can cause sudden death from heart attack. Humans lose electrolytes through sweat and urine which also need to be replaced along with fluids.
While anyone can be affected by hyperthermia, some populations are especially susceptible to heat illness and injury. As noted by Joseph Rampulla in the The Health Care of Homeless Persons :
Heat illness most seriously affects the poor, urban dwellers, young children, those with chronic physical and mental illnesses, substance abusers, the elderly, and people who engage in excessive physical activity under harsh conditions.