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 due to excessive exposure to heat. The heat-regulating mechanisms of the body eventually become overwhelmed and unable to effectively deal with the heat, and body temperature climbs uncontrollably. This is a medical emergency that requires immediate medical attention.
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.
Hyperthermia is the opposite of hypothermia.
Progression
Body temperatures above 40°C (104 °F) are life-threatening. This compares to normal body temperature of 36-37°C (97-98°F). At 41°C (106 °F), brain death begins, and at 45°C (113°F) death is nearly certain. Internal temperatures above 50°C (122°F) will cause rigidity in the muscles and certain, immediate death.
Heat stroke may come on suddenly, but usually follows a less-threatening condition commonly referred to as heat exhaustion or heat prostration.
 

Signs and symptoms
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.
Under very rare circumstances a person may exhibit symptoms similar to heat stroke without suffering a heat stroke.
 

First aid
Heat stroke is a medical emergency requiring hospitalization, and the local emergency services should be notified as soon as possible.
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, or the person may be wrapped in a cool wet towel. 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). Ice and very cold water can produce hypothermia; they should be used only when there are means to monitor the victim's temperature continuously.
Immersing a victim into a bathtub of 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.
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 authorities are 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 victim should be placed into the recovery position to ensure that the person's airway remains open.
 

Prevention
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 or sports drinks are the most effective in replacing lost fluids.
 

Especially susceptible populations
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.

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