Hyperthermia

Hyperthermia

Overview
Hyperthermia is an elevated body temperature due to failed thermoregulation
Thermoregulation
Thermoregulation is the ability of an organism to keep its body temperature within certain boundaries, even when the surrounding temperature is very different...

. Hyperthermia occurs when the body produces or absorbs more heat
Heat
In physics and thermodynamics, heat is energy transferred from one body, region, or thermodynamic system to another due to thermal contact or thermal radiation when the systems are at different temperatures. It is often described as one of the fundamental processes of energy transfer between...

 than it can dissipate. When the elevated body temperatures are sufficiently high, hyperthermia is a medical emergency
Medical emergency
A medical emergency is an injury or illness that is acute and poses an immediate risk to a person's life or long term health. These emergencies may require assistance from another person, who should ideally be suitably qualified to do so, although some of these emergencies can be dealt with by the...

 and requires immediate treatment to prevent disability or death.

The most common causes are heat stroke and adverse reactions to drugs. Heat stroke is an acute condition of hyperthermia that is caused by prolonged exposure to excessive heat or heat and humidity.
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Encyclopedia
Hyperthermia is an elevated body temperature due to failed thermoregulation
Thermoregulation
Thermoregulation is the ability of an organism to keep its body temperature within certain boundaries, even when the surrounding temperature is very different...

. Hyperthermia occurs when the body produces or absorbs more heat
Heat
In physics and thermodynamics, heat is energy transferred from one body, region, or thermodynamic system to another due to thermal contact or thermal radiation when the systems are at different temperatures. It is often described as one of the fundamental processes of energy transfer between...

 than it can dissipate. When the elevated body temperatures are sufficiently high, hyperthermia is a medical emergency
Medical emergency
A medical emergency is an injury or illness that is acute and poses an immediate risk to a person's life or long term health. These emergencies may require assistance from another person, who should ideally be suitably qualified to do so, although some of these emergencies can be dealt with by the...

 and requires immediate treatment to prevent disability or death.

The most common causes are heat stroke and adverse reactions to drugs. Heat stroke is an acute condition of hyperthermia that is caused by prolonged exposure to excessive heat or heat and humidity. 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 relatively rare side effect of many drugs, particularly those that affect the central nervous system
Central nervous system
The central nervous system is the part of the nervous system that integrates the information that it receives from, and coordinates the activity of, all parts of the bodies of bilaterian animals—that is, all multicellular animals except sponges and radially symmetric animals such as jellyfish...

. Malignant hyperthermia
Malignant hyperthermia
Malignant hyperthermia or malignant hyperpyrexia is a rare life-threatening condition that is usually triggered by exposure to certain drugs used for general anesthesia; specifically, the volatile anesthetic agents and the neuromuscular blocking agent, succinylcholine...

 is a rare complication of some types of general anesthesia.

Hyperthermia can be created artificially by drugs or medical devices. Hyperthermia therapy
Hyperthermia therapy
Hyperthermia therapy is a type of medical treatment in which body tissue is exposed to slightly higher temperatures to damage and kill cancer cells or to make cancer cells more sensitive to the effects of radiation and certain anti-cancer drugs...

 may be used to treat some kinds of cancer
Cancer
Cancer , known medically as a malignant neoplasm, is a large group of different diseases, all involving unregulated cell growth. In cancer, cells divide and grow uncontrollably, forming malignant tumors, and invade nearby parts of the body. The cancer may also spread to more distant parts of the...

 and other conditions, most commonly in conjunction with radiotherapy.

Hyperthermia differs from fever
Fever
Fever is a common medical sign characterized by an elevation of temperature above the normal range of due to an increase in the body temperature regulatory set-point. This increase in set-point triggers increased muscle tone and shivering.As a person's temperature increases, there is, in...

 in the mechanism that causes the elevated body temperatures: a fever is caused by a change in the body's temperature set-point.

The opposite of hyperthermia is hypothermia
Hypothermia
Hypothermia is a condition in which core temperature drops below the required temperature for normal metabolism and body functions which is defined as . Body temperature is usually maintained near a constant level of through biologic homeostasis or thermoregulation...

, which occurs 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.

Classification



Hyperthermia is defined as a temperature greater than 37.5 –, depending on the reference, that occurs without a change in the body's temperature set-point.

The normal human body temperature
Normal human body temperature
Normal human body temperature, also known as normothermia or euthermia, is a concept that depends upon the place in the body at which the measurement is made, and the time of day and level of activity of the person...

 in a healthy adult can be as high as 37.7 °C (99.9 °F) in the late afternoon. Hyperthermia requires an elevation from the temperature that would otherwise be expected. Such elevations range from mild to extreme; body temperatures above 40 °C (104 °F) can be life-threatening.

Signs and symptoms


Hot, dry skin is a typical sign of hyperthermia. The skin may become red and hot as blood vessel
Blood vessel
The blood vessels are the part of the circulatory system that transports blood throughout the body. There are three major types of blood vessels: the arteries, which carry the blood away from the heart; the capillaries, which enable the actual exchange of water and chemicals between the blood and...

s dilate in an attempt to increase heat dissipation, sometimes leading to swollen lips. An inability to cool the body through perspiration causes the skin to feel dry.

Other signs and symptoms vary depending on the cause. The dehydration associated with heat stroke can produce nausea
Nausea
Nausea , is a sensation of unease and discomfort in the upper stomach with an involuntary urge to vomit. It often, but not always, precedes vomiting...

, vomiting, headaches, and low blood pressure. This can lead to fainting or dizziness
Dizziness
Dizziness refers to an impairment in spatial perception and stability. The term is somewhat imprecise. It can be used to mean vertigo, presyncope, disequilibrium, or a non-specific feeling such as giddiness or foolishness....

, especially if the person stands suddenly.

In the case of severe heat stroke, the person may become confused or hostile, and may seem intoxicated. Heart rate and respiration rate will increase (tachycardia
Tachycardia
Tachycardia comes from the Greek words tachys and kardia . Tachycardia typically refers to a heart rate that exceeds the normal range for a resting heart rate...

 and tachypnea
Tachypnea
Tachypnea means rapid breathing. Any rate between 12-20 breaths per minute is normal. Tachypnea is a respiration rate greater than 20 breaths per minute. - Distinction from other breathing terms :...

) as blood pressure drops and the heart attempts to supply enough oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

 to the body. The decrease in blood pressure can then cause blood vessels to contract, resulting in a pale or bluish skin color in advanced cases of heat stroke. Some people, especially young children, may have seizures. Eventually, as body organs begin to fail, unconsciousness
Unconsciousness
Unconsciousness is the condition of being not conscious—in a mental state that involves complete or near-complete lack of responsiveness to people and other environmental stimuli. Being in a comatose state or coma is a type of unconsciousness. Fainting due to a drop in blood pressure and a...

 and death
Death
Death is the permanent termination of the biological functions that sustain a living organism. Phenomena which commonly bring about death include old age, predation, malnutrition, disease, and accidents or trauma resulting in terminal injury....

 will result.

Heat stroke


Heat stroke is due to an environmental exposure to heat, resulting in an abnormally high body temperature. In severe cases, temperatures can exceed 40 °C (104 °F). Heat stroke may be non-exertional (classic) or exertional, depending on whether the person has been exercising in the heat. Significant physical exertion on a very hot day can generate heat beyond a healthy body's ability to cool itself, because the heat and humidity of the environment reduce the efficiency of the body's normal cooling mechanisms. Other factors, such as drinking too little water, drinking alcohol or lack of air conditioning, can exacerbate the condition. Non-exertional heat stroke is predominant in the young and the elderly. It can be precipitated by medications that reduce vasodilation, sweating, and other heat-loss mechanisms, such as anticholinergic
Anticholinergic
An anticholinergic agent is a substance that blocks the neurotransmitter acetylcholine in the central and the peripheral nervous system. An example of an anticholinergic is dicycloverine, and the classic example is atropine....

 drugs, antihistamines, and diuretics. In this situation, the body's tolerance for the excessive environmental temperatures can be too limited to cope with the heat, even while resting.

Drugs


Some drugs cause excessive internal heat production, even in normal temperature environments. The rate of drug-induced hyperthermia is higher where use of these drugs is higher.
  • Many psychotropic medications, such as selective serotonin reuptake inhibitors (SSRIs), monoamine oxidase inhibitors (MAOIs), and tricyclic antidepressants, can cause hyperthermia. Serotonin syndrome
    Serotonin syndrome
    Serotonin syndrome is a potentially life-threatening adverse drug reaction that may occur following therapeutic drug use, inadvertent interactions between drugs, overdose of particular drugs, or the recreational use of certain drugs...

     is a rare adverse reaction to overdose of these medications or the use of several simultaneously. Similarly, neuroleptic malignant syndrome
    Neuroleptic malignant syndrome
    Neuroleptic malignant syndrome is a life- threatening neurological disorder most often caused by an adverse reaction to neuroleptic or antipsychotic drugs...

     is an uncommon reaction to neuroleptic agents. These syndromes are differentiated by the other associated symptoms, such as tremor in serotonin syndrome and "lead-pipe" muscle rigidity in neuroleptic malignant syndrome.
  • Many illicit drugs, including amphetamines, cocaine
    Cocaine
    Cocaine is a crystalline tropane alkaloid that is obtained from the leaves of the coca plant. The name comes from "coca" in addition to the alkaloid suffix -ine, forming cocaine. It is a stimulant of the central nervous system, an appetite suppressant, and a topical anesthetic...

    , PCP
    Phencyclidine
    Phencyclidine , commonly initialized as PCP and known colloquially as angel dust, is a recreational dissociative drug...

    , LSD
    LSD
    Lysergic acid diethylamide, abbreviated LSD or LSD-25, also known as lysergide and colloquially as acid, is a semisynthetic psychedelic drug of the ergoline family, well known for its psychological effects which can include altered thinking processes, closed and open eye visuals, synaesthesia, an...

    , and MDMA can produce hyperthermia as an adverse effect
    Adverse effect
    In medicine, an adverse effect is a harmful and undesired effect resulting from a medication or other intervention such as surgery.An adverse effect may be termed a "side effect", when judged to be secondary to a main or therapeutic effect. If it results from an unsuitable or incorrect dosage or...

    .
  • Malignant hyperthermia
    Malignant hyperthermia
    Malignant hyperthermia or malignant hyperpyrexia is a rare life-threatening condition that is usually triggered by exposure to certain drugs used for general anesthesia; specifically, the volatile anesthetic agents and the neuromuscular blocking agent, succinylcholine...

     is a rare reaction to common anesthetic agents (such as halothane
    Halothane
    Halothane is an inhalational general anesthetic. Its IUPAC name is 2-bromo-2-chloro-1,1,1-trifluoroethane. It is the only inhalational anesthetic agent containing a bromine atom; there are several other halogenated anesthesia agents which lack the bromine atom and do contain the fluorine and...

    ) or a reaction to the paralytic agent succinylcholine. Malignant hyperthermia is a genetic condition, and can be fatal.

Personal protective equipment


People working in industry, the military and first responder
Certified first responder
A certified first responder is a person who has completed a course and received certification in providing pre-hospital care for medical emergencies. They have more skill than someone who is trained in basic first aid but they are not a substitute for advanced medical care rendered by emergency...

s must wear Personal Protective Equipment (PPE) to protect themselves from hazardous threats such as chemical agents, gases, fire, small arms and even Improvised Explosive Devices (IEDs)
Improvised explosive device
An improvised explosive device , also known as a roadside bomb, is a homemade bomb constructed and deployed in ways other than in conventional military action...

. This PPE can include a range of hazmat suits, firefighting turnout gear, body armor and bomb suits, among many other forms. Depending on its design, PPE often ‘encapsulates’ the wearer from a threat and creates what is known as a microclimate, due to an increase in thermal resistance and decrease in vapor permeability. As a person performs physical work, the body’s natural method of thermoregulation (i.e., sweating) becomes ineffective. This is compounded by increased work rates, high ambient temperatures and humidity levels, and direct exposure to the sun. The net effect is that protection from one or more environmental threats inadvertently brings on the threat of heat stress.

Other


Other possible, but rare, causes of hyperthermia are thyrotoxicosis and the presence of a tumor on the adrenal gland
Adrenal gland
In mammals, the adrenal glands are endocrine glands that sit atop the kidneys; in humans, the right suprarenal gland is triangular shaped, while the left suprarenal gland is semilunar shaped...

, called a pheochromocytoma
Pheochromocytoma
A pheochromocytoma or phaeochromocytoma is a neuroendocrine tumor of the medulla of the adrenal glands , or extra-adrenal chromaffin tissue that failed to involute after birth and secretes excessive amounts of catecholamines, usually noradrenaline , and adrenaline to a lesser extent...

, both of which can cause increased heat production. Damage to the central nervous system, such as from a brain hemorrhage, status epilepticus
Status epilepticus
Status epilepticus is a life-threatening condition in which the brain is in a state of persistent seizure. Definitions vary, but traditionally it is defined as one continuous unremitting seizure lasting longer than 5 minutes, or recurrent seizures without regaining consciousness between seizures...

, and other kinds of damage to the hypothalamus can also cause hyperthermia.

Pathophysiology


A fever
Fever
Fever is a common medical sign characterized by an elevation of temperature above the normal range of due to an increase in the body temperature regulatory set-point. This increase in set-point triggers increased muscle tone and shivering.As a person's temperature increases, there is, in...

 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 bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

l or viral
Virus
A virus is a small infectious agent that can replicate only inside the living cells of organisms. Viruses infect all types of organisms, from animals and plants to bacteria and archaea...

 infection, the body will raise its temperature, much like raising the temperature setting on a thermostat
Thermostat
A thermostat is the component of a control system which regulates the temperature of a system so that the system's temperature is maintained near a desired setpoint temperature. The thermostat does this by switching heating or cooling devices on or off, or regulating the flow of a heat transfer...

.

In contrast, hyperthermia occurs when the body temperature rises without a change in the heat control centers.

Some of the gastrointestinal symptoms of acute exertional heat stroke, such as vomiting, diarrhea, and gastrointestinal bleeding, may be caused by barrier dysfunction and subsequent endotoxemia. Ultraendurance athletes have been found to have significantly increased plasma endotoxin levels. Endotoxin stimulates many inflammatory cytokines, which in turn may cause multiorgan dysfunction. Furthermore, monkeys treated with oral antibiotics prior to induction of heat stroke do not become endotoxemic.

Diagnosis


Hyperthermia is generally diagnosed in the presence of an unexpectedly high body temperature and a history that suggests hyperthermia instead of a fever. Most commonly this means that the elevated temperature has appeared in a person who was working in a hot, humid environment (heat stroke) or who was taking a drug for which hyperthermia is a known side effect (drug-induced hyperthermia). The presence of other signs and symptoms related to hyperthermia syndromes, such as the extrapyramidal symptoms that are characteristic of neuroleptic malginant syndrome, and the absence of signs and symptoms more commonly related to infection-related fevers, are also considered in making the diagnosis.

If fever-reducing drugs
Antipyretic
Antipyretics ; an-tee-pahy-ret-iks; from the Greek anti, against, and pyreticus, are drugs or herbs that reduce fever. Normally, they will not lower body temperature if one does not have a fever. Antipyretics cause the hypothalamus to override an interleukin-induced increase in temperature...

 lower the body temperature, even if the temperature does not return entirely to normal, then hyperthermia is excluded.

Prevention


Exposure limits to heat stress are usually set by indices based on the wet bulb globe temperature
Wet Bulb Globe Temperature
The Wet Bulb Globe Temperature is a composite temperature used to estimate the effect of temperature, humidity, wind speed and solar radiation on humans. It is used by industrial hygienists, athletes, and the military to determine appropriate exposure levels to high temperatures...

.

In cases where heat stress is caused by physical exertion, hot environments or wearing protective equipment it can be prevented or mitigated by taking frequent rest breaks, staying hydrated and carefully monitoring body temperature. However, in situations demanding prolonged exposure to a hot environment or wearing protective equipment, a personal cooling system is required as a matter of health and safety. A variety of active or passive technologies personal cooling systems exist which can be categorized by their power sources and whether they are man or vehicle-mounted.

Due to the broad variety of operating conditions, a personal cooling system must meet specific requirements, such as the rate and duration of cooling, need for physical mobility and autonomy, access to power, and conformance with health & safety regulations. For example, active liquid systems operate on the basis of chilling water and circulating it through a garment that cools the skin surface area that it covers through conduction. This type of system has proven successful in certain Military, Law Enforcement and Industrial applications. Bomb disposal technicians wearing bomb suits to protect against an Improvised Explosive Device (IED) use a small, ice-based chiller unit strapped to their leg with a Liquid Circulating Garment, usually a vest, worn over their torso to maintain their core temperature at safe levels. By contrast, soldiers traveling in combat vehicles can face microclimate temperatures in excesss of 150 degrees Fahrenheit and require a multiple-user vehicle-powered cooling system with rapid connection capabilities. Requirements for Hazmat teams, the medical community and workers in heavy-industry will vary further.

Treatment


Treatment for hyperthermia depends on its cause, as the underlying cause must be corrected. Mild hyperthemia caused by exertion on a hot day might be adequately treated through self-care measures, such as drinking water and resting in a cool place. Hyperthermia that results from drug exposures is frequently treated by cessation of that drug, and occasionally by other drugs to counteract them. Fever-reducing drugs
Antipyretic
Antipyretics ; an-tee-pahy-ret-iks; from the Greek anti, against, and pyreticus, are drugs or herbs that reduce fever. Normally, they will not lower body temperature if one does not have a fever. Antipyretics cause the hypothalamus to override an interleukin-induced increase in temperature...

 such as paracetamol
Paracetamol
Paracetamol INN , or acetaminophen USAN , is a widely used over-the-counter analgesic and antipyretic . It is commonly used for the relief of headaches and other minor aches and pains and is a major ingredient in numerous cold and flu remedies...

 and aspirin
Aspirin
Aspirin , also known as acetylsalicylic acid , is a salicylate drug, often used as an analgesic to relieve minor aches and pains, as an antipyretic to reduce fever, and as an anti-inflammatory medication. It was discovered by Arthur Eichengrun, a chemist with the German company Bayer...

 have no value in treating hyperthermia.

When the body temperature is significantly elevated, mechanical methods of cooling are used to remove heat from the body and to restore the body's ability to regulate its own temperatures. Passive cooling techniques, such as resting in a cool, shady area and removing clothing can be applied immediately. Active cooling methods, such as sponging the head, neck, and trunk with cool water, remove heat from the body and thereby speed the body's return to normal temperatures. Drinking water and turning a fan or dehumidifying air conditioning
Air conditioning
An air conditioner is a home appliance, system, or mechanism designed to dehumidify and extract heat from an area. The cooling is done using a simple refrigeration cycle...

 unit on the affected person may improve the effectiveness of the body's evaporative cooling mechanisms (sweating).

Sitting in a bathtub of tepid or cool water (immersion method) can remove a significant amount of heat in a relatively short period of time. However, immersion in very cold water is counterproductive, as it causes vasoconstriction
Vasoconstriction
Vasoconstriction is the narrowing of the blood vessels resulting from contraction of the muscular wall of the vessels, particularly the large arteries, small arterioles and veins. The process is the opposite of vasodilation, the widening of blood vessels. The process is particularly important in...

 in the skin and thereby prevents heat from escaping the body core. In exertional heat stroke, studies have shown that although there are practical limitations, cool water immersion is the most effective cooling technique and the biggest predictor of outcome is degree and duration of hyperthermia. No superior cooling method has been found for nonexertional heat stroke.

When the body temperature reaches about 40 C, or if the affected person is unconscious or showing signs of confusion, hyperthermia is considered a medical emergency
Medical emergency
A medical emergency is an injury or illness that is acute and poses an immediate risk to a person's life or long term health. These emergencies may require assistance from another person, who should ideally be suitably qualified to do so, although some of these emergencies can be dealt with by the...

 that requires treatment in a proper medical facility. In a hospital, more aggressive cooling measures are available, including intravenous hydration, gastric lavage
Gastric lavage
Gastric lavage, also commonly called stomach pumping or Gastric irrigation, is the process of cleaning out the contents of the stomach. It has been used for over 200 years as a means of eliminating poisons from the stomach. Such devices are normally used on a person who has ingested a poison or...

 with iced saline, and even hemodialysis
Hemodialysis
In medicine, hemodialysis is a method for removing waste products such as creatinine and urea, as well as free water from the blood when the kidneys are in renal failure. Hemodialysis is one of three renal replacement therapies .Hemodialysis can be an outpatient or inpatient therapy...

 to cool the blood.

Epidemiology


The frequency of environmental hyperthermia can vary significantly from year to year depending on factors such as heat wave
Heat wave
A heat wave is a prolonged period of excessively hot weather, which may be accompanied by high humidity. There is no universal definition of a heat wave; the term is relative to the usual weather in the area...

s.

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