Hyperthermia is elevated body temperature due to failed thermoregulation that occurs
when a body produces or absorbs more heat than it dissipates. Extreme temperature elevation
then becomes a medical emergency requiring immediate treatment to prevent disability
or death. The most common causes include heat stroke
and adverse reactions to drugs. The former is an acute temperature elevation caused by
exposure to excessive heat, or combination of heat and humidity, that overwhelms the
heat-regulating mechanisms. The latter is a relatively rare side effect of many drugs,
particularly those that affect the central nervous system. Malignant hyperthermia is
a rare complication of some types of general anesthesia.
Hyperthermia can also be deliberately induced using drugs or medical devices and may be
used in the treatment of some kinds of cancer and other conditions, most commonly in conjunction
with radiotherapy. Hyperthermia differs from fever in that the
body's temperature set point remains unchanged. The opposite is hypothermia, which occurs
when the temperature drops below that required to maintain normal metabolism.
Classification Hyperthermia is defined as a temperature greater
than 37.5–38.3 °C, depending on the reference used, that occurs without a change in the
body's temperature set point. The normal human body temperature in health
can be as high as 37.7 °C 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 can be life-threatening. Signs and symptoms
Hot, dry, skin is typical as blood vessels dilate in an attempt to increase heat loss.
An inability to cool the body through perspiration may cause the skin to feel dry.
Other signs and symptoms vary. Accompanying dehydration can produce nausea, vomiting,
headaches, and low blood pressure and the latter can lead to fainting or dizziness,
especially if the standing position is assumed quickly.
In severe heat stroke, there may be confused, hostile, or seemingly intoxicated behavior.
Heart rate and respiration rate will increase as blood pressure drops and the heart attempts
to maintain adequate circulation. The decrease in blood pressure can then cause blood vessels
to contract reflexly, resulting in a pale or bluish skin color in advanced cases. Young
children, in particular, may have seizures. Eventually, organ failure, unconsciousness
and death will result. Causes
Heat stroke Heat stroke occurs when thermoregulation is
overwhelmed by a combination of excessive metabolic production of heat, excessive environmental
heat, and insufficient or impaired heat loss, resulting in an abnormally high body temperature.
In severe cases, temperatures can exceed 40 °C. Heat stroke may be non-exertional or exertional.
Significant physical exertion in hot conditions can generate heat beyond the ability to cool,
because, in addition to the heat, humidity of the environment may reduce the efficiency
of the body's normal cooling mechanisms. Heat loss mechanisms are limited to vasodilation
of skin vessels and increased rate of sweating. Vasodilation dissipates heat by convection
and sweating by evaporation. However, thermoregulation can be assisted with shade or fans. Other
factors, such as insufficient water intake, consuming alcohol, or lack of air conditioning,
can worsen the problem. The principles of physics involved include:
Newton's law of cooling which states that dry heat loss is proportional to temperature
difference between the human body and surroundings; Stefan-Boltzmann law which states that the
higher the temperature of an object, the more it radiates, and the energy radiating from
an object and received by the human body is proportional to temperature difference between
object and skin. The increase in body temperature that results
from the break down in thermoregulation affects the body biochemically. Enzymes involved in
metabolic pathways within the body such as cellular respiration fail to work as effectively
at higher temperatures and further increases can lead them to denature, reducing their
ability to catalyse essential chemical reactions. This loss of enzymatic control affects the
functioning of major organs with high energy demands such as the heart and brain.
Non-exertional heat stroke mostly affects the young and elderly. In the elderly in particular,
it can be precipitated by medications such as anticholinergic drugs, antihistamines,
and diuretics that reduce vasodilation, sweating, and other heat-loss mechanisms. In this situation,
the body's tolerance for high environmental temperature may be insufficient, even at rest.
Heat waves in the United States are followed by a rise in the death rate and these 'classical
hyperthermia' deaths involve the elderly and infirm. This is partly because thermoregulation
involves cardiovascular, respiratory and renal systems which may be inadequate for the additional
stress because of the existing burden of aging and disease, further compromised by medications.
During the July 1995 heat wave in Chicago, there were at least 700 heat-related deaths.
The strongest risk factors were being confined to bed, and living alone, while the risk was
reduced for those with working air conditioners and those with access to transportation. Even
then, reported deaths may be underestimates as diagnosis can be misclassified as stroke
or heart attack. Drugs
Some drugs cause excessive internal heat production. The rate of drug-induced hyperthermia is higher
where use of these drugs is higher. Many psychotropic medications, such as selective
serotonin reuptake inhibitors, monoamine oxidase inhibitors, and tricyclic antidepressants,
can cause hyperthermia. Serotonin syndrome is a rare adverse reaction to overdose of
these medications or the use of several simultaneously. Similarly, neuroleptic malignant syndrome
is an uncommon reaction to neuroleptic agents. These syndromes are differentiated by other
associated symptoms, such as tremor in serotonin syndrome and "lead-pipe" muscle rigidity in
neuroleptic malignant syndrome. Various stimulant drugs, including amphetamines,
cocaine, PCP, LSD, and MDMA can produce hyperthermia as an adverse effect.
Malignant hyperthermia is a rare reaction to common anesthetic agents or the paralytic
agent succinylcholine. Those who suffer this reaction, which is potentially fatal, have
a genetic predisposition. The use of anticholinergics, more specifically
muscarinic antagonists are thought to cause mild hyperthermic episodes due to its parasympatholytic
effects. The sympathetic nervous system a.k.a the "Fight or Flight Response" dominates by
raising catecholamine levels by the blocked action of the Rest and Digest System.
Personal protective equipment Those working in industry, the military and
first responders, may be required to wear personal protective equipment against hazards
such as chemical agents, gases, fire, small arms and even Improvised Explosive Devices.
PPE includes a range of hazmat suits, firefighting turnout gear, body armor and bomb suits, amongst
others. Depending on design, the wearer may be encapsulated in a microclimate, due to
an increase in thermal resistance and decrease in vapor permeability. As physical work is
performed, the body’s natural thermoregulation becomes ineffective. This is compounded by
increased work rates, high ambient temperature and humidity levels, and direct exposure to
the sun. The net effect is that desired protection from some environmental threats inadvertently
increases the threat of heat stress. Other
Other rare causes of hyperthermia include thyrotoxicosis and an adrenal gland tumor,
called pheochromocytoma, both of which can cause increased heat production. Damage to
the central nervous system, from brain hemorrhage, status epilepticus, and other kinds of injury
to the hypothalamus can also cause hyperthermia. Pathophysiology
A fever occurs when the core temperature is set higher, through the action of the pre-optic
region of the anterior hypothalamus. For example, in response to a bacterial or viral infection,
certain white blood cells within the blood will release pyrogens which have a direct
effect on the anterior hypothalamus, causing body temperature to rise, much like raising
the temperature setting on a thermostat. 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. Experimentally, monkeys treated with oral antibiotics prior
to induction of heat stroke do not become endotoxemic.
There is scientific support for the concept of a temperature set point - that is, maintenance
of an optimal temperature for the metabolic processes that life depends on. Nervous activity
in the preoptic-anterior hypothalamus of the brain triggers heat losing or heat generating
activities through stimulation of the autonomic nervous system. The pre-optic anterior hypothalamus
has been shown to contain warm sensitive, cool sensitive, and temperature insensitive
neurons, to determine the body's temperature setpoint. As the temperature that these neurons
are exposed to rises above 37 °C, the rate of electrical discharge of the warm-sensitive
neurons increases progressively. Cold-sensitive neurons increase their rate of electrical
discharge progressively below 37 °C. Diagnosis
Hyperthermia is generally diagnosed by the combination of unexpectedly high body temperature
and a history that supports hyperthermia instead of a fever. Most commonly this means that
the elevated temperature has occurred in a hot, humid environment or in someone taking
a drug for which hyperthermia is a known side effect. The presence of signs and symptoms
related to hyperthermia syndromes, such as extrapyramidal symptoms characteristic of
neuroleptic malignant 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 lower the body temperature, even if the temperature does not return entirely
to normal, then hyperthermia is excluded. Prevention
When ambient temperature is excessive, humans and many animals cool themselves below ambient
by evaporative cooling of sweat; this helps prevent potentially fatal hyperthermia. The
effectiveness of evaporative cooling depends upon humidity. Wet-bulb temperature, which
takes humidity into account, or more complex calculated quantities such as wet-bulb globe
temperature, which also takes solar radiation into account, give useful indications of the
degree of heat stress and are used by several agencies as the basis for heat-stress prevention
guidelines. A sustained wet-bulb temperature exceeding
35 °C is likely to be fatal even to fit and healthy people unclothed in the shade
next to a fan; at this temperature, environmental heat gain instead of loss occurs. As of 2012
wet-bulb temperatures only very rarely exceeded 30 °C anywhere, although significant global
warming may change this. In cases of heat stress caused by physical
exertion, hot environments, or protective equipment, prevention or mitigation by frequent
rest breaks, careful hydration, and monitoring body temperature should be attempted. However,
in situations demanding one is exposed to a hot environment for a prolonged period or
must wear protective equipment, a personal cooling system is required as a matter of
health and safety. There is a variety of active or passive personal cooling systems; these
can be categorized by their power sources and whether they are person- or vehicle-mounted.
Because of the broad variety of operating conditions, these devices must meet specific
requirements concerning their rate and duration of cooling, their power source, and their
adherence to health and safety regulations. Among other criteria are the user's need for
physical mobility and autonomy. For example, active-liquid systems operate by chilling
water and circulating it through a garment; the skin surface area is thereby cooled through
conduction. This type of system has proven successful in certain military, law enforcement,
and industrial applications. Bomb-disposal technicians wearing special suits to protect
against improvised explosive devices use a small, ice-based chiller unit that is strapped
to one leg; a liquid-circulating garment, usually a vest, is worn over the torso to
maintain a safe core body temperature. By contrast, soldiers traveling in combat vehicles
can face microclimate temperatures in excess of 65 °C 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 vary further.
Treatment The underlying cause must be removed. Mild
hyperthemia caused by exertion on a hot day may be adequately treated through self-care
measures, such as increased water consumption and resting in a cool place. Hyperthermia
that results from drug exposure requires prompt cessation of that drug, and occasionally the
use of other drugs as counter measures. Fever-reducing drugs such as paracetamol and aspirin have
value in treating hyperthermia. When body temperature is significantly elevated,
mechanical cooling methods are used to remove heat 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 unit on the affected person may improve the effectiveness
of the body's evaporative cooling mechanisms. Sitting in a bathtub of tepid or cool water
can remove a significant amount of heat in a relatively short period of time. It is thought
by some that immersion in very cold water is counterproductive, as it causes vasoconstriction
in the skin and thereby prevents heat from escaping the body core. However one British
analysis of various studies stated "supporters of other cooling methods suggest that iced
water immersion may cause peripheral vasoconstriction and therefore slow cooling, although this
has never been proven experimentally. Indeed, a recent study using normal volunteers has
shown that cooling rates were fastest when the coldest water was used". 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 that requires treatment in a proper medical facility. In
a hospital, more aggressive cooling measures are available, including intravenous hydration,
gastric lavage with iced saline, and even hemodialysis to cool the blood.
Epidemiology The frequency of environmental hyperthermia
can vary significantly from year to year depending on factors such as heat waves. Statistically,
outdoor workers, including agricultural workers, are at increased risk of experiencing heat
stress and the resulting negative health effects. Between 1992 and 2006, 68 crop workers died
from heat stroke, representing a rate 20 times that of US civilian workers overall.
See also Heat cramps
Ice sock Rhabdomyolysis
References
External links American Red Cross Tips to Beat the Heat
CDC Emergency Preparedness and Response: Extreme Heat
CDC - NIOSH Workplace Safety and Health Topics: Heat Stress
CDC - NIOSH Preventing Heat-related Illness or Death of Outdoor Workers
OSHA - NIOSH Infosheet: Protecting Workers from Heath Illness
Excessive Heat Events Guidebook, from the United States' Environmental Protection Agency
Cold Water Immersion: The Gold Standard for Exertional Heatstroke Treatment
Physiological Responses to Exercise in the Heat—Chapter 3 of Nutritional Needs in Hot
Environments by the Institute of Medicine of the U.S. National Academies