Hypothermia and Frostbite
Temperature Control in Mammals
Mammals are said to be homeothermic because they regulate their body
temperature at a set point, although poikilothermic animals (so-called
cold blooded animals) also attempt to regulate their body temperatures
at a set point as well. The principle difference being that homeothermic
organisms regulate their temperature by the generation of heat through
metabolic processes (cooling is also accomplished through metabolic
processes, but to a lesser extent since most homeotherms live in an
evironment that is cooler than body temperature) while poikilotherms
rely on behavior to regulate their body temperature (e.g. basking in the
sun to warm up and seeking shade to cool down).
Human body temperature is 37°C but this number is really just an average
temperature for a healthy individual who is not sleeping. Normal
temperature varies from 35.8°C to 37.8° with activity and sleep. Body
temperature is normally lowest in the early morning, before awakening;
Napoleon's "4 o'clock in the morning courage" is aptly named, as
mountaineers who have to make an "alpine start" are all too aware.
During sustained and vigorous exercise, especially in hot, humid
weather, body temperatures can easily rise to 40°C, where heat stroke
One of the original Fermi problems actually looked at the power required
to maintain a human at 37°C. Fermi's reasoning went something like the
"From reading many detective novels, I know that a body will cool by
about 10°C in 6 hours. If we consider the body to be a 100 kg bag of water,
then this heat loss corresponds to 1.5°C/min or 1°C/40s. We know that a
calorie is the amount of heat required to raise the temperature of 1 g
of water by 1°C, so we have 100 kcal/40s. This gives 4180 J/40s or 104
J/s. So we can deduce that a human puts out about 100 watts of heating
He was pretty well on the mark, too (as he usually was). An adult human
consumes about 2000 kcal of food/day, corresponding to about 100 watts.
Extreme activity might support about 600 watts for a short time, but
only in a very fit individual. Room temperature is the temperature that
a fully clothed person will not have to expend excess energy to maintain their body
temperature. A nude human requires somewhat warmer temperatures, around
Most heat loss occurs through the skin, with perhaps 5% occuring through
the lungs, except during aerobic exercise, where there is more heat loss
from the lungs. Heat is moved around the body in the blood, and the
subcutaneous blood flow can be regulated over a range of about 100
times, allowing heat loss to be controlled through vasoconstriction and
vasodilation. Heat loss is also facilitated by evaporation of water from
the surface of the skin (sweating).
Heat production comes from the work done by muscles and
nerves, and from chemical reactions in the liver.
Thermoregulation is accomplished in the hypothalamus, although there are
also subcutaneous thermoregulatory components that work in denervated
tissue. The posterior part of the hypothalamus controls heat retention
and production when the organism is placed in a cold environment and the
anterior region controls heat loss when in a hot environment. These
systems operate independently of each other.
Humans have a limited capacity to adapt to extreme temperatures. Those
who live at very cold temperatures are able to function more readily in
the cold than those who are recently exposed. Similarly, people who live
in very hot regions are able to function better in the heat than others.
Cold or hot adaptation seems to impair function at the opposite extreme
but the mechanisms are not well understood.
Heat is lost from the surface of the body by conduction, convection,
evaporation, and radiation. At room temperature, most heat loss is
through evaporation and radiation. Evaporative heat loss can only be
controlled by controlling the humidity of the environment.
Paradoxically, evaporative heat loss can increase in cold weather due to
the low humidity. Air taken into the lungs must be brought to 100%
humidity and there is always some residual moistening of the surface of
the skin ("insensible" perspiration). Anything that leads to more rapid
breathing, such as exercise or the lowering of pO2 at
altitude, will increase evaporative cooling. Radiative
heat loss is controlled by wearing mirrored clothing (space blankets) or
by putting radiative barriers between ourselves and cold black-bodies
(space, at 3K, is the coldest that we usually see). In a cold
environment, however, conductive and convective heat loss will usually
dominate, unless specialized clothing is worn to prevent it.
Convection and conduction refer to the same mechanism of heat loss, the
familiar transfer of kinetic energy from one medium to another that it
is in contact with through molecular motion. Convection is simply
conduction in which the heat lost is only partially transferred to the
cold body. For example, when two bodies of different temperatures are
brought in contact with each other, the hot body loses heat to the cold
body at a rate that is proportional to the temperature difference at
the interface. Since heat diffusion has to occur within the cold
body in order to bring it to equilibrium, the rate of this process may
limit heat transfer from the hot body by the development of a thermal
gradient within the cold body. Convective heat loss refers to a
situation in which the cold body is a fluid with an infinite resevoir at
the cold temperature. The faster the fluid in contact with the hot body
is replaced, the faster the rate of heat transfer, up to the limit
specified by the thermal conductivity of the materials.
For a human in the cold, convective heat loss occurs when a wind is
blowing. At low wind speeds, heat diffusion within the air limits
cooling, but it is still faster than with no wind. The rate of cooling
reaches a maximum at wind speeds between 30 to 50 km/h and then begins
to fall off because the air is not in contact with the skin long enough
to heat up appreciably. Paul Siple, working in Antarctica, studied the
rate of freezing of water at various temperatures and wind speeds. He
published the now infamous Wind Chill Chart in 1945 as a rough indicator
of how wind might speed up tissue freezing. Unfortunately, we see this
chart abused every night on the evening news, where the weather person
intimates that the apparent temperature from the wind chill chart is an
indication of how cold it should feel. This might be true for nude
humans, but very few people are willing to make the comparison between
-40°C with no wind and a -40°C windchill while utterly naked.
Fig. 11.1.1 Wind Chill - Apparent temperature as a function of wind velocity.
Convective heat loss in water is a more serious problem, due to the high
thermal conductivity and the larger specific heat capacity of water (it
takes a lot of heat to warm up water and heat loss is proportional to
the temperature difference). In fact, an active person in cold water will lose
heat faster than someone who moves as little as possible because the
metabolic heat production cannot keep pace with convective heat loss.
Though there is a significant thermal output from chemical reactions in
the body during normal metabolism, it is not controllable for purposeful
heat production in humans. Changes in heat production can only be
brought about by muscular activity, either voluntary (work or activity)
or involuntary (shivering). Voluntary activity will always produce more
heat than simply shivering.
In order to produce heat, the body must have metabolic substrates
available for burning. Sugars and fats are the body's fuel and without a
proper supply, its ability to do work, and thereby generate heat, will
be limited. Although the body can metabolize its stores of fat, as well
as functional components such as muscle, to generate energy, this
process takes a long time and cannot be used to fight hypothermic heat
Metabolic activity will increase by a factor of about 3-4 fold during
conditions of significant heat loss without apparently increasing the
activity of the individual. This has the effect of lowering the maximum
activity that the individual can achieve to generate heat or to move to
a warmer environment.
Physiological Response to Cooling
Low temperatures slow down the rates of chemical reactions due to the
lower kinetic energy of the molecules. Even though the free energy of
the products of a reaction is less than that of the reactants, and hence
should drive the reaction forward, there is an activation energy
associated with a chemical reaction that must be overcome for the
reaction to proceed. When the molecules are less energetic, it takes
longer for the activation energy to be reached.
Fig. 11.1.2 Energy diagram for a chemical reaction.
The rate of the reaction is given by an Arrhenius relation which is
dependent on temperature and the activation energy
Hypothermia is the technical name for the condition in which the core
temperature of a homeothermic organism falls below its setpoint. We have
seen that there is variation in the core temperature with activity but
hypothermia refers to a situation in which the temperature drops below
the normal range.
The injury associated with hypothermia is often death, hence its
classification as a serious condition despite the fact that almost all
the changes associated with a lowering of body temperature are
completely reversible. It seems that the cause of death is usually from
the heart beating slower and slower until it eventually doesn't pump
enough blood to itself to allow the energy to be produced for another
Factors that hasten hypothermia in otherwise healthy people are:
- Immersion - Immersion in cold water rapidly lowers the core
temperature. In shipping accidents, this is a serious concern. Temporary
immersion in cold water during cold weather can also lead to
hypothermia. Dry clothing and shelter should be sought as quickly as
- Injury - An injured person can lose heat faster due to shock and
- Immobilization - Heat production is severely curtailed if activity
cannot be maintained.
- Dehydration - An increase in blood viscosity leads to a decrease in
the transport of oxygen and glucose to the muscles.
Survival and Treatment
Hypothermia is diagnosed by measuring the core body temperature. If it
is below 32°C, then it is classified as profound hypothermia
which is considered a medical emergency requiring immediate intensive
care. Since most people don't have thermometers with them in situations
where hypothermia develops, profound hypothermia is more easily
diagnosed by noticing unexpected stupidity, slurred speech, and a lack
of coordination. Hypothermia caused by immersion in cold water is sometimes
mistaken for death, as the symptoms are identical. Due to the decreased
requirement for oxygen in the hypothermic state, a pronouncement of death
should await rewarming to 37°C.
Treatment of mild hypothermia consists of adding heat to the victim and
reducing heat loss. Wet clothing must be removed and replaced with dry
clothing or further heat loss will occur. Warm liquids can be consumed
to make the victim feel better but will have little effect on the core
temperature. Interestingly, tea can make a substantial difference in the
rewarming process due to its content of theophylline, a structural
analog of caffeine. Theophylline acts on the cyclic
AMP/phosphodiesterase signalling pathway to make more metabolic
substrates available to the cells. The asthma medication aminophylline,
two theophyllines joined by an amide bond, is even more effective. These
drugs are better used as a prophylactic, though, since hypothermia
usually diminishes the body's ability to generate heat.
Alcoholic beverages, though they may make the victim feel
better, will not assist recovery. Another person in a sleeping bag will
be sufficient for treatment, but it is slow and can be either pleasant
or unpleasant depending on circumstances. If it is certain that
the victim is not suffering from profound hypothermia, then the quickest
way to treat the condition is with a hot bath.
Mortality for profound hypothermia varies between 50 to 80%, even with
treatment in a hospital environment but there is no reason for any
previously healthy person to die from hypothermia provided they can be
transported to a hospital; the current mortality
is principally due to innapropriate treatment.
The treatment for profound hypothermia consists of preventing (or
reversing) ventricular fibrillation while slowly rewarming the
victim. Some of the events that are known to trigger ventricular
fibrillation in a profoundly hypothermic patient are:
- Mouth-to-mouth resuscitation
- Alkalosis (sodium bicarb in the blood)
- Precordial thump
- Insertion of a large bore needle into central veins
- Physical exertion
- Rough handling
- Rapid external warming
No warming of a profoundly hypothermic victim should be attempted
outside of a hospital. The victim should be transported as gently as
possible and the core temperature should be maintained at the level upon
discovery (further heat loss should be prevented).
The victim should be prevented from any physical activity
that will lead to exertion (i.e. the victim should not be allowed to
move under his own power).
Some of the complications associated with hypothermia are the following,
although they are more indicative of pre-existing health problems that
are likely to correlate with hypothermia (e.g. homeless people get cold
because they don't have a home).
- Acute Pancreatitis
- Thrombosis (myocardial infarcts and stroke)
- Pulmonary Edema
- Acute Renal Failure
- Depressed bone marrow function
- Decreased blood clotting
- Low serum phosphorous
- Hematuria (blood in urine)
- Simian deformity of the hand
- Temporary adrenal insufficiency
Hypothermia is used extensively to lower metabolic function during organ
transplantation. For example, when performing a heart transplant,
cutting the blood supply during the transfer of the pumping function
from the heart to an artificial pump is inevitable. At normal body
temperature, brain injury would necessarily result. By cooling the body
before the surgery, the time that the body can go without fresh blood is
increased. This allows the surgeon to perform these procedures without
any apparent brain damage. The chest cavity is usually packed with ice
during these procedures.
A Cryobiologist in the Cold: A personal
encounter with hypothermia.
Frostbite, or accidental cryosurgery, refers to the condition that
occurs when ice crystals form inside the organism. This is almost always
confined to the outer extremities, the feet and hands, as well as the
face. Frostbite often occurs in conjunction with hypothermia, as the
constriction of the peripheral blood vessels and thickening of the blood
contribute to the inability to keep the extremities warm. Constrictive
clothing can also play a role if it cuts down the circulation. Smokers,
and those who have an impaired circulatory system, are more prone to
frostbite than healthy people.
An under-appreciated mechanism of frostbite results from the low melting
point of alcohol and its high thermal conductivity. Residents of cold
climates often leave their favourite beverage outdoors to save space in
the refrigerator. Unfortunately, a quick slash can instantly freeze the
lips, tongue, and other parts of the mouth. Fatalities have resulted
from freezing of the esophagus.
Although it is common for toes, fingers, and even feet to go numb in
cold weather, and stay numb for hours, it is possible for freezing of
the tissue to occur under these conditions without any further warning.
Intracellular freezing does not usually occur in frostbite, although
skin cells can freeze intracellularly if they contact a cold metal.
Direct cellular injury, as in cryosurgery, is due to the osmotic effects
and dehydration that accompanies freezing.
The primary mechanism of injury from frostbite is due to the vascular
injury that accompanies tissue freezing. After thawing, the blood
vessels become highly permeable and both solutes and water begin leaking
into the interstitial compartment (edema). The remaining blood becomes
viscous and the damaged endothelial cells release factors that promote
clotting. Once clots begin to form, occluding the capillary beds
completely, then the blood supply to parts of the tissue is stopped.
Unlike cryosurgery, which often must be performed next to critical
organs or tissues, frostbite injury almost always occurs in tissues that
are not necessary for life. Following the injury, swelling and
discoloration of the skin will occur (much of the area turns purple from sludging
of the blood), accompanied by the formation of
large blisters. Necrosis will begin to show within a few days of the
Fortunately, severe frostbite also kills the nerve cells at the site of
injury, so there is no acute pain associated with it (mild frostbite can
be quite painful, though). There is a low-level chronic pain that can
last for years afterward.
Frostbite is easily diagnosed as the frozen tissue has the hardness of
wood, is pale and almost waxy in appearance, and is cold.
Severe hypothermia, which sometimes accompanies frostbite, must be treated
immediately, if present. Otherwise,
the treatment for frostbite is to warm the extremity in a 40°C water
bath (hand hot). Thawing should be as rapid as possible, but control over the
temperature is paramount. An overly warm, dry heat, such as that
produced by a campfire, will cause much additional injury and should not
be used. It was due to the devastation of fireside warming during the
Napoleonic campaigns that led to
the ridiculous technique of treating frostbite by rubbing the exposed
areas with snow. Frostbitten regions should never be massaged at all,
especially not with snow!
Frequently frostbite occurs in situations where the victim must travel a
great distance to receive help. Since there may be excessive swelling
upon thawing, it may not be possible for the victim to put his footwear
back on after thawing. In such cases, the extremity should not be thawed
as re-freezing can significantly increase the severity of the injury
(compared to walking on the frozen tissue). This must be weighed against
the possibility of increasing the amount of frozen tissue. Since the
victim is unlikely to have thermocouples inside his extremities, a
choice must be made. If thawing is decided upon, it should be as fast as
possible without risking the drying/burning injury that can accompany a
fireside warming (contact with the skin of a warm person may be the only
After thawing, the skin over the injured area will be dead and will have
to be debrided or will eventually slough off. The risk of infection is
severe at this point, so care should be taken to prevent any tearing of
the skin or blisters until proper, aseptic care can be administered.
During this lengthy rehabilitation, the line of demarcation between
necrotic and live tissue may move either way significantly, thus if
intravenous antibiotics are available, it is best to wait until this
line stabilizes (weeks, at least) before amputations are performed.
Although the necrotic tissue will fall off on its own (not quite as
aesthetic as the Autumn colours in New England when the leaves turn and
begin to fall off), but failure to amputate
may result in gangrene, necessitating more aggressive amputations or
death. The prognosis, however, is notoriously difficult in the early
Treatment for frostbite is strictly palliative, at this point in time. One
suggestion for active treatment, however, comes from the finding that
there are prostaglandins and other clotting factors found in the
blisters that appear following thawing. These factors are secreted by
injured endothelial cells lining the blood vessels. Keeping a residual
level of aspirin in the blood might counteract these factors and result
in a better recovery. At any rate, there seems to be no indication that
this would impair recovery. As always, the best treatment is avoidance.
A Cryobiologist gets even Colder: A personal
encounter with frostbite.
Trenchfoot, Paddyfoot, and the Chilblains
Trenchfoot primarily refers to a condition suffered extensively by
soldiers in World War I, where they developed frostbite-like injury to
their feet from being constantly immersed in cold water in the trenches.
Freezing was not involved in the injury.
Paddyfoot refers to a condition endemic to more recent military
conflicts that were fought in locations where low-level flooding was
common (such as rice paddies). It is not a cold injury but simply due to
constant immersion for 48-72 hours.
The chilblains is a charmingly named condition that seems to crop up in
older literature. It is a drying and reddening of the skin caused by
exposure to cold, wet, and windy conditions. Lee's Priceless
Recipies, published in 1899 gives a treatment: "Flexible colodion 4
drams, castor oil 4 drams, spirits of turpentine 4 drams. Use 3 times
daily with camel's hair brush." Just above this entry, a cure for cancer
is provided (boiled red oak bark mixed with sheep fat).
Document last updated Feb. 22, 1999.
Copyright © 1999, Ken Muldrew.