Evaporation — Body heat
causes perspiration which
is lost from the body surface
when changed from liquid
Body heat is lost
to nearby objects
Convection — Body heat is lost
to surrounding air, which becomes
warmer, rises, and is replaced
with cooler air.
Radiation — Body heat
is lost to nearby objects
Figure 1: Mechanisms of heat loss
in which the body can lose heat to the environment: conduction, convection, radiation
and evaporation. (See Figure 1.)
In conduction, heat is lost from direct contact with a solid or liquid that’s colder than
the patient’s body, such as when a patient is
in cold water or has fallen and cannot get
up from a cold tile floor. To stop conductive
heat loss, simply insulate the patient from the
cold medium. For example, replace their wet
clothing with blankets or move them from the
ground to the stretcher.
Convection is just an extension of conduction, when air or liquid heated by the patient’s
body flows away and is replaced by colder
matter. A blanket keeps a person warm by
limiting convective heat loss—it traps a layer
of air around the body that then gets warmed
up by conduction. Limit convective heat loss
with clothing and blankets, or by moving the
patient into an ambulance or building.
Using a space blanket (known by many
other names, including foil blanket, survival
blanket and heat sheet), decreases heat loss due
to radiation. Humans emit energy in the form
of infrared light, a form of radiation, which
doesn’t require physical contact to transfer heat.
Under most conditions, radiation accounts for
around 60% of lost heat. 6 Even if special reflective blankets aren’t available, normal cloth can
significantly decrease radiation losses.
The final mechanism of heat loss, evaporation, is most noticeable in the form of sweat.
Even in colder temperatures, evaporation takes
place in the respiratory system. This lost moisture is what appears as foggy breath. Though
it’s more commonly an in-hospital intervention, evaporative heat loss can be limited by the
administration of heated humidified oxygen.
COMPENSATION FOR HEAT LOSS
In order to maintain a constant temperature in
even mildly cold conditions, the body has multiple ways to increase the amount of heat that it
produces. If fuel is available, the rate of metabolism increases and shivering generates heat
via the rapid action of skeletal muscles. 7 Since
infants can’t shiver, they compensate by metabolizing brown fat (which is unique to infants)
and entering a mildly hyperthyroid state. 8
Perhaps the most important physiologic
response to cold is vasoconstriction in the
skin and extremities. Distal and superficial
vasoconstriction serves two major purposes.
First, it keeps more warm blood around the
vital organs in the head and torso. Second, it
insulates a larger portion of the blood supply
from the cooling effect of running through the
extremities and skin, each of which can dissi-
pate heat by acting like a vehicle’s radiator. 9
As a person gets cold, they also exhibit a
standard set of behavioral responses to avoid
hypothermia. These include seeking shelter, putting on warm clothes, consuming hot
food and drink, engaging in physical activity
(e.g., rubbing hands or arms to create friction,
jumping in place), and changing position to
minimize the amount of exposed skin (e.g.,
crouching, crossing arms over chest). 8 Our
unresponsive patient found outside is losing
heat through all four mechanisms, and can’t
act to protect himself from the cold.
Before discussing what hypothermia does to
the body, it’s useful to be able to categorize its
severity. The most common scheme1, 10 recognizes four categories of hypothermia based
on core temperature: mild (90–95 degrees F;
32–35 degrees C), moderate (82–90 degrees
F; 28–32 degrees C), severe (68–82 degrees
F; 20–28 degrees C), and profound (below
68 degrees F; below 20 degrees C). There are
other systems, 11 but memorization isn’t critical; general familiarity is more important.
Clinical management should be guided
by the patient’s overall presentation, especially since the prehospital determination of
core body temperature is often problematic.
Although each category of hypothermia generally correlates with certain symptoms, the
correspondence isn’t absolute.
Hypothermia affects every organ system of
the human body. The central nervous system
(CNS) is initially protected from hypothermia
by the body’s autoregulatory mechanisms, as
described previously, but the CNS becomes
depressed as core temperature falls. Even in
mild hypothermia, patients can experience
slurred speech, confusion, impaired judgement and amnesia. 12 As hypothermia worsens,
patients progress from lethargic to comatose,
their reflexes disappear, and the CNS stops
regulating the cardiovascular system.
The cognitive symptoms are especially
dangerous, since they prevent patients from
taking the actions necessary to save themselves. The most extreme example of bizarre
decision-making in hypothermia is paradoxical undressing. A significant number (more
than 20% in one case series) of moderately to
severely hypothermic patients begin to feel
strangely warm and take off their clothes while
still exposed to the elements. 13
As tissue temperature drops, cellular metabolism slows down. A cold brain is sluggish and
doesn’t function optimally, but it also requires
less oxygen and metabolic fuel to stay alive.
It’s been recognized for decades that hypothermia can slow the progression of anoxic
brain injury. 14
Targeted temperature management (T TM),
also called therapeutic hypothermia, is now a
standard part of in-hospital care for cardiac
arrest patients who are comatose after return of
spontaneous circulation. T TM has been shown