Evaluation and Treatment of Hypothermia, Colin K. Grissom, M.D.
page 2
Definition of Hypothermia
Hypothermia is defined as the lowering of the body's core temperature to less than
35°C (95°F). It can be classified as mild 35°C to 32°C (95°F to 89.6°F); moderate 32°C
to 28°C (89.6°F to 82.4°F); or severe below 28°C (Giesbrecht 2001)(see Table 2). This
classification system for determining the level of hypothermia relies on measurement of
core body temperature. This may be impractical or unavailable in the field. The Swiss
Mountain Medicine Clinical Classification of Hypothermia (Table 1) provides a useful
system for estimating the severity of hypothermia based on clinical findings rather than
measurement of core body temperature (Durrer 2003).
Table 1. Swiss Mountain Medicine Clinical Classification of Hypothermia
(does not require field core body temperature measurement)(Durrer 2003)
Hypothermia I: patient alert, shivering (35 to 32°C, 95 to 89.6°F)
Hypothermia II: patient drowsy, non-shivering (32 to 28°C, 89.6 to 82.4°F)
Hypothermia III: patient unconscious (28 to 24°C, 82.4 to 75.2°F)
Hypothermia IV: patient not breathing (<24°C, <75.2°F)
Physiologically, hypothermia occurs when heat loss exceeds metabolic heat
production and heat conservation. Factors predisposing to hypothermia include:
1)
Decreased heat production (endocrine failure, insufficient fuel)
2)
Impaired thermoregulation (impaired shivering, neuromuscular disease,
critical illness, multi-system trauma)
3)
Increased heat loss due to environmental exposure (inadequate insulation)
Primary hypothermia occurs due to environmental exposure and is divided into
immersion and non-immersion (accidental hypothermia). Secondary hypothermia occurs
with systemic disorders and is frequently associated with traumatized or critically-ill
patients
Mechanisms of Heat Loss
Convection is a process whereby particles of air or water that have been warmed
by the body's heat are carried away and replaced by cooler particles. Wind exposure and
immersion in moving cold water are examples of convective heat loss.
Radiation is the direct transfer of non-particulate heat energy into the
environment. Convective and radiant heat losses of the body are directly related to skin
blood flow and the amount of skin surface exposed.
Conduction is the transfer of heat by direct contact of the body with a substance
that is a good heat conductor, such as water, snow, metal, or a damp ground surface.
Conductive heat loss can increase 5 times in wet clothing and 25 times in water.
Evaporation is a process in which heat loss occurs due to the conversion of water
from a liquid to gaseous state. In warm weather radiant and convective heat losses are
markedly reduced and evaporation becomes the major component in heat loss.
