Hypothermia is defined as a core, or internal, body temperature of less than 95 F (35 C). Normal body core temperature ranges from about 98 F to 100 F (36.6 C to 37.7 C); core temperature is best measured by a rectal thermometer; do not rely on an oral, ear, axillary (under the armpit), or skin temperature if hypothermia is suspected. Core body temperatures of 95 F (35 C) and lower can cause the heart and nervous system to begin to malfunction and can, in many instances, lead to severe heart, respiratory and other problems that can result in organ damage and death.
Hypothermia has been a military problem ever since Hannibal lost nearly half of his troops while crossing the Pyrenees Alps in 218 B.C., and has continued to plague military campaigns through both world wars and the Korean War. The tragic tales of people falling into icy lakes are poignant examples of hypothermia. Anyone exposed to cold temperatures, whether for work or recreation, may be at risk of becoming hypothermic.
Today, with the popularity of an expanding number of winter sports and increasing at-risk populations, hypothermia has slowly become a civilian, urban problem.
Hypothermia has been used as a technique to help improve neurologic recovery for people in cardiac arrest. This topic can be further examined by readers looking into the references 2 and 3 as these topics are not covered in this article.
Normal body temperature is the reflection of a delicate balance between heat production and heat loss. Many of the chemical reactions necessary for human survival can occur only in specific temperature ranges. The human brain has a number of ways to maintain vital temperature. When these mechanisms are overwhelmed, heat loss happens faster than heat production, which results in hypothermia.
Primary hypothermia is due to exposure to a cold or frigid environment, with no underlying medical condition, causing disruption in temperature regulation:
The body loses heat by several major mechanisms that may occur at the same time.
- 55% go 65% is lost to the environment via radiation.
- Conduction only accounts for 2% to 3% in dry conditions, but this figure can increase to 50% if the victim is immersed in cold water.
- Convection accounts for 10%, while 2% to 9% is lost to heating inspired air.
- Twenty percent to twenty-seven percent is lost as a result of evaporation from the skin and lungs.
- Children cool quicker than adults because their skin provides a larger surface area compared to body mass.
The body also has a variety of methods to increase heat production. But at a certain low level, the body cannot continue heat production, and core body temperature drops quickly. From 98.6 F to 89.6 F (30 C to 32 C), the body begins to shiver, blood vessels contract, and hormones are released to facilitate the generation of heat.
- Shivering can increase heat generation about two to five times the normal body rate of 40 to 60 kcal per square meter of skin. However, this can only last a few hours under mild to moderate freezing conditions and far less in cold water immersion, the time depending on the water temperature and core body temperature. Eventually fatigue sets in, and the body exhausts its fuel stores.
- Blood vessels contract or narrow in the arms and legs, which allows warm blood to remain internal and somewhat protected from the cold temperatures to which the skin is subjected.
- Hormones and other small proteins are released in order to speed up the basal metabolic rate, essentially eating stored fuels in the hopes of producing heat as a byproduct.
When the core body temperature is 89.6 F to 75.2 F (32 C to 24 C), shivering stops and basic metabolism progressively slows down. At a body temperature lower than 75.2 F, almost every mechanism for heat conservation becomes inactive. Core body temperature continues to plummet. In primary hypothermia, the body is unable to generate heat fast enough to compensate for ongoing heat losses. This primarily is a disease of exposure.
- In general, in cold, dry environments, hypothermia occurs over a period of hours.
- In cold water, core temperature can drop to dangerous levels in a matter of minutes.
- The elderly, because of their impaired ability to produce and retain heat, may become hypothermic over a period of days while living in indoor, regulated conditions that other people would find comfortable.
- The homeless, alcoholics, and mentally ill are prone to hypothermia because they are often unable to find adequate shelter or are unable to recognize when it is time to come in from the cold.
Sometimes the body's temperature control can be altered by disease. In this case, core body temperature can decrease in almost any environment. This condition is called secondary hypothermia. In secondary hypothermia, something goes wrong with the body's heat-balancing mechanisms. People with such diseases as stroke, spinal cord injury, low blood sugar, and a variety of skin disorders can become hypothermic in only mildly cool air.
Medically Reviewed by a Doctor on 3/26/2015
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