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Symptoms and Signs of Hypothermia

Doctor's Notes on Hypothermia

Hypothermia is defined as a core or internal body temperature (best measured with a rectal thermometer) of less than 95 F or 35 C. Signs and symptoms of hypothermia vary with the decreasing core temperature. Shivering, along with an increase in heart rate, breathing rate and blood pressure are seen at a core temperature of about 95 F or somewhat below; as temperature decreases toward 89.9, heart rate, breathing rate and blood pressure decrease and the person becomes clumsy, confused, speech slurred and apathetic; below 89.9, shivering stops and oxygen consumption drops while the person may develop an irregular heartbeat and may become stuporous. At core temperatures below 82.4, reflexes are lost, cardiac output falls, chances of a dangerous heart rhythm increase, brain activity slows and the patient appears comatose or dead. However, some patients can be revived. Almost all individuals with hypothermia should be evaluated by an emergency medical caregiver (call 911).

The cause of hypothermia is exposure to a cold or frigid environment long enough to not allow the body to sustain a normal core temperature (98-100 F). Cold water immersion can drop core temperatures to dangerous levels in minutes (falling through ice in a lake or river); it may take hours for hypothermia to develop in cold dry environments (homeless, elderly in an unheated house).

Medical Author:
Medically Reviewed on 3/11/2019

Hypothermia Symptoms

Although the distinctions among mild, moderate, and severe hypothermia are not often clear, a somewhat constant sequence of events occurs as core body temperatures continue to decline.

  • At temperatures below 95 F (35 C), shivering is seen. Heart rate, breathing rate, and blood pressure increase.
  • As the temperature drops further, pulse, breathing rate, and blood pressure all decrease. People may experience some clumsiness, apathy, confusion, and slurred speech.
  • As core temperature drops lower than 89.9 F (32.2 C), shivering stops and oxygen consumption begins to drop. The victim may be in a stupor. The heart rhythm may become irregular.
  • At temperatures below 82.4 F (28 C), reflexes are lost and cardiac output continues to fall. The risk of dangerously irregular heart rhythms increases, and brain activity is seriously slowed. The pupils are dilated, and the victim appears comatose or dead.

Hypothermia Causes

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.

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REFERENCE:

Kasper, D.L., et al., eds. Harrison's Principles of Internal Medicine, 19th Ed. United States: McGraw-Hill Education, 2015.

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