United States Search and Rescue Task Force

Hypothermia and Cold Weather Injuries

How We Lose Heat to the Environment

How we lose heat

Example: Generally conductive heat loss accounts for only about 2% of overall loss. However, with wet clothes the loss is increased five times.

Response to Cold

Cold Challenge - (negative factors)

Heat Retention - (positive factors)

Heat Production - (positive factors)

Heat Retention + Heat Production less than Cold Challenge = Hypothermia
Body Fat
Surface to Volume ratio
Shell to Core shunting

Your Body Core Temperature

1. Heat is both required and produced at the cellular level. The environment acts as either a heating or a cooling force on the body. The body must be able to generate heat, retain heat, and discharge heat depending on the body activity and ambient external temperature.

2. Body temperature is a measure of the metabolism - the general level of chemical activity within the body.

3. The hypothalamus is the major center of the brain for regulating body temperature. It is sensitive to blood temperature changes of as little as 0.5 degrees Celsius and also reacts to nerve impulses received from nerve endings in the skin.

4. The optimum temperature for chemical reactions to take place in the body is 98.6 degrees F. Above 105 F many body enzymes become denatured and chemical reactions cannot take place leading to death. Below 98.6 F chemical reactions slow down with various complications which can lead to death.

5. Core = the internal body organs, particularly the heart, lungs, and brain.
Periphery = the appendages, skin, and muscle tissue.

6. Core temperature is the temperature that is essential to the overall metabolic rate of the body.  The temperature of the periphery is not critical.

How Your Body Regulates Core Temperature

1. Vasodilation - increases surface blood flow, increases heat loss (when ambient temperature is less that body temperature). Maximal vasodilation can increase cutaneous blood flow to 3000 ml/minute (average flow is 300-500 ml/minute).

2. Vasoconstriction - decreases blood flow to periphery, decreases heat loss. Maximal vasoconstriction can decrease cutaneous blood flow to 30 ml/minute.

3. Sweating - cools body through evaporative cooling

4. Shivering - generates heat through increase in chemical reactions required for muscle activity. Visible shivering can maximally increase surface heat production by 500%. However, this is limited to a few hours because of depletion of muscle glucose and the onset of fatigue.

5. Increasing/Decreasing Activity will cause corresponding increases in heat production and decreases in heat production.

6. Behavioral Responses - putting on or taking off layers of clothing will result in heat regulation


1. Hypothermia - "a decrease in the core body temperature to a level at which normal muscular and cerebral functions are impaired." 

2. Conditions Leading to Hypothermia

3. What are "hypothermia" temperatures

4. Signs and Symptoms of Hypothermia

a. Watch for the "Umbles" - stumbles, mumbles, fumbles, and grumbles which show changes in motor coordination and levels of consciousness.

b. Mild Hypothermia - core temperature 98.6 - 96 degrees F

c. Moderate Hypothermia - core temperature 95 - 93 degrees F

d. Severe Hypothermia - core temperature 92 - 86 degrees and below (immediately life threatening)

e. Death from Hypothermia

5. How to Assess if someone is Hypothermic

Stage Core Temperature Signs & Symptoms
Mild Hypothermia 99 - 97F Normal, shivering can begin
97 - 95F Cold sensation, goose bumps, unable to perform complex tasks with hands, shiver can be mild to severe, hands numb
Moderate Hypothermia 95 - 93F Shivering, intense, muscle incoordination becomes apparent, movements slow and labored, stumbling pace, mild confusion, may appear alert. Use sobriety test, if unable to walk a 30 foot straight line, the person is hypothermic.
93 - 90F Violent shivering persists, difficulty speaking, sluggish thinking, amnesia starts to appear, gross muscle movements sluggish, unable to use hands, stumbles frequently, difficulty speaking, signs of depression, withdrawn.
Severe Hypothermia 90 - 86F Shivering stops, exposed skin blue of puffy, muscle coordination very poor, inability to walk, confusion, incoherent/irrational behavior, but may be able to maintain posture and appearance of awareness
86 - 82F Muscle rigidity, semiconscious, stupor, loss of awareness of others, pulse and respiration rate decrease, possible heart fibrillation
82 - 78F Unconscious, heart beat and respiration erractic, pulse may not be palpable
78 - 75F Pulmonary edema, cardiac and respiratory failure,death. Death may occur before this temperature is reached.

Treating Hypothermia

The basic principles of rewarming a hypothermic victim are to conserve the heat they have and replace the body fuel they are burning up to generate that heat. If a person is shivering, they have the ability to rewarm themselves at a rate of 2 degrees C per hour.

Mild - Moderate Hypothermia

1. Reduce Heat Loss

2. Add Fuel & Fluids

It is essential to keep a hypothermic person adequately hydrated and fueled.

a. Food types

b. Food intake

c. Things to avoid

3. Add Heat

Severe Hypothermia

1. Reduce Heat Loss

2. Add Fuel & Fluids

3. Add Heat

Heat can be applied to transfer heat to major arteries - at the neck for the carotid, at the armpits for the brachial, at the groin for the femoral, at the palms of the hands for the arterial arch.

Hypothermia Wrap


Is a situation in which the core temperature actually decreases during rewarming. This is caused by peripheral vessels in the arms and legs dilating if they are rewarmed. This dilation sends this very cold, stagnate blood from the periphery to the core further decreasing core temperature which can lead to death. In addition, this blood also is very acetic which may lead to cardiac arrythmias and death. Afterdrop can best be avoided by not rewarming the periphery. Rewarm the core only! Do not expose a severely hypothermic victim to extremes of heat.

CPR & Hypothermia

When a person is in severe hypothermia they may demonstrate all the accepted clinical signs of death:

But they still may be alive in a "metabolic icebox" and can be revived. You job as a rescuer is to rewarm the person and do CPR if indicated. A hypothermia victim is never cold and dead only warm and dead. During severe hypothermia the heart is hyperexcitable and mechanical stimulation (such as CPR, moving them or Afterdrop) may result in fibrillation leading to death. As a result CPR may be contraindicated for some hypothermia situations:

1. Make sure you do a complete assessment of heart rate before beginning CPR. Remember, the heart rate may be 2-3/minute and the breathing rate 1/30 seconds. Instituting cardiac compressions at this point may lead to life-threatening arrythmias. Check the carotid pulse for a longer time period (up to a minute) to ascertain if there is some slow heartbeat. Also, even though the heart is beating very slowly, it is filling completely and distributing blood fairly effectively. External cardiac compressions only are 20-30% effective. Thus, with its severely decreased demands, the body may be able to satisfy its circulatory needs with only 2-3 beats per minute. Be sure the pulse is absent before beginning CPR. You will need to continue to do CPR as you rewarm the person.

2. Ventilation may have stopped but respiration may continue - the oxygen demands for the body have been so diminished with hypothermia that the body may be able to survive for some time using only the oxygen that is already in the body. If ventilation has stopped, artificial ventilation may be started to increase available oxygen. In addition, blowing warm air into the persons lungs may assist in internal rewarming.

3. CPR Procedures

Cold Injuries

Tissue temperature in cold weather is regulated by two factors, the external temperature and the internal heat flow. All cold injuries described below are intimately connected with the degree of peripheral circulation. As peripheral circulation is reduced to prevent heat loss to the core these conditions are more likely to occur.

1. Factors influencing cold injuries

2. Cold-induced Vasodilation - When a hand or foot is cooled to 59 degrees F, maximal vasoconstriction and minimal blood flow occur. If cooling continues to 50 degrees, vasoconstriction is interrupted by periods of vasodilation with an increase in blood and heat flow. This "hunting" response recurs in 5-10 minute cycles to provide some protection from cold. Prolonged, repeated exposure increases this response and offers some degree of acclimatization. Ex. Eskimos have a strong response with short intervals in between.

3. Pathophysiology of Tissue Freezing - As tissue begins to freeze, ice crystals are formed within the cells. As intracellular fluids freeze, extracellular fluid enters the cell and there is an increase in the levels of extracellular salts due to the water transfer. Cells may rupture due to the increased water and/or from tearing by the ice crystals. Do not rub tissue; it causes cell tearing from the ice crystals. As the ice melts there is an influx of salts into the tissue further damaging the cell membranes. Cell destruction results in tissue death and loss of tissue. Tissue can't freeze if the temperature is above 32 degrees F. It has to be below 28 degrees F because of the salt content in body fluids. Distal areas of the body and areas with a high surface to volume ratio are the most susceptible (e.g ears, nose, fingers and toes - this little rhyme should help remind you what to watch out for in yourself and others).

  Cold Response Mild Frostnip Superficial Frostbite Deep Frostbite
Sensation Painful May have sensation Numb Numb
Feels Normal Normal Soft Hard
Color Red White White White

4. Cold Response

5. Frostnip


6. Frostbite


7. Rewarming of Frostbite

8. Special Considerations for Frostbite

9. Trench Foot - Immersion Foot

Trench foot is a process similar to chilblains. It is caused by prolonged exposure of the feet to cool, wet conditions. This can occur at temperatures as high as 60 degrees F if the feet are constantly wet. This can happen with wet feet in winter conditions or wet feet in much warmed conditions (ex. sea kayaking). The mechanism of injury is as follows: wet feet lose heat 25x faster than dry, therefore the body uses vasoconstriction to shut down peripheral circulation in the foot to prevent heat loss. Skin tissue begins to die because of lack of oxygen and nutrients and due to buildup of toxic products. The skin is initially reddened with numbness, tingling pain, and itching then becomes pale and mottled and finally dark purple, gray or blue. The effected tissue generally dies and sluffs off. In severe cases trench foot can involve the toes, heels, or the entire foot. If circulation is impaired for > 6 hours there will be permanent damage to tissue. If circulation is impaired for > 24 hours the victim may lose the entire foot. Trench Foot causes permanent damage to the circulatory system making the person more prone to cold related injuries in that area. A similar phenomenon can occur when hands are kept wet for long periods of time such as kayaking with wet gloves or pogies. The damage to the circulatory system is known as Reynaud's Phenomenon.

Treatment and Prevention of Trench foot

10. Chilblains

11. Avoiding Frostbite and Cold related Injuries

12. Eye Injuries

a. Freezing of Cornea

b. Eyelashes freezing together

c. Snowblindness



Thanks to Rick Curtis for the majority of the above information.

How Much Have You Learned About Hypothermia?

Take The Test!


1. Cold weather can affect older adults because many take medications that can interfere with the body's ability to regulate temperature.          T          F

2. Shivering makes your body lose heat.          T          F

3. Sleepiness and slow, slurred speech are symptoms of hypothermia.          T          F

4. The best way to find out if someone is suffering from hypothermia is to use a thermometer.          T          F

5. If you suspect hypothermia and emergency aid is not available right away, move the person to a warmer location and wrap them in a warm blanket to stop heat loss.     T         F

6. Hypothermia affects younger people more often than older people.          T          F

7. Rubbing a hypothermia victim's arms and legs will help.          T          F

8. Some illnesses place people at risk because they affect the way the body handles cold temperatures.          T          F

9. Wind chill plays no role in hypothermia.         T          F

10. Hot meals may prevent hypothermia.         T         F


Answers To The Test:

1. True

2. False

3. True

4. True

5. True

6. False

7. False

8. True

9. False

10. True