Cibola
Search and Rescue

Hypothermia

by Mike Dugger

This information has been compiled from several sources, and the summary below represents a non-professional's interpretation of what he has learned. In particular, I made use of a document compiled and presented by Reed Burnett, one of our former members who is a physician's assistant, during a winter bivy on Mount Taylor. The Search and Rescue Society of British Columbia has one of the best compilations on the subject I have ever seen at http://www.sarbc.org/hypo.html. Hey, I'm no doctor. As always, the reader should consult a professional for an expert opinion.

Introduction

We require a body core temperature of 96 to 101 degrees F to sustain life. Maintaining body temperature in this narrow range is a balance between heat production and heat loss. Our ability to survive in cold temperatures is due to behavior, not biology. We've used everything from animal skins to wool to fancy, high-tech waterproof and breathable fabrics to keep warm. The body's thermostat is sensitive to less than 1 degree F change in core temperature, and the body responds to this cooling in a variety of ways, depending upon the temperature and rate of cooling. Hypothermia is defined as a body core temperature less than 95 degrees F. Hypothermia can most definitely be fatal if untreated.

Heat Production

We generate heat by burning fuel. For our bodies, this means metabolizing food. In order to generate this heat by metabolism, we need food for fuel, air for oxygen, and water for the chemical processes of digestion. Here are some interesting numbers on the effect of activity the body's rate of energy consumption. Just sitting around, we burn an average of 100 calories per hour. While shivering, we may burn 500 cal/hour, and hiking uphill with a 40 pound pack burns about 1000 cal/hour! All these calories can come from metabolizing food, or burning stored energy in the form of fat.

Even if we have plenty of food and water available, there are limits to how much heat our bodies can produce. Our ability to generate heat by physical processes is limited by our level of fitness. Fit people can supply oxygen to the bloodstream much more efficiently than unfit people, and oxygen is critical for our bodies to metabolize food. Availability of oxygen can also be affected by altitude. Oxygen depletion in the blood and tissues (hypoxia) will also limit heat production. Depletion of glycogen, a starchy substance converted easily to sugar by the body, can reduce the body's ability to generate heat through shivering and aerobic exercise.

Heat Loss Factors

There are several basic methods by which energy may be transmitted from one body to another. All of these apply to the human body to some degree, but some more than others.

Conduction is the flow of energy (or heat) from a warm surface to a cold surface by direct contact. Hold a metal rod at one end and put the other end in cold water, and the end you're holding eventually gets cold. This occurs by conduction of heat down the rod away from your hand. The same thing happens when you hold a glass of iced tea, or sit on a cold rock.

Convection occurs when heat is transferred by a moving fluid, such as air flowing over your skin or clothing. This is why there is such a thing as "wind chill." Water can also be the heat transfer medium, and heat can be carried away much faster by cold water flowing over the body than by air. The faster the fluid is moving, the faster the heat loss will be. The rate of heat loss also depends upon the surface area exposed to the fluid. In this case, the surface is the skin, and the surface area of our skin is about 2 square meters. Convective heat loss can be reduced by wearing wind-proof garments and a hat.

Radiation is the method by which the sun heats the earth. In this case, energy is transmitted as electromagnetic waves, without any direct contact of the surfaces or exposure to any heat transfer fluid. Thermal energy is radiated primarily at infrared wavelengths. You can experience radiative heat transfer by holding your hand facing a bright light bulb. Without contacting the bulb, and even if there is a slight breeze blowing from your hand to the light, you can feel the heat on your hand. This is also why the side of a house sitting in the sun all day feels warm, even if you don't touch it. Again, clothing can help minimize heat losses by radiation.

Energy can also be transferred by a phase change in a material. For example, it takes energy to boil water on the stove. As long as the water is boiling, the temperature of the water does not change. We are simply using energy to change the phase of the water from liquid to gas. On the body, this method of heat transfer manifests itself as evaporation. It takes energy to evaporate water, just like it takes energy to boil water on the stove. The energy to evaporate sweat comes from our bodies. The rate of energy loss by evaporative cooling depends on the wind (carries moist air away), humidity (how much additional moisture the air can hold), and temperature. About 2/3 of energy lost by evaporation is from the skin, and 1/3 from the lung during respiration. Energy loss through the lungs by evaporation during breathing obviously increases as the respiratory rate increases.

Risk Factors

The risk of heat loss is increased by the use of drugs and alcohol, which affect circulation and blood flow. Impaired consciousness also increases risk, since the affected person may not feel cold or take precautions to prevent heat loss. Exhaustion also leads to increased heat loss because of decreased metabolism and cardiac output. Hunger obviously signals the lack of food to burn for energy, and anemia interferes with the metabolism and delivery of oxygen to burn fuel.

Infants and elderly people are most susceptible to hypothermia [1-3]. Infants have a larger body surface area to mass ratio than adults, allowing greater heat loss. Infants also cannot produce as much heat as adults through muscle activity. Metabolism decreases as we age, so elderly people have more difficulty maintaining body temperature through metabolism in cold climates.

Responses to Decreased Body Core Temperature

Arteries, veins and blood vessels in the extremities contract to keep warm blood in the core (trunk) and head, where the organs critical to sustaining life are located. The body does not waste energy heating the arms and legs. Metabolism increases as the body tries to burn fuel faster to generate heat. This also leads to dehydration due to increased water use. Shivering begins. The body attempts to generate heat by doing work. Rapid muscle contraction generates heat through friction.

As body temperature drops, brain function slows down. Higher functions like logic, reasoning, and the ability to solve problems are the first to go, and decline as the core temperature drops below 95 degrees F. Cerebral metabolism decreases by 3.5% for every 1 degree F drop in core temperature. This explains why hypothermic people may appear to be drunk or incoherent, not making sense. The EEG is flat (no brain activity) at 70 degrees F.

The heart rate initially increases as the core temperature drops, in an attempt to deliver more oxygenated blood to the tissues and fuel the increase in metabolism. Below 92 degrees F the heart rate decreases, and abnormal heart rhythms, or arrhythmia, may occur below 90 degrees F. Cardiac output decreases rapidly with decreasing core temperature, and is about 50% of normal at 77 degrees F. The body's ability to assimilate oxygen decreases due to reduced lung capacity by muscle constriction in the chest, resulting in decreased oxygen consumption.

What to Look for in the Stages of Hypothermia

In mild hypothermia, 95 to 92 degrees F, the subject will shiver in an attempt to generate heat. Muscle tone increases. When shivering ceases the body can no longer keep up with the rate of heat loss and core temperature falls rapidly. Moderate hypothermia, between about 92 and 86 degrees F, is signaled by the subject ceasing to shiver. The body is beginning to shut down. A MAJOR MEDICAL EMERGENCY IS IMMINENT!! The subject will exhibit lethargic behavior, characterized by apathy. The subject may also exhibit dysarthria (slurred, slow or effortful speech, perhaps accompanied by changes in pitch, loudness or rhythm due to paralysis or weakness of the muscles used in speech) or ataxia (inability to coordinate voluntary muscle movements). Arrhythmia and unconsciousness may follow.

In severe hypothermia, 86 to 82 degrees F, the subject will appear to be in a stupor, and may have fixed, dilated pupils and no reflexes. Respiratory arrest may follow.

Death occurs at a core temperature of about 70 degrees F, when brain activity stops. Be aware that even though a hypothermia victim may appear dead, full resuscitation and recovery is possible, although unusual [1]. Always act on the premise that "no one is dead until warm and dead" [4]. Now what about cold water drownings, you ask? Subjects exposed to very cold water cool off so fast that the brain's need for oxygen is dramatically reduced before damage occurs. With proper treatment, cold water drowning victims have been resuscitated after an hour in the water.

Differential Diagnosis

There are several medical and behavioral conditions that may have symptoms similar to those seen in the stages of hypothermia. A qualified medical professional should assess the subject to distinguish between these conditions and hypothermia. Some of these conditions are altitude illness, exhaustion, dehydration, fright, loss of will, carbon monoxide poisoning, and the influence of alcohol or drugs.

Treatment

If assessment of the subject indicates hypothermia, ABC's (airway, breathing, circulation) are still the first order of business. For a conscious subject, warm by any means and proceed with evacuation. An unconscious subject with a pulse should be transported to emergency care as rapidly as possible, again while attempting to warm by any means. If the subject has no pulse, CPR should obviously be started immediately while providing external heat.

[Ed. Note: Since this article was written, there may have been important changes in medical protocols regarding beginning CPR in severely hypothermic subjects. It is not in the scope of this web site to provide this updated information --- If you have medical training you should be keeping current through continuing education training. The information provided on this web site is not a substitute for proper medical training, licensure, and continuing education. Cibola Search and Rescue encourages those with an interest in providing medical service to obtain proper training and licensure, and specifically disclaims liability for the misuse of information provided here by individuals who are not qualified to provide medical treatment.]

To fight hypothermia, continued heat loss must be prevented, and steps taken to increase heat production by the subject. Heat loss can be prevented by interrupting the heat flow away from the subject by the pathways discussed above. To stop conduction, insulate the subject from the ground or other cold surfaces. Stop convective heat loss by providing shielding from the wind. Prevent radiative heat loss by making sure that all exposed areas of the body are covered with insulation. Finally, stop heat loss by evaporation by making sure the subject is dressed in dry clothing.

In mild hypothermia, the subject's own heat production rate can be boosted by increased physical activity and oxygen use. Provide the subject with water and food, to increase metabolism. Hot chocolate or other warm, sweetened liquids (no caffeine) can be offered to a conscious subject. At high altitude, administering oxygen can help the subject's body generate heat. If field evacuation is not imminent, external heat should also be provided. This may be as simple as building a fire or getting the subject into a heated (and properly ventilated) tent. Chemical heat packs or hot water bottles may be applied to armpits, groin, and neck to heat the body, but be careful. These can get hot enough to cause burns. Direct body to body contact with the subject is a very effective method of warming. Make sure to replace fluids in order to prevent dehydration.

Rewarming must be done carefully for subjects having moderate-to-severe hypothermia which developed over a prolonged period of time. For example, rewarming by application of external heat may be hazardous because it is likely to cause sudden dilation of blood vessels close to the skin (vascodilation), allowing cold, lactic acid-rich blood to return to the core. This cold blood flowing into the core will reduce the core temperature even further (convective afterdrop), and change the blood pH [5-7]. Cold blood returning to the heart may be enough to put the patient into ventricular fibrillation. To prevent vasodilation, it is important that the patient's core be rewarmed before the extremities. For the same reason, even a conscious patient having moderate hypothermia must be handled very gently and not be allowed to exercise, as muscular action can pump cold blood to the heart.

Equipment for proper core rewarming of a severely hypothermic subject may not be readily available in the field. In this case, the best course of action may be to insulate the subject to prevent further heat loss, and transport them as quickly as possible to facilities with proper equipment.

If CPR is necessary, it is important to be aware that it may be more difficult on a hypothermic subject. Decreased core temperature and muscle constriction may make the heart and thorax stiff. Blood flow may be poor due to vascoconstriction, and will exhibit increased resistance to flow through the body. A cold heart muscle may not return to a normal rhythm as easily as one at normal temperature. There will also probably be additional challenges to effective CPR technique, such as the need to transport, and environmental factors such as wind and snow. Also be aware that the pulse may be very slow and difficult to detect in a subject suffering from severe hypothermia.

[Ed. Note: Since this article was written, there may have been important changes in medical protocols regarding beginning CPR in severely hypothermic subjects. It is not in the scope of this web site to provide this updated information --- If you have medical training you should be keeping current through continuing education training. The information provided on this web site is not a substitute for proper medical training, licensure, and continuing education. Cibola Search and Rescue encourages those with an interest in providing medical service to obtain proper training and licensure, and specifically disclaims liability for the misuse of information provided here by individuals who are not qualified to provide medical treatment.]

References

  1. Fox, R.H., et al., Body Temperature In The Elderly: A National Study Of Physiological, Social And Environmental Conditions. Br Med J 1 (1973) pp. 200-206.
  2. Coleman, A., et al., A Pilot Study Of Low Body Temperatures In Old People Admitted To Hospital. J R Coll Physicians Lond 11 (1977) pp. 291-306.
  3. Edlich, R.F., et al., Cold Injuries And Disorders. Current Concepts Trauma Care (1986) pp. 4-11.
  4. From hypothermia training material on SARBC's web site at http://www.sarbc.org/hypo.html.
  5. Stedha, J.A., Efficacy And Safety Of Prehospital Rewarming Techniques To Treat Accidental Hypothermia. Ann Emerg Med 20 (1991) pp. 896-901.
  6. Webb, P., Afterdrop Of Body Temperature: An Alternative Explanation. J. Appl Physiol 60 (1986) pp. 385-390.
  7. Romett, T.T., Mechanism Of Afterdrop After Cold Water Immersion. J. Appl Physiol 65 (1988) pp. 1535-1538.

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