convective

His ordeal was not over yet. The hypothermia caused his heart to stop, and only after three hours of rewarming and CPR did he begin to recover. His story was presented by the media as one of "miraculous" survival. He was lucky, and he knows it. Today, this man is a strong advocate of prevention.

Human cells, tissues and organs operate efficiently only within narrow temperature limits. If our temperature rises 2°F above the normal of 98.6°F, we become ill. If it rises 7°F, we become critically ill. If our temperature decreases 2°F, we feel cold. A 7°F decrease puts our life in jeopardy.

Human beings are designed to live in tropical climates, so our heat loss mechanisms are highly developed. Our insulation mechanisms, however, are less efficient. To adapt structurally to cold, our bodies would have to grow thick insulating hair all over and develop greater reserves of fat. Rather than remaining angular and cylindrical, which promotes heat loss, our body shape would become rounder and shorter to prevent heat loss. This would especially affect our ability to tolerate lower body temperatures and near-freezing temperatures in our fingers and toes.

As it is, human beings can live in the cold because our intellectual responses enable us to deal effectively with environmental stress. Much of what students learn on NOLS courses is how to live comfortably in extreme environmental conditions by employing skill, disciplined habits and quality equipment. We compensate for our physical deficiencies with behavioral responses such as eating and drinking and creating microclimates through the use of clothing, fire and shelter. The diminished intellectual response evident in early stages of hypothermia, as well as altitude sickness, heat illness and dehydration, dangerously impairs our ability to react to the environment.

Exercise is an important method of heat production. Muscles, which make up 50 percent of our body weight, produce 73 percent of our heat during work. Short bursts of hard physical effort can generate tremendous amounts of heat, while moderate levels of exercise can be sustained for long periods. This valuable source of heat does have its limitations. Physical conditioning, strength, stamina and fuel in the form of food and water are necessary to sustain activity.

As with all forms of work, the price of shivering is fuel. How long and how effectively we shiver is limited by the amount of carbohydrates stored in muscles and by the amount of water and oxygen available. In order to shiver, we have to pump blood into our muscles. Warm blood flowing close to the sur-face reduces our natural insulation and increases heat loss.

The core of the body contains the organs necessary for survival: the heart, brain, lungs, liver and kidneys. The shell consists of the muscles, skin and superficial tissues. The ebb and flow of blood from core to superficial tissues is a constant process. As our temperature rises, blood volume shifts and carries heat to the outer layers of the skin. As we cool, less blood flows to the periphery, preserving heat for the vital organs.

Our mechanisms for heat loss are so well developed that we lose heat in all but the hottest and most humid conditions. If on a warm day we do not lose most of the heat our bodies produce, our temperature rises. The primary means of heat loss is through the skin. Warm, flushed skin can dispose of the heat through radiation, convection, conduction or evaporation.

Conduction is transfer of heat through direct contact between a hot and a cold object. Heat moves from the warmer to the colder object. We lose heat when we lie on the cold, wet ground. We gain heat when we lie on a hot beach or rock. The rate of heat transfer is determined by the temperature difference between the two objects, the surface area exposed to the cold surface and the effectiveness of the insulation between the body and the cold surface. The more efficient the insulation, the less heat is transferred. Warm, still air is an effective insulator. Water is a good conductor. Immersion in cold water is a profound threat to temperature balance.

Convective heat transfer occurs when the medium of transfer moves. Whether through moving air or water, heat escapes from the surface of the body by convection. Moving air (wind chill), besides cooling us directly, strips us of the microclimate of air heated by the body. The loss of this insulating layer next to the body further accelerates heat loss.

Moving water carries heat directly from the surface of the body. To discover the cooling power of moving water, place your fingers in a bowl of cold water. Slowly swirl your fingers. The increase in heat loss is immediately perceptible.

When exposed to the environment, the skin acts as a radiator. Unlike in the rest of the body, the blood vessels in the head do not constrict and reduce the blood supply flowing to the scalp. The head is therefore an excellent radiator of heat, eliminating from 35 to 50 percent of our total heat production. The effectiveness of garments designed to reflect and conserve radiative heat is not agreed upon universally, but the effectiveness of dry insulation, especially on the head, is undeniable.

Heat is necessary to the evaporation of perspiration from the skin's surface. Evaporative heat loss accounts for 20 percent of the body's normal total heat loss. When we become overheated, though, evaporation becomes our major mechanism for heat loss. Evaporation can liberate as much as 1,000 kilocalories an hour.

Sweating accounts for roughly two thirds of our evaporative heat loss. The remaining one third is lost through breathing. Inhalation humidifies air and warms it to body temperature. During exhalation, evaporation of moisture from the surface of the lungs and airways uses heat and cools the body. The rate and depth of breathing and humidity of the air determine the amount of heat and moisture lost. The colder and dryer the air and the faster the breathing rate, the greater the heat loss. To reduce heat loss through evaporation, avoid hard breathing and sweating. Sweating in cold environments is a bad habit. It wets insulation and cools the body.

A constant balance of heat gain and loss is required to maintain a stable body temperature. The adjustments the body makes are designed to keep our vital organs-heart, brain, lungs, kidneys and liver-within a temperature range in which they operate effectively. If core temperature rises above normal, potentially life-threatening conditions-heat stroke or high fever-develop. When core temperature drops below normal, hypothermia may be the result.

Hypothermia occurs when body temperature drops to 95°F or lower, a condition that is not exclusive to the winter environment. Hypothermia can develop whenever heat loss exceeds heat gain and is as common during the wind, rain and hail of summer as it is during winter. Immersion in cold water can cause hypothermia. If body temperature drops as low as 80 degrees F, death is likely.

The signs and symptoms of hypothermia change as body temperature falls. Mental functions tend to go first, and the patient loses his ability to respond appropriately to the environment. Muscular functions deteriorate until he is too clumsy to walk or stand. Biochemical processes become slow and deficient as the body cools.

Hypothermia in which body temperatures remain above 90°F is classified as mild to moderate. Hypothermia below 90°F is severe. A healthy adult with a body temperature of 93°F is dangerously cold, but chances are good that rewarming will be successful. If the same person has a temperature of 90°F or less, rewarming in the backcountry can be difficult, and the patient's life may be in grave danger.

Early signs and symptoms of hypothermia can be difficult to recognize and may easily go undiagnosed. The patient does not feel well. You may assume he is tired, not hypothermic. Yet this is the stage in which successful rewarming in the wilderness is possible if our awareness is such that we catch the problem.

In the early stages of hypothermia the patient feels chilled. The skin may be numb with goose bumps. Minor impairment of muscular performance is evident in stiff and clumsy fingers. Shivering begins. Mental deterioration occurs at the same time. Responses are slow and/or improper, such as not changing into dry clothes or failing to wear a rain jacket, wind garments or hat.

Shivering is the first response to cold. It reaches its maximum when body temperature has fallen to 95° to 93°F. Shivering stops when the temperature falls to 92° to 90°F. During the fast cooling phase, the pulse rate increases to as high as 150 per minute. Later, as the body becomes cooler, pulse rate and blood pressure falls. Respirations slow and may finally cease around 78°F.

As body temperature drops into the mid-nineties, muscular coordination deteriorates. The patient may stumble, walk slowly, lack energy and become apathetic, and lethargic. He talks less and may become uncooperative and complaining. Responses to questions may be inappropriate; the patient may exhibit slurred speech and confusion about time or place.

The most important diagnostic tools in the backcountry are the first-aider's awareness of and suspicion concerning the condition and his attention to the patient's mental state. Oral or axillary temperatures may not reflect the status of the core organs; a rectal temperature is the most accurate temperature available in the field. However, obtaining a rectal temperature reading on a cold and confused patient can be awkward. Also, exposing the patient in order to obtain a rectal reading may cause further cooling. Whether you can obtain a rectal temperature or not, if you suspect hypothermia, treat it immediately and aggressively.

A mildly hypothermic patient may be rewarmed in the field. In the absence of a serious underlying medical condition, the chances for successful rewarming are good. The patient in early hypothermia may respond well to removal of the cold stress. While we cannot change the air temperature, we can replace wet clothing with dry, protect the patient from the wind, add layers of insulation and apply heat.

Hot drinks are a good source of heat, fluid and sugar. Again, be careful of burning the patient. The patient must be conscious and alert to drink. Hydrate and feed with hot drinks and simple foods, such as candy bars, followed by a good meal after he is rewarmed. A rewarmed patient should not return to the cold until his energy and fluid reserves have been replenished. A fatigued or dehydrated patient is a strong candidate for another episode of hypothermia.

A sleeping bag is the backcountry's most tried and true rewarming tool. Place the patient in one or more bags with at least one other person as a heat source. If the patient is left alone in the bag, he will only be insulated at his current body temperature, since the hypothermic patient has lost the ability to produce heat himself. In fact, further cooling may occur.

Hot water bottles applied to the chest, abdomen, neck and groin--areas close to the core and containing large blood vessels--are an excellent source of heat. Be careful not to burn the patient. Apply the hot water bottles to yourself before applying them to your patient. Wrap them in socks to insulate them from direct contact with the patient.

Fires are an excellent source of heat. Position the patient in the sleeping bag next to or between two fires. Use a space blanket as a reflector. If you are without a sleeping bag, dress the patient in dry clothes for insulation. One or more individuals huddling around or hugging the patient will provide insulation and heat. A windproof outer layer will reduce the patient's convective and evaporative heat loss.

It may be difficult to find the pulse or respiration rate of a cold patient. A severely cold patient may have a heart rate of 20 to 30 beats per minute and be breathing only three to four times a minute. Cold reduces metabolic demands; a patient can sustain life with these abnormally low rates. Take your time during assessment.

Frostbite occurs when tissue is frozen. As blood flow declines, cooling can progress to freezing. The fluid between cells freezes. The formation of ice crystals draws water out of the cells, dehydrating them. Mechanical cell damage also occurs as the crystals rub together. Blood clots in small vessels and circulation stops, further damaging cells.

Low temperatures, contact with moisture and wind chill accelerate heat loss and increase the likelihood of frostbite. Metal and petroleum products can cool well below the point of freezing. Skin contact with metal or supercooled gasoline will cause immediate freezing. Constriction of an extremity, as caused by tight boots, gaiters or watchbands, or confinement in a cramped position may reduce blood flow and increase the likelihood of frostbite.

With frostnip only the outer layer of skin is frozen. It appears white and waxy or possibly gray or mottled. Frostnip may occur from contact with a cold metal or a supercooled liquid or from exposure to severe windchill. High winds together with cold temperatures create conditions for frostnip on exposed areas of the face, nose, ears and cheeks.

Frostnip is similar in physiology to a first degree burn and is sometimes called first degree frostbite. After the nipped area is rewarmed, the layer of frozen skin becomes red. Over a period of several days the dead skin will peel. As it heals, the appearance of the injury is similar to that of sunburn, a first degree burn.

Superficial frostbite injures a partial thickness of the skin, similar to a second degree burn. This injury has progressed from frostnip into the underlying tissues. Externally it appears as a white, mottled or gray area. It feels hard on the surface, soft and resilient below. Blisters usually appear within 24 hours after rewarming.

Treatment for second degree or superficial frostbite is rapid rewarming by immersion in warm (101° to 108°F) water. This injury extends into the underlying tissues and is more extensive than frostnip. Unlike frostnip, the injury should not be rewarmed by simple application of heat. Proper rewarming is crucial to healing.

The most serious form of frostbite is deep or third degree frostbite. The injury extends from the skin into the underlying tissues and muscles. The external appearance is the same as frostnip and superficial frostbite, but the frozen area feels hard. After thawing the area may not blister or may blister only where deep frostbite borders on more superficial damage.

Differentiating superficial from deep frostbite before thawing is difficult. Blisters containing clear fluid, extending to the tips of the digits and forming within 48 hours of rewarming suggest superficial frostbite. Blood-filled blisters that don't reach the tips of the digits, delayed blisters or the lack of blisters indicates deep frostbite. Like superficial frostbite, deep frostbite is rewarmed by immersion in warm water.

Try to keep the injury frozen until rewarming can be carried out correctly. Many people, including this author, have traveled long distances with frozen feet in order to reach a place where rewarming could be done once and done well. How long the area can be kept frozen without increasing the damage is a matter of controversy. Tissue damage does seem related to the length of time the tissue stays frozen.

There are several problems with keeping a frostbitten extremity frozen while evacuation takes place. If the injury occurred from exposure to extreme cold, lack of proper clothing or in conjunction with hypothermia, the frostbitten area may rewarm as the problem that caused it is corrected. The activity of traveling may generate enough heat to begin thawing, thus increasing the possibility of further injury from refreezing or bruising. Unintentional slow rewarming is common.

If the injury is confined to a small area of the body, tips of toes or fingers, slow thawing is likely, and field rewarming should be started. If the injury is extensive, thawing will be difficult. Try to keep the area frozen. Adjustments in clothing and work rate will be necessary.

Treatment for frostbite, best done in a hospital, is rapid rewarming in water between 100° to 108°F. Use a thermometer to ensure that the water is the proper temperature. For a rough estimate, 105°F is hot tap water. Water colder than 100°F will not thaw frostbite rapidly. Water hotter than 108°F may burn the patient.

Water temperature should remain constant throughout the procedure. This requires a source of hot water and several containers large enough to contain the entire frozen part. Do not pour hot water over the frozen tissue. Rather, immerse the frozen area, being careful not to let it touch the sides or bottom of the container. When the water cools, remove the frostbitten part, quickly rewarm the water and re-immerse the part.

Thawing frozen fingers generally takes 45 minutes. There is no danger of overthawing, but underthawing can leave tissue permanently damaged. A flush of pink indicates blood returning to the affected site. Rewarming frostbite is generally very painful. Aspirin or ibuprofen are appropriate for pain relief. If hypothermia is present, it takes priority in treatment.

Air dry the extremity carefully; don't rub. Swelling will occur, along with blister formation. Inserting gauze between the fingers or toes will keep these areas dry as swelling occurs. Blisters may be drained with a sterile syringe and then dressed with aloe vera ointment. Once the tissue is thawed, it is extremely delicate and seemingly minor trauma can damage it.

Immersion foot is a local, non-freezing cold injury that occurs in cold, wet conditions, usually in temperatures of 30° to 40°F. At least 12 hours' exposure to cold, wet conditions is necessary to produce the injury. People have contracted immersion foot in hip waders and vapor barrier boots. Dry socks and feet provide total protection.

The extremity appears cold, swollen and mottled. Cyanosis is usually present. Tactile sensitivity is reduced, as is capillary refill time. The foot may look shiny. The patient may describe the foot as feeling wooden.

When the extremity rewarms, the skin becomes warm, dry and red. The pulse is bounding. The injury is painful. The injured area may itch, tingle and exhibit increased sensitivity to cold, possibly permanently. The recovery period can last weeks. Nerve damage may be permanent. The development of blisters, ulcers and gangrene is possible. Loss of a foot or lower leg is also possible.

Warm an immersion foot slowly at room temperature. In serious cases swelling, pain and blister formation will prevent walking. In most cases the extremity will be sore. Avoid walking on injured feet, and elevate the feet to reduce the swelling. Bed rest, along with avoiding trauma, is necessary until the injury heals.

Hypothermia is a lowering of the core body temperature occurring when heat loss exceeds heat production. It is a dangerous disturbance of body function. Mild hypothermia (above 93°F-patient conscious, shivering, able to walk) is treatable in the field. Severe hypothermia (below 93° F -- patient unconscious, not shivering, unable to walk) requires rewarming in a hospital.

Frostbite is a local freezing injury classified as frostnip, superficial or deep. Frostbitten tissue is cold, gray, white or mottled. Frostnip only affects the skin and is easily treated with immediate rewarming. Superficial and deep frostbite progress into underlying tissue layers and should be rewarmed rapidly in warm water.

convective

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