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WILDERNESS COLD INJURIES
Objectives :
• Describe the manner in which the human body loses heat in cold environments and how heat loss can be prevented, the circumstances in which hypothermia occurs, the features of mild and severe hypothermia, preventive and therapeutic measures, and the limitations of cardiopulmonary resuscitation for severely hypothermic individuals.
• Describe the circumstances, in which frostbite occurs, the pathogenesis of tissue lesions, the clinical features of frostbite including how to estimate its extent and severity, and techniques for early and late therapy. • Describe the circumstances in which trenchfoot appears, the presumed pathogenesis of the lesions, the clinical features of this disorder, its treatment, and late effects of trench foot.
Hypothermia occurs in many different locations and in all seasons. In one multicenter survey, sixty-nine of 428 cases of accidental hypothermia in North America occurred in Florida. Annually, from 1979 through 1995, the average number of deaths attributed to hypothermia in the United States was 723. Most accidental hypothermia in industrialized countries occurs in urban settings. However, this discussion is directed largely to accidental hypothermia in wilderness situations.
HOW HEAT IS LOST
An understanding of hypothermia requires knowledge of the way heat is lost from the human body. Heat is dissipated to the environment in four ways: radiation, evaporation, convection, and conduction. (In hot climates heat can be gained through radiation and conduction.)
RADIATION
Heat radiates continuously from warmer objects to colder objects, including the sky. Infrared radiation that can be visualized by “night vision” devices is a form of such radiation. In a calm, temperate climate of about 70ºF (21ºC) a clothed, sedentary person loses approximately 60 percent of his body heat through radiation.
Radiative heat loss is poorly understood by many. An example is found in old churches with thick stone walls. To save the cost of fuel, these buildings would not be heated during the week. Only on Friday or Saturday, in preparation for Sunday services, would the heat be turned on. On Sunday morning the air temperature inside the church would be at a comfortable level, but members of the congregation would be cold and often needed to wear heavy coats. The brief period of heating would warm the air in the church, but would not warm the thick walls. Individuals in the church were cold because they were losing so much heat by radiation to the walls.
Clothing has little effect on heat loss by radiation. Heat radiates from the body to the clothing and from there to the atmosphere. Efforts to develop clothing materials that reflect heat back to the body have met with little success. However, radiant heat loss becomes a major problem only in extremely cold situations (below -20 ° to -30 ° F or -29 ° to -35 ° C). If clothing adequately limits heat loss by other routes, particularly convection, physiologic mechanisms can compensate for the increased radiant heat loss encountered in most cold environments.
EVAPORATION
Evaporation goes on constantly from the skin and from the lungs. Inhaled air is humidified to 100 percent humidity, and water is lost through the skin through sensible and insensible perspiration. In temperate resting conditions about 500 cc of water is lost each day by these processes. During exercise in most climates, perspiration and its subsequent evaporation is the major mechanism for heat loss. In hot climates greater water volumes are lost through perspiration: at high altitudes much more water is lost through the lungs. Exercise also increases water loss through the lungs.
The conversion of 1 cc of liquid water to vapor absorbs approximately 580 calories of heat (0.6 kcal). Evaporation of 1 liter of water would extract approximately 580 kilocalories. The amount of heat lost by a sedentary person in a temperate climate through normal evaporative processes is approximately 300 kcal, or about 15 percent of his average daily food caloric intake.
Heat loss from the respiratory tract cannot be reduced in any practical manner. Mouth breathing increases fluid and heat loss somewhat, but the amount is insignificant in comparison with the quantity of heat lost through other sources. Winter sportsmen, particularly high altitude climbers, must be aware that this heat and water loss is occurring, must eat enough food to regenerate the heat, and must drink enough liquids to replace the water. Respiratory water losses at altitudes over 20,000 feet can be as great as four liters a day.
Heat loss from the skin through insensible perspiration also cannot be effectively limited. Vapor barrier systems, which consist of a layer of material impermeable to water vapor (usually plastic) between or beneath layers of insulation, have been tried. Theoretically, because the barrier prevents water vapor loss, perspiration would not evaporate, and heat loss by that route should be eliminated. However, perspiration does not cease, and the clothing underneath the barrier becomes wet and no longer limits convective and conductive heat loss.
No vapor barrier system works well at temperatures above freezing because too much water accumulates. Even at lower temperatures the only vapor barrier system widely used is has been in footwear (Korean or “Mickey Mouse” boots) that is no longer being manufactured. Although such boots do keep feet warm, application of antiperspirants to the feet with or without frequent sock changes are needed to avoid injury by perpetual wetness.
CONVECTION
Convection refers to heat loss that occurs when air on the skin surface is warmed to skin temperature and then moves away. As the cold air that replaces it is warmed, more heat is extracted. When the wind is blowing, warm air against the skin is constantly being removed. (Convection is the mechanism by which a spoonful of soup is cooled by blowing on it. The warmed air above the soup is constantly being replaced by cooler air.)
Convective heat loss is an almost continuous process. In a temperate environment approximately 40 percent of heat loss is convective. Even in still air the becomes lighter when it is warmed, rises, and is replaced by cooler air. In a cold atmosphere convective heat loss is greater because more heat is required to warm the colder air. However, the greatest convective heat losses occur when air is moving. Even a mild breeze greatly increases heat loss because the layer of warmed air next to the skin is constantly being replaced with cooler air.
If the ambient air is cold and is moving rapidly, tremendous amounts of heat can be lost. Wind-chill charts indicate the increased heat loss that occurs as wind speed increases. In calm conditions, 20 ° F is rather warm for skiing, but a ten mile-per-hour (mph) wind produces cooling equivalent to a temperature of 2 ° F, which is as cold as many skiers enjoy. With a twenty mph wind the equivalent temperature is –9 ° F, and a thirty mph wind produces cooling equivalent to –18 ° F, which should drive the most ardent skiers into the lodges. (At wind speeds above 40 mph cooling does not increase because the air does not remain in contact with the skin long enough to be warmed to skin temperature.) The attached wind chill chart was created for the Canadian armed forces in 2003 because equivalent cooling in previously published charts was too high. The Canadian chart was published with metric values (centigrade temperatures and wind speed in kilometers per hour). The Fahrenheit and mile per hour figures have been calculated.
Because convective heat loss can increase so enormously, it is the major cause of terrestrial hypothermia in the wilderness. Fortunately, clothing can greatly reduce this type of heat loss. Insulating clothing forms a myriad of small pockets in which air is trapped—the essence of thermal insulation. Windproof outer garments prevent displacement of the air within and between layers.
CONDUCTION
Conduction refers to the direct transfer of heat from the body by material in contact with the skin. Air conducts heat poorly, and conductive heat losses on land are usually small, although sleeping on the ground without adequate insulation, contact with a cold stone, or contact with ice and snow can result in significant conductive heat loss. (Because air conducts heat so poorly, it is used as insulation in clothing.) Water has about twenty-five times the conductivity of air. More importantly, water has about 3,500 times the volumetric heat capacity of air. (To raise the temperature of a specific volume of water a specific amount requires 3,500 times as much heat as would be required to raise an identical volume of air the same amount.) Conductive heat losses are quite high for anyone immersed in cold water without protective clothing. Conductive heat losses can become significant on land when clothing is wet.
Pads composed of small foam cells eliminate most of the conductive heat for individuals lying on the ground. (Air mattresses, which allow air to circulate freely, provide much less insulation than foam pads.) Although conductive heat loss alone is rarely a major cause of hypothermia, heat loss by this route can aggravate convective heat losses and should be avoided. Some heat can be lost through the soles of boot in contact with snow or ice if they are thin and not well insulated.
PREVENTING COLD INJURIES
Water And Food
Avoiding hypothermia in a cold climate requires water, food, and clothing. Failure to replace normal water losses through the kidneys, skin, and lungs, or abnormal losses by other routes, results in dehydration, which decreases the blood volume and, in a cold environment, impairs heat production by exercise. Dehydration can be accompanied by weakness, fatigue, dizziness, and even a tendency to faint when standing, which impede efforts to deal rationally with a threatening environment.
Dehydration also contributes to other problems. Constriction of peripheral blood vessels so the smaller volume of blood goes to vital organs increases the risk of frostbite. Shock may develop following relatively mi-nor injuries. Clots tend to form in the legs and have resulted in pulmonary embolism, even though in a severely hypothermic patient the coagulation cascade is impaired.
In a dehydrated state the sensation of thirst is diminished or absent, and a conscious effort to consume adequate fluids must be made. Water intake with mild exertion should be at least two quarts per day. With heavier exertion or at high altitude, three to five quarts are needed. In a world of snow and ice, fuel is required to melt snow for drinking water. Eating snow does not provide an adequate volume of water, and body heat is lost in warming ingested snow to body temperature.
An adequate fluid intake is indicated by urine that has a light yellow color and a volume of at least one liter every twenty-four hours. Few outdoorsmen would measure urine volume, but they should be able to appreciate a reduced frequency for voiding, particularly the absence of a need to void after a night's sleep. They certainly can recognize the deep yellow or orange color of concentrated urine indicative of dehydration. When voiding into snow, orange “snow flowers” are an ominous sign. Food is needed for physical activity and heat production. Eating small amounts of food at frequent intervals helps prevent depletion of energy stores during the day. Some experienced outdoorsmen seem to be munching almost continuously, and often have developed mixtures of nuts, dried fruits, candies, and other high calorie food. Such mixtures are sold as “gorp” or “trail mix.”
In a survival situation, experience has demonstrated that food is one of the most important ingredients of success. Any source of food, even wild animals such as birds or rodents, which may have to be eaten uncooked, is preferable to the fatigue and depression that result from not eating and that can contribute significantly to hypothermia.
Physiologic Responses
Body temperature is controlled by regulatory centers located in the hypothalamus. This area contains receptors that sense blood temperature and also receives input from cutaneous nerves that sense skin temperature. In response to a fall in temperature, the heat centers cause the blood vessels in the skin and in the extremities to constrict, particularly arteriovenous anastomoses, reducing the temperature of the skin and heat loss to the environment. Blood flow to the skin can vary as much as one hundred fold between the extremes of vasodilatation in a hot climate and vasoconstriction in a cold climate. In severe cold conditions, skin temperatures can fall to surprisingly low levels with no reduction in core temperature. This reduction in skin temperature reduces heat loss by convection and radiation.
When the core temperature continues to fall, shivering is initiated. Shivering is an involuntary muscular action that accomplishes no useful work, but can increase heat production as much as five times, if glycogen stores have not been depleted. Much more heat can be produced by voluntary exercise, particularly exercise that uses the large muscles of the legs and back (such as quickly hiking out of a cold environment.)
Clothing
Human physiologic responses to cold are of limited effectiveness in a severely cold environment, and man is dependent upon his intellect to devise protection—clothing and shelter.
Clothing for cold climates must not only protect from the cold, it must be able to compensate for changes in environmental temperature and for heat production by exercise. The most flexible cold weather clothing systems are composed of three layers: an inner layer (underwear), one or more middle insulating layers, and an outer windproof (and perhaps water repellent) shell. The middle and outer layers can be opened or removed when environmental temperature or heat production increases. Additional insulating layers can be added as the temperature falls or the person becomes inactive.
In a multi layered clothing system each succeeding layer must be slightly larger than the one beneath. If the layers are the same size, the outer layers compress the deeper layers and reduce their insulation value. Each layer should provide a one-quarter inch air space between it and the layer beneath.
However, the outer layer must not be too large. It must be snug enough to be warmed by the wearer's body so that its inner surface temperature does not fall to the dew point. If that occurs, moisture collects on the inner surface and wets the insulating layers, greatly reducing their effectiveness.
Sweating must be avoided. Sweat moistens the clothing, greatly reducing its insulation value, and more heat is lost as the perspiration evaporates. Outer layers must be opened or taken off soon after activity begins, not after the individual has become hot and begun to perspire. These layers must be put back on or zipped soon after activity ceases, not after the individual has become cold and requires more heat to warm him again.
Clothing Materials
No third-party testing facility, such as Underwriters' Laboratory, exists for clothing. Buyers are at the mercy of advertising agents serving the manufacturers and sellers. The most reliable indicator of a fabric or garment's performance is its persistence in the market place for two or three years or more. Effective cold weather clothing must have two properties: insulation and permeability. Insulation basically is the ability of the clothing to entrap air and prevent convective heat loss. The insulating properties of a material are dependent upon its thickness and how well it inhibits the movement of air.
Permeability refers to the ability of water vapor to move through the fabric. Water vapor resulting from the evaporation of sensible or insensible perspiration on the skin must be able to move through clothing to the atmosphere. If the clothing is impermeable, the vapor condenses and the clothing becomes wet.
A variety of natural materials are used in clothing for outdoor wear. Wool, cotton, silk, and goose down are the most popular. All but down are usually combined with synthetic materials such as nylon, polypropylene, polyester, and acrylics to impart specific desirable characteristics to the fabric.
As discussed above, clothing for cold weather should be layered. The materials best suited for underwear or the inner layer, insulation or the middle layer or layers, and the shell or outer layer are described below.
Underwear
Polypropylene became popular for underwear in the 1970's. Because it is hydrophobic and allows moisture to wick from the skin surface to the surface of the fabric where it evaporates without cooling the skin, it provides a greater sensation of warmth. Furthermore, polypropylene retains most of its insulating properties when wet. The disadvantages of polypropylene include its retention of body odors—it can become quite foul after repeated use—its tendencies to become brittle when heated and to pill when dried in a clothes dryer, and its tendency to become baggy. The last problem has been corrected to some extent by adding nylon to the fabric.
Polyester fabrics have to a large extent replaced polypropylene because, in spite of being slightly more expensive, they do not have the same disadvantages. Capilene®, Coolmax®, Thermax®, and Thermostat® are the trademarks of polyester fabrics.
These fabrics keep the skin cool and dry through a wide range of activity, but to achieve this effect these fabrics must be worn next to the skin, not over cotton underwear or as a jacket.
Wool remains an excellent fabric for underwear, but has fallen from popularity and is difficult to find. Adding a small amount (tablespoon) of olive oil to the wash water can eliminate the scratchiness that wool tends to develop after repeating washings. Some people like the feel of dry silk, but when silk becomes wet it feels unpleasant. The fabric holds 25 percent of its weight in water.
Insulation
Wool is the oldest and one of the best insulating materials for cold weather clothing. Its major disadvantage is its greater weight than pile or fleece, but it is still popular for caps and gloves. Wool is one of the few materials that maintains its insulating properties when wet. However, with prolonged wetness the inner core of wool fibers can absorb water, which makes the fabric considerably heavier. Totally drying this core requires a large amount of heat—generally extracted from the wearer's body.
Synthetic fabrics have largely replaced wool. Pile fabrics, introduced in the 1970's, are lighter and are hydrophobic. However, they tend to loose their pile and to pill badly with wear and laundering, and have largely been replaced by fleece.
Fleece is a similar polyester fabric, but with stiffer fibers than pile, and superior qualities. It is produced in different thicknesses: microfleece for underwear and 100, 200, and 300 weights for outer garments. This material breathes well, and is lightweight, durable, and fast drying. It is easy to cut and sew and has largely replaced pile as a fabric for outdoor clothing.
Goose down is the best available insulating material for its weight—when it is dry. True down is the philoplume of geese or ducks and historically was handpicked from those animals, but that material is no longer available. Down now comes from killed animals and is composed of less mature plumes that do not loft as well. However, down garments drape well, do not constrict movement, loft after compression, and are comfortable. When wet, down mats together and loses most of its insulating properties. However, when precipitation is in the form of dry snow, which is typical of high altitudes, down is the insulating material of choice.
Synthetic fibers can provide insulation similar to down and retain their insulating properties when wet. Although various materials have been tried, manufacturers currently are using a blend of three different deniers (thicknesses) to gain high loft. Primaloft ® and Lightloft ® are the tradenames of two down replacements. The disadvantages of such materials are their greater weight (about 50 percent heavier than down) and their bulk or lack of compressibility.
Mat materials are extruded, densely packed fine fibers. Because the fibers are so thin, they slightly limit radiant heat loss, but lose this advantage when laundered. These materials resist compression and do not drape well. Thinsulate ® is the dominant brand of this type of fabric, the use of which is limited largely to ski clothing.
Shell
The outer shell must be windproof in order to protect the insulating qualities of the underlying clothing. Tightly woven fabrics made of synthetic fibers are most commonly used. The shell usually must be water repellant also. The ideal fabric that would allow all water vapor to pass through freely but keep out all liquid water has yet to be developed. The best available fabrics are laminates such as Gore-Tex ® and urethane-coated materials, which have small pores close enough together to resist penetration by liquid water, but large enough for most water vapor to pass through. Soiling limits the functionality of these fabrics.
Protecting Hands And Feet
When the body is cool the blood vessels in the hands and feet constrict, reducing heat loss through those tissues, but also reducing their temperature and commonly causing severe discomfort. The most effective way to prevent cold extremities is to keep the body warm, a lesson some outdoor enthusiasts seem to have difficulty learning.
For the hands, mittens are much warmer than gloves. Radiant heat is lost from the surface of protective garments; the larger the surface area the more heat that is lost. Because the fingers are such narrow cylinders, increasing the thickness of gloves by more than one-quarter inch increases the surface area to such an extent that the increased heat loss eliminates any benefit from the increased insulation. Because mittens do not have such a large relative surface area, their thickness can be increased to a much greater extent without a concomitant increase in heat loss. In mittens the skin is not in close contact with the fabric and a thicker layer of insulating air is present.
The basic components of mittens are an outer shell and an insulating layer. A third inner layers of a thin fabric such as nylon or silk—usually a glove—is useful if the mittens have to be removed to manipulate clothing or equipment. The outer shell should be an abrasion-resistant material, typically nylon. Wool works well for insulation, but down and synthetic materials are also used. Many different types of one-piece mittens have been developed, particularly for skiing. Many are quite expensive but are not more effective than a simple, inexpensive wool mitten with an outer nylon shell.
One of the warmest types of footgear yet devised is the U.S. Army double vapor barrier boot known as the white Korean boot. However, this type of footwear, which is no longer manufactured and is rarely obtainable, is too soft for kicking steps in hard snow and is difficult to fit with crampons.
For severely cold climates, particularly for high altitude mountaineering that requires ice climbing, double or triple boots are best. The outer boot is constructed of hard, protective plastic. The inner boot or boots are made of softer insulating material—felt or similar substances. The insulating material also covers the bottom of the foot, and the soles are quite thick. Another type of cold weather boot popular with dog mushers is constructed with a 1.5 inch layer of polyurethane foam insulation and covered by Cordura nylon. This boot not only has a thick layer of insulation over the sides and soles, it “breathes”— allows moisture to escape. It is advertised as being effective at temperatures as low as –60ºF (–52ºC). Information about this boot is available at http://www.northernoutfitters.com/ .
Older double boots, which are no longer available unless custom made, were made of leather, which is entirely adequate but is heavier than plastic. Leather also breathes and can expand to accommodate swelling of feet and ankles due to an upright position or altitude. It remains the best material for single boots in moderately cold climates.
Heat Loss From The Head
The brain receives about one-fourth of the cardiac output during resting conditions, but this organ is surrounded only by a thin shell of bone that conducts heat well and a layer of skin that contains little fat. Heat loss from the head can be a significant portion of the body's total heat loss. Snug fitting, insulating caps made of wool or polypropylene limit heat loss best. Balaclavas that cover the neck and lower face are desirable for severe conditions. Hoods do not fit snugly enough to provide similar protection, even hoods thickly layered with insulating material. However, a combination of both works even better.
The neck carries this large volume of blood in vessels that are close to the surface and through which large quantities of heat can be lost. A scarf, the value of which has been known for centuries, can very effectively limit such loss. Neck gaiters have largely replaced scarves and also are effective.
HYPOTHERMIA
Hypothermia is a body core temperature that is lower than normal. By definition the core temperature must be below 95 ° F (35 ° C), but as discussed below, that criterion has no practical value.
Participants in the conference following the 1986 Mount Hood disaster agreed that hypothermia need only be divided into “mild” and “severe” categories. The basis for distinguishing the two categories is the body's ability to rewarm itself, which usually is lost when shivering stops at body temperatures of 30 ° to 33 ° C (86 ° to 91 ° F). (Great individual variation exists.) A “moderate” category was considered to have no practical value, particularly in a wilderness situation. Others have separated severe hypothermia as defined above into “moderate” and “profound” categories on the basis of whether consciousness has been lost.
Treatment for hypothermia obviously can be divided into prehospital and hospital therapy. Not so apparent is the need to distinguish between the care needed for severe hypothermia in an urban environment (where most North American hypothermia occurs) and that needed in a remote wilderness area. In most urban environments severely hypothermic individuals are rarely more than an hour from a hospital, and the care they need consists of little more than protection from the environment and rapid transport, even when they have no cardiac activity. Urban hypothermia is often (typically?) complicated by drug abuse In contrast, a severely hypothermic individual in a remote area may be in a hopeless situation unless he can be transported by helicopter.
Hypothermia results from inadequacies. In a wilderness environment, hypothermia results from:
• Inadequate protection from the cold, including inadequate clothing • Inadequate food for metabolic fuel to be burned during exercise • Inadequate fluid intake resulting in dehydration
In an urban environment, where hypothermia is significantly more common, the causes are:
• Inadequate youth (old age) • Inadequate money for housing, heat, clothing, and food • Inadequate sobriety
In both situations, inadequate “smarts” is an almost universal component of the cascade of events that leads to hypothermia!
MILD HYPOTHERMIA
Mild hypothermia, a core temperature above 90 ° to 92 ° F (30 ° C), can be subtle and difficult to diagnose; in contrast, it usually is easy to treat, at least in comparison with treating moderate or severe hypothermia. Individuals with mild hypothermia may feel cold, but that sensation is not an indication that core temperature has fallen. Painful hands and feet should serve as a warning that the body is not being kept warm.
Signs of a definite reduction in core temperature—mild hypothermia—include slowing of pace, greater fatigue than other members of the group, muscular incoordination that leads to stumbling or falling, and intellectual and personality changes, particularly uncharacteristic irritability.
As body temperature falls to about 93 ° F, (34ºC) uncontrollable shivering develops, and is quite easy to recognize.
A mnemonic for remembering the early stages of hypothermia is “umbles.” The mildly hypothermic individual:
• Mumbles • Grumbles • Fumbles • Stumbles • Tumbles
The first two reflect intellectual impairment; the last three physical impairment.
The treatment of mild terrestrial hypothermia consists of “all those things your mother told you to do when you went out in the cold.” More clothes to retain heat, a fire or other external heat source if possible, vigorous exercise—particularly with large muscles—to generate heat, and shelter from the environment. Food and drink are necessary to correct nutritional and fluid inadequacies. Warm beverages make mild hypothermia victims feel better, even though they provide few calories.
Individuals with mild hypothermia who are shivering vigorously may recover faster if they are not externally warmed by hot water bottles, heated stones, or body-to-body contact. Shivering generates much more heat than skin-warming devices can impart, particularly the warming devices available in a wilderness situation, and warming the skin stops shivering.
Victims of mild hypothermia can return to a cold environment if they are provided with additional clothing. They must not be sent out into the same conditions in which they became hypothermic without additional protection .
STAGES OF HYPOTHERMIA
Mild Hypothermia
98 ° - 95 ° F Sensation of chilliness, skin numbness; minor impairment 37 ° - 35 ° C in muscular performance, particularly in fine movements with the hands; shivering begins. 95 ° - 93 ° F More obvious in coordination and weakness; stumbling; 35 ° - 34 ° C slow pace; mild confusion and apathy. 93 ° - 90 ° F Gross incoordination with frequent stumbling, falling, and inability to use hands; mental sluggishness with slow 34 ° - 32 ° C thought and speech; retrograde amnesia.
Moderate Hypothermia
90 ° - 86 ° F Cessation of shivering; severe incoordination with 32 ° - 30 ° C stiffness and inability to walk or stand; incoherence, confusion, irrationality. 86 ° - 82 ° F Severe muscular rigidity; semiconsciousness; dilatation 30 ° - 28 ° C of pupils; inapparent heart beat or respirations.
Profound Hypothermia
Below 82 ° F Unconsciousness; eventually death due to cessation of heart Below 28 ° C Action at temperatures approximating 68 ° F (20 ° C) or below.
MODERATE AND SEVERE HYPOTHERMIA
Diagnosis
Moderate and severe hypothermia are usually easy to diagnose, but may be difficult or impossible to treat in the wilderness. Moderate hypothermia has been defined as a core temperature below 90 ° to 92 ° F or 30 ° C, but that is another valueless criterion in a wilderness environment. (It is not valueless in a hospital environment.)
As core temperatures fall to about 90 ° F, shivering ceases, an ominous sign considered the first indication of severe hypothermia. Cold nerves no longer can carry the impulses that produce shivering, and cold muscles can no longer respond. It should sound an alarm for anyone who is not too hypothermic to respond, but if one member of a group has been so severely chilled, others may be also.
In the absence of other injuries or illnesses, (including drug ingestion) an individual's cerebral function is a reliable indicator of the depth of severe hypothermia. Intellect begins to decline with mild hypothermia, and individuals develop personality changes, usually increased irritability or silliness. As their temperature continues to fall, their intellectual function deteriorates and they become confused or disoriented. Severely hypothermic individuals often behave in odd ways. Typically they do no protect themselves from the cold. They do not put on hats or mittens, coats are not zipped, and extra clothing is left in packs. Urinating in their clothing is common. The British aptly call this condition the “stumbling slobbers.” Muscular control also fails. From simply stumbling or falling, severely hypothermic individuals become unable to walk without assistance. Eventually they cannot stand upright. Such disability is indicative of a temperature in the low 30's Celsius (mid 80's to low 90's Fahrenheit).
Finally profoundly hypothermic persons become stuporous and lapse into unconsciousness or severe hypothermia, which indicates an even lower body temperature and the need for aggressive rewarming. Individuals with a core temperature in the mid-eighties Fahrenheit (upper twenties Centigrade) are usually stuporous or comatose, but people vary widely in their response to low body temperatures. Some individuals remain surprisingly alert. One man with a measured rectal temperature of 86 ° F (30 0 C) was not only conscious but drove his car a number of miles on an Interstate Freeway to get to a hospital emergency room.
The heart rate and respirations are slowed by hypothermia. In individuals cold enough to be unconscious, pulse may be as quite slow, as low as twelve per minute. Respirations can be in the range of three per minute and so shallow they are undetectable. (Such slow rates can help confirm hypothermia and not some other disorder such as diabetic ketoacidosis as the cause of unconsciousness.)
Paradoxical undressing is a bizarre event that may occur just before an individual loses consciousness. Apparently the blood vessels in the skin dilate at this stage, producing such a sensation of warmth that individuals take off their clothes or climb out of their sleeping bags. As a result, they cool more rapidly and unconsciousness quickly follows.
Temperature Measurement
Measuring the temperature of a hypothermic individual in a wilderness situation is of little value. It has no role in diagnosing or treating mild hypothermia. A person who feels cold should be protected from the environment even if the body temperature is above 95 ° F (35 ° C.) The body temperatures of mildly hypothermic individuals play no role in decisions about rewarming.
Because people vary so much in their responses to lowering their body temperatures, measuring that temperature is of no value in distinguishing between mild and severe hypothermia. Cessation of shivering and mental incapacitation—in an appropriate situation—should be considered diagnostic of severe hypothermia.
The temperature of an unconscious hypothermic individual often cannot be measured. Tightly clinched jaws may prevent oral measurements. An unconscious hypothermia victim should not be moved to measure rectal temperature because that could precipitate ventricular fibrillation. Treatment should be based on the individual's condition, not his temperature. Ventricular Fibrillation
Severely hypothermic hearts have a pronounced tendency to fibrillate. (Death from hypothermia is usually cardiac: either ventricular fibrillation or asystole.) Fibrillation can be initiated by almost insignificant trauma, such as rolling an individual onto his back from his side, or transferring him from a stretcher to a hospital gurney. It often occurs without an identifiable initiating event. Anyone who is severely hypothermic, particularly persons cold enough to be stuporous or unconscious, must be protected from any bumps or jarring.
TREATMENT
If a mildly hypothermic individual is protected from the environment, the heat generated by his metabolism and by exercise can rewarm him. With severe hypothermia metabolism falls to very low levels, victims do not shiver, and they cannot exercise to generate heat. As a result, severely hypothermic individuals cannot rewarm themselves. They must be rewarmed by heat from an external source. When an individual who is severely hypothermic, particularly one who is stuporous, is first found, he should not be allowed to move. (Obviously, an individual so hypothermic he is comatose would not be able to move.) To minimize the risk of ventricular fibrillation, he should not be allowed to sit up, or even roll over unassisted. If the individual is in water, he should be passively removed as gently as possible. If he is covered with snow, the snow should be brushed away with as little movement of the person as possible. The person should be examined briefly to establish a diagnosis of hypothermia and to ensure injuries that need treatment are identified.
If shelter is available, the person should be moved into it—carried in a flat or prone position if possible to avoid pooling of blood in the lower portion of the body as the result of cardiovascular instability. If the shelter is a tent, it should be pitched at a site that minimizes the distance the individual must be transported. Snow caves are warmer. The shelter can be warmed with heated stones or similar objects. A gasoline stove should not be lighted inside the shelter unless it is well ventilated. Carbon monoxide accumulates in poorly ventilated structures when gasoline stoves are burning. If the structure is well ventilated and a stove is lit, the flame should be maintained at a high level. Low level flames, such as those used to simmer liquids, produce significantly more carbon monoxide.
An ideal way to heat a well-ventilated structure is to boil water. The increased humidity reduces heat loss by evaporation from the respiratory tract.
Wet clothing should be removed, cut off if necessary to avoid moving the subject. If no dry clothing is available, the wet clothing should be wrung out and replaced. Some fabrics, particularly wool, pile or fleece, and garments filled with man-made fibers such as polyesters, retain much of their insulating capabilities when wet. Saturated down-filled garments are almost worthless.
The hypothermic individual should be insulated from the ground, snow, or whatever surface he is lying on. Insulating pads are ideal, but clothing, leaves, grass, boughs from evergreen trees, newspaper—whatever is available—should be used.
The person must be protected from the environment—low temperature, wind, and wetness—as well as possible. Sleeping bags and insulating clothing are ideal. Anything available can be used. Constricting clothing, particularly boots, should be loosened—not removed—to reduce the risk of frostbite.
A fire can be built if materials are available (the person is below the tree line.) A reflector placed behind the fire directs more heat to the individual being treated.
Field Rewarming
With the exception of the Heatpac, no effective methods for field rewarming are available in most remote areas. Putting a severely hypothermic individual in a sleeping bag is not adequate because sleeping bags only insulate. They do not generate heat. Heated stones, hot water bottles, even another individual in the sleeping bag add only a minimal amount of heat, not nearly enough to rewarm a person who is cold enough to be unconscious. A rough estimate of the amount of heat required to rewarm an unconscious hypothermic individual can be gained from the following calculations. The human body has a specific heat of 0.83. To warm an 80 kg person 1ºC requires 66.4 kcal (80 X 0.83). To rewarm that individual from 80ºF (26.7ºC) to 90ºF (32.2ºC) (the approximate temperature at which he would begin shivering and his metabolism would have increased to a level at which his body could rewarm itself) would require 365 kcal (66.4 X 5.5). (In most situations, even inside a heated building, significantly more heat would be required because his body would still be losing heat to his environment. Few buildings are heated to temperatures higher than 70ºF [21ºC]) The maximum amount of heat that could be imparted to that theoretical individual by three liters of intravenous fluid heated to 104ºF (40ºC) is 39.9 kcal (3 X 13.3). The maximum amount of heat that could be imparted by a two-liter hot water bottle heated to 104ºF is 26.6 kcal (2 X 13.3). The maximum amount of heat that could be exchanged (heat gained and heat loss prevented) by heated, humidified aerosols at a body temperature of 80ºF is about 13 kcal an hour. However, much of the heat available from these sources is lost to the environment and actual heat transfer is much lower, even when adequate precautions are taken.
Heated aerosols have been advocated for rewarming, but such devices can transfer only a limited amount of heat. Such aerosols are less effective at altitudes where air is less dense. Most of the heat transfer results from condensation of water in the respiratory tract. If the inhaled gas, preferably oxygen, is not humidified to saturation, evaporation of water in the respiratory tract causes more heat to be lost than is imparted by the aerosol.
Heat transfer by such devices is low because the specific heat of air is so small and respiratory rates and volumes are also quite low. However, mechanically increasing respiratory exchange would increase carbon dioxide loss and produce alkalosis, which aggravates the tendency of a severely hypothermic heart to fibrillate.
Individuals employing heated, humidified aerosols have been impressed with their benefits, which appear greater than can be explained by the small heat exchange. Most of the heat transfer takes place in the upper airway, not in the lungs where it would warm the heart. Investigators have speculated that warming the base of the brain and the brain stem may be responsible for the salutary effects.
The Heatpac is an effective rewarming device. It contains charcoal that is slowly burned and a fan to disseminate the heat. It directly contributes 250 watts of heat per hour to the skin of the chest and requires only a D cell battery for the fan. It must be used in association with insulation such as a sleeping bag, and would be routinely carried only by rescue groups.
Transport
Because no effective method for field rewarming is available, most individuals with severe hypothermia must be evacuated. Even if the person can be rewarmed, twenty-four hours or more are required. Rescuers must have the equipment—tents, sleeping bags, and protective clothing—and the supplies—food and water—they need to survive unharmed for that period of time. Severe weather, such as a snowstorm with high winds, is a threat to rescuers as well as hypothermic subjects.
Hypothermic individuals must be transported without jarring to avoid ventricular fibrillation. Ground transport, whether hand carrying a litter, or mechanized conveyances, such as snow cats, snowmobiles, or four-wheel-drive automobiles, almost inevitably produce enough jolts to initiate ventricular fibrillation. Because hypothermia protects the brain from anoxia, severely hypothermic individuals can usually tolerate an hour without cardiac function without sustaining irreversible central nervous system damage. However, a person in a wilderness environment who is unconscious as the result of severe hypothermia, who cannot be evacuated by helicopter, and who is more than an hour by land from sophisticated rewarming may well be in a hopeless situation. The inevitable jarring associated with evacuation by litter or snow cat would induce ventricular fibrillation, but CPR cannot be performed during litter evacuation. (Intermittent CPR during litter transport may be beneficial.) Ventricular fibrillation in a hypothermic heart is highly resistant to electrical conversion. Even in surgical theaters, electrical shocks almost never restore a normal heartbeat until the heart has been rewarmed to about 90 ° F. (32.2ºC)
As an example, Tuckerman Ravine is a spring ski area on Mount Washington, the highest of New Hampshire's White Mountains. It attracts as many as 3,500 skiers a day during good weather even though no lifts have been built, accessing the Ravine requires a 2.9 mile hike, and the shelters are unheated. Many visitors are from urban areas and know little about avoiding cold injuries. Hypothermia is common. However, helicopter transport has not been available, and severely hypothermic individuals have had to be evacuated by ground transport, usually snowcats, over a trail and a rough fire road. (From the base of the trail hospital care is still a half-hour or more away.) For many years, no one sufficiently hypothermic to be unconscious survived. Severe hypothermia in a cold terrestrial environment may be untreatable. Rewarming an individual who is so hypothermic he is unconscious, without removing him from the cold environment, may be impossible because no effective field method for rewarming is available. Transporting an individual who is unconscious as the result of hypothermia by any means other than a helicopter without precipitating lethal ventricular fibrillation may be impossible.
CPR FOR SEVERE HYPOTHERMIA
CPR for a severely hypothermic individual should be initiated only in a few narrowly defined situations.
• CPR must not delay evacuation to a hospital. If the individual can be transported to a hospital in an hour or less, CPR should not be started, or should be started only after he is in the transport vehicle.
• CPR must be performed only in situations that are safe for the rescuers. The probability of success is sufficiently small that placing others at risk of an avalanche or similar hazard can not be justified.
• CPR must not be contraindicated by other conditions such as associated severe illness or injury, a non-compressible chest, or witnessed prolonged cardiac inactivity.
• CPR must not be initiated if the individual shows any signs of life. The subject must clearly not have any effective heart action.
CPR for a severely hypothermic person with a heartbeat would usually induce ventricular fibrillation. (Any heartbeat, regardless of how slow, is a contraindication to CPR.) A portable EKG monitor can demonstrate the absence of cardiac electrical activity, and should be carried by rescue teams. (A portable EKG cannot distinguish between fibrillation, absent heartbeat, and artifacts, but can identify QRS complexes.) If an EKG monitor is not available, at least one minute (preferably three) must be spent trying to palpate a carotid pulse or listening to the chest. The pulse may be as slow as twelve to fifteen; blood pressure usually is undetectable.
If CPR is attempted, it should be administered at one-half the usual rate. A severely hypothermic person needs less than one-half the oxygen a normothermic person needs, and his metabolism is so low he produces much less carbon dioxide. CPR at a normal rate could lead to excessive CO 2 loss and alkalosis that can precipitate fibrillation. In addition, hypothermic myocardium loses much of its compliance. The bradycardia of hypothermia is characterized by prolongation of systole. Slower CPR allows more time for the heart to fill between compressions.
A multicenter survey found that nine of twenty-seven individuals for whom CPR was started in the field survived, as did six of fourteen subjects for whom CPR was initiated in the emergency department. Successful resuscitation has been achieved after CPR as long as 220 and 390 minutes.
Unfortunately, no reliable indication that a hypothermic individual is irrecoverably dead is available. The adage “no one is dead until they are warm and dead” is highly respectable. Even apparent rigor mortis, dependent lividity, or dilated and fixed pupils are not indicators that CPR should be withheld. In several studies, individuals with serum potassium of more than 10 mmol/L have been unresuscitatable. Some centers use that value, an arterial pH lower than 6.5, and a core temperature below 12ºC (53.6ºF) as criteria that cardiopulmonary bypass rewarming should not be started. However, those parameters cannot be measured in a wilderness situation.
Even intermittent CPR has been shown to be beneficial. Therefore CPR probably should be started if the listed criteria can be met, if enough rescue personnel to carry out prolonged CPR without becoming exhausted are available, and if the subject is not in such a remote location that litter evacuation is impossible. Once CPR is initiated, deciding when to stop may be a difficult decision. Every situation is so different that no standards can be established. Continuing CPR for several hours does not appear unreasonable in view of the successes described above; neither does discontinuing it after that time.
DRUG AND FLUID THERAPY
The role of drugs in preventing fibrillation is unclear; perhaps no role exists. The effectiveness of drugs at low body temperatures has not been evaluated. (The limited use for this information does not repay the expense.) Evaluation of the activity at low body temperatures of widely used drugs is badly needed.
Parenteral administration of drugs may be impossible due to collapse of peripheral veins and limited circulation in muscle and subcutaneous tissue. Metabolism of drugs is greatly reduced in hypothermia, and the accumulation of unmetabolized drugs can lead to toxicity during rewarming.
Partial correction of the almost universal dehydration associated with severe hypothermia with intravenous fluids prior to moving an individual is desirable. Accessing a vein may be difficult when vasoconstriction has greatly reduced peripheral blood flow. Any solution that contains glucose would be satisfactory, although theoretically a hypothermic liver's ability to metabolize lactate would be impaired.
If the victim is breathing, oxygen administration at a generous flow rate prior to transport may reduce the risk of fibrillation. Dry oxygen is suitable if the victim can be transported to a hospital in minutes. Humidified oxygen heated to 40º to 45ºC (104º to 113ºF) could transfer a minimal amount of heat to the patient and help to limit any further temperature drop. IMMERSION HYPOTHERMIA
Hypothermia resulting from immersion in cold water differs from hypothermia developing on land, “terrestrial hypothermia,” in several significant aspects. Hypothermia almost always develops faster in water that on land, but it does not develop in minutes. A person not protected by special garments dropped into near freezing water cools rapidly enough to begin losing muscular coordination after about ten minutes. After twenty minutes coordination usually is so impaired the individual can no longer swim effectively. At that time the person may drown if he is not wearing a flotation device.
However, even in water at 32°F (0°C) about an hour is required for an adult to cool enough to develop lethal hypothermia. (Individuals who drown in cold water appear to aspirate or ventilate water and cool much faster.) The speed with which hypothermia develops depends on body size and characteristics. Small bodies cool faster because their body surface area is proportionately greater. Fat is insulating and fat persons cool more slowly than slim ones.
Movement increases heat loss in cold water. Cooling is slowed by holding as still as possible. If more than one subject is immersed, they can reduce heat loss by huddling together. Swimming should not be attempted unless the shore or a flotation device on which the individual can pull himself out of the water is quite close. Individuals removed from the water after a period of immersion tend to suffer a severe drop in blood pressure that often results in cardiac fibrillation or asystole and death. The cause has not been completely ascertained, but persons immersed in cold water are known to rapidly become dehydrated. Part of the cause is cold induced diuresis. Investigators have speculated that pressure imposed by the water in which the individual is immersed activates internal pressure receptors and also increases renal fluid loss. As a result, blood volume is significantly reduced. When removed from the water, subjects suffer a severe fall in blood pressure that frequently results in ventricular fibrillation or asystole.
This event has been labeled “circumrescue collapse.” To prevent such collapse, which often results in death, individuals rescued from cold water must be kept flat to avoid blood pooling in the lower extremities and limits the drop in blood pressure that occurs when they are removed from water.
HOSPITAL TREATMENT FOR HYPOTHERMIA
The coldest accidental hypothermia victim to be successfully resuscitated, an adult, had a core temperature of 13.7ºC, (56.5ºF) Neurosurgery patients have been routinely cooled to 9 ° C (48.2 ° F) and subsequently rewarmed with no significant brain damage, so rewarming of colder accidental hypothermia victims may be possible.
MONITORING TEMPERATURE
Within a hospital, temperature monitoring is best accomplished with an esophageal temperature probe inserted (gently) 24 cm below the larynx. The airway must be protected. Rectal temperature measurements are not reliable, particularly if the rectum contains feces that would warm much more slowly than the body core. Even if feces is not present, rectal temperatures may lag behind core temperatures after rewarming has started.
A tympanic membrane temperature probe would be reliable, but usually can only be used while the subject is unconscious. Investigative studies have found infrared tympanic membrane temperature measurements to be accurate only when the ear canal is plugged. Temperature probes in Foley catheters are not reliable. Central intravascular temperature probes are reliable, but must remain outside the right atrium to avoid precipitating cardiac disrhythmias. Pulmonary artery temperature probes carry the risk of producing a disrhythmia, and also can perforate the cold pulmonary artery.
average of 2.4ºC (4.3ºF) an hour. When combined with heated intravenous fluids and heated, humidified aerosols afterdrop, a decline in core temperature after rewarming is started, has been avoided. This device is effective even when used beneath bed sheets.
Immersion in a 40ºC (104ºF) water bath can also rewarm an individual effectively. Theoretically water baths pose the risk of dilating peripheral blood vessels more than central vessels, and dumping cold, acidotic blood into a cold heart, greatly increasing the risk of fibrillation. In addition, monitoring, treatment of other injuries, and resuscitation are much more difficult, and appropriate water baths may not be available.
Arteriovenous anstomoses rewarming consists of immersing the distal portions of the extremities (forearms and hand, calves and feet) in water heated to 44º to 45 ºC (111.2º to 113ºF). The heat opens arteriovenous anstomoses located 1 mm below the skin surface in the digits, which increases the flow of warmed venous blood directly to the heart. The effectiveness and safety of this technique have not been clearly demonstrated. Whether water hotter than is comfortable for a bath would superficially burn severely vasoconstricted skin, and whether the semi-upright position required for this procedure would precipitate hypotension in volume depleted individuals remains to be determined.
Heated Irrigation
For gastrointestinal irrigation, heated fluids can be introduced into the stomach or colon. To avoid fluid and electrolyte disturbances the fluids should be instilled into balloons. Aliquots of 200 to 300 ml of heated normal saline or Ringer's lactate are inserted into the balloon, allowed to remain about fifteen minutes, and are then replaced. Rewarming rates of 1º to 2ºC (1.8º to 3.6ºF) per hour have been achieved. The technique cannot be continued during CPR.
A modification of this technique consists of irrigating the urinary bladder, which would avoid fluid and electrolyte changes that occur with direct gastric irrigation.
Mediastinal irrigation has been used following thoracotomy to warm a fibrillating, hypothermic heart. Electrical defibrillation may not be possible until the heart is warmed to about 90ºF (32.2ºC). Pleural (thoracic) lavage requires the placement of one or two large-bore thoracostomy tubes in one or both pleural spaces. One tube is placed anteriorly for installation of heated fluid. A second tube, if utilized, is placed posteriorly for drainage. (If a second tube is not inserted, fluid instilled through the first tube must subsequently be aspirated.) Only limited clinical studies have been reported, but rewarming rates as high as 3º to 6ºC (5.4º to 10.8ºF) have been found. This technique appears most suitable for rewarming during CPR. A tube in the left pleural cavity may precipitate fibrillation. Pleural adhesions greatly interfere with rewarming by this technique.
For peritoneal lavage, a tube can be inserted (through a minilaparoscopy incision or over a guide wire previously inserted with a needle) and advanced into one of the pelvic gutters. Up to two liters (10 to 20 ml/kg) of heated normal saline, Ringer's lactate, or standard dextrose dialysate solution is infused, allowed to remain twenty to thirty minutes, and then aspirated. An exchange rate of 6 L/hr rewarms 1º to 3ºC (1.8º to 5.4ºF) an hour. If adhesions are present, peritoneal lavage is less effective, is associated with more complications, and may not be possible.
This technique has the advantage of providing some dialysis for drug overdose and rhabdomyolysis toxicity. Direct hepatic warming reactivates the metabolic functions of that organ. However, dialysis worsens preexisting hypokalemia, and electrolytes must be closely monitored.
Extracorporeal Blood Rewarming Cardiopulmonary bypass rewarming is a valuable rewarming technique, particularly when spontaneous cardiac activity is absent and circulation must be restored promptly to avoid neurologic damage or the heart must be rewarmed to allow electrical defibrillation. Femoral vein to femoral artery bypass that incorporates an oxygenator and heat exchanger is the technique of choice and avoids the complications of thoracotomy. A catheter inserted through the femoral vein into the right atrium to actively aspirate the venous return allows higher flow rates and improved decompression of the myocardium. Rewarming rates of 1º to 2ºC (1.8º to 2.6ºF) every three to five minutes or 9.5ºC (17.1ºF) per hour have been achieved. One of the disadvantages of bypass is the passage of fluid through endothelium damaged by hypothermia. Compartment pressures can be raised, and frostbite damage may be increased.
Arteriovenous rewarming is another extracorporeal technique. Catheters are inserted into a femoral vein and the contralateral femoral artery, and blood passes from the arterial catheter, through a heat exchanger, and back into the femoral vein. Cardiac function and a blood pressure of at least 60 mm Hg must be present.
For venovenous rewarming blood is removed from a vein, usually a large central vein, heated to 40º C (104ºF), and returned through another catheter. In a modification of this technique the blood is heparinized, pumped through a heat exchanger, treated with protamine to neutralize the heparin, and returned through a subclavian or jugular vein to preferentially warm the heart.
Routine hemodialysis using a heat exchanger is another technique for extracorporeal blood rewarming. It is particular useful for individuals with renal failure, electrolyte abnormalities, or intoxication with a dialyzable substence. TEMPERATURE “CORRECTION” OF BLOOD GAS DETERMINATIONS
Normal blood gas concentrations and pH levels at any other temperature are not the same as at 37ºC (98.6ºF). (The neutral pH of water is 7.0 at 25ºC but 6.8 at 37ºC.) Blood gas and pH determinations for hypothermic patients must not be “corrected” for temperature because tables of normal values for lower (or higher) body temperatures are almost never available. Evaluating the patient's status by comparing his “uncorrected” pH and blood gas levels with normal values at 37ºC is entirely appropriate, is much easier, and is far less likely to lead to erroneous therapy. (Blood samples are warmed to 37ºC in the laboratory before the determinations are made.)
COAGULOPATHIES IN HYPOTHERMIA
Coagulopathies should be anticipated in hypothermia because clotting components do not function normally at low temperatures. Prothrombin time (PT) and partial thromboplastin time (PTT) are both prolonged by hypothermia alone. (Evaluation of these functions may be difficult because blood specimens are warmed to 37ºC before the determinations are performed.) Heparinization is required for bypass if heparin-bonded tubing is not available. Disseminated intravascular coagulation (DIC) should be anticipated during and following that procedure. </ |
