Management of Cold Injuries
Linda Laskowski-Jones
Lawrence J. Jones
INTRODUCTION TO COLD INJURIES
Wilderness emergency medical services (WEMS) providers are exposed to a wide range of weather and environmental conditions. Extremes of cold, heat, rain, snow, humidity, wind, and solar energy can impact provider safety and performance, as well as the capability to care for team members and patients. For this reason, WEMS providers must have a keen awareness of the risk factors that lead to cold injuries. Equally important is developing the knowledge base to accurately discern clinical manifestations of the various cold injuries, so that appropriate medical management can be planned and executed. Failure to consider the physiologic consequences of cold injuries can lead to significant morbidity and mortality in a wilderness setting.
Having a fundamental understanding of the mechanisms and pathophysiology of cold injuries enables WEMS providers to better prepare for outdoor deployment, as well as employ best practice strategies for assuring personal safety and well-being. Scene safety is of significant importance and begins with appropriate trip preparation. The objective is to remain an asset in a rescue rather than become a casualty of the environment. Attention to weather forecasts, avalanche danger, thin ice, dangerous road and trail conditions, and the potential for falling tree limbs heavy with snow or ice are essential for planning risk mitigation strategies.
SCOPE OF DISCUSSION
Cold injuries encompass a spectrum of disorders from minor, superficial manifestations without long-term consequences, to profound physiologic derangements that can ultimately lead to permanent disability and death. This chapter covers commonly encountered conditions that stem from exposure to cold and presents out-of-hospital treatment approaches that are pertinent to an austere or wilderness environment. Perhaps most vital is information that will enable EMS providers to plan and take the necessary actions to prevent these conditions from occurring in themselves and others.
PRINCIPLES OF THERMOREGULATION AND HOMEOSTATIC PHYSIOLOGY
The human body strives to maintain a narrow window of body temperature, at or near 37°C (98.6°F), through a dynamic balance of heat production and heat loss. Enzymatic functions required for normal human physiology are temperature dependent. If body temperature drops below the normal set point, metabolism increases to generate more heat and peripheral blood vessels constrict to reduce heat loss at the body surface and in the extremities. Shivering is a highly effective involuntary mechanism for thermogenesis, or heat production, through muscular activity. Cognitively intact humans also make conscious choices to promote warmth such as adding clothing, increasing activity, seeking shelter and heat sources, and consuming food. When cognition is impaired—which, ironically, is an early effect of cold illness itself—the risk of heat loss due to maladaptive behaviors is significant.
Heat is lost from the body in four ways. The methods of heat transfer (loss) include conduction, convection, radiation, and evaporation.
Conduction is the heat transfer between two objects through direct contact. Body heat will move from a warmer object to a colder object. For example, sitting on a cold rock or lying on snow will result in conductive heat loss from the areas of the body that are in contact with the cold surface. Wet clothes or immersion in cold water exacerbates the heat transfer.
Convection is the process in which heat is carried away from the body by currents of air or water movement. The body warms the air or water in contact with the skin; the heat is carried away by the movement of the air or liquid and then is replaced by an unheated source. The heat loss is related to the relative temperature and the speed of the air or water. This is the reason why the outdoor temperature feels colder when the wind speed is high and is the basis for the concept of wind chill. The use of appropriate insulating clothing and windproof outerwear can decrease heat loss from convection.
Radiation is the loss of heat through infrared radiation. Heat from the warmer body radiates to the surrounding cooler environment. Radiant heat loss is much more significant in a colder environment because of the difference in temperature between the heat source and the surrounding environment. Insulating clothing, including wearing a hat, reduces radiant heat loss.
Evaporation is the energy lost through the process of converting liquid on the surface of the skin to a gas. Body heat is necessary to evaporate liquid (sweat) to a gas, thereby cooling the body and releasing that heat to the air. This process is much more effective in a dry environment, which is why it is so useful for cooling in the desert. Heat is also lost through warming and humidifying air in the lungs during the process of breathing or respiration. This form of heat loss can increase at high altitude because of the deeper and more rapid respiratory pattern that is common at elevation.
The pathophysiologic effects of cold injury are important to recognize to enable timely identification of cold-induced conditions. Cold that produces a lower than normal body temperature (hypothermia) acts as a central nervous system depressant. Effects worsen because body temperature continues to drop. Manifestations include impaired judgment, incoordination, slurred speech, and a decreased level of consciousness. In a wilderness environment, these pathophysiologic effects lead to maladaptive behaviors and a subsequent a high risk for injury.
The respiratory system initially responds to the cold through an increased respiratory rate. However, a progressive drop in body temperature leads to respiratory depression with a corresponding decrease in carbon dioxide production.1 Cold-induced pulmonary edema can develop. With severe cold, the thorax can become stiff and noncompliant, furthering respiratory failure.
The cardiovascular system is initially stimulated via the autonomic nervous system and responds with tachycardia, peripheral vasoconstriction to conserve heat, and blood pressure elevation. However, as body temperature drops, cardiac conduction slows and contractility decreases, producing bradycardia, decreased cardiac output, and hypotension. Because cold induces cardiac conduction system impairment and irritability, all manner of atrial and ventricular dysrhythmias can occur. A characteristic electrocardiographic finding is the appearance of an Osborn or J wave at the junction of the QRS complex and the ST segment when body temperature falls to 32.2°C (90°F) or below.1 These waves become more pronounced as body temperature drops further and can mimic the appearance of a myocardial infarction.
Peripheral vasoconstriction is a heat conservation response to shunt peripheral blood from the cold extremities to the core. Conversely, if an individual is rewarmed, particularly through heat application to the skin, peripheral vasodilation occurs. When the cold peripheral blood mixes with the warmer core blood as circulation is stimulated during active rewarming, core temperature afterdrop ensues. In afterdrop, body temperature will continue to drop for a period of time even after rewarming interventions because of this counter-current mixing effect of cold peripheral blood and warm core blood. Keep in mind that cardiac irritability increases with this drop in temperature.
The renal system is also impacted by peripheral vasoconstriction. Because vasoconstriction in the extremities pushes more blood volume into the core, there is greater blood flow to the kidneys. This increased volume load produces a significant diuresis of dilute urine, even in dehydrated individuals. Consuming alcohol can double the volume of this diuresis.1
Cold also impacts the ability of blood to clot normally. Blood clotting is a temperature-dependent process; the enzymes responsible for normal coagulation are able to function only across a very narrow temperature range. A core body temperature less than 36°C (96.8°F) produces a coagulopathy that is unresponsive to the administration of blood products.2 This coagulopathy is especially detrimental to individuals with injuries because they will suffer increased blood loss.
Cold can also affect the body at the tissue level, particularly in skin without adequate protection from cold and wet environmental conditions. Skin that is exposed to cold can suffer various forms of damage, including nonfreezing injuries as well as superficial to deep tissue freezing with ice crystal formation. Adequate blood circulation plays a key role in maintaining warmth. When the skin is exposed to temperatures below 15°C (59°F), vasoconstriction starts to occur, which can promote a further drop in skin temperature from the reduction in circulation.3 If the skin becomes numb, the individual can lose awareness of the cold sensation and fail to recognize the early stages of tissue freezing. Without timely preventive measures at this stage, blood circulation will be further reduced, the tissue temperature will continue to drop and, ultimately, the skin can take on the temperature of the environment.
The pathophysiology of frostbite includes extracellular and intracellular ice crystal formation, cellular dehydration and shrinkage, derangement of intracellular electrolyte concentrations, endothelial damage, vasoconstriction, thrombosis, ischemia, and ultimately tissue necrosis.4
Inflammatory mediators such as thromboxane, arachidonic acid, and prostaglandins play a significant role in tissue destruction. Both the degree of tissue damage and the prognosis for recovery are related to the depth of the cold injury.
EPIDEMIOLOGY
Cold injuries occur when heat production by the body or heat conservation is inadequate to compensate for heat loss. Freezing temperatures are not necessary for these conditions to develop. Cold injuries are possible even in moderate temperature climates. Predisposing environmental factors in a wilderness setting include an ambient temperature below 26.6°C (80°F), wet conditions, altitude, and wind chill. Wind chill, a function of wind velocity and air temperature, can exacerbate cold effects as a result of increased air movement across the body surface.
Physical factors in the at-risk individual include fatigue or exhaustion, inadequate clothing, wet clothing from perspiration, immersion, or weather conditions, poor conditioning, and insufficient food or fluid intake. Alcohol consumption and intoxication, illicit drug use, and certain medications (eg, antidepressants, opioids, and antianxiety agents) also promote cold injuries. Other significant precipitating factors include trauma, burns, dehydration, cardiovascular disease, infection, psychiatric illness, and any disease state that slows the metabolic rate or impairs thermoregulation such as diabetes hypothyroidism, adrenal insufficiency, and stroke. The use of nicotine products elevates the incidence of cold-induced tissue injury because of its vasoconstriction effects.
PREVENTION
Cold injuries are best prevented through appropriate physical preparation and trip planning. WEMS providers are in an ideal position to educate members of medical teams who provide wilderness rescue services as well as outdoor enthusiasts about the important of personal cold injury prevention.
Food
In order to function efficiently, the human body requires energy. Adequate food and fluid intake are essential to meet physiologic demands for heat production, especially in a cold environment. This is particularly important during EMS operations in a cold weather environment, where low temperature, wind, rain, snow, and the physical and mental stresses involved in an extended wilderness operation increase the body’s energy requirements. Although the WEMS provider must maintain a high level of physical fitness and good nutrition at all times, consideration must be given to providing supplemental nutrition for personal and patient consumption during field operations that may go on for hours to days or weeks.
For short-term operations, WEMS providers should carry enough food to sustain them for 24 hours or more. This food should be carried in the provider’s personal pack in the event that the rescue team is separated. Meal-type food that requires a fire, stove, or preparation is not recommended because the opportunity to prepare full meals during a rescue operation is seldom available, unless such meals are prepared at a command center during a truly extended operation lasting a day or more, such as a wildland fire operation or prolonged search.
Excellent food choices include nonperishable energy bars, dried fruit, nuts, peanut butter, chocolate, and other snack type foods that can be consumed easily while working or during brief breaks. Some groups will carry military “Meals Ready to Eat” (MREs) for extended operations. Food should be calorie/energy-dense, nutritionally sound, with an adequate mix of carbohydrates, fats, and protein, and should be nonperishable.
Hydration is equally important in a cold weather environment because dehydration can increase the risk of hypothermia and frostbite, and lead to poor physical and mental performance. The perception of thirst can decrease when skin temperature falls, leading to lower fluid intake. Exercise at high altitude and in cold dry air increases respiratory water loss. Increased diuresis occurs due to peripheral vasoconstriction, pushing fluid to the central core. In addition, some medications, such as acetazolamide (Diamox) for altitude illness prophylaxis, will increase diuresis.
An adequate water supply must be carried or procured by the WEMS provider. Typical water requirements for an adult are about 2.5 L/day but can increase to 4 to 5 L/day or more during strenuous activity in a cold dry environment. Avoid eating snow as a hydration method. Snow can cause freezing injuries to the mouth, lower body temperature, and provides very little usable water. A means of melting snow such as a portable stove is essential in a snowy environment. Alternatively, where water is available from a stream or other nonfrozen source, a means of purifying water must be used, such as boiling, filtering, chemical, or UV methods. See Chapter 7 (WEMS Equipment) for further information on water purification equipment.
Clothing
Clothing choices for cold temperatures should allow wicking and evaporation of perspiration as well as insulation. The old adage “cotton kills” is true in a cold environment: When cotton clothing gets wet, it holds the moisture, takes a long time to dry, and loses its insulating ability. Synthetic materials including technical fabrics that are breathable and enable wicking of moisture should be sought for the base layer of clothing. For those who prefer natural fabrics, wool is a good choice over cotton. It maintains its insulating ability even when wet.
Layering clothing is an excellent strategy to prevent the loss of body heat to the environment. Heavier weight wool or synthetic fabric, including inexpensive polyester fleece, can be worn over the base layer that is next to the skin to retain warmth. Colder temperatures call for more layers. The outer layer should be a fabric that is both breathable and wind- and waterproof. This layer prevents the heat that is close to the body from being lost to the environment, blocks the chilling effects of the wind, and keeps the individual dry. During physical exertion, layers can be removed to prevent overheating, and then added when the activity level decreases or stops.
Footwear
For those traveling on foot, it is vitally important to keep feet warm and dry to prevent cold injuries such as trench foot. Select footwear that fits well and is nonconstricting. If vapor barrier (eg, rubberized or impermeable) boots are used, special attention must be directed at preventing the accumulation of sweat.5 Inspect the feet frequently and allow them to air dry.
Cotton socks are not good footwear choices for outdoor travel because they retain moisture to a greater degree than socks made from synthetic material or wool. Avoid wearing multiple layers of socks, which can actually decrease circulation to the feet and can increase the risk of cold injury. More than two layers of socks are rarely necessary: a synthetic liner and a synthetic or wool sock. Many individuals prefer a single sock layer made of synthetic knit or wool fabric. Change wet socks 2 to 3 times/day as needed. If necessary, employ methods to decrease perspiration of the feet such as applying antiperspirants.6 Maintain activity to preserve blood circulation to the feet.
Skin Protection
Regularly applying a hydrating lotion or cream moisturizer to exposed areas of skin is beneficial to keep skin supple and to prevent skin from becoming dry and forming cracks or fissures, which can predispose individuals to pain, open wounds, and infection. Moisturizers are especially recommended for those who are prone to chilblains or pernio as a possible prevention strategy.
Shelter
Shelter from the elements is a primary survival strategy, particularly in a cold environment. Although the typical goal is rapid evacuation of sick in injured patients in most WEMS operations, wilderness providers may wind up in a situation that requires them to shelter in place for some period of time because of severe weather that makes travel even more treacherous. Survival depends on the ability of WEMS providers to adequately protect themselves and their patients from wind, rain, snow, and low temperatures. Staying dry and getting out of the wind increases the insulating ability of clothing and the effectiveness of preserving body heat.
Shelters in a WEMS operation can range from very simple (heavy-walled plastic trash bag), to commercial (a tarp or mountaineering tent), to an improvised structure (debris shelter, snow cave). The choice of shelter may be mission-specific and depends on the scope of the operation, expected time in the field, and the probability of needing to make a safe camp for 1 or more days. Are rescuers expecting to care for a patient for an extended period of time, or will they carry only a simple emergency shelter for the unexpected night out?
There are several types of shelters to consider: a heavy-duty 3- to-4-mm plastic trash bag pulled over a person’s head makes an instant wind and waterproof emergency shelter. Cut a small opening big enough to fit the face through near the bottom of the bag. A second bag can be pulled up over the legs for a complete emergency shelter. Make sure that the upper bag overlaps outside the bottom bag so precipitation will not run inside.
A space blanket is made of a lightweight Mylar material that reflects body heat. Although it fits into a pocket, it is very prone to tearing and puncturing. A bivouac (bivy) sack is smaller and lighter than a full tent, and is generally designed for one person. It can be used with a sleeping bag inside and makes a good emergency shelter. Lightweight and inexpensive, tarpaulins (tarps) come in a variety of sizes and have grommets to allow tying to trees and other natural features. Tarps can be rigged in a number of ways such as an A-frame or diamond shape. Depending on how they are set up, tarps can offer some protection from precipitation and wind. Tarps can also be a useful addition to other shelters such as snow trenches, tree wells, and debris shelters.
Heavier, but offering more substantial protection than a tarp, mountaineering tents that are easy to set-up make a superior choice for extended field operations. Many are designed for high winds and adverse weather conditions. Tents come in a variety of sizes and features for sheltering an overnight rescue group and patient.
In an emergency when other forms of shelter are limited or not available, natural shelters such as caves and rock overhangs can provide limited protection from precipitation and wind. They can be improved upon by using other natural materials (eg, tree branches and rocks) to increase the windbreak and reflect the heat of a fire. Another option is to build a debris shelter. These are constructed from natural materials, commonly using a fallen tree as a ridge pole, and stacking branches against the ridgepole to create an A-frame shape, then covering it with tree branches, leaves, grasses, and other debris to provide some insulation. Debris shelters can be time-consuming and labor intensive to construct.
In a snow-covered landscape, snow caves can be excavated into deep snow on the side of a hill, or into a large pile of snow (ie, a quinzee) as an emergency shelter. They are more durable
structures and are useful if a situation calls for sheltering in place for several days to wait out severe weather conditions. Snow caves do take a considerable amount of time and energy to construct; the builders will get wet in the process.
structures and are useful if a situation calls for sheltering in place for several days to wait out severe weather conditions. Snow caves do take a considerable amount of time and energy to construct; the builders will get wet in the process.
If a snow cave or quinzee is not feasible given the time and energy requirements, one of the simplest and quickest snow shelters to construct is a snow trench. Excavate a trench in the snow approximately 3 ft deep, 7 ft long and wide enough for the occupants. Place skis and ski poles across the top and cover with a tarp. Weight the tarp edges with snow and throw snow onto of the tarp for extra insulation. Simpler yet is the use of a tree well. Tree wells can be natural emergency shelters that take advantage of the void space in the snow found beneath trees. Enhanced with skis and a tarp and some insulating material on the ground, a tree well can quickly offer considerable wind protection.
An important factor to consider in selection of an appropriate shelter includes the time it takes to construct. An improvised tarp shelter can be rigged in minutes but may offer minimal protection, whereas a snow cave or quinzee hut will provide much better protection from the elements but requires several hours to build.
The number of people using the shelter will determine the size. Avoid the temptation to build a huge shelter because it will take much more time and energy to construct, and the shelter will not be warmed by body heat if it is too large. All shelters should include a means of insulating the individual from the cold ground to reduce conductive heat loss. Foam sleeping pads, backpacks, pine boughs, and dry leaves and grasses will offer additional insulation between the body and a cold- or snow-covered surface. The use of portable stoves or fires inside a shelter is not recommended because of the asphyxiation risk; however, if their use is unavoidable, make sure to provide adequate ventilation.
Whether they are planned shelters or emergency shelters, locate the shelter to take advantage of terrain features as natural wind blocks whenever possible. Ridges, stands of trees, and rock outcrops may provide protection from prevailing wind. Warm air rises—hillsides may be partially protected from the wind. Avoid the very bottom of a valley because cold air sinks, and avoid the very top of the hill or ridge because they tend to be windiest. Avoid sheltering beneath dead or unstable trees, loose rock, cliffs and steep drops, potential avalanche areas or other hazardous locations, and avoid creek beds, where flash floods are possible. Whenever possible, establish the shelter in daylight hours so hazards can be better identified.
HYPOTHERMIA
Definition
Hypothermia represents a core body temperature less than the normal range (centered at 37°C or 98.6°F and ranging down to 35°C or 95°F). The etiology can be medically induced or accidental. The focus of this chapter is on accidental hypothermia that occurs in a wilderness environment when there is a 2 degree or greater decrease in the normal core body temperature as a result of exposure to the elements. Cold and wet conditions are the predominant precipitating factors. Accidental hypothermia can occur during any season of the year, even in temperate climates when environmental conditions pose risks. The Wilderness Medical Society defines hypothermia as “an unintentional drop in core temperature to 35°C or below.”7
Accidental hypothermia may be primary or secondary in nature. Primary hypothermia can arise as the principle clinical problem in a wilderness environment from an imbalance between heat production and heat loss, or secondary to a medical disorder that predisposes an individual to the condition, such as burn wounds, trauma, alcohol intoxication, or a systemic disease state. Another subclassification of accidental hypothermia is immersion hypothermia, which results when patients are immersed in cold water and subsequently experience a drop in body temperature, as opposed to nonimmersion hypothermia that is unrelated to cold water exposure.
Identification
The medical literature varies widely in regard to the specific temperature ranges that characterize the severity levels of hypothermia, with many sources listing subtle differences in the numeric values. For the purpose of this chapter, hypothermia will be categorized as defined by the Wilderness Medical Society: mild (35°C to 32°C [95°F to 89.6°F]), moderate (32°C to 28°C [89.6°F to 82.4°F]), and severe or profound (less than 28°C [82.4°F]).7
Each severity level of hypothermia is typically associated with clinical manifestations that correspond with the pathophysiologic changes that occur in the body due to cold injury. Not all patients will exhibit these manifestations in a predictable manner, however. There may be significant variation from person to person. It is also critical to recognize that there are multiple disease and injury entities that may mimic the clinical presentation of hypothermia such as head injury, stroke, alcoholism, hypothyroidism, and hypoglycemia. Any of these conditions can occur together with hypothermia. For that reason, maintain a high index of suspicion that the patient may be experiencing multiple disorders and require care beyond that which is indicated for hypothermia alone.
Prior to the development of true accidental hypothermia at the threshold of 35°C (95°F), an individual can experience cold stress from the effects of cold temperature on the skin and may start to shiver. Of note, this person will have a body temperature of 35°C (95°F) to 37°C (98.6°F) and a normal mental status.7 Early recognition is essential: If heat production
remains inadequate and heat loss continues, accidental hypothermia will ensue.
remains inadequate and heat loss continues, accidental hypothermia will ensue.
Mild hypothermia (35°C [95°F] to 32°C [89.6°F]) results in autonomic nervous system stimulation along with depression of the central nervous system.1 Patients can often be identified at this stage when their skin is cold and pale and they exhibit the “umbles”: They stumble, mumble, fumble, and grumble. In a wilderness environment, these individuals are more likely to lose their footing when walking or climbing, become difficult to understand when speaking because of trouble forming words, be unable to perform fine motor functions with their hands (such as closing a zipper), and show evidence of mental status changes. These behaviors include becoming inappropriately argumentative, apathetic, moody, or flat in affect. The key take-away point is that these individuals are at high risk of injury due to significantly impaired coordination as well as the loss of intact judgment. As temperature continues to drop, the individual can become incapacitated and unable to function.
Shivering is pronounced and uncontrollable in mild hypothermia; it serves as an important compensatory mechanism that generates heat through muscle activity. Shivering requires calories to burn for heat production. Inadequate nutrition will negatively impact this essential physiologic response and will promote worsening of the hypothermic condition.
Vital signs in mild hypothermia reflect increased levels of circulating catecholamines, which produce tachycardia, elevated blood pressure, and increased respiratory rate. Because cold-induced peripheral vasoconstriction shunts blood to the core of the body, the kidneys respond by producing large quantities of dilute urine. Patients subsequently experience a cold-induced diuresis that will result in frequent urination, worsen any preexisting dehydration, and promote further cold injury.
Moderate hypothermia (32°C [89.6°F] to 28°C [82.4°F]) is characterized by cardiopulmonary depression and further central nervous system impairment. Cellular energy stores are depleted. Patients develop hypotension, bradycardia, and have a slow respiratory rate. Atrial and ventricular cardiac dysrhythmias are common and can be further exacerbated by rough handling. Shivering may still occur, but ultimately stops altogether. Speech becomes slurred to incomprehensible, and the level of consciousness declines to unresponsiveness as the condition worsens.
It is at this stage of hypothermia that the unusual phenomenon of paradoxical undressing can occur: Patients may shed their clothing in a profoundly cold environment because they suddenly feel very hot. Although this action seems completely counter to what a hypothermic patient should do, this phenomenon is best explained by understanding that the sensation of hot or cold is related to the perception of temperature at the skin surface.8 Because cellular energy is required to maintain peripheral vasoconstriction in hypothermia, when the body no longer has enough energy to maintain peripheral vasoconstriction, widespread peripheral vasodilation occurs.8 Therefore, the individual with an impaired mental status experiences a rush of warmth as blood flows into the cold extremities. Because the person is unable to process what is happening and respond in a logical way to this critical survival situation, he or she removes articles of clothing and casts them aside in an attempt to cool down. The resulting clothing trail serves as an ominous preterminal sign for rescue parties who are searching for a missing person in cold weather. Rescuers have found such missing persons naked in the snow with a predictably poor outcome for survival.
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