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Similar to heat illness, cold illness represents a significant public health issue that is often preventable. In the United States, cold exposure leads to a total of 16,911 deaths from 1999 to 2011, which is an average of 1301 per year. 2010 saw the highest yearly total of hypothermia-related deaths with 1536 deaths.28 Hypothermia is defined as an unintentional drop in core body temperature of <35°C. Hypothermia can be classified into three categories: mild (core body temperature 35–32°C), moderate (core body temperature 32–30°C), and severe (core body temperature <30°C).29 A low body temperature may develop as a result of exposure to low ambient temperature or may be secondary to a disease process (Table 139-4).
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Once again, similar to heat-related illness, pediatric patients are included in the group that is at greater risk for having cold illness. Age is an important factor in determining the susceptibility to hypothermia and the morbidity and mortality associated with it. Neonates are particularly at high risk for developing hypothermia due to their large surface area compared with body mass and the relative paucity of subcutaneous tissue.30 Neonates have also been postulated to have poorly developed thermoregulatory systems. The newly born is particularly susceptible to hypothermia due to the evaporation of amniotic fluid from the skin. Throughout infancy and young childhood, children remain susceptible to hypothermia with exposure to cold, although less so with advancing age. Most cases of accidental hypothermia in older children and adolescents are associated with submersion events in cold water.31 Recently, there has been an increase in exposure-related hypothermia in older children and adolescents, which is believed to be associated with the increased popularity of winter sports.31 The greater the environmental stress, the higher the potential for development of hypothermia.29 Inexperience and lack of caution, which are common among adolescents, increase the likelihood of their becoming victims of hypothermia. Appropriate supervision, education, and adequate preparation have the potential to positively affect the number of hypothermia cases.
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Thermoregulatory measures control the normal body's temperature over a narrow range usually. The primary mechanisms of heat loss related to cold illness are outlined in Table 139-5.
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Exposure to cold stimulates skin receptors, resulting in peripheral vasoconstriction and conservation of heat. As the temperature of the blood declines, the preoptic anterior hypothalamus is stimulated to initiate mechanisms to generate more heat. Shivering is one of the initial mechanisms appreciated, but metabolic and endocrine means of thermogenesis, primarily mediated by thyroid hormones and adrenal axis, also play a role (Fig. 139-2).32
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Cold stress potentially affects all organ systems (Fig. 139-3). The most prominent effects are seen in the cardiovascular, central nervous, respiratory, renal, and gastrointestinal systems.29,33 The effects of hypothermia on the cardiovascular system are often the most noticeable. After an initial tachycardia, the heart rate falls as temperature falls. Mean arterial pressure also falls progressively, along with cardiac output.29,34,35 Atrial dysrhythmias commonly appear at temperatures below 32°C but are usually considered innocent because the ventricular response is slow. Ventricular ectopy is seen with temperatures <30°C and the risk of ventricular fibrillation is greatly increased. Electrocardiogram may demonstrate J waves (Osborn waves) at the junction of the QRS complex and ST segment (Fig. 139-4).29,34 Although considered pathognomonic for hypothermia, the Osborn or J wave has no prognostic or predictive value in cases of hypothermia.29 As hypothermia worsens, the cardiovascular system continues to be adversely affected and at 19°C asystole can occur.
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Central nervous system is also greatly affected by cold stress. As the core body temperature drops below 33°C, the patient becomes confused and ataxic. Brain enzymes are less functional with declining temperature, resulting in a linear decrease in cerebral metabolism. Cerebral perfusion is maintained until autoregulation fails at approximately 25°C. At 20°C, the electroencephalogram shows a flat line.
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While hypothermia may affect the respiratory system through influencing the central nervous system, it can also directly impact the pulmonary system. Cold initially stimulates the respiratory drive, but as temperature falls, a progressive decline in minute ventilation supervenes. Bronchorrhea, because of the local effect of cold air, can be severe, simulating pulmonary edema29,35
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In addition to cardiac, central nervous system, and pulmonary injury, cold illness also affects other organ systems. Vasoconstriction in the extremities results in an initial central hypervolemia. The kidney responds rapidly, producing a large “cold diuresis” of dilute glomerular filtrate. Ethanol and immersion in cold water increase this early diuresis. Gastrointestinal motility is decreased and gastric dilatation, ileus, constipation, and poor rectal tone commonly result. Inflammatory changes in the pancreas are also often found. When managing a patient with hypothermia, all organ systems need to be considered and evaluated.
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The diagnosis of hypothermia may be obvious when a history of exposure is known; however, hypothermia may develop insidiously because of causes other than environmental exposure or to exposure in relatively warm environments. Unfortunately, the hypothermic patient is often not able to give an adequate history, so other sources of information should be sought. Attempts should be made to obtain a thorough history of the exposure, including the circumstances, location, ambient temperature, the length of exposure, and presence or absence of submersion or wet skin. If significant exposure is unlikely, an extensive history is required to search for clues for other causes of hypothermia (Table 139-4).
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The core temperature defines the presence and severity of hypothermia. Most thermometers for routine clinical use will record a temperature down to only 34.4°C.35 Special glass or electronic thermometers are required for accurate measurement of temperatures in hypothermic patients. Continuous monitoring of rectal, esophageal, or tympanic temperature is very useful during treatment.29,34
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The physical examination will help reveal the severity of the hypothermia. The key physical findings in patients with hypothermia, and the temperature level at which they occur, are depicted in Table 139-6. The skin is typically cold, firm, pale, or mottled. Localized damage due to frostbite may be present.36 Shivering will often be present in the older child or adolescent but ceases by the time the temperature reaches 31°C. Shivering can increase heat production by four to five times. Shivering is absent in neonates, making them entirely dependent on care from others, vasoconstriction, and heat generated by lipolysis.32 Behavioral responses, such as seeking a warm environment or putting on protective clothing, are major preventive mechanisms that are entirely absent in the infant.
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In older children and adolescents, the affects on the central nervous system become a prominent clinical feature. Early neurologic signs of hypothermia include confusion, apathy, poor judgment, slurred speech, and ataxia. Focal neurologic defects may also be present. Coma usually supervenes by the time the temperature reaches 27°C. Due to the significant central nervous system effects, other pathology can be masked. It is imperative to actively search for signs of trauma, toxic ingestion, and endocrine disturbance. Serial physical examinations repeated at intervals during treatment can aid in the discovery of clues to problems that were initially masked by the hypothermia.
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Since hypothermia impacts all organ systems, a high suspicion should be maintained for end-organ injury. Often extensive diagnostic testing may be indicated (Table 139-7).30 In hypothermia, there is decreased tissue perfusion and the oxyhemoglobin dissociation curve is shifted to the left. Arterial blood gases are useful for the evaluation of oxygenation, ventilation, and acid–base status. Although some authorities have recommended correcting blood gas results for body temperature, correction can lead to false elevation of Po2. Metabolic acidosis is usually present and the buffering capacity of the blood is markedly reduced.
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Typically, the hematocrit increases 2% for each 1°C drop in temperature. A baseline complete blood count is useful to help assess trends. In addition, the hemoglobin level may be decreased due to blood loss or chronic illness. Occult trauma should always be considered as a cause of low hemoglobin levels. Hypothermia can also lead to alterations in the white blood count. It may be reduced by sequestration and bone marrow depression. Even in the presence of severe infection, leukocytosis may not be seen.
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Basic chemistry panels can also be useful in the evaluation of the patient with cold-related injury. Renal function tests are useful for establishing baseline renal function but are poor indicators of fluid status in hypothermia. During continued care, acute tubular necrosis may develop after rewarming. Serum glucose values may be elevated due to catecholamine effect and insulin inactivity below 30°C. Persistently elevated levels suggest pancreatitis or diabetic ketoacidosis. Hypoglycemia may develop due to inadequate glycogen stores in neonates and malnourished children.
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Considering that hypothermia can also lead to clotting dysfunction, platelet count, coagulation studies, and fibrinogen level can be helpful in moderate-to-severe hypothermia. Cold injury induces thrombocytopenia and prolongs clotting times. Persistent changes after rewarming suggest the development of disseminated intravascular coagulation.
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In addition, numerous other studies may prove to be beneficial. Amylase and lipase may be elevated and, because of the unreliability of the abdominal examination, may be the only indicators of the development of pancreatitis, which is associated with poor outcomes in the setting of hypothermia. Urinalysis will demonstrate a low specific gravity due to cold diuresis. Toxicologic studies are frequently indicated to detect causative or predisposing agents. Cultures of body fluids are indicated in all cases of moderate-to-severe hypothermia. Sepsis is a common cause of hypothermia in infants and may also develop as a complication of hypothermia due to other causes.
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Radiologic imaging should include a chest radiograph in all cases of significant hypothermia. Pulmonary edema may develop during rewarming and aspiration is relatively common. Cervical spine films may be indicated if there is suspicion of trauma. Cranial computed tomographic scanning may be indicated in the setting of trauma or to search for other etiologic factors, especially when mental status does not clear with rewarming. An electrocardiogram is indicated for all patients with a core temperature <32°C to detect dysrhythmias or evidence of myocardial ischemia. The J wave (Osborn wave) (Fig. 139-4) is usually seen when the temperature falls below 32°C.
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While the diagnosis of cold injury can be difficult to make in the emergency department, it requires a high index of suspicion to diagnose hypothermia in the field. Prehospital providers should presume hypothermia in situations where exposure, even at moderate temperatures, has occurred.37
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One particular instance that requires consideration is the transport of the newly born or the neonate. Great caution is needed to prevent hypothermia and to initiate its early treatment in neonates. The neonate should immediately be dried and wrapped in warm blankets. Alternatively, after drying, the neonate can be placed against the body of the mother and then covered.
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For other potentially hypothermic patients, simple strategies can be very helpful. All wet clothing should be removed, and dry blankets applied. Resuscitation fluids should be warmed whenever possible. While supporting ventilation, heated, humidified oxygen, if available, may minimize further core temperature loss and significantly add to other rewarming techniques. Cardiac monitoring is indicated to detect dysrhythmias. Because pulse and respiratory rates may be very slow in severe hypothermia, assessment for breathing and pulselessness is carried out over a 30- to 45-second period. If there is no pulse, chest compressions are started immediately. Basic life support measures should not be withheld while the patient is being rewarmed.
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Emergency Department Management
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The management of previously healthy patients who is only mildly hypothermic (35–33°C) is generally straightforward. Passive external rewarming with placing the patient in a dry, warm environment and providing dry insulating coverings is usually sufficient to safely reheat the patient. For these patients, if there is no obvious environmental exposure, it is important to consider other causes of hypothermia, particularly infectious etiologies in the very young or immunocompromised.
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For the patient with moderate or severe hypothermia, the initial approach to the patient is the same as that for any seriously ill patient, with evaluation and stabilization of the airway, breathing, and circulation before moving to other aspects of treatment.34 The path of further management of hypothermia is directed by the results of the assessment of the patient's airway, breathing, and circulation (Fig. 139-5). Since the spectrum of illness ranges from mild symptoms to profound illness, the approach will need to be tailored for the individual. Patients without protective airway reflexes require endotracheal intubation, which should not be withheld for fear of precipitating ventricular fibrillation. As those patients with hypothermia are prone to arrhythmia, cardiac monitoring should be initiated and vascular access obtained. In addition, continuous monitoring of temperature is very helpful during treatment. A urinary catheter is also necessary to adequately monitor output.
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In the unfortunate circumstance of a patient arriving without pulses, cardiopulmonary resuscitation should be started. Focus should be on ensuring appropriate and adequate chest compressions. The cold myocardium may be resistant to defibrillation and to pharmacologic agents. During hypothermia, protein binding of drugs is increased, and most drugs will be ineffective in normal doses. Pharmacologic attempts to alter the pulse or blood pressure are to be avoided because drugs can accumulate in the peripheral circulation and subsequently lead to toxicity as rewarming occurs. If the initial, three defibrillation attempts fail to establish a rhythm, CPR should be resumed with minimal interruptions. Additional defibrillation attempts are unlikely to be successful until the patient is rewarmed to 30°C. Many patients spontaneously convert to an organized rhythm once achieving a core temperature of 32 to 35°C. It is important to remember that infants and children who have sustained prolonged hypothermic cardiac arrest have recovered with little or no neurologic impairment. In general, resuscitative efforts should continue until the hypothermic child is warmed to at least 30°C.29,30,35 Unfortunately there are no consistently reliable prognostic laboratory values that can help guide the reasonable duration of resuscitative measures.
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While prompt basic life-supporting techniques are paramount, moderate-to-severe cases of hypothermia require active rewarming to be initiated as soon as possible. Heated, humidified oxygen and intravenous fluids warmed to 40°C have been shown to be safe and efficacious and are used from the onset of therapy. For neonates and infants, using radiant warmers prevents further heat loss. The older child and adolescent should be covered with dry, warm blankets. Attention should be paid to trying to limit heat loss from the head and often a warm blanket covering the scalp will help the rewarming process while not hindering resuscitation. Forced air rewarming systems are also very useful and have been described as effective, noninvasive, and not associated with an after-drop phenomenon. Another noninvasive means of rewarming involves applying subatmospheric pressure to the hand and forearm, along with heat. This technique is described as immediately resulting in subcutaneous vasodilatation and rapid heat acquisition, rapidly eliminating shivering, and subjective improvement. Further studies of these techniques are needed.29,38 Hot packs and electric blankets can be dangerous, because cold, vasoconstricted skin is very susceptible to thermal injury.
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Simple rewarming measures like warm intravenous fluids, blankets, and warm, humidified oxygen are all effective strategies, but active core rewarming is used in most cases of moderate-to-severe hypothermia.24,29,30 Active core rewarming can be accomplished by irrigation of the stomach, bladder, and colon. Heat transfer by these techniques is somewhat limited. Rapid rewarming by irrigation of the mediastinum or pleural cavity via a thoracostomy tube is effective but very invasive. Peritoneal lavage with heated fluid (40–45°C) is probably a more effective method of active core rewarming. Another viable option for rewarming in severe conditions is extracorporeal rewarming, which is the most rapid method. Extracorporeal rewarming is indicated in patients with hypothermic cardiac arrest and with patients who present with completely frozen extremities. Using this technique, young, otherwise healthy people have survived deep hypothermia with no or minimal cerebral impairment.29,30,34
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A special consideration in pediatric hypothermia is submersion events in cold water. (See Chapter 136.)
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Another special consideration for cold-related injury is frostbite, which may occur in conjunction with hypothermia or as an isolated localized injury. Once primarily a military problem, frostbite is now more prevalent in the civilian population as a result of occupational and recreational exposures. In frostbite, the body parts most susceptible are those areas farthest from the body's core; the earlobes, nose, hands, and feet. Predisposing factors to the development of frostbite include environmental, individual, behavioral, and occasion-linked factors (Fig. 139-6).36,39
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The pathophysiology of frostbite includes three distinct pathways of tissue freezing:1 extracellular formation of ice crystals,2 hypoxia secondary to cold-induced local vasoconstriction, and3 release of inflammatory mediators such as prostaglandins PGF2 and thromboxane A2. All pathways can occur simultaneously thereby intensifying tissue damage. Cold exposure also increases blood viscosity, promotes vasospasm, and precipitates microthrombus formation. The release of inflammatory mediators has been shown to peak during the rewarming process and cycles of recurrent freezing and rewarming further increase the tissue levels.29,40
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Classification of Frostbite Injuries
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The clinical signs and symptoms of frostbite differ according to the depth of injury. Traditionally, frostbite has been classified by degree. First-degree frostbite is limited to the superficial epidermis. Erythema and edema occur and resolve without sequelae. Second-degree frostbite results in deeper epidermal involvement. Third-degree injury consists of full thickness skin injury. Recent studies have suggested that frostbite may be more usefully classified as superficial or deep (Table 139-8).29,30,36,39 With superficial frostbite, the rewarmed skin develops clear blisters in contrast to the hemorrhagic blisters seen upon rewarming with deep frostbite (Figs. 139-7 and 139-8).
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The treatment of frostbite is rapid rewarming. The preferred initial technique is immersion of the affected part in circulating warm water (40–42°C). Narcotic analgesics are often required to control pain duringrewarming. A number of adjunctive therapies, such as vasodilators, hyperbaric oxygen, and sympathectomy, have been recommended, but definitive evidence for their effectiveness is still lacking. Recently, the utility of thrombolytic therapy has been reported.41,42 While this is as of yet a novel approach, advances in imaging and interventional radiology have led to this technique being potentially viable on a larger scale.41
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It is very difficult to determine tissue viability after significant hypothermic injury. Debridement of nonviable tissue is traditionally delayed for several days to weeks to preserve as much tissue as possible; however, recent improvements in radiologic assessment of tissue viability have led to the possibility of earlier surgery and more rapid rehabilitation times.29,36 Topical aloe vera cream and ibuprofen may be used for outpatient treatment after rewarming. More extensively injured patients will require continued inpatient treatment and pain control.
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Rewarmed body parts are highly susceptible to refreezing, leading to even greater tissue loss. If exposure is anticipated, it is better not to rewarm the tissue.
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Patients with mild accidental hypothermia (35–32°C) may be rewarmed and discharged to a safe environment if there is no evidence of underlying disease.
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Most patients with hypothermia will require hospitalization for further treatment and evaluation. Those patients with a core temperature of <32°C will require cardiac monitoring. Profoundly, hypothermic patients with cardiac arrest and those with completely frozen extremities are candidates for extracorporeal rewarming and may require transfer to a tertiary care facility with this capability.
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Proper preparation and planning in conjunction with adequate supervision can prevent most injuries. The first step is to be aware of cold risks. While little true acclimatization to the cold probably occurs, there is some value to practicing tasks that are to be performed in the cold and building up endurance before beginning a major cold activity. Good nutrition and hydration are helpful in resisting the stresses of cold weather activity. Sufficient clothing, either in layers or made of good insulating materials, is essential. Spare clothing is also essential since it may be necessary to change into dry clothing if clothing becomes wet. Going slowly and avoiding exhaustion or excessive sweating is also helpful. Alcohol and tobacco should be strictly avoided, as should contact with metallic objects. Appropriate education about the potential hazards of cold exposure is vital to the effort to reduce the incidence of cold-related injuries.