Chapter 65 With an increase in outdoor activities, changing weather patterns, and the growing epidemic of homelessness in our country, issues pertaining to hypothermia remain in the forefront. Hypothermia is not only a common diagnosis in rural areas but has also become more commonplace in urban centers across the nation secondary to inadequate housing or lack of preparation for cold weather changes.1 It is also important to note that numerous cases of accidental hypothermia (AH) are reported each year in areas typically considered warm weather locales such as Florida, Texas, California, and Alabama.2,3 Every year, many recreational and elite athletes participate in outdoor sporting events. The higher the environmental stress, the greater the potential for failure in performance and the development of hypothermia.4 The high-altitude expeditions on Mt. Everest in 1996, Mt. Denali in 2003, and Mt. Hood in 2006 are reminders that even well-protected, acclimatized individuals can succumb to cold-related fatalities. Optimal treatment of hypothermia remains controversial. It is a well-accepted practice to carry out resuscitation of these individuals for extended periods. The medical literature contains numerous anecdotal reports of profoundly hypothermic individuals who are successfully resuscitated and discharged neurologically intact.5–8 Despite these spectacular reports of survival, both morbidity and mortality from hypothermia are common. Between 1972 and 2002, 16,555 deaths in the United States were attributed to hypothermia, which equates to 689 deaths per year.9 Between 1999 and 2002 alone, 4607 death certificates in the United States had hypothermia-related diagnoses listed as the underlying cause of death.9 The actual number of patients seen in emergency departments (EDs) with hypothermia is unknown. Poverty, homelessness, alcoholism, and psychiatric illnesses are commonly associated conditions. This chapter critically reviews approaches and procedures appropriate to the management of several categories of hypothermic patients. The recommendations combine treatment efficacy with safety. Before describing procedures and making recommendations, essential terms are defined and the pathophysiology of hypothermia is briefly reviewed. Accidental hypothermia has been defined as an unintentional decrease in core (vital organ) temperature to below 35°C (<95°F).5 Victims of hypothermia can be separated into the following categories: mild hypothermia, 35°C to 32°C (95.0°F to 90.0°F); moderate hypothermia, lower than 32°C to 30°C (<90.0°F to 86.0°F); and severe hypothermia, colder than 30°C (<86.0°F). Other factors that may be useful in separating groups of patients with AH include the presence of underlying illness,1,10–15 altered neurologic state on arrival at the ED, hypotension, and the need for prehospital cardiopulmonary resuscitation (CPR). A hypothermia outcome score has been developed that incorporates some of these factors and may permit comparison of outcomes in patient groups treated with different modalities.16 Risk factors for the development of AH include burn injuries, extremes of age, ethanol intoxication, dehydration, major psychiatric illness, trauma, use of intoxicants, significant blood loss, sleep deprivation, malnutrition, and concomitant medical illnesses.17,18 Risk factors for the development of hypothermia indoors include advanced age, coexisting medical conditions, being alone at the time of illness, being found on the floor, and abnormal perception or regulation of temperature.13 Unlike healthy exposed outdoor enthusiasts, such as skiers or mountaineers,19 hypothermia in urban populations is most often associated with conditions that impair either thermoregulation or the ability to seek shelter. In the majority of studies of urban hypothermia, death has been attributed to the severity of the underlying disease.1 Because signs and symptoms may be vague and nonspecific, mild to moderate hypothermia may easily be overlooked in the ED. A common error is failure to routinely obtain an accurate core temperature in all patients at risk. The diagnosis is frequently delayed because of false reliance on standard oral temperatures. Symptoms such as confusion in the elderly and combativeness in intoxicated patients might not initially be recognized as symptoms of hypothermia. Hypothermic patients frequently will not feel cold or shiver, particularly the elderly population, who have impaired thermoregulatory responses because of their advanced age.20–22 Paradoxical undressing, a cold-induced psychiatric dysfunction, has been described in confused patients in whom a sensation of heat develops at lowered body temperatures. It occurs as a result of constricted blood vessels near the surface of the body that suddenly dilate. In many cases these patients are mislabeled as psychotic, thereby leading to further delays in appropriate treatment.23 Because of the nonspecific nature of the symptoms of hypothermia, accurate assessment of temperature is a necessity when considering this diagnosis. It is of paramount importance not only for confirmation of the diagnosis but also for guidance in further diagnostic and therapeutic decisions. Any thermometer that does not record temperatures in the hypothermic range is inappropriate for evaluating significant hypothermia. Standard glass/mercury thermometers generally cannot record temperatures lower than 34°C (<93.2°F), although some models are available that record temperatures as low as 24°C (75.2°F) (Dynamed, Inc., Carlsbad, CA). An electronic probe with accompanying calibrated thermometer is recommended when monitoring this vital sign. Examples of thermometers with accompanying accuracy at various temperature ranges are shown in Figure 65-1A and B. Core temperature is traditionally estimated with a rectal probe, but rectal temperature often lags behind core temperature because of large gradients within the body.22 Esophageal probes may be used, although they may be affected by warm humidified air therapy. Other possible sites for measurement of temperature include the tympanic membrane, nasopharyngeal tract, and urinary bladder.1,24,25 Fresh urine temperature can closely approximate core temperature.26 “Deep forehead” temperatures measured with a Coretemp thermometer (Teramo, Tokyo) have also demonstrated excellent accuracy and approximation of core temperatures.27 For continuous monitoring purposes, rectal or bladder probes are preferred. Infrared tympanic temperatures have demonstrated excellent correlation with core temperatures. However, studies show that although easier to use and faster, infrared tympanic temperatures can be inaccurate at extremes of temperature by underestimating higher temperatures and overestimating lower temperatures.28 When a rectal probe is used, it should be inserted at least 15 cm beyond the anal sphincter and its position verified frequently.6 One should remember that temperature gradients exist in the human body and therefore consistency of monitoring at one or more sites is mandatory. A chart and formula that convert centigrade to Fahrenheit temperatures will assist the clinician in assessing the severity of hypothermia (see Fig. 65-1C). AH results from failure of the body’s thermoregulatory responses to generate enough heat to compensate for heat losses. These thermoregulatory responses include shivering, tachycardia, tachypnea, increased gluconeogenesis, peripheral vasoconstriction, and shunting of blood to central organs (Fig. 65-2).29,30 As core temperature drops despite these compensatory mechanisms, the patient becomes poikilothermic and cools to ambient temperature. Four methods of heat loss affect the body: radiation, conduction, convection, and evaporation. Radiation involves transfer of heat from a warmer body to a cooler environment and accounts for approximately 60% of heat loss in a normothermic individual. Conduction refers to loss of heat from direct contact with a cooler surface. These losses are most profound with immersion hypothermia. Convection occurs when cool air currents pass by the body and accounts for 15% of heat loss, especially with a wind chill factor. Evaporation refers to significant loss of heat through sweating and insensible water loss.21,30 With hypothermia, the enzymatic rate of metabolism decreases twofold to threefold with each 10°C (18°F) drop, and cerebral blood flow decreases 6% to 7% per 1°C (1.8°F) drop. Signs and symptoms of hypothermia vary according to the core temperature. The overall functioning of all organ systems is impaired by the cold.31 The greatest effects are seen in the cardiovascular, neurologic, and respiratory systems (Table 65-1). As core body temperature drops below 33°C (<91.4°F), the patient becomes confused and ataxic.32 The initiation of involuntary motor activity (shivering) prevents the reduction in core temperature.33 Shivering thermogenesis in skeletal muscle operates on acute cold stress. In a malnourished patient, the mechanism may be rendered ineffective secondary to reduced muscle mass.34 Shivering stops at about 32°C (89.6°F), and shivering artifact on an electrocardiogram has been associated with increased survival of individuals with severe hypothermia.35 Atrial fibrillation occurs frequently as the temperature continues to drop and the patient loses consciousness. A J wave on the electrocardiogram often appears before ventricular fibrillation (Fig. 65-3).36,37 Though classically considered pathognomonic for hypothermia, the J or “Osborne” wave has no prognostic or predictive value in cases of hypothermia. Studies have found that Osborne waves are present in 36% of AH survivors and in 38% of nonsurvivors.35,38 Ventricular fibrillation may occur below 29°C (<84.2°F) and becomes common as the core temperature drops to 25°C (77°F).39 The electroencephalogram flattens at 19°C to 20°C (65.2°F to 68°F).40 Asystole commonly develops at 18°C (64.4°F) but has been seen at higher temperatures. Initial core temperature does not necessarily correlate with patient outcome.42 The lowest recorded temperature in a survivor of AH is 9.0°C (43.7°F).30 Treatment of hypothermia can be divided into prehospital care and ED management. In the prehospital setting, focus primarily on removing the patient from the current environment to prevent further decreases in core temperature. Studies have shown that oral temperatures are sufficiently accurate for field use41; however, infrared tympanic thermometers may not be reliable in the prehospital setting.42 Handle these patients with special care and anticipate the presence of an irritable myocardium because aggressive measures can inadvertently trigger cardiac dysrhythmias. Hypovolemia and a large temperature gradient often exist between the periphery and the core in a hypothermic patient.6 Avoid aggressive field management and prolonged transport times.43,44 After removing the patient’s wet clothing, wrap the patient in dry blankets or sleeping bags. “Field rewarming” is a misnomer because adding significant heat to a hypothermic patient in the field is extremely difficult. Studies have shown that for mild hypothermia, resistive heating (e.g., warming blankets) can be used safely in the prehospital setting. Resistive heating augments thermal comfort, increases core temperature by approximately 0.8°C/hr (33.4°F/hr), and reduces patient pain and anxiety during transport.45 In one study, resistive heating more than doubled the rewarming rate when compared with passive insulation and did not produce an afterdrop.46 With longer transport times, use active rewarming methods limited to heated inhalation and truncal heat application. Place insulated hot water bottles near the patient’s axilla or groin. The Res-Q-Air device (CF Electronics, Inc., Commack, NY) is lightweight and portable and delivers heated humidified air or oxygen at temperatures ranging from 42°C to 44°C (107.6°F to 111.2°F) and down to ambient conditions of −20°C (−4°F). In more remote settings, another option is to use a modified forced-air warming system in the field. The Portable Rigid Forced-Air Cover is heated with a Bair Hugger heater/blower (Augustine Medical, Inc., Eden Prairie, MN). It covers the patient’s trunk and thighs and can adapt to various transport vehicle power sources.6 Immobilize patients with potential traumatic injuries to the spine or extremities before transport. Pay continuous attention to airway maintenance. Initiate fluid resuscitation with intravenous (IV) crystalloid, preferably 5% dextrose in normal saline (D5NS). Alternatively, give warmed oral glucose-containing drinks to a patient who is awake and alert. Most hypothermic patients are dehydrated because fluid intake is reduced and cold causes diuresis. Avoid using lactated Ringer’s solution because it can theoretically decrease the metabolism of lactate by cold-induced hepatic dysfunction. If possible, use warmed IV fluids because they are generally well tolerated.47,48 If available, use a flameless heater, which is currently being used by military medical units and provides an easy and expedient means of warming fluids in the prehospital setting.49 Intubate unresponsive patients, but recognize that there is no universal agreement on when to intubate a hypothermic patient who has detectable vital signs. Pulse oximetry is not usually helpful because vasoconstriction limits blood flow to the periphery and readings may be inaccurate or not possible. Some authors suggest that pulseless victims with core temperatures below 32°C (<89.6°F) should be transported with continuous CPR.47 Other authors believe that it is unnecessary to perform CPR on a patient who has any perfusing cardiac rhythm because it may precipitate ventricular fibrillation.50 There is no universally accepted standard for intubation or CPR in hypothermic patients with detectable vital signs.51 Transport cold, stiff, cyanotic patients with fixed and dilated pupils because the treatment dictum for prehospital personnel remains, “No one is dead until warm and dead.” A succinct summary of the prehospital care of a hypothermic patient is rescue, examine, insulate, and transport.6 Treatment priorities in the ED setting are to prevent further decreases in core body temperature; establish a steady, safe rewarming rate; maintain stability of the cardiopulmonary system; and provide sufficient physiologic support. Adjust the rate of rewarming and the techniques used according to the degree of hypothermia and the severity of the patient’s clinical condition (Table 65-2). Anticipate and prevent complications. TABLE 65-2 ACR, active core rewarming; AER, active external rewarming; PER, passive external rewarming. With a mild to moderate reduction in core temperature, the level of mentation correlates with the severity of the AH, associated illnesses, or both. Noteworthy exceptions are alcoholics and diabetics, who can be in a coma at higher core temperatures because of concomitant hypoglycemia. Perform bedside glucose measurements on patients when they arrive in the ED. A high correlation exists between alcohol consumption and the development of hypothermia, especially in colder climates.1 A review of 68 cases of hypothermic deaths in Jefferson County, Alabama, found that a significant number of cases involved middle-aged men who had consumed alcohol.3 In the 22 cases of AH reviewed by Fitzgerald,52 all but 2 patients were alcoholics. The serum glucose level was less than 50 mg/dL in 41% (nine patients). This study noted glycosuria in two patients, even when low serum glucose values were evident, and described a renal tubular glycosuria in patients with AH. Such glycosuria may worsen or cause hypoglycemia. Glycosuria in AH is no guarantee of an adequate serum glucose concentration. This supports the routine use of supplemental IV glucose unless a normal serum glucose value can be quickly ensured. Consider administering IV thiamine (100 mg) and a trial dose of 0.4 to 2 mg of IV naloxone (Narcan) in a comatose patient. Although failure to rewarm spontaneously has been noted in victims with hypothyroidism and other endocrine deficiencies, reserve the use of thyroid hormones and corticosteroids for patients with suspected thyroid and adrenal insufficiency, respectively. The thermoregulatory vasoconstriction caused by hypothermia significantly decreases subcutaneous oxygen tension.21 Good correlation exists between the incidence of wound infection and subcutaneous oxygen tension. As core temperatures decrease from 41°C to 26°C (105.8°F to 78.8°F), neutrophil function is significantly impaired.21 In animal models, hypothermia appears to decrease leukocyte sequestration within the brain parenchyma, thus offering some resistance to meningitis.53 Although antibiotics are not routinely indicated for victims of uncomplicated mild hypothermia, some authors advocate the routine empirical initiation of broad-spectrum antibiotic therapy on admission of severely hypothermic patients. In this setting, detection and treatment of the underlying cause, such as infection, may be more critical than treatment of the hypothermia.1 Hypothermia affects virtually every organ system because of generalized slowing of the body (see Fig. 65-2). Management goals depend on the severity of the hypothermia, but in all cases the primary goal is to increase core temperature and prevent further loss. In a patient with mild hypothermia, a conservative approach to rewarming is generally advocated. Overly aggressive methods may be more harmful to the patient by causing worsening hypotension, a paradoxical decrease in core temperature, and cardiac dysrhythmias. Other complications may include bleeding and infection of surgical incisions.21 The optimal rewarming rate remains unclear and varies with each case. Standard rewarming rates are a 0.5°C/hr to 2.0°C/hr (0.9°F/hr to 3.6°F/hr) rise in temperature in an otherwise stable patient (Table 65-3). Carefully consider and individualize invasive therapy to the severity of the hypothermia and the condition of the patient. Avoid overtreating and overusing invasive techniques in an otherwise stable hypothermic patient. In patients with severe underlying problems such as hypoglycemia, hyperglycemia, sepsis, adrenal crisis, drug overdose, or hypothyroidism, treat these conditions appropriately in addition to treating the hypothermia. Long-term outcome may depend more on treatment of the underlying illness than on treating the hypothermia.1,55 TABLE 65-3 Note: Thoracic lavage had a median rewarming rate of 2.95°C/hr (see Plaisier54). From Danzl D, Pozos RS. Multicenter hypothermia study. Ann Emerg Med. 1987;16:1042. The cornerstone of the effectiveness of passive external rewarming relies on the body’s ability to restore normal body temperature through its own mechanisms for heat production. Stop further heat loss with insulation and manipulation of the environment. Give warm fluids containing glucose to patients who are fully alert. For patients with mild AH, remove wet clothing and then provide passive external rewarming with blankets. The technique is simple, but the patient must be capable of generating enough body heat for this method to be successful. Give warmed IV fluids to counteract the cold-induced diuresis. Internal heat generation is required for rewarming, and this effect will be relatively slow. In an otherwise stable patient, aggressive intervention with drugs and invasive monitoring might be more harmful than beneficial. Patients who cannot shiver, those who are hypotensive, or those who are intoxicated or malnourished may not have this capability. Survival rates with passive external rewarming have ranged from 55% to 100%.56–59 Indications: Although there is some suggestion that active external rewarming of profoundly hypothermic patients by immersion may be associated with an increase in mortality over other treatments,16,60 more recent studies suggest that this technique is highly effective for mild hypothermia.32,61 Use it selectively and limit it to the trunk. Other forms of active external rewarming are increasingly being used in the ED as adjunctive care of moderately hypothermic, otherwise healthy individuals. Vasoconstriction limits the ability to increase core temperature with techniques that primarily warm the skin.62 Active external rewarming is most beneficial when the heat supplied by the external source is greater than the loss of rewarming heat incurred by the cessation of shivering. In more remote wilderness settings where more aggressive warming techniques are precluded because of the lack of equipment or personnel, active external rewarming with body-to-body contact may be the only option. The rewarming contribution of body-to-body contact appears to be limited, however.63 Equipment: Traditionally, immersion therapy has used a heated (40°C to 42°C [104.0°F to 107.6°F]) water tank of the type present in most burn units. Generally, immerse a hypothermic patient entirely except for the extremities and head, but immersion of the extremities may hasten rewarming.64,65 A major drawback is the inability to closely monitor patients undergoing immersion. Alternatively, use a warm water–filled heat exchange blanket (e.g., Blanketrol, Cincinnati Sub-Zero Products, Cincinnati, OH) for conduction warming. Intraoperative studies have demonstrated excellent results.66 A forced warm air convection system (Bair Hugger, Augustine Medical, Eden Prairie, MN; Snuggle Warm Convective Warming System, Sims Level 1, Inc., Rockland, MA) has been used for postsurgical rewarming.66,67 This approach has also been used successfully for ED-based AH therapy. Rewarming by warm air convection permits continued monitoring in the ED and is better tolerated than immersion because of the less rapid development of vasodilation in peripheral tissues. Technique: Because profound fluid shifts can occur with conduction warming, give the patient supplemental IV fluid warmed to 40°C (104.0°F; Hotline Fluid Warmer, Sims Level 1, Inc., Rockland, MA) at a rate sufficient to generate a urinary output of 0.5 to 1.0 mL/kg/hr. Give an initial fluid bolus of 500 mL of D5NS. Note that blood pressure is not an accurate means of gauging fluid resuscitation since serious hypothermia is always accompanied by “physiologic” hypotension. Because patients requiring mechanical ventilation have rarely been subjected to tank immersion, it cannot be recommended for hypothermic patients who require intubation. Rewarming rates ranging from 0.9°C to 8.8°C (1.6°F to 15.8°F) per hour have been reported with immersion therapy.6,68 A heat exchange blanket allows the patient to receive other treatments that may be difficult or impossible to carry out in a tub, such as defibrillation, CPR, or more invasive warming techniques. Place the heating blanket and overlying cloth sheet underneath the patient. Set the blanket temperature to 40°C to 42°C (104.0°F to 107.6°F), and initiate the measures described in the section “Passive Rewarming Techniques.” Forced-air rewarming (convection) uses a blanket cradle to create an environment through which heated air is blown. Access to the patient is quite good with this system because the overlying blankets can be raised temporarily to evaluate the patient or perform procedures. Experience with mild immersion-induced hypothermia in volunteers suggests that the forced-air technique warms at a rate comparable to that of vigorous shivering, but with less metabolic stress and less afterdrop.69 Arteriovenous Anastomoses Rewarming: Arteriovenous anastomoses rewarming (AVR) involves immersion of the distal end of the extremities (hands, forearms, feet, and lower part of the legs). Advantages include rapid rewarming rates. A study of AVR immersion at temperatures of 45°C and 42°C (113.0°F and 107.6°F) in healthy volunteers demonstrated rewarming rates of 9.9°C/hr (±3.2°C/hr) for the former and 6.1°C/hr (±1.2°C/hr) (43.0°F ± 34.2°F/hr) for the latter.70 There was also a decrease in postcooling afterdrop. AVR is well tolerated by patients because of the rapid rise in core temperature and the shortened period of shivering.64,71 Complications: There is concern that surface warming with accompanying vasodilation may produce relative hypovolemia in a hypothermic patient. Other complications described with the active external rewarming method include core temperature afterdrop and rewarming acidosis. In core temperature afterdrop, colder peripheral blood is transported to the warmer core organs, thereby further reducing core temperature. In rewarming acidosis, colder blood and lactic acid return to the core organs and worsen the acidosis. To limit these complications in patients with moderate hypothermia, some authors advocate using active external warming only after active internal techniques have been initiated.71 There is evidence that active core rewarming may decrease mortality from severe hypothermic exposure when compared with other techniques. In the face of circulatory failure, often the best chance of survival is treatment with extracorporeal circulation (ECC) and warming of the blood.72 Several methods have been described, including the use of warm humidified air through an endotracheal tube or mask, peritoneal lavage, gastric or bladder lavage with warm fluid, thoracic tube lavage, cardiopulmonary bypass, AVR, peripheral vascular extracorporeal warming, hemodialysis, and thoracotomy with mediastinal lavage. These techniques transfer heat actively to the body core and achieve varying rewarming rates. The specific techniques and some of the advantages and disadvantages for each procedure follow. Emergency Warming of Saline in a Microwave: Under ideal circumstances, keep saline in a standard warming device. When large amounts of saline are required for such procedures as peritoneal lavage, warm 1-L saline bags rapidly in a standard microwave oven (Fig. 65-4).73 Although devices vary, a 650-W microwave oven has been demonstrated to warm 1 L of room-temperature non–dextrose-containing saline from 21.1°C to 38.3°C (70°F to 101°F) in 120 seconds on the high setting. At midcycle (i.e., after 60 seconds), interrupt the heating with agitation, and repeat the agitation at the end of the cycle before infusion. Inhalation of Heated Humidified Oxygen or Air: The use of warm humidified oxygen to treat hypothermia has been well established. Average rates of rewarming of 1°C/hr (33.8°F/hr) via mask and 1.5°C/hr to 2.0°C/hr (34.7°F/hr to 35.6°F/hr) via endotracheal tube with heated aerosol at 40°C (104.0°F) can be obtained.6,32 Faster rewarming rates may be accomplished with a maximum safe aerosol temperature of 45°C (113°F). Core rewarming with this technique occurs through the following mechanisms. The warmed alveolar blood returns to the heart and warms the myocardium. The heated, humidified air delivered to the alveoli also warms contiguous structures in the mediastinum by conduction. Warming the inhaled air or oxygen eliminates a major source of heat loss. Indications and Contraindications.: The use of heated humidified air or oxygen is a simple technique that should be used routinely in all patients with hypothermia, regardless of severity. If the correct equipment is available, it can be used in the field and in the hospital.44,45 One must address the risk for burns during the inhalation of warm air in the field environment.74 Mouth-to-tube ventilation in an intubated hypothermic prehospital patient has the theoretical advantage of providing warm humidified air without special equipment. A ventilating rescuer can inhale oxygen before expiring into the patient’s endotracheal tube to provide air with increased oxygen content. There are no contraindications to or reported complications from the use of warm humidified air for hypothermia, and there is no afterdrop.75 Technique.: Use a heated cascade nebulizer with a mask for patients with spontaneous respirations. Use a volume ventilator for intubated patients. Monitor the inspired air to maintain a temperature of approximately 45°C (≈113.0°F).76 Temperatures higher than 50°C (122°F) may burn the mucosa, and temperatures lower than 45°C (<113°F) do not deliver the maximum heat. Humidify the air or oxygen and note that the heater module may need modification because many units have feedback mechanisms that shut off at a given temperature. It may be difficult to deliver oxygen at the recommended temperature because of equipment limitations. In many cases the air temperature is only 30°C (86°F). Summary.: Inhalation of warm humidified air or oxygen results in gradual rewarming of the core and should be the mainstay of all rewarming therapy. Studies have suggested that the rewarming rate of inhalation therapy is inferior to that of peritoneal lavage, thoracic lavage, and bath rewarming.6 Inhalation therapy can be combined with any and all other methods of rewarming and is relatively noninvasive and inexpensive. This therapy should be considered as the initial treatment of choice for hypothermic patients. Peritoneal Dialysis (Lavage): Peritoneal dialysis (lavage) is an attractive treatment of severe hypothermia because it is available in most hospitals and does not require any unusual equipment or training. Rewarming rates of 2°C to 3°C (3.6°F to 5.4°F) per hour, depending on the dialysis rate, can be achieved without sophisticated equipment that may delay therapy or require transfer of the patient to a tertiary care facility.77 This technique can also be used to help correct electrolyte imbalances. Rewarming by peritoneal dialysis was first used successfully in a patient in ventricular fibrillation with a temperature of 21°C (69.8°F).78 Since that time, there have been reports of successful rewarming with peritoneal lavage in stable, severely hypothermic patients and unstable hypothermic patients in cardiac arrest.79,80 Peritoneal lavage works via transfer of heat from lavage fluid to the peritoneal cavity. The peritoneal great vessels and abdominal organs provide a large surface area for exchange of heat. The use of warmed peritoneal lavage fluid is an effective approach to rewarming.81 There have been reports in the literature of success with rapid high-volume peritoneal lavage in pediatric patients. The technique involves the use of an infraumbilical “mini-laparotomy” incision followed by placement of a large silicone peritoneal dialysis catheter. The catheter is connected to a rapid infusion device with delivery of 1 L of warmed normal saline every 90 seconds.81 Indications and Contraindications.: Peritoneal dialysis is appropriate therapy in a severely hypothermic patient. In practice, it is often omitted if other measures appear to be successful. There are no universally established criteria for performing peritoneal lavage in hypothermic patients who have detectable vital signs. Though theoretically less effective than other techniques that directly warm the thorax in the setting of cardiac arrest, it has been used successfully in that situation. It is theoretically useful in hypothermic patients who have overdosed with a dialyzable toxin. Other less invasive methods, such as gastric or bladder lavage or warm nebulized air or oxygen inhalation, may be preferred in stable patients with temperatures higher than 26°C to 28°C (>78.7°F to 82.4°F). Peritoneal dialysis should not be performed on patients with previous abdominal surgery. It should be used with extreme caution in patients with a coagulopathy. Equipment.: We recommend using the Seldinger technique with a commercially available disposable kit (e.g., Arrow Peritoneal Lavage Kit, product no. AK-09000, Arrow International, Inc., Reading, PA) because of the ease of performance and minimal morbidity associated with this procedure. Technique.: In a noncritical patient, obtain a coagulation profile before the procedure, but in life-threatening situations, initiate the procedure immediately before laboratory studies. Place the patient in the supine position with a Foley catheter and nasogastric tube in place. After infiltrating with lidocaine, make an infraumbilical stab incision with a No. 11 scalpel blade, and place an 18-gauge needle into the peritoneal cavity directed toward the pelvis at a 45-degree angle. Insert a standard flexible J wire through the needle, and then remove the needle. Pass the 8-Fr dialysis catheter over the wire with a twisting motion, and then remove the wire. Lavage rates of 4 to 12 L/hr can be achieved with two catheters. Warm the fluid with a standard blood warmer to 40°C to 45°C (104.0°F to 113.0°F). Use a standard 1.5% dextrose dialysate solution. Add potassium (4 mmol/L) if the patient becomes hypokalemic. Saline has also been used successfully. The rate should be at least 6 L/hr and preferably 10 L/hr.80 Complications.: The Seldinger method has a complication rate of less than 1%.82 A “mini-lap” performed via direct dissection may also be used but might have a higher complication rate.83 Further discussion of potential complications is provided in Chapter 43. Summary.: Peritoneal dialysis is a useful method because it entails readily available fluid and can be done with a self-contained disposable kit.83 If a hospital also treats trauma victims, the lavage kit can be the same as that used for evaluation of abdominal trauma. If this technique is combined with warm nebulized inhalation, warming rates of 4°C/hr (7.2°F/hr) can be achieved.84 Peritoneal lavage rewarms the liver and restores its synthetic and metabolic properties.85 Gastrointestinal and Bladder Rewarming: Gastric or bladder irrigation offers some of the same advantages as peritoneal dialysis without invading the peritoneal cavity. Heat is delivered to structures in close proximity to the core. In the Multicenter Hypothermia Study, gastric/bladder/colon lavage had a first-hour rewarming rate of 1.0°C to 1.5°C/hr (33.8°F/hr to 34.7°F/hr) and a second-hour rewarming rate of 1.5°C/hr to 2.0°C/hr (34.7°F/hr to 35.6°F/hr) for severe hypothermia.83,86 In a multifactorial analysis of the Multicenter Hypothermia Study there was a trend toward improved survival in patients treated in this manner.16 Although the amount of heat delivered with gastric lavage appears to be less than that delivered with peritoneal dialysis, it is somewhat easier to use and less invasive. When combined with other methods, gastric or bladder lavage provides significant warming.83,84 Serum electrolyte levels should be monitored if large volumes of tap water are used because dilutional electrolyte disturbances may occur. Children and geriatric patients might be more susceptible to electrolyte changes with tap water irrigation.87 Indications and Contraindications.: Warmed gastric or bladder lavage may be used as adjunctive therapy for moderate or severe hypothermia. It can be combined with other warming techniques when rapid rewarming is needed. Patients who are obtunded and lack protective airway reflexes should undergo endotracheal intubation before gastric lavage to prevent aspiration of gastric contents. Refer to the appropriate chapters concerning nasogastric tube placement (see Chapter 40), gastric lavage (see Chapter 42), and urethral catheterization for specific contraindications to these procedures. Equipment.: Use a large-diameter 32- to 40-Fr lavage tube with normal saline solution warmed to 40°C to 45°C (104.0°F to 113.0°F) in a microwave or blood warmer with verification of temperature before use. Although smaller tubes are easily passed nasally, use oral placement of the large lavage tubes. A modified Sengstaken tube with gastric and esophageal balloons may also be used. Technique.: Instill 200- to 300-mL aliquots of fluid into the stomach before removal by gravity drainage. For bladder irrigation the optimal volume is not known, but avoid distention of the bladder (100- to 200-mL aliquots should be sufficient). The amount of time that the irrigant should be left in place before removal is not known, but use of rapid exchanges with a dwell time of 1 to 2 minutes is suggested. Complications.: Complications of lavage include trauma to the nasal turbinates, gastric and esophageal perforation, dilutional hyponatremia, inadvertent placement of the tube in the lungs, and pulmonary aspiration, all of which can be minimized by careful, proper technique. Fluid overload and electrolyte disturbances when using tap water are potential complications in pediatric and geriatric patients. Summary.: Gastrointestinal and bladder lavage with heated fluids is easily performed with equipment and solutions available in any hospital. The stomach, colon, and bladder are poor sites for body cavity lavage as a result of the small surface area for heat exchange.85 Because of its ease and availability, it can be started early in the resuscitation and be combined with any other rewarming method to significantly add heat,69 although its specific effect on morbidity and mortality is not known. Thoracic Cavity Lavage: Thoracic cavity lavage can be performed either by closed means, through chest tubes placed in one hemithorax,88,89 or in open fashion, after resuscitative thoracotomy.90 The former approach offers the advantages of being less invasive and is an effective form of treatment in hospitals not equipped for cardiopulmonary bypass.89 Furthermore, closed-chest CPR can be continued while this technique is used. The open thorax approach offers the theoretical advantage of direct warming of the heart and the option of open-chest cardiac massage. Rapid warming rates of 6°C to 7°C (42.8°F to 44.6°F) in 20 minutes have been described.88,89 Pleural irrigation results in cardiac rewarming and might be the method of choice, particularly in patients with an arrhythmia.85 Indications and Contraindications.: Thoracic cavity lavage should be considered for patients requiring rapid core rewarming in the setting of cardiac arrest or inadequate perfusion (e.g., shock, during CPR) when cardiac bypass is not available. Open thoracic lavage should be considered in patients who will receive open-chest massage or thoracotomy for other reasons (e.g., hypothermic arrest with penetrating trauma). Thoracic lavage is not necessary for patients with mild or moderate hypothermia who can be rewarmed by other less invasive methods. Avoid the technique in patients with a coagulopathy unless required as a lifesaving measure. Closed Thoracic Lavage.: An alternative that is more practical in the ED is pleural rewarming by repeatedly using warmed saline placed intermittently and then withdrawn through a chest tube. Place two large-bore thoracostomy tubes (e.g., 36 to 38 Fr in 70-kg adults) in one hemithorax. Infuse one chest tube with 3-L bags of heated normal saline (40°C to 41°C [104°F to 105.8°F]) via a high-flow fluid infuser (e.g., Level-1 Fluid Warmer, Technologies, Inc., Marshfield, MA). Collect the effluent with an autotransfusion thoracostomy drainage set (e.g., Pleur-evac, Deknatel A-5000-ATS, Fall River, MA). Empty the removable reservoir as needed. Alternatively, use a single–chest tube system with a Y-connector arrangement similar to that used for gastric lavage. Place aliquots of 200 to 300 mL with a 2-minute dwell time followed by suction drainage (at 20 cm H2O). Open Thoracic Lavage.: Perform a left thoracotomy and pour saline warmed to 40°C to 41°C (104.0°F to 105.8°F) continuously into the thoracic cavity to bathe the heart while an assistant suctions the excess fluid from the lateral edge of the thoracotomy. Alternatively, add fluid to the thorax and mediastinum intermittently and suction after several minutes. Follow this with more warmed saline and repeat. This technique also allows direct monitoring of myocardial temperature. Perform direct cardiac massage until adequate spontaneous perfusion occurs. Perform direct cardiac defibrillation in a patient warmed to 30°C (86°F) with persistent ventricular fibrillation. When defibrillation is successful, continue direct myocardial warming until the patient’s temperature approaches 35°C (95°F). If defibrillation is unsuccessful at a core temperature of 30°C (86°F), continue warming while oxygenation, perfusion, and other physiologic parameters are optimized before further attempts at defibrillation. Summary.: Thoracic lavage is an effective form of active core rewarming that is usually reserved for hypothermic arrest patients.54,89,90 Thoracic lavage may be considered when vital signs are inadequate or unstable enough to severely limit perfusion. Precise indications have not been clarified beyond patients in cardiac arrest. Cardiac Bypass: The use of cardiac bypass or an extracorporeal shunt through either the femoral artery–femoral vein or the aortocaval procedure can result in rapid rewarming but requires surgical expertise, the availability of appropriate equipment, and technical support.5,50,91 This procedure has not been compared with other rewarming methods in a controlled fashion, and few centers have this modality available in a time frame that would affect survival rates. Its main advantages appear to be the rapid rate of warming that it produces and optimal patient oxygenation and perfusion. Femoral flow rates of 2 to 3 L/min with the warmer set at 38°C to 40°C (100.4°F to 104°F) will raise the core temperature 1°C to 2°C (33.8°F to 35.6°F) every 3 to 5 minutes.6 Drawbacks include potential delays in assembling the appropriate team and equipment, delays because of the time necessary to complete the operation, complications from the operation, the expense of the procedure and bypass equipment, and the potential for infection. Its use in extreme situations that may include cardiac arrest should be based on individual characteristics of the patient, clinician team, and hospital resources. If readily available, it should be strongly considered in hypothermic patients with asystole or ventricular fibrillation.41 If oxygenation is not a consideration, venovenous rewarming with an extracorporeal venovenous rewarmer can achieve rapid rewarming rates (2°C/hr to 3°C/hr [3.6°F/hr to 5.4°F/hr]), although they are slower than rates with cardiopulmonary bypass.71 Such a device is relatively easy to use, involves readily available technology, and probably does not require heparin. This equipment needs to be assembled before patients with hypothermia arrive.24 In severely hypothermic patients, extracorporeal rewarming using venovenous hemofiltration has also been reported to be successful.70,71 When compared with adults, children, especially smaller ones, require special consideration with regard to IV cannulation because drainage can be inadequate with femoral-femoral cannulation. In smaller hypothermic children, some sources recommend a more aggressive emergency median sternotomy for cardiopulmonary bypass.92 Cardiopulmonary bypass is indicated in the following situations: (1) cardiac arrest or hemodynamic instability with a temperature lower than 32°C (<89.6°F), (2) no response to less invasive techniques, (3) completely frozen extremities, or (4) rhabdomyolysis with severe hyperkalemia.2 A 47% long-term survival rate was obtained in a Swiss study of 32 young, otherwise healthy individuals, including mountain climbers, hikers, and victims of suicide attempts. Cardiopulmonary bypass is unlikely to confer similar benefit in older, poorly conditioned populations with underlying chronic diseases.93 Hemodialysis: Hemodialysis was first described for the management of AH in 1965.94 It is a rapid and efficient modality for rapid internal rewarming of patients with moderate to severe AH, but it is uncommonly used in clinical practice. One study reported that 26 patients with AH combined with circulatory arrest or severe circulatory failure were rewarmed to normothermia with the use of ECC.95 Core rewarming by hemodialysis has been achieved after placement of a dialysis catheter or with the use of an existing shunt. Some of the potential advantages and drawbacks of cardiac bypass also apply to this procedure, although slower warming rates have been reported. A range from 0.6°C/hr (33.1°F/hr) to rates as high as 4.5°C/hr (40.1°F/hr) have been achieved with fluid warmed to 40°C (104.0°F).87 For patients who have ingested a dialyzable toxin (such as barbiturates and toxic alcohols), hemodialysis can be used to both remove the toxin and rewarm the blood.94 In such cases its use may be appropriate. Experimental Techniques: Ultrasonic, radiowave, and low-frequency microwave diathermy rewarming appears to be a rapid, safe, noninvasive technique that has shown promise in animal studies.67,96 Frequencies of 13.6 to 40.7 MHz are typically used. In a volunteer study the technique seemed to be less effective than immersion therapy and equivalent to passive rewarming techniques.96,97 Total liquid ventilation with warmed oxygenated perfluorocarbon is currently being studied in animals as a method of rapidly rewarming the core. Benefits include shorter rewarming times than with warm humidified oxygen (1.98 ± 0.5 hours versus 8.61 ± 1.6 hours; P < 0.0001), no afterdrop phenomenon, and no increase in lactate dehydrogenase and aspartate transaminase.75 Very hot IV fluids (65°C [149°F]) have been used in animals with little vascular damage or hemolysis. Trials in humans undergoing burn débridement have been very successful in preventing hypothermia during operative procedures. Saline heated to 60°C (140°F) with modified fluid warmers was infused through central venous access. There was no evidence of intravascular hemolysis or coagulopathy after the infusions.98 The role of hot IV fluids in the management of AH is currently undefined. Cardiac arrest secondary to AH requires immediate treatment for the best chance of a successful outcome. Rapid rewarming and restoration of cardiac rhythm are essential for patients in cardiopulmonary arrest and can best be achieved with a combination of passive and multiple active core rewarming techniques. Because of numerous cases of survival from hypothermic cardiac arrest with prolonged external cardiac compression,5,41,99 thoracotomy is not mandatory. Thoracotomy does offer some theoretical advantages, however, such as increased cardiac output with open-chest massage,90 direct observation of cardiac activity, and direct warming of cardiac tissue with thoracic cavity lavage of warm fluid. Cardiopulmonary bypass is an effective technique for rapid rewarming. Blunt trauma and head trauma victims were previously not ideal candidates for cardiac bypass because of the anticoagulation requirement, but some authors have advocated this technique with heparin-bonded tubing even in the setting of known traumatic injury.5 A review of outcomes after hypothermic cardiac arrest from one institution found that the average time from thoracotomy to the development of a perfusing rhythm was 38 minutes (range, 10 to 90 minutes).5 The optimal rate of cardiac compressions in hypothermic patients is not known. Because of decreased oxygen consumption by vital organs, the rate required in hypothermic cardiac arrest is less than that recommended for normothermic cardiac arrest. Cardiac compressions should be initiated at half the normal rate in profoundly hypothermic patients. Guidelines developed by the American Heart Association and the Wilderness Medical Society recommend that CPR be initiated in patients with AH unless any of the following conditions exist: a “do-not-resuscitate” status is documented and verified, obvious lethal injuries are present, chest wall depression is impossible, no signs of life are present, or rescuers are endangered by delays in evacuation and altered triage conditions.6 The duration of CPR depends on the time required to raise the core temperature to a level at which defibrillation should be successful (i.e., >30°C [>86°F]). Previously, it was recommended that patients not receive a set of three countershocks until a core temperature above 30°C (>86°F) could be attained. There have been reports of successful defibrillation in patients with profound hypothermia with core temperatures of 25.6°C (78.1°F).86 The decision to terminate resuscitative efforts remains a clinical one, but there are certain poor prognostic factors. Certainly, survival is unlikely in patients who persist in asystole or go from ventricular fibrillation to asystole as they are warmed past 32°C (>89.6°F). Prognostic markers for patients with severe hypothermia and cardiac arrest have been proposed as contraindications to ED thoracotomy and cardiac bypass by some authors.5 Such markers include potassium levels elevated to above 10 mmol/L (mEq/L) and pH levels below 6.5. Nonetheless, there are reports of survival in patients with higher potassium levels and a pH as low as 6.29.92 The decision to continue resuscitative efforts should not be based solely on specific laboratory values or the initial core temperature. Isolated reports of survival of hypothermic patients with prolonged CPR make extended efforts to resuscitate such patients reasonable. Children may be the best candidates for heroic measures.50 Under ideal conditions, hypothermic cardiac arrest patients may reasonably be admitted to an intensive care unit for a 4- to 5-hour trial of rewarming with CPR in progress. Manual CPR should be replaced by mechanical methods if the equipment is available. The oxygen-powered “thumper” has been successful during prolonged hypothermic resuscitation. Absence of responsiveness to treatment, in conjunction with a highly elevated potassium level, is an indication for termination of resuscitative efforts. Maintain a secure functioning airway for hypothermic patients, just as in any critically ill patient. With mild hypothermia, deliver heated, humidified oxygen by face mask. Recognize that a hypothermic patient can be combative and uncooperative and may require arm restraints if a mask is used. Intubate patients with decreased sensorium who cannot reliably maintain their airway or hypothermic patients who may be hypoxic. Endotracheal intubation may be performed safely without the added risk of ventricular dysrhythmias.15 The technique for endotracheal intubation depends on the specific circumstances and the expertise of the operator. Once an endotracheal tube has been placed and secured, use it to provide warm humidified oxygen. There is no evidence that tracheal intubation is detrimental in severely hypothermic patients, and it should be considered if indicated for ventilation, oxygenation, or airway protection. Acid-base disturbances are variable and can lead to metabolic acidosis from carbon dioxide retention and to lactic acidosis or metabolic alkalosis from decreased carbon dioxide production or hyperventilation. Interpretation of arterial blood gases in a hypothermic patient has been the cause of some confusion. Previously, it was suggested that all blood gases be corrected for temperature with correlation factors. With a decrease in temperature of 1°C (33.8°F), pH rises 0.015, carbon dioxide pressure (Pco2) drops by 4.4%, and oxygen pressure (Po2) drops 7.2% relative to values that would be obtained with blood analyzed under normal conditions. Despite the conversion guide, optimal or normal values in hypothermia have not been well documented.32 Other recent literature supports the use of uncorrected arterial blood gas values to guide therapy with bicarbonate or hyperventilation.30 This approach appears appropriate to support optimal enzymatic function. Gradual correction of acid-base imbalance will allow increased efficiency of the bicarbonate buffering system as the body warms. Arterial pH did not correlate with patient death in the Multicenter Hypothermia Study and should not be used as a prognostic guide to resuscitation.84 Abnormal clotting occurs frequently in hypothermic patients, probably because cold inhibits the enzymatic coagulation cascade.100–102 Hypothermia-induced coagulopathy does not result from excessive clot lysis, but rather from impaired clot formation.16,21 Platelet function is also impaired during hypothermia because production of thromboxane B2 is inhibited. Hypothermia-induced platelet aggregation with or without neutrophil involvement has been associated with neurologic dysfunction in patients undergoing surgical procedures.91 Hypercoagulability with a risk for thromboembolism may also occur, but the importance of cold-induced coagulopathy mainly involves patients with coincidental trauma. Such victims often have bleeding that is difficult to control. Replace appropriate clotting factors and use warm blood to limit further blood loss and worsening of the hypothermia. Mortality is increased in trauma patients with temperatures below 32°C (<89.6°F). It is not clear whether this increased mortality is actually a result of the hypothermia or whether the hypothermia is merely an indicator of severe injury and response to a massive transfusion of cold fluid.21,103 Patients with severe trauma are prone to hypothermia because their injuries often expose them to environmental heat loss. Concurrent alcohol intoxication may add to the heat loss as a result of its vasodilatory effects on cutaneous vasculature and the prolonged cold exposure secondary to altered mental status. Victims of severe injury also lose heat because of exposure during resuscitation and rapid administration of cold fluids. It is unknown to what degree correcting the hypothermia improves outcome. Nevertheless, devices to rapidly infuse warm fluids such as the Level 1 fluid warmer (Level 1 Technologies, Rockland, MA) and the Thermostat 900 (Arrow International, Reading, PA) are frequently used to warm large-volume fluid transfusions. Use of these devices seems reasonable to prevent the hypothermia associated with massive transfusions (see Chapter 28). Their use for hypothermia not associated with severe trauma is limited by the relatively low fluid requirements of patients with environmental exposure. Another Thermostat device (Aquarius Medical Corp., Phoenix, AZ) is used to accelerate recovery from hypothermia by mechanically distending blood vessels in the hand, thereby increasing transfer of exogenous heat to the body core. One article found that this particular rewarming device was not very effective in accelerating rewarming in hypothermic surgical patients after general anethesia.104 Hypothermia alters the pharmacodynamics of various drugs. It markedly alters drug kinetics, but not enough is known about this phenomenon to define specific therapeutic guidelines. Administer drugs with caution to hypothermic patients (Table 65-4). Because of the negative effects of hypothermia on both hepatic and renal metabolism, toxic levels of medications can accumulate rapidly after repeated use.105 Avoid certain drugs, such as digitalis. Sinus bradycardia and most atrial arrhythmias do not require pharmacologic treatment because the majority resolve with rewarming. Transient ventricular dysrhythmias also do not require treatment. Bretylium is the preferred agent for patients requiring medication for ventricular dysrhythmias, but lidocaine, magnesium, propranolol, and amiodarone have also been used.30 For severe acidosis (pH <7.1), IV sodium bicarbonate can be used with extreme caution. Vasopressors should be used with care, perhaps in much smaller doses than usual, because of the arrhythmogenic potential and the delayed metabolism of these agents. A review of intensive care unit admission of hypothermic patients found that treatment with vasoactive drugs was an independent risk factor for mortality, but this phenomenon remains poorly understood.41 In animal studies, use of epinephrine impaired myocardial efficiency in cases of moderate hypothermia.106 There was no advantage to repeated doses of epinephrine or high-dose epinephrine in hypothermic cardiac arrest animal models.107 The use of inamrinone, formerly known as amrinone, has been investigated in cases of deliberate mild hypothermia. Initial results indicate that amrinone accelerates the cooling rate of the core temperature, thereby potentially limiting its usefulness in management of AH.108
Procedures Pertaining to Hypothermia and Hyperthermia
Procedures Pertaining to Hypothermia
Definitions
Measurement of Core Temperature
Pathophysiology
Initial Evaluation and Stabilization of Hypothermic Patients
Prehospital Care
ED Management
Management Guidelines
Passive External Rewarming
Active External Rewarming
Active Core Rewarming
Special Situations
Airway Management
Acid-Base Disturbances
Coagulopathies
Trauma and Hypothermia
Pharmacotherapy and Monitoring
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Procedures Pertaining to Hypothermia and Hyperthermia
