Accidental hypothermia in adults: Management

INTRODUCTION — 

Death from accidental hypothermia (sometimes called "exposure") occurs worldwide and can present significant management problems [1-4]. The management of hypothermia requires prevention of further heat loss, resuscitation, rewarming, and treatment of complications. Complete neurologic recovery in patients with hypothermia and cardiac arrest despite prolonged resuscitation has been well documented.

The management of accidental hypothermia will be discussed here. A table and algorithm outlining the emergency management of hypothermia in adults are provided (table 1 and algorithm 1). A temperature unit conversion calculator is provided (calculator 1). The following related content is discussed separately:

●Clinical manifestations and evaluation of accidental hypothermia in adults (see "Accidental hypothermia in adults: Clinical manifestations and evaluation")

●Drowning, often complicated by hypothermia (see "Drowning (submersion injuries)")

●External injuries from cold (see "Frostbite: Acute care and prevention" and "Nonfreezing cold water (trench foot) and warm water immersion injuries")

●Accidental hypothermia in children (see "Hypothermia in children: Clinical manifestations and diagnosis" and "Hypothermia in children: Management")

STAGES OF HYPOTHERMIA — 

Hypothermia is defined as a core temperature below 35°C (95°F). The stage of hypothermia, defined by core temperature, has a large impact on management. The most commonly used definitions are the following [5,6] (table 2):

●Mild hypothermia – Core temperature 32 to 35°C (90 to 95°F)

●Moderate hypothermia – Core temperature 28 to 32°C (82 to 90°F)

●Severe hypothermia – Core temperature <28°C (82°F)

Other staging schemes are discussed separately. (See "Accidental hypothermia in adults: Clinical manifestations and evaluation", section on 'Definitions and stages of hypothermia'.)

RESUSCITATION — 

Management of hypothermia requires prevention of further heat loss; evaluation and support of the airway, breathing, and circulation; initiation of rewarming appropriate to the degree of hypothermia; and treatment of complications [6-8]. A table and algorithm outlining the emergency management of hypothermia in adults are provided (table 1 and algorithm 1).

Prevent core temperature afterdrop/rescue collapse — Unless the individual has only mild hypothermia and is already walking, they should not be allowed to stand or walk and should be extricated from the cold environment in a horizontal position whenever possible. Even low-intensity use of peripheral muscles should be avoided, as muscle perfusion and core temperature afterdrop can be increased by exertion [5]. We do not allow a patient to stand or walk if they may have moderate or severe hypothermia and have been in a horizontal position until insulated and shivering is supported (ie, caloric intake) for 30 minutes [8].

Afterdrop refers to the continued decrease of core temperature during rewarming. It primarily occurs due to increased perfusion of cold extremities occurring from peripheral vasodilation associated with removal from the cold environment or rewarming. Countercurrent cooling of blood continues until the gradient is eliminated. With peripheral vasodilation, cold, acidemic blood that has pooled in the vasoconstricted extremities returns to the core circulation, causing a drop in temperature and pH. Another mechanism is temperature equilibration between the warmer core and the cooler periphery. These can contribute to precipitous hypotension, inadequate coronary perfusion, and ventricular arrhythmias ("rescue collapse") [9].

Avoid further cold exposure and precipitating arrhythmias — Wet clothing should be removed and the patient should be insulated from further cold exposure. Ambient temperature in treatment areas should be maintained at approximately 28°C (82°F) if possible.

While managing a patient with significant hypothermia, keep in mind that the hypothermic heart is very sensitive to movement. Rough handling can precipitate arrhythmias, including ventricular fibrillation, that may be unresponsive to defibrillation and medications. Take care to avoid jostling the patient during removal of clothing, physical examination, or the performance of procedures.

Airway and breathing — Tracheal intubation should be performed in a patient with respiratory distress or who cannot protect their airway. Atropine is not indicated as a pretreatment agent to reduce bronchorrhea. (See "Advanced cardiac life support (ACLS) in adults" and "Rapid sequence intubation in adults for emergency medicine and critical care".)

Circulation

Assessing perfusion — Before we determine that a patient has a nonperfusing rhythm, we check for a central pulse for up to a full minute using a continuous-wave Doppler or perform a bedside echocardiogram, if available. Transesophageal echocardiography (TEE) is more sensitive than standard bedside echocardiography and preferred if available at the bedside and the patient is tracheally intubated. A perfusing rhythm on a cardiac monitor can be easily misdiagnosed as pulseless electrical activity (PEA) because peripheral pulses can be difficult to palpate in a vasoconstricted bradycardic patient.

Perfusion not detected — Cardiopulmonary resuscitation (CPR), including chest compressions, should be initiated in a patient with accidental hypothermia who sustains a cardiac arrest. Fixed and dilated pupils are not a contraindication to starting CPR. Apparent rigor mortis is also not a reliable sign of death. Mandibular rigidity can simulate rigor mortis [10].

However, we believe that chest compressions should not be performed in the following circumstances:

●There is an organized rhythm on a cardiac monitor without a palpable pulse or other signs of life (see 'Organized cardiac rhythm and role of chest compressions' below)

●Cardiac contractions can be seen on bedside echocardiogram or TEE

●A pulse can be detected with blood pressure monitoring or with Doppler ultrasound

●Verified "do not resuscitate" (DNR) status

●Obviously lethal injuries

●A frozen chest wall that is not compressible

●Avalanche burial >35 minutes and airway obstructed by ice or snow

●The presence of any signs of life (indicating that there is perfusion)

If pulses are not found after checking for one minute and none of the contraindications immediately above exists, chest compressions should be started immediately and provided continuously, if possible. (See "Adult basic life support (BLS) for health care providers", section on 'Chest compressions'.)

If continuous chest compressions are not possible because of safety considerations or during evacuation, they can be delayed up to 10 minutes while rescuers move the patient to a safer location. In a patient with a core temperature of 20 to 28°C (68 to 82°F), or if the core temperature is not known, perform chest compressions continuously for periods of at least five minutes, alternating with periods of no longer than five minutes without compressions. In a patient with a core temperature below 20°C (68°F), perform chest compressions continuously for periods of at least five minutes, alternating with periods of no longer than 10 minutes without compressions [6,11]. Resume continuous compressions as soon as possible.

Mechanical chest-compression devices should be used, if available, to avoid interruption during transport or prolonged CPR. (See "Therapies of uncertain benefit in basic and advanced cardiac life support", section on 'Mechanical compression devices'.)

Organized cardiac rhythm and role of chest compressions — Although evidence is scant, we believe that chest compressions should not be performed in patients who have an organized rhythm on a cardiac monitor (PEA) even if they have no palpable pulses and no other signs of life. Our reasoning is that such rhythms may reflect successful perfusion that is difficult to detect but that could be disrupted by chest compressions, and that any PEA is likely to be transient. We start chest compressions if the patient is rewarmed to >28°C (82°F) and still has PEA. There is little downside to withholding chest compressions briefly in apparently lifeless hypothermic patients. If PEA becomes asystole or PEA without cardiac activity on point-of-care ultrasound, chest compressions should be started immediately.

We do not administer epinephrine to patients with a core temperature <30°C (86°F) [6,12]. After the patient is rewarmed to >30°C (86°F), we administer epinephrine 1 mg intravenously but double the interval as compared with normothermia (every 6 to 10 minutes), and then resume normal intervals when the temperature is ≥35°C (95°F). The evidence is discussed immediately below. (See "Advanced cardiac life support (ACLS) in adults", section on 'Asystole and pulseless electrical activity'.)

Ventricular arrhythmias or asystole — In hypothermic patients in cardiac arrest, ventricular arrhythmias and asystole may be refractory to conventional therapy until the patient has been rewarmed. Because resuscitating a patient who is not in cardiac arrest is easier than resuscitating a patient requiring ongoing circulatory support, it is reasonable to attempt defibrillation. The definitive management of ventricular arrhythmia is focused on aggressively rewarming the patient in conjunction with advanced cardiac life support (ACLS), most importantly basic cardiopulmonary resuscitation. (See "Advanced cardiac life support (ACLS) in adults" and "Adult basic life support (BLS) for health care providers".)

●Defibrillation – In a patient with nonperfusing ventricular arrhythmias (even in severe hypothermia), attempt defibrillation with up to three shocks. In a patient with a core temperature <30°C (86°F), we withhold further defibrillation attempts [6,12]. After the patient is rewarmed to >30°C (86°F), we follow ACLS guidelines for normothermic patients. Some experts will reattempt a shock with every 1 to 2°C of rewarming [13], although cold myocardium can potentially be injured with too many shocks. Evidence is lacking on the effectiveness of defibrillation in hypothermia, but it may be effective below 30°C (86°F) [14-16]. Most published cases of successful defibrillation below 30°C (86°F) involve core temperatures in the mid-20s (mid-70s °F), although there is a case report of successful defibrillation by an automatic implantable cardioverter defibrillator in a patient with a core temperature of 18.2°C (64.8°F) [17].

●Medications – We do not administer ACLS medications (epinephrine, amiodarone) to patients with a core temperature <30°C (86°F) [6,12]. After the patient is rewarmed to >30°C (86°F), we administer ACLS medications but with an increased interval for epinephrine as discussed above. Some experts will administer a single dose of epinephrine in patients with a core temperature below 30°C (86°F). (See 'Organized cardiac rhythm and role of chest compressions' above.)

Evidence to support drug therapy prior to successful rewarming is limited and consists primarily of animal studies, which have not demonstrated improved survival with amiodarone or epinephrine [18-20]. Medications are metabolized more slowly during hypothermia, potentially leading to toxic concentrations with repeat dosing. This is a particular concern with epinephrine as it can cause myocardial injury [6]. A ventricular arrhythmia is likely to recur when the temperature is <30°C (86°F) even if epinephrine successfully converts the rhythm.

Perfusing arrhythmia or bradycardia — Bradycardia may be physiologic in severe hypothermia. Cardiac pacing generally is not required unless there is hypotension or the bradycardia persists despite rewarming to 32 to 35°C (90 to 95°F). Atrial fibrillation or atrial flutter does not generally cause a rapid ventricular response and often resolves spontaneously with rewarming. The management of ventricular arrhythmias can be problematic [21-24]. Transcutaneous pacing is less hazardous than transvenous intracardiac pacing for severe bradycardia associated with hypotension that is profoundly disproportionate to temperature [25]. (See "Advanced cardiac life support (ACLS) in adults", section on 'Bradycardia'.)

Hypotension — Patients with moderate or severe hypothermia frequently become disproportionately hypotensive during rewarming because of severe dehydration and fluid shifts [5,21,22]. Two large (14- or 16-gauge) peripheral IV lines should be placed. Blood pressure is supported with warmed (40 to 42°C [104 to 108°F]) infusions of isotonic crystalloid. Large infusions may be necessary. Use of warmed crystalloid is critical since infusion of room-temperature fluids can worsen hypothermia. An indwelling bladder catheter may be helpful to assess urinary output and fluid shifts despite a cold-induced diuresis.

Intraosseous (IO) access may be easier to obtain than intravenous (IV) access in cold, vasoconstricted patients. IO lines should be primed with a 10 mL bolus of isotonic crystalloid with or without lidocaine immediately after insertion to open the marrow space and to help ensure good flow. (See "Intraosseous infusion".)

A central venous catheter may aid with fluid resuscitation. Temporary placement of a femoral venous catheter is preferred to avoid precipitating an arrhythmia from irritation of the right atrium. (See "Placement of femoral venous catheters".)

Norepinephrine should be used to maintain blood pressure in cases refractory to volume resuscitation. If norepinephrine is not available, other vasopressors are acceptable. Evidence is limited to animal studies, and low-dose dopamine was used in both [26,27]. (See "Use of vasopressors and inotropes".)

Duration of resuscitation — We continue CPR until the patient is rewarmed to 30°C (86°F), at which point we make renewed attempts at defibrillation and resuscitation with ACLS medications. If the patient does not achieve return of spontaneous circulation (ROSC), we then continue warming the patient to 32°C (90°F), if possible. At that point, if there has been no ROSC, we terminate resuscitative efforts. Because of the neuroprotective effects of hypothermia, complete recovery of patients with hypothermic cardiac arrest has been well documented despite prolonged resuscitation [28-34]. There are case reports in which patients have made complete recoveries after hours of CPR followed by more hours of extracorporeal life support (ECLS). In one case, a 65-year-old female survived neurologically intact after cardiac arrest for over seven hours; during the resuscitation, she received almost five hours of CPR followed by almost four hours of cardiopulmonary bypass [35].

Resuscitative efforts should only be withheld if there is a nonsurvivable injury or fatal illness, if the body is so frozen that chest compressions are impossible, or if the nose and mouth are blocked with snow or ice [6,36]. Efforts should be continued, sometimes for several hours [21].

Various clinical and biochemical markers can help inform the decision whether to continue resuscitation [37-39]. However, a single marker should not be used alone to decide whether to continue resuscitation.

●Hyperkalemia – There are no reported cases of survival when the patient's serum potassium concentration exceeds 12 mEq/L (mmol/L). When available, a central venous blood sample should be used to measure the serum potassium [40]. Extreme hyperkalemia reflects cell lysis and may predict a futile resuscitation. Case reports suggest that lower cutoffs, such as 8 mEq/L (mmol/L) should not be used [41], except for avalanche victims. Cardiac arrest in avalanche victims is usually caused by asphyxia rather than hypothermia. For avalanche victims, a retrospective study reported that the optimal cutoff for potassium is 7 mEq/L (mmol/L) [42]. However, potassium measurements may vary by over 3 mEq/L (mmol/L) depending on the sampling site (central venous is more accurate compared with peripheral venous or arterial) and may be falsely elevated because of hemolysis [40].

●Other blood markers – Evidence of intravascular thrombosis (fibrinogen concentration below 50 mg/dL [1.5 mmol/L]), ammonia concentrations above 420 mcg/dL (250 mmol/L), and elevated blood lactate or serum sodium or creatinine are additional indicators of a poor prognosis [43].

●Limited role for end-tidal CO₂ (EtCO₂) – Do not terminate resuscitation for low EtCO₂ (less than 10 mmHg) in hypothermic patients with cardiac arrest. Unlike in normothermic patients with cardiac arrest, low EtCO₂ may reflect a low metabolic rate rather than poor perfusion [44]. (See "Carbon dioxide monitoring (capnography)", section on 'Prognosis in cardiac arrest'.)

REWARMING — 

Rewarming should begin as soon as possible. The degree of hypothermia determines which rewarming technique should be implemented; these are divided into passive external rewarming, active external rewarming, and active internal core rewarming [28,45]. (See 'Stages of hypothermia' above.)

A table and algorithm outlining the emergency management of hypothermia in adults are provided (table 1 and algorithm 1).

Patient with cold-stress or mild-moderate hypothermia

●Cold-stress or mild hypothermia – This can be treated with passive external rewarming, but active external rewarming should be added, if possible, to decrease metabolic load and increase comfort. Active external rewarming may be necessary for older adults, malnourished patients, refractory mild hypothermia, and patients with cardiovascular disease or other comorbidities. (See 'Passive external rewarming' below and 'Active external rewarming' below.)

●Moderate hypothermia – This requires active rewarming, usually with active external rewarming. Patients with refractory moderate hypothermia should be treated with active internal rewarming or extracorporeal rewarming. (See 'Active external rewarming' below and 'Active internal (core) rewarming' below and 'Extracorporeal life support' below.)

Passive external rewarming — During passive external rewarming, wet clothing should be removed and the patient covered with blankets or other types of insulation. The resulting reduction in heat loss combines with the patient's intrinsic heat production to cause rewarming. Room temperature should be maintained at approximately 28°C (82°F), if feasible. If available, active external rewarming is routinely added to passive rewarming for patient comfort and to decrease cardiovascular energy requirements. (See 'Active external rewarming' below.)

Passive external rewarming requires physiologic reserves sufficient to generate heat by shivering or by increasing the metabolic rate. Passive external rewarming alone may be unsuccessful in the setting of glycogen depletion, sepsis, or hypovolemia, especially in older adults. Many older adults lack capacity for normal metabolic and cardiovascular homeostasis and require active rewarming.

The recommended rate of rewarming varies between 0.5 and 2°C/hour (1 and 4°F/hour). Failure to rewarm should alert the clinician to the possibility of additional causes of hypothermia other than environmental exposure (table 3) [46]. (See 'Failure to rewarm' below and "Accidental hypothermia in adults: Clinical manifestations and evaluation", section on 'Differential diagnosis'.)

Active external rewarming — During active external rewarming, some combination of warm blankets, heating pads, radiant heat, or forced warm air can be applied directly to the patient's skin. We prefer forced air rewarming because it is simple and effective. It is not necessary to leave the extremities uncovered when using forced air rewarming [47]. Temperature regulation devices traditionally used for therapeutic hypothermia have been used successfully but are more expensive than forced air warming systems [48].

Rewarming should not be performed in a warm bath since the associated peripheral vasodilation can contribute to afterdrop. (See 'Prevent core temperature afterdrop/rescue collapse' above.)

Active external rewarming is an effective initial strategy for most spontaneously perfusing hypothermic patients. Combining active core rewarming techniques with active external rewarming (eg, forced air rewarming) can minimize afterdrop in patients with severe hypothermia [49]. There is no evidence that prehospital external rewarming is deleterious for patients with accidental hypothermia [47].

Body surface burns are a possible complication of rewarming with heating pads. The combination of decreased sensation and reduced blood flow, minimizing local heat dissipation, predisposes a hypothermic patient to burns from heating pads. Heating pads should not be used without a barrier, such as a sheet or thin towel. The skin should be frequently assessed for developing burns during active external rewarming with heating pads. Burns are not a concern with forced air rewarming.

The core temperature of a patient rewarmed using either active external or active internal rewarming techniques generally increases at a rate of at least 2°C/hour (4°F/hour). Colder patients tend to rewarm at faster rates [50]. There is no evidence that increasing the rate of rewarming improves outcomes in spontaneously perfusing patients.

Role of airway rewarming and heated IV fluids — Airway rewarming and heated IV or IO fluids may help to decrease heat loss when active rewarming methods are used but should not be primary rewarming methods.

Airway rewarming with heated, humidified oxygen decreases insensible heat loss but provides limited benefit. Most humidifiers cannot exceed 41°C (106°F) without modification. The ideal setting is 45°C (113°F).

The benefit of administering warm IV fluid is limited. Even large volumes have minimal effect on raising temperature. However, a patient receiving IV fluids while being rewarmed should receive fluids warmed to 40 to 42°C (104 to 108°F) to avoid inadvertent lowering of core temperature.

Patient with severe hypothermia — Severe hypothermia without cardiac arrest should be treated with active internal rewarming or with extracorporeal rewarming (ECLS). We initiate treatment with less invasive rewarming techniques, progressively adding more invasive techniques as needed. (See 'Active internal (core) rewarming' below and 'Extracorporeal life support' below.)

For a patient in cardiac arrest or with cardiovascular instability, and in special cases such as frozen limbs preventing IV or IO access, rewarming should be performed with ECLS, if available. ECLS should also be used when rewarming is inadequate despite the other measures. Transferring patients to tertiary care centers is generally unnecessary unless they require ECLS. (See 'Extracorporeal life support' below.)

The evidence for ECLS in severe hypothermia and the Hypothermia Outcome Prediction after ECLS (HOPE) survival probability score, which provides a probability of survival in patients with hypothermic cardiac arrest treated with ECLS, are discussed further below. (See 'Severe hypothermia without cardiac arrest treated with extracorporeal life support' below and 'Benefit of extracorporeal life support' below and 'HOPE survival probability score' below.)

Active internal rewarming can be used alone or combined with active external rewarming. (See 'Active external rewarming' above.)

Active internal (core) rewarming — Active internal rewarming (also called active core rewarming) provides internal heat transfer. Endovascular rewarming (which requires an endovascular temperature control catheter) is the method of choice for patients not requiring ECLS or if ECLS is not available. If an endovascular temperature control catheter is not available, alternative rewarming methods include irrigation with warmed isotonic crystalloid of the thorax via the pleural space or the peritoneum. We generally prefer thoracic lavage because it is easier to exchange fluid with thoracic lavage than with peritoneal lavage. However, the choice between thoracic irrigation and peritoneal irrigation depends on the circumstances and usual practice at a given center. If a chest tube is indicated, thoracic lavage is preferred.

●Endovascular rewarming – Endovascular temperature-control catheters are effective and less invasive alternatives to extracorporeal blood rewarming in patients who are not in cardiocirculatory arrest [51,52]. These devices are designed to administer therapeutic hypothermia. They use a femoral catheter that circulates temperature-controlled water inside a closed catheter tip in the femoral vein, warming or cooling blood as it flows past the tip. The thermostat is connected to an esophageal temperature probe, and the machine is set to rewarm rapidly until it approaches the target temperature. The rewarming rate is then slowed to avoid overshooting. Some devices have a fail-safe mechanism that shuts the machine down when the core temperature sensor reads <30°C (86°F). This mechanism can be overridden by connecting the machine to a probe in warm water at 30°C (86°F) until esophageal temperature reaches 30°C (86°F). Rewarming rates have been reported in the range of 2 to 3°C/hour (4 to 5°F/hour). However, endovascular rewarming has not been demonstrated to be more rapid than other rewarming techniques [53].

●Peritoneal and pleural irrigation – When endovascular warming devices are unavailable, alternative approaches include peritoneal and pleural irrigation with warmed isotonic fluid. Peritoneal irrigation can be performed by infusing 10 to 20 mL/kg of isotonic saline warmed to approximately 42°C (108°F). The fluid is left in the peritoneal cavity for 20 minutes and then removed. The usual exchange rate is 6 L/hour, most easily accomplished with two catheters, one for instillation and one for drainage [22]. Catheter placement is similar to that used for diagnostic peritoneal lavage. (See "Diagnostic peritoneal lavage (DPL) or aspiration (DPA)", section on 'DPL/DPA technique'.)

Pleural irrigation should be reserved for the severely hypothermic patient who is not rewarming unless the patient has an alternative indication for a chest tube. Pleural irrigation can be accomplished by placing two large bore (ie, 36 to 40 French) thoracostomy tubes in the same hemithorax, one cephalad and anterior and a second caudad and posterior in the chest cavity. This should be performed in the right hemithorax in a perfusing patient, as left-sided chest tubes may precipitate myocardial irritability. Either hemithorax may be used in a patient in cardiac arrest [54]. Warm isotonic saline at a temperature of 40 to 42°C (104 to 108°F) is infused in 200 to 300 mL amounts through the anterior tube and allowed to drain posteriorly [55]. Leave the inferior tube for drainage after rewarming. (See "Thoracostomy tubes and catheters: Placement techniques and complications".)

●Esophageal heat transfer device – A minimally invasive method of rewarming in a case of severe accidental hypothermia, using an esophageal heat transfer device, has been described [56]. The device is an orogastric tube with additional lumens designed to circulate temperature-controlled water in a closed circuit that was developed for cooling in targeted temperature management. This device shows promise for use in accidental hypothermia. As with endovascular warming devices, use of an esophageal heat transfer device avoids volume overload and electrolyte shifts.

●Techniques to avoid – Gastric or colonic irrigation can cause fluid and electrolyte fluctuations, and both are best avoided. In addition, gastric lavage risks pulmonary aspiration in critically ill patients. The surface available for heat exchange in the bladder is too small to be of much benefit.

Extracorporeal life support — Several ECLS techniques can be used to treat hypothermic patients by rewarming blood outside the body: venovenous rewarming, hemodialysis, continuous arteriovenous rewarming (CAVR), extracorporeal membrane oxygenation (ECMO), and cardiopulmonary bypass (CPB) [30,57-63]. Venoarterial ECMO and CPB are usually reserved for patients with hemodynamic instability or cardiac arrest, patients who do not rewarm with less invasive active internal rewarming techniques, and those with completely frozen extremities or severe rhabdomyolysis with hyperkalemia [64]. The technique selected depends upon clinical circumstances and available resources.

Evidence is insufficient to determine the optimal rewarming rate with ECLS. Slower rewarming rates during ECLS have been associated with improved survival and neurologic outcomes. A secondary analysis of a meta-analysis of 658 patients with accidental hypothermia treated with ECLS-assisted rewarming found the optimal cutoff value for good neurological outcomes was less than 5°C/hour (9°F/hour) [65].

●Venovenous rewarming/hemodialysis – A venovenous rewarming circuit (without ECMO) requires two IV lines, one of which must be a central venous catheter. A femoral catheter is preferred to avoid precipitating an arrhythmia from irritation of the right atrium. Flow rates average 150 to 400 mL/minute and rewarming occurs at a rate of 2 to 3°C/hour (4 to 5°F/hour), but does not provide direct oxygenation or circulatory support. Hemodialysis can achieve similar rewarming rates [66-68]. (See "Extracorporeal life support in adults in the intensive care unit: Overview" and "Acute hemodialysis prescription".)

●Continuous arteriovenous rewarming – CAVR can rewarm at 3 to 4°C/hour (5 to 7°F/hour) but requires a systolic blood pressure of 60 mmHg [57]. CAVR utilizes percutaneous 8.5 French femoral catheters to direct blood through a countercurrent heat exchanger. Venovenous ECMO is similar to CAVR but can perform both oxygenation and rewarming. (See "Extracorporeal life support in adults: Management of venovenous extracorporeal membrane oxygenation (V-V ECMO)".)

●Extracorporeal membrane oxygenation – Venoarterial ECMO provides oxygenation as well as circulatory support, treats noncardiogenic pulmonary edema (a common complication of hypothermia), and appears to improve survival [60,69-71]. Venoarterial ECMO is generally preferred to CPB because it allows prolonged support and requires less surgical resources. ECMO requires the administration of heparin, limiting its use in cases of hypothermia associated with bleeding, such as due to trauma [72]. In a meta-analysis (23 observational studies, 464 patients), overall survival after hypothermic cardiac arrest was better with ECMO compared with CPB (44 versus 31 percent, RR 1.41, 95% CI 1.11-1.80). In survivors, 80 percent had good neurologic outcomes that were slightly worse with ECMO compared with CPB (75 versus 87 percent, relative risk [RR] 0.86, 95% CI 0.75-0.99) [73]. (See "Extracorporeal life support in adults: Management of venoarterial extracorporeal membrane oxygenation (V-A ECMO)".)

●Cardiopulmonary bypass – CPB can rewarm at rates up to 9.5°C/hour (17°F/hour) but requires significant surgical resources [58,74]. Portable CPB units have been used to treat hypothermia [75]. In a retrospective series of 32 patients with severe hypothermia and cardiac arrest treated with CPB, the long-term survival rate was 47 percent with minimal sequelae [30]. This relatively high survival rate likely reflects that many patients in the series were young and previously healthy, and asphyxia and anoxic encephalopathy rarely preceded the development of hypothermia. (See "Initiation of cardiopulmonary bypass".)

Monitoring during rewarming

Temperature — Core temperature should be monitored closely to assess the adequacy of therapy and to prevent iatrogenic hyperthermia.

Esophageal temperature is the most accurate method to track the progress of rewarming, especially in mechanically ventilated patients. An esophageal probe inserted into the lower third of the esophagus (approximately 24 cm below the larynx in adults) provides a near approximation of cardiac temperature [50]. An instructional video demonstrating esophageal temperature measurement is available in two references [76,77]. Rough movement and activity should be avoided because they may provoke ventricular fibrillation.

Use of rectal temperature is reasonable in conscious patients. Rectal probe readings may rise following peritoneal lavage or fall if adjacent to cold feces. Bladder temperatures are commonly used and are adequate in mild to moderate hypothermia but should not be used in critical patients during rewarming. Changes in rectal and bladder temperatures significantly lag behind core temperature changes during rewarming. Core temperature may be increasing in response to rewarming while rectal and bladder temperatures are still dropping.

Oxygenation — Oxygenation should be monitored continuously. However, the response of pulse oximeters placed on the finger is slowed by hypothermia [78]. Topical nitroglycerin may help. Probes placed on the ears or forehead appear to be less affected by decreased body temperature and the associated peripheral vasoconstriction.

Anticipated complications — Complications, especially pulmonary, renal, and neurologic, are common during and after rewarming. These can occur late and can be fatal. Patients with significant hypothermia should be monitored closely in an intensive care setting throughout their resuscitation, including cardiac monitoring and serial laboratory measurements. Issues with interpretation of hematocrit, tests of coagulation, and blood gasses in hypothermia are discussed separately. (See "Accidental hypothermia in adults: Clinical manifestations and evaluation", section on 'Laboratory studies'.)

●Hypotension – Patients with moderate or severe hypothermia frequently become disproportionately hypotensive from severe dehydration and fluid shifts during rewarming [5,21,22]. Aggressive fluid resuscitation with isotonic crystalloid is first-line treatment. Use of vasopressors may be necessary. (See "Treatment of severe hypovolemia or hypovolemic shock in adults" and "Use of vasopressors and inotropes".)

●Cardiac arrest – Bradycardia, hypoxemia, and ventricular arrhythmias were the strongest predictors of developing hypothermic cardiac arrest in a study of 182 patients with severe accidental hypothermia presenting with spontaneous circulation [79]. Fifty-two patients (29 percent) developed hypothermic cardiac arrest, while 130 were warmed to normothermia without cardiac arrest. No single parameter could be used to predict hypothermic cardiac arrest. Afterdrop can also precipitate cardiac arrest [9]. (See 'Prevent core temperature afterdrop/rescue collapse' above.)

●Electrolyte abnormalities – Rewarming can be accompanied by rapid or unpredictable changes in electrolyte concentrations. We reassess electrolytes approximately every four hours in moderate and severe hypothermia [80]. Hypothermia may obscure normal premonitory ECG changes commonly associated with hyperkalemia. Electrolyte abnormalities should be treated as in normothermic patients. (See "Clinical manifestations and treatment of hypokalemia in adults" and "Treatment and prevention of hyperkalemia in adults" and "Treatment of hypocalcemia" and "Treatment of hypercalcemia" and "Overview of the treatment of hyponatremia in adults" and "Treatment of hypernatremia in adults".)

●Atrial arrhythmias – Atrial arrhythmias often develop during rewarming but are generally benign and do not require treatment except in rare cases of rapid ventricular response. They usually resolve spontaneously during or after rewarming. (See "Overview of the acute management of tachyarrhythmias" and "Control of ventricular rate in patients with atrial fibrillation who do not have heart failure: Pharmacologic therapy".)

●Rhabdomyolysis and multiple organ dysfunction syndrome – These can develop during rewarming. (See "Rhabdomyolysis: Clinical manifestations and diagnosis" and "Sepsis syndromes in adults: Epidemiology, definitions, clinical presentation, diagnosis, and prognosis", section on 'Multiple organ dysfunction syndrome'.)

●Hyperglycemia and pancreatitis – Hyperglycemia persisting during rewarming suggests pancreatitis or diabetic ketoacidosis. Pancreatitis is common in hypothermia [81]. Pancreatitis may lead to diabetic ketoacidosis in hypothermic patients [82]. Insulin is ineffective below 30°C (86°F). (See "Clinical manifestations, diagnosis, and natural history of acute pancreatitis" and "Diabetic ketoacidosis and hyperosmolar hyperglycemic state in adults: Clinical features, evaluation, and diagnosis" and "Diabetic ketoacidosis in adults: Treatment".)

●Coagulopathy – Treatment of hypothermia-induced coagulopathy consists of rewarming. Administration of clotting factors is ineffective.

●Stress cardiomyopathy – Takotsubo cardiomyopathy has been reported [83]. (See "Clinical manifestations and diagnosis of stress (takotsubo) cardiomyopathy" and "Management and prognosis of stress (takotsubo) cardiomyopathy".)

Failure to rewarm — We perform the following measures in a patient who fails to rewarm:

●First, we ensure that appropriately aggressive rewarming techniques are in progress.

●We address readily reversible contributing causes, such as hypoglycemia.

●For a patient who fails to increase their core temperature faster than 0.7°C/hour (1.3°F/hour) despite appropriate rewarming efforts, we treat empirically with broad-spectrum IV antibiotics, even if there is no obvious source of infection.

●We investigate and address other potential causes of failure to rewarm, including sepsis, endocrine dysfunction such as adrenocortical insufficiency or hypothyroidism, toxins, and lesions of the central nervous system (table 3). A patient who fails to respond to aggressive rewarming measures may also have impaired thermogenesis from any of the following causes:

•Infection/sepsis – A hypothermic patient with a likely source of infection should receive broad-spectrum IV antibiotics. Infection compromises the inherent capacity for thermogenesis and often manifests with slower rewarming rates. In a study of 88 hypothermic patients treated at a single urban emergency department, patients with hypothermia and an underlying infection had slower rewarming rates (<0.67°C/hour [1.2°F/hour]) compared with patients without infection (>1.67°C/hour [3°F/hour]) despite the use of similar rewarming techniques [46]. Patients with infection accounted for the most deaths. No patient died from a hypothermia-induced arrhythmia.

•Adrenocortical insufficiency – Although no evidence exists for this practice, we treat potential adrenocortical insufficiency with a single dose of glucocorticoid in a patient with moderate or severe hypothermia who fails to rewarm at an expected rate despite aggressive rewarming measures. We administer dexamethasone 4 to 6 mg IV or hydrocortisone 100 mg IV. Hydrocortisone has both glucocorticoid and mineralocorticoid activity. Dexamethasone is not measured in serum cortisol assays, thus is preferable if testing of adrenal function is performed. (See "Treatment of adrenal insufficiency in adults", section on 'Adrenal crisis'.)

•Hypothyroidism – We empirically treat hypothyroidism if we think it may be contributing to a patient's failure to rewarm. Clues include a history of hypothyroidism, a medication list that includes thyroid hormone, or presence of a surgical scar over the anterior neck. Severe hypothyroidism (myxedema coma) may be treated empirically with levothyroxine 200 to 400 mcg IV (table 4) after blood is drawn for thyroid function studies. Concurrent treatment for adrenal insufficiency is also required. (See "Myxedema coma", section on 'Treatment'.)

PROGNOSIS AND NEUROLOGIC OUTCOME

General outcomes — Otherwise healthy patients who develop accidental hypothermia and are hemodynamically stable at presentation usually survive neurologically intact [84,85]. Evidence is limited and conflicting regarding the prognosis of patients with severe hypothermia [63,86,87]. Factors associated with a worse prognosis are summarized in the table (table 5).

In a large retrospective study of hypothermia outcomes, factors associated with death within 24 hours of presentation included prehospital cardiac arrest, low or absent blood pressure on presentation, elevated blood urea nitrogen (BUN), and the need for tracheal intubation [88]. Outcome did not correlate with core temperature at presentation. However, these results should be interpreted with caution as patients were not followed beyond 24 hours, no validation study was performed, and results reflect pre-1990s management.

Outcomes of accidental hypothermia sustained during outdoor activities have not been systematically studied. In the International Hypothermia Registry, including 201 nonconsecutive cases, the main cause of hypothermia was "mountain accidents," mostly in young males [89]. Survival in patients without cardiac arrest was 95 percent. Another study found that younger age, outdoor cooling, and higher blood pressures were associated with increased survival [90].

There are a few observational studies of outcomes among urban dwellers. In a registry study involving 358 patients treated at 12 Japanese emergency departments, factors associated with increased in-hospital mortality from accidental hypothermia included age ≥75 years, frailty, hemodynamic instability, and hyperkalemia [91]. A retrospective, single-center study of 67 patients found that increased in-hospital mortality was associated with age ≥70 years, mean arterial pressure <90 mm Hg, pH <7.35, creatinine >1.5 mg/dL (133 micromol/L), and confusion [92].

Severe hypothermia without cardiac arrest treated with extracorporeal life support — Evidence is mixed regarding the benefit of extracorporeal life support (ECLS) in patients with severe hypothermia who do not sustain cardiac arrest.

A multicenter, prospective study in Japan (ICE-CRASH) of mostly older adult patients (mean age 81 years) with severe accidental hypothermia found that in 185 patients with circulatory instability without cardiac arrest (systolic blood pressure <60 mm Hg or heart rate <50 beats per minute), extracorporeal membrane oxygenation (ECMO) did not significantly improve 28-day survival (77 versus 76 percent) or favorable neurologic outcomes (71 versus 60 percent) and led to more adverse events with an increased frequency of bleeding complications [71]. These results may not be generalizable to other populations because patients were older and hypothermia occurred predominantly indoors.

In a multicenter, retrospective study of severely hypothermic patients (core temperature <28°C [82°F]) with preserved spontaneous circulation in Europe, rewarming with ECLS (50 patients) compared with other less invasive methods (85 patients) was associated with improved survival (79 versus 59 percent) [90]. In a propensity-score matched cohort analysis, less invasive rewarming methods were associated with increased risk of death (41 versus 21 percent, RR 2.0, 95% CI 0.99-7.37).

Patients who sustained cardiac arrest — Witnessed cardiac arrest is associated with relatively favorable neurologic outcomes, while unwitnessed cardiac arrest and asphyxia are associated with worse outcomes (table 5) [85]. When cardiac arrest occurs during rescue, it is referred to as rescue collapse ("circumrescue collapse" in older literature). Afterdrop can be a contributing factor. (See 'Prevent core temperature afterdrop/rescue collapse' above.)

A systematic review of 214 patients with accidental hypothermia and witnessed cardiac arrest found survival to hospital discharge was approximately 73 percent with favorable neurologic outcomes in 89 percent (102 of 115 discharged patients) for whom outcome data was available [93]. The mean core temperature at arrest of 24±3°C (75.2±5.4°F), with a range of 15.2 to 32.2°C (59.4 to 90°F). The highest core temperature in a survivor was 29.4°C (84.9°F).

Survival in 73 patients with cardiac arrest was 36 percent in the International Hypothermia Registry [89]. Patients with witnessed cardiac arrest were more likely to survive (71 versus 30 percent). Other predictors of survival included return of spontaneous circulation before rewarming, normal potassium, and absence of asphyxia.

In a Japanese registry study of 754 hypothermic patients with out-of-hospital cardiac arrest, one-month survival was approximately 6 percent [94]. Factors associated with increased mortality included pH <6.9, lactate >13.3 mmol/L (120 mg/dL), and potassium >7 mmol/L (7 mEq/L).

In general, hypothermia associated with asphyxia by drowning or avalanche burial has a poor prognosis [60,85,95]. Good neurologic outcome after unwitnessed cardiac arrest in an avalanche is very rare [96]. However, witnessed cardiac arrest after extrication from an avalanche in patients with a core temperature below 24°C (75°C/hour) is associated with relatively favorable neurologic outcomes [97].

Benefit of extracorporeal life support — ECLS may improve survival and neurologically intact survival in select patients with hypothermic cardiac arrest [60]. The rate of neurologically intact survival in patients with hypothermic cardiac arrest treated with ECLS (ECMO or cardiopulmonary bypass [CPB]) has been estimated to be 40 to 63 percent [28,73,98,99]. However, there are also many reports of neurologically intact survival without the use of ECLS following cardiac arrest associated with hypothermia [55,84,100-102]. (See 'Extracorporeal life support' above.)

In a meta-analysis of 44 observational studies and 40 case reports, including a total of 658 patients with accidental hypothermia treated with ECLS, the rate of survival with good neurologic outcome was 40 percent (compared with a historical survival rate of 10 to 37 percent without ECLS) [63]. Absence of asphyxiation, serum potassium <5 mmol, and female sex were associated with increased survival with good neurologic outcome. Slower rewarming rates using extracorporeal circulation were also associated with higher rates of neurologically intact survival, but the effect size was small.

Other studies have found similar results. A multicenter, prospective study in Japan (ICE-CRASH) found that in 57 patients with cardiac arrest, ECLS was associated with improved 28-day survival (58 versus 21 percent, odds ratio [OR] 0.17, 95% CI 0.05-0.58) and favorable neurologic outcome (42 versus 15 percent, OR 0.22, 95% CI 0.06-0.81) [71]. In a study of patients initially treated by the Danish Air Force Search and Rescue Service, 39 percent (21 of 54 patients) treated with ECLS survived to hospital discharge [103]. A retrospective registry study of 98 nonasphyxiated patients in hypothermic cardiac arrest treated with ECLS reported a survival rate of 53 percent, with 94 percent of survivors having good neurologic outcomes; lower age and lower serum lactate concentration were favorable prognostic factors [104]. Another registry study of 127 patients in hypothermic cardiac arrest treated with ECLS reported a survival rate of 49 percent, with 89 percent of survivors having good neurologic outcomes; lower age and lower serum potassium and lactate concentrations were associated with a higher likelihood of survival [105]. A systematic review of 221 patients with unwitnessed hypothermic cardiac arrest rewarmed with ECLS found a survival rate of 27 percent. Favorable prognostic factors included female sex, nonasphyxial mechanism of cooling, pulseless electrical activity as the initial rhythm, and normal serum potassium [106]. Most of the survivors (83 percent) had no neurologic deficit, including patients whose presenting rhythm was asystole.

The reported benefit for neurologically intact survival from ECLS compared with other treatments is likely overestimated as patient populations differ substantially among studies. Some studies included hypothermic patients with associated trauma or asphyxia, such as avalanche victims [84], while others focused on isolated hypothermia [104]. Studies of treatment with ECLS have included predominantly young, healthy, nonasphyxiated patients, while studies involving other treatments typically include older patients with more comorbidities. The benefits of ECLS are limited by rescue collapse, which is a witnessed cardiac arrest. For example, in a study of 45 matched pairs of patients with severe hypothermia without cardiac arrest initially rewarmed with ECLS, survival was lower in patients who ultimately developed cardiac arrest (24 versus 48 percent) [107].

HOPE survival probability score — The Hypothermia Outcome Prediction after ECLS (HOPE) survival probability score yields an expected chance of survival as a percentage. The HOPE score is the most widely used instrument in Europe to determine eligibility for ECLS in patients with hypothermic cardiac arrest [86]. The score involves a complex equation that can be calculated online at www.hypothermiascore.org.

We agree with other experts that patients with a HOPE score <10 percent should not be rewarmed [108]. This threshold is estimated to result in a false positive rate <40 percent (patients who die in spite of being rewarmed) and false negative rate <0.5 percent (patients who would have survived if they had been rewarmed). This threshold creates a reasonable compromise that resembles the patient populations from the initial derivation and validation sets of the original HOPE study. Using a lower threshold would have greatly increased the false positive rate while not significantly decreasing the false negative rate, which depends on the distribution of HOPE scores in a population.

The HOPE score was developed based on the outcomes of 286 patients: 237 patients from 18 published studies and 49 additional patients from hospital data. Overall survival was 37 percent. Favorable prognostic factors at hospital admission were female sex, mechanism other than asphyxiation (ie, exposure or immersion rather than submersion or avalanche burial), greater age (in contrast to HELP registry study [104]), lower serum potassium concentration, shorter duration of cardiopulmonary resuscitation (CPR), and lower core temperature. Witnessed arrest and cardiac activity (ie, pulseless electrical activity or ventricular fibrillation rather than asystole) were associated with improved survival but were not included in the score. In a retrospective, external validation study of the HOPE score (122 patients), overall survival was 42 percent while the average HOPE scores were 38 percent and the negative predictive value of HOPE scores <10 percent was 97 percent [87].

Individual factors such as age or serum potassium (a criterion commonly used to determine eligibility for ECLS) have limited prognostic value. In the original HOPE cohort, the median age of survivors was 40 (range 18 to 56), while the median age of nonsurvivors was 29.5 (range 13 to 54) [86]. Compared with serum potassium alone, use of the HOPE score would have avoided unsuccessful rewarming in 27 percent of patients with a potassium ≤12 mmol/L without any additional deaths.

The HOPE score should be interpreted with caution since it is based on data from retrospective observational studies and has methodologic limitations. Possible problems include selection bias and substantial overlap between groups for some parameters, such as age, duration of CPR (median 106 minutes for survivors and 120 minutes for nonsurvivors), and core temperature (median 23°C for survivors and 25°C for nonsurvivors).

SOCIETY GUIDELINE LINKS — 

Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Management of environmental emergencies" and "Society guideline links: Hypothermia".)

INFORMATION FOR PATIENTS — 

UpToDate offers two types of patient education materials, "The Basics" and "Beyond the Basics." The Basics patient education pieces are written in plain language, at the 5^th to 6^th grade reading level, and they answer the four or five key questions a patient might have about a given condition. These articles are best for patients who want a general overview and who prefer short, easy-to-read materials. Beyond the Basics patient education pieces are longer, more sophisticated, and more detailed. These articles are written at the 10^th to 12^th grade reading level and are best for patients who want in-depth information and are comfortable with some medical jargon.

Here are the patient education articles that are relevant to this topic. We encourage you to print or e-mail these topics to your patients. (You can also locate patient education articles on a variety of subjects by searching on "patient info" and the keyword(s) of interest.)

●Basics topic (see "Patient education: Hypothermia (The Basics)")

SUMMARY AND RECOMMENDATIONS

●Stages and initial management – Hypothermia is defined as a core temperature below 35°C (95°F) and can be further classified by severity, which has a large impact on management (table 2). (See 'Stages of hypothermia' above.)

The initial management of hypothermia is directed toward prevention of further heat loss, resuscitation, and rewarming (table 1 and algorithm 1). Tracheal intubation should be performed in patients with respiratory distress or who cannot protect their airway. (See 'Resuscitation' above.)

●Prevent core temperature afterdrop and rescue collapse – Unless the individual has only mild hypothermia and is already walking, they should not be allowed to stand or walk and should be extricated from the cold environment in a horizontal position whenever possible. Afterdrop refers to the continued decrease of core temperature during rewarming, which occurs primarily from peripheral vasodilation and can contribute to precipitous hypotension, inadequate coronary perfusion, and ventricular arrhythmias ("rescue collapse"). (See 'Prevent core temperature afterdrop/rescue collapse' above.)

●Avoid further cold exposure and precipitating arrhythmias – Remove wet clothing and insulate from further cold exposure. Ambient temperature of the treatment area should be maintained at approximately 28°C (82°F), if feasible. Avoid rough handling of the moderate or severe hypothermic patient since that can precipitate arrhythmias, including ventricular fibrillation. (See 'Avoid further cold exposure and precipitating arrhythmias' above.)

●Assess perfusion – Before we decide that a patient has a nonperfusing rhythm, we check for a central pulse for up to a full minute using a continuous-wave Doppler, if available. (See 'Assessing perfusion' above.)

●Patient not perfusing/cardiac arrest – In a patient without perfusion who has an organized rhythm on a cardiac monitor or cardiac contractions seen on echocardiogram, we suggest withholding chest compressions (Grade 2C). If the patient develops asystole, does not have cardiac activity on bedside ultrasound, or is rewarmed to >28°C (82°F) and still has pulseless electrical activity (PEA), chest compressions should be started immediately. Cardiopulmonary resuscitation (CPR), including chest compressions, should be initiated in patients with accidental hypothermia and cardiac arrest except in circumstances described above. (See 'Perfusion not detected' above.)

In a patient with a core temperature <30°C (86°F), we suggest not administering advanced cardiac life support (ACLS) medications (epinephrine, amiodarone) (Grade 2C) and if they have a ventricular arrhythmia, we suggest attempting defibrillation up to three times (Grade 2C). CPR should continue until the patient is rewarmed to 30°C (86°F), at which point renewed attempts at defibrillation and resuscitation with ACLS medications are undertaken. (See 'Organized cardiac rhythm and role of chest compressions' above and 'Ventricular arrhythmias or asystole' above.)

The neuroprotective effects of low temperature may allow recovery following prolonged cardiac arrest. We terminate resuscitative efforts if the patient does not achieve return of spontaneous circulation after warming to 32°C (90°F). (See 'Duration of resuscitation' above.)

●Patient with cold-stress or mild hypothermia – We treat with passive external rewarming and active external rewarming, if available, to decrease metabolic load and increase comfort, especially in patients with refractory mild hypothermia or comorbidities. (See 'Patient with cold-stress or mild-moderate hypothermia' above and 'Passive external rewarming' above.)

●Patient with moderate hypothermia – Recovery from moderate hypothermia requires the addition of exogenous heat, thus passive external rewarming alone will be unsuccessful since patients cannot generate sufficient heat by shivering or by increasing the metabolic rate. In a patient with moderate hypothermia, we suggest rewarming with active external rewarming rather than active internal rewarming or extracorporeal life support (ECLS) (Grade 2C). We prefer forced air rewarming because it is simple and effective. A patient with refractory moderate hypothermia should be treated with active internal rewarming or possibly extracorporeal rewarming. (See 'Patient with cold-stress or mild-moderate hypothermia' above and 'Active external rewarming' above.)

●Patient with severe hypothermia – In a patient with severe hypothermia and a perfusing rhythm, we recommend rewarming with active internal (core) rewarming or ECLS rather than active external rewarming (Grade 1C). If available, endovascular rewarming catheters are effective, less invasive, and more easily monitored than alternative techniques (eg, thoracic or peritoneal irrigation with warmed isotonic fluid). We initiate treatment with less invasive rewarming techniques, progressively adding more invasive techniques (eg, ECLS) as needed. (See 'Patient with severe hypothermia' above.)

In a salvageable patient with severe hypothermia and a nonperfusing cardiac rhythm or frozen extremities, we suggest treatment with ECLS rather than active internal rewarming (Grade 2C). A Hypothermia Outcome Prediction after ECLS (HOPE) survival probability score ≥10 percent is a reasonable criterion to determine salvageability, and can be calculated online at www.hypothermiascore.org. We prefer venoarterial extracorporeal membrane oxygenation (ECMO), but cardiopulmonary bypass (CPB) is an alternative if available. Active internal rewarming should be performed if ECLS is unavailable and transfer is not feasible. (See 'Extracorporeal life support' above and 'Patients who sustained cardiac arrest' above.)

●Monitoring and anticipated complications – Core temperature (esophageal probe in unconscious patients, rectal probe in conscious patients) should be monitored closely to assess the adequacy of rewarming and to prevent iatrogenic hyperthermia. Patients with moderate or severe hypothermia frequently become hypotensive and require aggressive fluid resuscitation. Complications are common during and after rewarming, including arrhythmia, electrolyte abnormalities, rhabdomyolysis, and bleeding diathesis. (See 'Hypotension' above and 'Monitoring during rewarming' above.)

●Failure to rewarm – For a patient who fails to rewarm appropriately (>0.7°C/hour) despite appropriately aggressive rewarming measures, we treat empirically for sepsis, adrenocortical insufficiency, and hypothyroidism, and address potential contributing causes, such as hypoglycemia. (See 'Failure to rewarm' above.)

ACKNOWLEDGMENT — 

The UpToDate editorial staff acknowledges C Crawford Mechem, MD, FACEP, who contributed to earlier versions of this topic review.