Chapter 110. Hypothermia

• Accidental hypothermia results from the unintentional decrease in core body temperature to lower than 35°C (95°F) and can be classified as mild (32.2°C to 35°C, or 90°F to 95°F), moderate (28°C to 32.2°C, 82°F to 90°F), or severe (<28°C, 82°F).
• Although hypothermia from environmental exposure is very common, several medical conditions may also predispose to hypothermia necessitating hospitalization and admission to the intensive care unit.
• Individuals at highest risk for hypothermia include the homeless, the mentally ill, trauma victims, outdoor workers, those at the extremes of age, those with serious underlying medical conditions, and those with ethanol or drug intoxication.
• Several organ systems are affected by hypothermia: clinical manifestations depend on the underlying cause and core body temperature. Below 30°C (86°F), shivering ceases, level of consciousness progressively declines, and cardiac arrhythmias become more common.
• In the initial stages, wet clothing should be removed promptly, continued heat loss must be prevented, and the underlying illness should be identified and treated.
• Rewarming is the primary treatment for moderate to severe hypothermia. For patients with a core body temperature above 32°C (90°F), passive and active external rewarming and supportive therapies are preferred; for patients with lower temperatures and those with hemodynamic instability, active core rewarming using body cavity lavage or extracorporeal blood warming may be considered.
• In addition to rewarming, all patients with hypothermia need continuous monitoring of cardiac status, intensive fluid resuscitation, and circulatory support.

Hypothermia is defined as a core body temperature lower than 35°C (<95°F). The severity of hypothermia is indicated by the degree to which the core body temperature is lowered and is classified as mild, moderate, or severe. Several medical conditions may increase the risk of hypothermia. Individuals at the extremes of age are at greatest risk. Risk of death from hypothermia is related to age, preexisting illnesses, nutritional status, and alcohol and drug intoxication.1 In cases of severe hypothermia, prompt intervention with rapid rewarming is crucial and may be life saving. This chapter discusses the pathophysiology, risk factors, clinical diagnosis, and management of hypothermia. The use of hypothermia as a treatment, such as after cardiac arrest, is covered in Chap. 16.

Body temperature is closely regulated through a balance between heat production and heat dissipation.2 The majority of endogenous heat production results from metabolic activity in the heart and liver. The skin accounts for 90% of heat loss and the lungs contribute the rest. Radiation cooling is the primary method of thermal load dissipation. The preoptic nucleus of the anterior hypothalamus is the thermal control center, which maintains body temperature at a given set value. In response to a decrease in core body temperature, the hypothalamus initiates mechanisms to conserve heat by cutaneous vasoconstriction and stimulation of muscular activity in the form of shivering.2,3 In a conscious individual, the appreciation of cold induces the individual to exercise, wear more clothes, or move to a warmer environment.

As the core temperature decreases below 35°C (95°F), the coordinated systems responsible for thermoregulation begin to fail because the physiologic responses to minimize heat loss are very limited.4Primary hypothermia (accidental hypothermia) refers to a spontaneous decrease of core body temperature, usually as result of exposure to cold environments without adequate protection. Environmental hypothermia results from a combination of heat loss by convection (degree of wind exposure), conduction, and radiation to the surrounding ambient air. Secondary hypothermia represents a complication of an underlying disorder. Some of the disorders and conditions that may predispose an individual to hypothermia by decreasing heat production, increasing heat loss, or interfering with the central or peripheral control of thermoregulation are listed in Table 110-1.1–9

From 1979 through 1995, 12,368 deaths were reported in the United States for which hypothermia was the underlying cause (an annual average of 723 and a rate of 0.3 deaths per 100,000 population).10 Approximately half of the deaths from hypothermia occurred in persons older than 65 years; the annual death rate from hypothermia in this age group was 1.2 per 100,000. The age-adjusted death rate for men was almost triple that for women. In addition, for persons older than 65 years, the death rate for men and women of black and other races was much higher than that for white men and women.11 Race-specific differences may reflect differences in socioeconomic determinants for factors that are important in the prevention of hypothermia, such as access to protective clothing, shelter, and medical care. Hypothermia may occur in any climate and during any season of the year. Hypothermia is most common among the elderly, the homeless or mentally ill, trauma victims, outdoor workers, children, and individuals with certain medical conditions as mentioned above.

Diagnosis

Historical facts do not always suggest hypothermia. When exposure to cold is obvious by history, the diagnosis is simple. Measurement of core body temperature is important in diagnosis of less overt presentations. Core temperature is best measured by using a cold-recording rectal probe thermometer capable of measuring temperatures as low as 25°C (77°F). Clinical manifestations differ with the severity of hypothermia, which is classified based on core temperature as mild (35°C to 32.2°C, 95°F to 90°F), moderate (<32.2°C to 28°C, <90°F to 82°F), and severe (<28°C, <82°F).2,3,5,7,12,13 This classification has implications for management because appropriate treatment depends on severity of the disorder, as described below. The onset of hypothermia is often insidious. Initial symptoms may be vague and include hunger, nausea, dizziness, chills, pruritus, or dyspnea. Extremity stiffness, weakness, and shivering may also be prominent. As core body temperature decreases, many patients no longer complain of cold, shivering disappears at temperatures below 32°C (<90°F), and muscles become rigid.5,14 At this point, the level of consciousness becomes markedly altered and systemic manifestations are readily evident. A severely hypothermic victim has a markedly decreased metabolic rate. As a consequence, the cerebral ischemic tolerance during cardiocirculatory arrest is considerably longer in contrast with the normothermic state.5,15,16 Therefore, one has to be very careful in assessing brain death while a patient remains hypothermic. Low temperatures cause the myocardium to become irritable and cardiovascular abnormalities are common.15 These may include initial tachycardia followed by progressive bradycardia with an increase in systemic vascular resistance. Arrhythmias are common at core temperatures below 32°C (90°F), and ventricular fibrillation may occur spontaneously when the temperature is below 28°C (82°F).14 Systemic blood pressure is often decreased in patients with severe hypothermia. Other clinical manifestations of hypothermia are listed in Table 110-2.

Laboratory Evaluation

Initial laboratory evaluations should be obtained to assess metabolic status and organ dysfunction. Recommended tests include blood glucose, electrolytes, renal and hepatic functions, complete blood count, and coagulation profile. Arterial blood gases should be obtained, and correction for temperature is not necessary.5,17,18 Electrolytes, hematocrit, and coagulation status change with rewarming, so frequent monitoring is necessary. Other laboratory tests such as thyroid function studies, cardiac isoenzymes, toxicologic screen, and cultures should be used selectively based on the clinical history and examination. Chest and abdominal radiographs should be obtained with the need for other radiographs dictated by the clinical situation. The following are common laboratory findings in patients with hypothermia.

Arterial Blood Gases

Typically, pH values increase as body temperature decreases (0.0147 and 1°C). Arterial pressures of O2 and CO2 also decrease with a decrease in temperature (7.2% and 4.4% and 1°C decrease in temperature), and the oxyhemoglobin dissociation curve is shifted to the left. However, because all arterial blood gas samples are warmed to 37°C (98.6°F) before values are measured, simply comparing uncorrected values measured at 37°C with the normal reference values at 37°C yields an accurate interpretation. Respiratory acidosis and metabolic acidosis are common findings in patients with moderate and severe hypothermia.17

Complete Blood Count

An increase in hematocrit secondary to decrease in plasma volume is common (2% for each 1°C decrease in temperature). White blood cell and platelet counts may decrease as temperature decreases.2,5

Coagulation Profile

A physiologic increase in coagulation occurs with hypothermia, and disseminated intravascular coagulation may occur with rewarming. Prolonged bleeding and clotting times are common. Prothrombin time and partial thromboplastin time may initially appear normal despite the presence of clinical coagulopathy because the values are measured after warming the blood sample to 37°C. Blood viscosity may be exacerbated in patients with cryoglobulinemia or cryofibrinogenemia, which is more common in elderly patients.

Serum Electrolytes

Recurrent evaluation of electrolytes is essential during rewarming because no consistent effect is present. Hypo- and hyperkalemia may complicate the course of hypothermia and either should be promptly corrected.

Serum Urea Nitrogen and Creatinine

These are almost always elevated because of decreased urinary clearance and decreased renal perfusion.

Blood Glucose

Especially when core body temperature is above 30°C (86°F), acute hypothermia may be associated with an initial elevation of blood glucose due to catecholamine-induced glycogenolysis, whereas subacute and chronic hypothermia produce glycogen depletion with subsequent hypoglycemia. Nevertheless, hypoglycemia is one of the most common causes of mild hypothermia in a hospitalized patient.

Other Laboratory Abnormalities

Hyperamylasemia is common and may be related to a preexisting pancreatitis or pancreatitis induced by hypothermia. Hyperamylasemia correlates with the severity of hypothermia and with mortality rate. Variable elevation in creatine phosphokinase levels may reflect underlying rhabdomyolysis.

Electrocardiographic Abnormalities

PR, QRS, and QT intervals may be prolonged. When body temperature decreases below 33°C (91.4°F), the J (Osborn) wave may be noted as a positive deflection in the left ventricular leads at the junction of the QRS and ST segments in 25% to 30% of patients.2,19–21 The presence of this wave is not pathognomonic and has no prognostic implication (Fig. 110-1).

The severity of hypothermia, clinical findings, and comorbid conditions of the patient are important factors for determining the aggressiveness of resuscitation techniques (Fig. 110-2). Actions in all patients should include prompt removal of wet clothing, protection against continued heat loss, continuous monitoring of cardiac status, and avoidance of rough movement and excess activity, which can precipitate ventricular fibrillation.20–25 Patients received in the hospital with moderate or severe hypothermia should be resuscitated until adequate rewarming has occurred or efforts are deemed unsuccessful.13,23,24

Initial Stabilization

Hypothermia should be confirmed by an accurate assessment of core temperature with a low reading thermometer. A rectal temperature is most practical, even though it may lag behind core changes. The probe should be inserted to an adequate depth but avoid cold fecal material. An esophageal temperature is an alternative to monitoring of rectal temperature, but readings may be falsely elevated in the intubated patient ventilated with warmed gas. In patients with moderate to severe hypothermia, temperature should be assessed continuously. The reliability of tympanic temperature devices has not been established in hypothermia.5 Other vital signs may be difficult to evaluate, but evidence of a spontaneous pulse or blood pressure should be aggressively sought. The use of a Doppler ultrasound device may be necessary. Pulse oximetry is unlikely to be accurate in the setting of hypothermia and poor perfusion.

Supplemental oxygen should be administered pending assessment of oxygenation. Endotracheal intubation is indicated unless the patient is alert and has intact airway reflexes. Orotracheal intubation is preferred due to the risk of traumatic bleeding with the nasal route. However, muscle rigidity of the jaw in moderate to severe hypothermia may preclude use of the oral route. Neuromuscular blockers are unlikely to be effective at temperatures below 30°C (86°F) and should be avoided. Topical vasoconstrictors and the use of a smaller endotracheal tube may facilitate blind nasotracheal intubation in the patient with some spontaneous respirations. Intubation is unlikely to induce dysrhythmias in hypothermic patients.8

A nasogastric or orogastric tube should be placed in patients with moderate or severe hypothermia to relieve gastric distention. A urinary catheter is also essential to monitor urine output and assess volume resuscitation efforts. Peripheral venous catheters are preferred over central venous access due to the potential to precipitate dysrhythmias. Needle electrodes are preferred for electrocardiographic monitoring. Intraarterial catheters for pressure monitoring should be used selectively. Pulmonary artery catheters are avoided due to potential dysrhythmias and the risk of vascular perforation. After rewarming, invasive monitoring may be warranted in complicated cases. Patients with moderate or severe hypothermia should be handled gently because movement and manipulation may precipitate dysrhythmias. Associated conditions requiring urgent intervention, such as traumatic injuries, hypoglycemia, or endocrinologic insufficiency, should be sought.

Volume Resuscitation

Patients with moderate or severe hypothermia are volume depleted and require rapid assessment of volume status and administration of fluids. A reasonable approach is to administer a 250- to 500-mL fluid challenge of warmed 5% dextrose in normal saline until further evaluation can be completed. During rewarming, increased fluid requirements are often necessary to prevent or treat hypotension. It has been recommended that lactated Ringer solution be avoided due to decreased ability of the liver to metabolize lactate.

In moderate or severe hypothermia, intravenous fluids heated to 40°C to 42°C (104°F to 107.6°F) should be administered. Heated intravenous fluids add little heat (unless very large volumes are infused) but do avoid the cooling effect of fluid therapy. Crystalloids can be heated in a microwave or commercial fluid warmer and should be mixed adequately before administration. Conductive heat loss can be minimized by using short segments of intravenous tubing and administering fluid as intermittent boluses.26 Alternatively, fluids can be heated to higher temperatures (60°C, 140°F) when long lengths of tubing cannot be avoided.

Hemoconcentration is usually present in the severely hypothermic patient. A low initial hematocrit suggests acute hemorrhage or preexisting anemia. Occasionally, transfusion of packed red blood cells may be necessary as part of acute resuscitation. Different blood warming devices are available to deliver warm red cell products. The patient should be monitored closely for clinical signs of fluid overload during rewarming.

Circulatory Support

Cardiopulmonary resuscitation is indicated in any patient with moderate to severe hypothermia in whom no pulse is present after appropriate evaluation or who has a nonperfusing rhythm such as ventricular fibrillation or asystole. The patient with a bradycardic pulse does not require pharmacologic manipulation of heart rate. Chest wall compressions are difficult and require more force in the hypothermic patient due to decreased chest wall elasticity. Frequent change of personnel performing chest compressions is necessary to prevent fatigue and ineffectiveness.

Initial defibrillation should be attempted for ventricular fibrillation or ventricular tachycardia without a pulse, although attempts may be unsuccessful at temperatures below 30°C to 32°C (86°F to 90°F). If initial defibrillation is unsuccessful, rewarming is instituted and defibrillation is attempted again after every 1°C to 2°C increase in temperature or when core temperature rises above 30°C to 32°C. Antiarrhythmic and vasoactive drugs are usually ineffective at temperatures below 30°C (86°F). These drugs should be avoided until rewarming to above 30°C (86°F), and then the lowest effective dose should be used. Excessive administration of resuscitation drugs may result in toxicity with rewarming due to altered metabolism.

Most hypothermia-induced dysrhythmias will convert spontaneously with rewarming. Atrial fibrillation is seen frequently and does not usually require specific treatment because the ventricle generally responds slowly. The best approach to ventricular dysrhythmias has not been determined. Bretylium tosylate has been effective in animal studies but is not currently used in resuscitation.27,28Lidocaine appears to be less effective, and procainamide may increase the incidence of ventricular fibrillation. The effects of amiodarone in hypothermic patients have not been evaluated. The optimal dose and infusion rate of bretylium and other vasoactive drugs are unknown in severe hypothermia.

Hypotension should be treated initially with volume replacement. Vasopressor agents have minimal effect on constricted vessels in moderate or severe hypothermia but may increase the risk of dysrhythmias.

Rewarming Methods

Although rewarming is the primary treatment for moderate or severe hypothermia, controversy exists as to the optimal method, duration, and rate of rewarming. No controlled studies comparing rewarming methods exist, and rigid treatment protocols cannot be recommended. Rapid rates of rewarming have not proved to improve outcome in patients with severe hypothermia. The clinical circumstances, availability of resources, and advantages and disadvantages of available methods should be taken into account when selecting specific interventions for the patient.3,5–7,13,14,18,23,29

Passive External Rewarming

Passive external rewarming involves covering the patient with an insulating material to prevent further heat loss. This method is the treatment of choice for most patients with mild hypothermia and is used as an adjunct in patients with moderate or severe hypothermia. Patients must be able to generate heat to rewarm themselves. Rewarming rates with passive external rewarming in mild hypothermia vary between 0.5°C to 2.0°C per hour.4

Active Rewarming

Active rewarming involves the transfer of exogenous heat to the patient by using external or internal techniques. Indications for active rewarming include cardiovascular instability, poikilothermia (<32°C), inadequate rewarming with other methods, endocrinologic insufficiency, and traumatic peripheral vasodilation (i.e., spinal core injury). Patients with endocrinologic diseases may have insufficient glycogen stores or insufficient endogenous thermogenesis.

Active External Rewarming

Different methods have been used for active external rewarming (AER), including immersion in a 40°C bath, warming blankets, heating pads, radiant heat, and forced air rewarming. Indications for use of these devices remain controversial. Concerns have been raised regarding AER because vasodilation in the extremities may facilitate transport of colder peripheral blood to the warmer core, thereby lowering the core temperature (“afterdrop”). Peripheral vasodilation may also potentially worsen hypotension. Immersion in a warm water bath can impede monitoring and active resuscitation. Thermal injury can occur with heating pads, warming blankets, and radiant heat sources. The most practical technique for AER is forced air rewarming, which transfers heat convectively and prevents heat loss. These devices are usually readily available from postoperative care units. They enable greater contact of warm air with the patient's body than traditional warming blankets. Successful use of forced air as the primary rewarming method in patients with severe hypothermia with and without cardiac arrest has been reported.30 Additional forced air warming systems are being evaluated.31

Warming rates of 1°C to 2.5°C per hour have been reported with AER after accidental hypothermia without any evidence of afterdrop.30,32 Circulatory problems may be minimized if AER is applied only to the trunk. Truncal AER may be combined successfully with other methods of active core rewarming such as warmed intravenous fluids and heated humidified oxygen. The advantages of AER are ease of institution, availability, low cost, and noninvasiveness.

Active Core Rewarming

Numerous alternatives are available for active core rewarming. Airway rewarming using heated humidified oxygen is relatively simple to institute and should be a part of management of most patients with moderate or severe hypothermia.33 The delivery of heated oxygen is more effective through an endotracheal tube than by mask. Oxygen should be warmed to 40°C to 45°C (104°F to 113°F) through modification of humidifier devices.34 A rewarming rate of 1°C to 2.5°C per hour can be expected.5 Although there are several proposed advantages of airway rewarming that include decreased respiratory heat loss, increased heat donation to the respiratory tract, and direct heat transfer to the hypothalamus, brain stem, and medulla, its efficacy remains equivocal.35

Heated irrigation has been used to transfer heat from fluids to internal areas with a variety of techniques. Peritoneal dialysis or lavage is probably the most widely recognized method of heated irrigation. Peritoneal dialysis can deliver fluid heated to 40°C to 45°C (104°F to 113°F) to the peritoneal cavity with flow rates of approximately 6 L/h. Potential advantages of this technique are hepatic rewarming, use during chest compressions, and the capability to simultaneously provide renal replacement when a dialysate is used. Rewarming rates average 1°C to 3°C per hour.4 This technique is not routinely advocated for stable patients but may be considered in combination with other rewarming methods for the patient with no evidence of perfusion.

Closed thoracic lavage also has been proposed for treatment of hypothermia.36–38 This technique uses a large thoracostomy tube inserted into the anterior second or third intercostal space in the midclavicular line. A second tube is inserted in the posterior axillary line in the fifth or sixth intercostal space. Sterile normal saline heated to 40°C to 42°C (104°F to 107.6°F) is infused through the anterior tube and allowed to drain passively from the posterior tube. This technique may have the advantage of warming the heart and great vessels. However, clinical experience is limited. Mediastinal irrigation and myocardial lavage could be considered in patients with severe hypothermia and no spontaneous perfusion. These techniques require expertise for thoracotomy and clinical experience is limited.

Irrigation of the stomach, bladder, or colon has limited utility because the surface area available for heat transfer is small. In addition, gastric lavage may predispose to aspiration and must be discontinued during chest compressions. Special double lumen esophageal tubes or modified Sengstaken tubes have had limited evaluation and use.39,40 These techniques are warranted only if no other methods are available for rewarming.

Four methods are available for extracorporeal blood rewarming. These include hemodialysis, arteriovenous rewarming, venovenous rewarming, and cardiopulmonary bypass (CPB). Hemodialysis uses a two-way flow catheter with percutaneous cannulation of a single vessel. The femoral vein is preferred over the subclavian vein to avoid myocardial irritation with the guidewire. This technique may be most appropriate in the patient without severe hemodynamic instability, although it has been used in unstable patients.41,42 Hemodialysis may be the preferred rewarming method when hypothermia is associated with severe renal dysfunction or intoxication with dialyzable substances. Relatively new alternatives to hemodialysis are arteriovenous and venovenous rewarming.43 These techniques use countercurrent fluid warming. Arteriovenous techniques depend on the patient's blood pressure to support flow, whereas venovenous techniques use a roller pump.

CPB using standard access through the femoral artery or femoral vein is the most invasive and labor-intensive technique for rewarming.44 It has the advantages of providing complete hemodynamic support during rewarming and rapid rewarming rates.15 Core temperature can increase 1°C to 2°C every 3 to 5 minutes with femoral flow rates of 2 to 3 L/min.4 Unfortunately, CPB may require considerable time to institute and is not available in all institutions. Systemic anticoagulation may be contraindicated in trauma victims or contribute to hypothermic coagulopathy. Heparin-bonded tubing, portable circuits, and methods using venovenous access may overcome some of these problems.45,46 These advances have allowed the institution of CPB in emergency departments and intensive care units.47 Long-term outcomes of patients with severe hypothermia treated with CPB have been favorable.48

Future Techniques

Techniques such as the use of very high-temperature intravenous fluids and diathermy are being explored for the treatment of moderate or severe hypothermia. Intravenous fluids heated to 65°C (149°F) have been used in animal studies and resulted in rewarming rates of 2.9°C to 3.7°C per hour with minimal intimal injuries.49,50

Diathermy involves the conversion of energy waves into heat. Ultrasound or low-frequency microwave radiation can deliver large amounts of heat to deep tissues. Although animal studies are promising, further investigation is needed to determine optimum clinical use.4,35

Supportive Care

Numerous complications are associated with moderate and severe hypothermia and should be anticipated by the clinician. Continuous monitoring and frequent reassessment of metabolic and hemodynamic parameters are essential to successful outcome. Rhabdomyolysis is frequent and may result in electrolyte disturbances and renal dysfunction. Compartment syndromes may become apparent several days after initial treatment. Acute respiratory distress syndrome, acute tubular necrosis, and disseminated intravascular coagulation may require intensive interventions.

The prognosis for hypothermia is improved by prompt recognition of the clinical presentation and institution of appropriate management strategies.51 The risk of death from hypothermia is related to age, preexisting illnesses, nutritional status, and alcohol and drug intoxication. Outcome from severe hypothermia is difficult to predict. Mortality rates vary in several cohorts between 40% and 75%.18 Clearly, resuscitation will be successful only if cardiac arrest is due to hypothermia and not a consequence of anoxia or other injuries. Underlying diseases, precipitating events, and duration and severity of exposure are important factors to be considered.52 Although the treatment dictum has been that “no one is dead until warm and dead,” clinical judgment is necessary for determining the most appropriate treatment and termination of efforts.18 Proposed indicators of prognosis on arrival include severe hyperkalemia (>10 mEq/L), which may indicate death before severe hypothermia, a venous pH below 6.5, and severe coagulopathy.33,53

Many episodes of severe hypothermia may be preventable by implementing public health strategies that include education programs targeting high-risk individuals such as the elderly and the homeless. Specific preventive measures include wearing adequate protective clothing, maintaining fluid and calorie intake, avoiding fatigue, ensuring heated shelter, and avoiding excessive alcohol consumption. Emergency departments and intensive care units should be prepared to manage victims with hypothermia especially in the winter months. Social services should be adequately staffed to provide counseling and shelter to homeless persons during periods of extreme cold. Family members, home health care workers, and social service providers should closely monitor the elderly and patients with medical conditions that may predispose to hypothermia.