Hypothermia

Background: For the sake of brevity, this discussion includes only accidental hypothermia, not intentional hypothermia, which is used in certain surgeries or in the intensive care unit.

Accidental hypothermia is defined as an unintentional decline in core temperature below 35°C (95°F). Primary hypothermia occurs because of accidental exposure to cold. Secondary hypothermia occurs when a disease state causes failure of thermoregulatory function.

At temperatures below 35°C (95°F), the patient becomes less capable of generating heat, and body temperature continues to fall unless some action is taken. At a core temperature less than 30°C (86°F), the body assumes the temperature of the surrounding environment. While a history of cold exposure makes the diagnosis of hypothermia easy, hypothermia confounding other more obvious medical problems makes treating patients with hypothermia a challenge.

Pathophysiology: Humans are homeothermic endotherms, in that we maintain a uniform body temperature by internal generation of heat. A stable body temperature results from a balance between internal heat production and heat loss to the environment. The CNS processes input from peripheral and central thermal sensors and regulates body temperature by maintaining this balance. If the core temperature begins to fall, voluntary and involuntary muscle activity (movement and shivering, respectively) can increase basal heat production by 2-5 times. Adaptive behaviors also play a role in thermoregulation. Disease states that affect cognition or motor function predispose patients to hypothermia when these behaviors fail to occur.

Primary hypothermia results from failure of heat generation in the face of a cold environment. The difference between ambient and core temperature does not have to be great. Since most heat generation occurs through muscle activity, as long as the level of muscle activity required to keep up with heat loss is sustained, the core temperature is maintained. As long as the ambient temperature remains low, fatigue eventually occurs and muscle activity declines or ceases, and the core temperature falls. Physical conditioning, dehydration, and lack of caloric intake necessary to feed the required muscle activity are examples of factors that exacerbate the problem (this often occurs in recreational situations by accident, misfortune, or stupidity). Medical conditions such as strokes or other injuries may prevent muscle activity or behavioral responses to cold, causing or worsening hypothermia.

Secondary hypothermia results when a disease state interferes with thermoregulation and subsequent failure of any of the multiple pathways that maintain heat balance. Onset of hypothermia can be triggered by events similar to the generation of primary hypothermia, and primary and secondary hypothermia may exist concurrently. For example, when a patient has a stroke (disruption of thermoregulation due to CNS injury) and falls to a concrete floor (heat loss through conduction) and cannot get up (failure of behavioral adaptations to cold), hypothermia may ensue.

A high index of suspicion is necessary to accurately diagnose and treat secondary hypothermia since many causes are possible, and treatment is predicated on identification and correction of the underlying abnormality.

A physiologic heat balance is a result of many variables, including the ability to generate heat, body size, age, insulation in the form of clothing, and the temperature of the environment to which the patient is exposed. Heat always flows from a warmer object to a colder object. Under most circumstances the body is warmer than the surrounding environment; thus, the natural flow of heat is out of the body.

Heat is lost or gained through several physical mechanisms, including radiation, conduction, convections, and evaporation. Radiation may account for 55% of loss, evaporation 30%, and conduction 15%, with convection being a relatively minor component. Being wet or immersed in water causes more rapid heat loss because water is 25 times more conductive than air. Concrete or stone is approximately 100 times more conductive, and direct exposure to this material results in even greater heat loss than with exposure to water. The ground is somewhere between stone and water, depending on its exact composition.

In summary, the body generally loses heat to the environment, and a drop in core temperature is the natural consequence of failed thermoregulation for any reason. Because many disease states interfere with thermoregulation, the complex interactions necessary for balanced thermoregulation can and do fail, resulting in a hypothermic patient.

Frequency:

• In the US: In the United States, 646 hypothermia-related deaths were reported in 2002, with an annual death rate of 0.2 per 100,000 population. Hypothermia may account for even more deaths, especially in older persons, because hypothermia as a comorbid condition may not be recognized or reported. Homeless people and people with psychiatric disorders are often susceptible to hypothermia. In addition, a recent surge in cold weather recreation by novices has increased the incidence of accidental hypothermia.

Mortality/Morbidity:

• The mortality rate varies by location with a low of .49 persons per 100,000 population in southern states and a high of 4.64 persons per 100,000 population in Alaska (CDC, 2006).

Race:

• Hypothermia affects all racial groups.

Sex:

• Males and females are equally susceptible to excessive cold; among civilian populations, most persons who die from hypothermia are male (DeGroot, 2003).

Age:

• Approximately half of the deaths for which hypothermia was the underlying cause occurred in adults older than 65 years. This is not surprising, since the diseases that predispose patients to hypothermia are more common in this age group. Also, many elderly patients live in relative poverty and may not have adequate heating systems in their older housing or may not be able to afford rising energy costs. Subsequently, elderly patients can fall victim to accidental hypothermia during extremely cold weather without leaving their homes.

• Older persons with preexisting medical conditions such as congestive heart failure, diabetes, or gait disturbance also are at increased risk for hypothermia (CDC, 2006).

Treatment
Medical Care: Treatment begins in the prehospital environment, with removal of wet clothing, passive rewarming of the victim, and removal from the cold environment. Associated injuries are stabilized, and the patient should be transported as soon as possible. Rough handling of the patient may precipitate ventricular arrhythmias and should be avoided. An axiom in treatment is that a patient with hypothermia may appear dead; therefore, a patient is not considered dead until they are warm and dead.

Attention to basic life support (BLS) principles, with attention to airway, breathing, and circulation, is the cornerstone of prehospital care. Since patients may be breathing slowly, BLS providers should assess breathing over at least 35-45 seconds.

The temperature at which defibrillation attempts should be made has not been established, but, if ventricular fibrillation or ventricular tachycardia is present, defibrillation should be attempted but limited to one shock. If that does not convert the rhythm, efforts should be focused on rewarming the patient prior to repeat defibrillation. Automated external defibrillator (AED) defibrillation is recommended for prehospital use in hypothermia.

• The procedure to follow upon arrival at the emergency department is as follows:

• The patient's vital signs, temperature, and ECG should be monitored continuously.
• Cardiopulmonary resuscitation (CPR) is started if no pulse is present.
• Advanced cardiac life support (ACLS) management focuses on aggressive core rewarming.
• Advanced trauma life support (ATLS) protocols should be followed if trauma is a factor in the development of the hypothermia.
• ecause many arrhythmias convert spontaneously upon rewarming, aggressive therapy of minor arrhythmias is not warranted. Transient ventricular arrhythmias should be ignored. This also is true of bradycardia or atrial arrhythmias.

• Defibrillate at 2 J/kg (or the biphasic equivalent) if the patient is in ventricular fibrillation or ventricular tachycardia.
• Amiodarone is a good choice for treatment of ventricular fibrillation. Since no controlled studies have been conducted on amiodarone as of 2005, this recommendation is extrapolated from standard ACLS protocols and recommendations by critical care specialists.
• Success rates of defibrillation are low if the core temperature is less than 32°C.

• If defibrillation fails, repeat attempts should be made after every 1°C rise in body temperature.
• Intravenous drugs are often withheld if the core temperature is less than 30°C; intravenous drugs are given at longer intervals if the temperature is above 30°C.
• Drugs that normally are used in arrest situations (eg, lidocaine) have little effect if core temperature is less than 30°C and should not be used until core temperature is above this value. Procainamide may precipitate ventricular fibrillation and should be avoided.
• Accurate assessment of core temperature is important. The rectal temperature is the most accurate. Tympanic measurements are not reliable.
• Oxygen therapy is started until blood gas results are available.
• Indications for intubation are the same as in a normothermic patient. Because coagulopathies are common in patients with hypothermia, methods of intubation that pose the least risk of trauma are recommended. Intubation with direct visualization of the cords, performed by an experienced operator, is optimal. Because hypocapnia can increase ventricular irritability, avoiding overzealous ventilation is imperative.
• Nasogastric tube placement may be warranted to relieve gastric distention.
• A large bore IV line is placed. Hypothermic patients have vasodilatation on rewarming, so volume administration is recommended. An initial bolus of 250-500 cc of 5% dextrose in isotonic sodium chloride solution is indicated in most patients with a core temperature of less than 32°C. The clinical situation determines if more or less fluid should be used. Intravenous fluids should be heated to 40-42°C if possible. Rapid fluid resuscitation usually is necessary if the clinical scenario dictates because cardiovascular efficiency improves with crystalloid administration.
• Hypotension is treated by volume replacement and rewarming. Vasopressors should be avoided because they have little effect on vasoconstriction secondary to hypothermia and can precipitate ventricular fibrillation.
• Routine use of steroids, barbiturates, and antibiotics has not been shown to increase survival rates.

• The cornerstone of treatment is rewarming the patient. Significant controversy exists regarding the best method to rewarm victims, and, consequently, the choice is based on the degree of temperature depression and availability. The practitioner should use the fastest method that is available and appropriate for the patient. The mechanical details of each method are not discussed in this text. The reader should decide which methods are available or practical in his or her institution and learn that method well.

• Passive external warming: The patient is insulated from heat loss and allowed to generate heat by themselves. This method is useful for mild cases with no underlying disease. It is a slow method, generating only 0.5-2°C/h.
• Active external warming: External heat is applied to the patient's skin in a noninvasive manner. It is useful in milder cases. Because the vasoconstricted extremities hold pooled blood, warming of the extremities may result in a reversal of the vasoconstriction and may release incompletely rewarmed blood back to the central circulation. This return of relatively cold blood to a warmer core may cause temperature after-drop or arrhythmias. Nevertheless, in 1 study, this method was used successfully in 16 patients. Rewarming the trunk alone may minimize this problem.
  • Immersion in water bath at 40°C: Monitoring and resuscitation are difficult.
  • Radiant heat sources from the typical hospital are warmers often used in nurseries. Heated blankets and heating pads also may be used.
  • Forced-air rewarming is practical and can rewarm as fast as 2.4°C/h. This should be used in conjunction with warmed oxygen and warmed IV fluids. One study showed no complications with this method
• Active core rewarming: Numerous methods exist and are dependent on availability and operator competence. They include the following:
  • Heated infusions and heated inhalation: These should be used in all patients with hypothermia as an adjunct to other methods.
  • Heated gastric and colonic lavage: This method is limited by surface area and should be used as an adjunct to other methods. Kits are commercially available and are convenient. Regurgitation is common. CPR must be stopped during fluid installation.
  • Mediastinal lavage: The heart is bathed with isotonic sodium chloride solution, heated to 40°C, through a sternotomy or left thoracotomy incision. The procedure is invasive and should be used only if cardiopulmonary bypass is immediately available or the patient is in full cardiac arrest.
  • Closed thoracic lavage: A large bore thoracotomy tube is placed anteriorly. A drain tube is placed posteriorly. Sterile isotonic sodium chloride solution is infused and not recycled. This method could induce ventricular fibrillation if placed in the left hemithorax. It should be used only if cardiopulmonary bypass is immediately available.
  • Peritoneal lavage: This is available at most hospitals. A standard 1.5% dextrose dialysate is heated to 40-45°C. This method may help detoxification in drug overdose and rewarms the liver faster than other methods. Rewarming averages 1-3°C/h. It is not routinely advocated for stable patients.
• Extracorporal blood rewarming: All of these methods take time to set up and have varying availabilities at each hospital. They require considerable skill to perform.
  • Hemodialysis
  • Atriovenous rewarming
  • Venovenous rewarming
  • Cardiopulmonary bypass: This may require systemic anticoagulation and may be contraindicated in trauma patients. It can worsen coagulopathies. Newer technology may permit use without anticoagulation. This method is fast; core temperature may rise 1-2°C every 3-5 minutes.
• Diathermy: Heat is delivered ultrasonically by conversion of energy. It is contraindicated in patients with frostbite burns, significant edema, and implanted metallic objects.

Consultations:

• Consultation should be obtained early with a critical care specialist if possible and with a cardiothoracic surgeon if invasive rewarming methods are being considered.

Medication

Drug Category: Antiarrhythmic agents -- Indicated for ventricular fibrillation experienced in hypothermia.

Drug Name	Amiodarone (Cordarone) -- Use of this drug in hypothermia is not supported by research. May inhibit AV conduction and sinus node function. Prolongs action potential and refractory period in myocardium and inhibits adrenergic stimulation. Prior to administration, control the ventricular rate and CHF (if present) with digoxin or calcium channel blockers.
Adult Dose	Loading dose: 800-1600 mg/d PO in 1-2 doses for 1-3 wk; decrease to 600-800 mg/d in 1-2 doses for 1 mo Maintenance dose: 400 mg/d PO; alternatively, 150 mg (10 mL) IV over first 10 min, followed by 360 mg (200 mL) over next 6 h, and then 540 mg over next 18 h
Pediatric Dose	10-15 mg/kg/d or 600-800 mg/1.73 m2/d PO for 4-14 d or until adequate control of arrhythmia is attained
Contraindications	Documented hypersensitivity, complete AV block, and intraventricular conduction defects; patients taking ritonavir or sparfloxacin
Interactions	Increases effect and blood levels of theophylline, quinidine, procainamide, phenytoin, methotrexate, flecainide, digoxin, cyclosporine, beta-blockers, and anticoagulants; cardiotoxicity is increased by ritonavir, sparfloxacin, and disopyramide; coadministration with calcium channel blockers may cause an additive effect and further decrease myocardial contractility; cimetidine may increase levels
Pregnancy	C - Safety for use during pregnancy has not been established.
Precautions	Caution in thyroid or liver disease