Hypothermia

Hypothermia

This will focus on accidental hypothermia; there are numerous secondary causes that will not be the focus of this review. 

Although typically associated with regions of the world with severe winters, hypothermia is also seen in areas with warmer climates, as well as during summer months and in hospitalized patients.  Even with modern supportive care, mortality for patients with moderate to severe accidental hypothermia approaches 40%.

Body Temperature reflects the balance between heat production (cellular metabolism) and heat loss (evaporation, radiation, conduction & convection).  The normal human core temperature is 98.6±0.9^oF (37±0.5^oC).  The human body has limited physiological capacity to respond to cold environmental conditions (basically, peripheral vasoconstriction), thus our behavioral adaptations of clothing and shelter.  In response to a cold stress, the hypothalamus attempts to stimulate heat production through shivering and increased thyroid, catecholamine and adrenal activity.  There is also sympathetic mediated vasoconstriction of peripheral tissues. 

Hypothermia is defined as a core temperature below 95^oF (35^oC).

•          Mild: 93.2 – 95^oF (34-35^oC)
•           Moderate: 86 – 93.2^oF (30-34^oC)
•           Severe:  less than 86^oF (less than 30^oC)

Mild hypothermia is characterized by tachypnea, tachycardia, initial hyperventilation, ataxia, dysarthria, impaired judgment, shivering and “cold diuresis.”  “Cold Diuresis” is renal-fluid wasting due to hypothermia-induced vasoconstriction and diminished release of anti-diuretic hormone.

Moderate hypothermia is characterized by a proportionate reduction in pulse rate and cardiac output, hypoventilation, central nervous system depression, hyporeflexia, decrease renal blood flow, and the loss of shivering.  Patients may begin to display paradoxical undressing.  Atrial fibrillation, junctional bradycardia and other cardiac arrhythmias may also occur.

Patients with severe hypothermia develop pulmonary edema, oliguria, areflexia, hypotension, bradycardia, coma, ventricular arrhythmias and asystole.

Risk Factors for hypothermia include: Age (infants and the elderly), Environmental (exposure, drowning, and an alpine environment), Poverty, Homeless, Drugs / Toxicology and Psychiatric disorders.

To measure the temperature in a hypothermic patient requires a low-reading thermometer.  Most standard thermometers only read down to 93^oF (34^oC).  If the patient is conscious, a rectal probe thermometer is practical (although, to be truly accurate, needs to be inserted 15cm).  If the patient is intubated, an esophageal probe is preferred and is most accurate (inserted into the lower 1/3 of the esophagus). 

What studies to obtain?

•  Finger stick glucose!  Do not miss the patient who is hypothermic secondary to hypoglycemia.  Remember, if the patient does not have glucose, the body cannot generate heat to help rewarm itself.  Also, insulin release is decreased in hypothermia, so hyperglycemia is common.
• ECG
•   Hypothermia causes characteristic ECG changes because of slowed impulse conduction through potassium channels.  This results in prolongation of all the ECG intervals.  There also maybe elevation of the J point, producing the characteristic Osborn J Wave (from the distortion of the earliest phase of membrane repolarization).                            
• BMET
• Resuscitation is futile if K >10
• Check frequently during the resuscitation
• CBC
• There is 2% drop in Hct for each 1^oC drop in temp
• Thrombocytopenia is common
• Lactate – elevated from shivering and poor tissue perfusion
• PT/PTT – often coagulopathy is clinically evident but laboratory studies appear normal as the test is run at 98.6F (37C)
•  Fibrinogen
• Creatine phosphokinase
• Arterial blood gas – acidosis often present due to severe respiratory depression and CO₂ retention as well as lactic acid production
• CXR – pneumonia (aspiration) is a common complication
• Toxicology screen
• ETOH

 

Management

• ABCs
• There is an alteration in ACLS algorithm for patients with severe hypothermia (less than 86^oF (less than 30^oC)).  In patients with severe hypothermia, begin CPR and attempt defibrillation once.  Withhold typical ACLS medications and any further defibrillation attempts until the patient’s core temp is >86^oF (>30^oC).  These patients will require active internal rewarming (information below).
• Peripheral pulses may/will be difficult to assess, check a central pulse for up to a minute and consider using doppler.
• Establish two large bore (14 or 16 gauge) peripheral intravenous lines and start an infusion of warmed (100.4 – 107.6^oF (38 – 42^oC)) isotonic crystalloid.  This will only really prevent further heat loss!  If central venous access is needed, use the femoral approach, if possible (to avoid the guide wire irritating the right atria and causing an arrhythmia, which can occur with the internal jugular or subclavian approach).
• Treatment of cardiac arrhythmia.  Handle these patients with care!  Movement has been reported to trigger arrhythmias, including lethal ventricular fibrillation.  Remember, bradycardia is expected and pacing is not required unless the bradycardia persists despite rewarming to 90-95^oF (32-35^oC).
• Typical progression is sinus bradycardia to atrial fibrillation to ventricular fibrillation to asystole.
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• Rewarming Therapies

o   Passive External Rewarming (PER).  This is the treatment of choice for patients with mild hypothermia.  Removal all wet and cold clothing and then cover the patient in blankets or other types of insulation (aluminum foil).  Set room temp to 82^oF. PER requires the patient to have a physiologic reserve sufficient to generate heat by shivering and an increased metabolic rate.  If the patient’s temperature does not rise by 0.5-2^oC/hr., reconsider the diagnosis (are they septic, hypoglycemic, hypovolemic, endocrine source, etc.) and consider starting AER.

o   Active External Rewarming (AER).  AER is indicated for moderate to severe hypothermia and for a patient with mild hypothermia who is unstable, lacks physiologic reserve or fails PER.  AER is a combination of warmed blankets, heating pads (watch for body surface burns from decreased sensation and reduced blood flow), radiant heat, warm baths, or forced warm air, which are applied to the patients skin. 

§  Core Temperature After Drop is a risk during AER.  This occurs when the truck and extremities are warmed simultaneously.  Cold, academic blood that has pooled in the extremities returns to the core and can cause a drop in temperature and pH.  This can trigger cardiac dysrhythmias. 

§  Rewarming shock can occur when peripheral vasodilation and venous pooling results in relative hypovolemia and hypotension.

o   Active Internal Rewarming is indicated for severe hypothermia and those who fail to respond to AER. 

§  Airway rewarming is utilized by use humidified air at 40-45^oC. 

§  Pleural irrigation can be accomplished by placing two thoracotomy tubes (36 to 40 French), one placed anterior and one posterior, and instilling warmed IVF into the anterior chest tube and allowed to drain out the posterior chest tube.  If the patient is pulseless, use the left thoracic (bath the heart in the warm fluid), if the patient has a pulse, use the right thoracic to avoid triggering an arrhythmia by irritating the heart with tube insertion.

§  Bladder irrigation is another option

o   Extreme options (not readily available in many EDs): ECMO, hemodialysis, cardiopulmonary bypass

• Medications

o   Glucose, if the patient is hypoglycemic

o   Naloxone

o   Thiamine, as patients are often alcoholics (also, Wernicke’s)

o   Hydrocortisone, if the patient has a history of adrenal insufficiency

o   Antibiotics, for suspected sepsis

Remember, the patient is not dead until they are “warm and dead.”  But, how warm is warm?  Target a temperature of 89.6^oF (32^oC) in adults and a temperature of 95^oF (35^oC) in children. 

If the body is frozen and chest compressions are impossible; or if the nose and mouth are blocked by ice; or if the patient’s potassium is >10, then resuscitation can be withheld. 

References:

Tintinalli Sixth Edition

Accidental hypothermia in Adults. Up-To-Date

Circulation. 2005;112:IV-136-IV-138.

Brown, et al. Accidental Hypothermia. N Engl J Med 2012;367:1930-8.