Hypothermia

Hypothermia can be defined as a drop in the core body temperature below 35°C (95°F) and is classified into mild, moderate, severe, and profound forms based on the degree of temperature decrease. Certain populations may be more vulnerable to accidental hypothermia, including extremes of age, homeless, mentally ill, and alcohol and drug abusers. Evaluation should include assessment for associated trauma and contributing medical conditions. Management involves rewarming the patient by different methods based on the severity of the hypothermia.

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Overview

Definition

Hypothermia is a decrease in core body temperature to below 35°C (95°F).

Epidemiology

  • More common in cold climates, but occurs throughout the world
  • In-hospital mortality for moderate and severe hypothermia is 40%.
  • Risk factors:
    • Homelessness
    • Alcohol intoxication
    • Substance abuse
    • Advanced age
    • Psychiatric disease

Etiology

  • Prolonged exposure to cold environmental temperatures:
    • Outdoor exposures
    • Cold water submersions
  • Predisposing factors include:
    • Decreased heat production:
      • Adrenal insufficiency
      • Hypothyroidism
      • Hypoglycemia
      • Immobility
      • Extremes of age (neonates, elderly)
    • Increased heat loss: 
      • Skin diseases (burns)
      • Ethanol (impaired responses to cold/shivering, vasodilation)
    • Impaired thermoregulation:
      • Central nervous system (CNS) pathology
      • Drugs (antidepressants, antipsychotics, sedatives)
      • Spinal cord injuries 
      • Polytrauma

Accidental hypothermia results from exposure to cold temperatures.

Image: “Skiing uphill” by Free-Photos. License: Pixabay License

Pathophysiology

Mechanisms of heat loss

  • Heat is lost mainly through the skin and lungs:
    • Evaporation
    • Radiation
    • Conduction
    • Convection
  • Most common in accidental hypothermia:
    • Convective loss to air
    • Conductive loss to water

Pathogenesis

  • Normal set point for human core temperature is 37°C ∓ 0.5°C (98.6°F).
  • Body uses autonomic mechanisms to conserve heat in response to a cold environment.
  • Hypothalamus stimulates heat production through:
    • Shivering:
      • ↑ Metabolism
      • ↑ Ventilation
      • ↑ Cardiac output
    • ↑ Thyroid hormone
    • ↑ Catecholamines
  • Peripheral vasoconstriction helps conserve heat.
  • Once the core temperature reaches 32°C (89.6°F), shivering becomes less effective.
  • Metabolism, ventilation, and cardiac output begin to decline.

Clinical Presentation

General manifestations

  • Cardiac (arrhythmias, bradycardia, asystole)
  • Vascular (peripheral vasoconstriction)
  • Neurologic (altered mental status, coma)
  • Hypovolemia (cold diuresis)
  • Respiratory depression

Stages of hypothermia

  • Mild hypothermia (32°C–35°C (89.6°F–95°F)):
    • Shivering is maximal.
    • Dysarthria and ataxia develop.
    • Patient is apathetic.
    • Tachycardia
    • Tachypnea
  • Moderate hypothermia (29°C–32°C (84.2°F–89.6°F)):
    • Shivering ceases.
    • Lethargic and stuporous, pupils dilate
    • Hyporeflexia
    • Bradycardia is universal and atrial dysrhythmias are common.
    • Respiratory depression begins.
  • Severe hypothermia (22°C–28°C (71.6°F–82.4°F)):
    • Coma develops, and reflexes and voluntary motion are absent.
    • Ventricular dysrhythmias (ventricular fibrillation)
    • Respiratory depression or apnea
    • Hypotension is expected.
    • Non-cardiogenic pulmonary edema
  • Profound hypothermia (< 22°C (< 71.6°F)):
    • All neurologic signs of life are absent.
    • Profound bradycardia/asystole and apnea are expected.

Diagnosis

Physical exam

  • Full body survey:
    • Assess for signs of trauma.
    • Assume the possibility of spinal cord injury until reliably ruled out.
    • Local temperature-related injuries (frostbite)
    • Patients need to be handled gently: Rough maneuvers may precipitate arrhythmias.
  • Temperature measurements:
    • Need to have a low-reading thermometer (below 34°C (93.2°F))
    • Esophageal probe: most reliable measurement of core temperature
    • Rectal or bladder probe: can be used in mild-to-moderate hypothermia

Laboratory studies

  • Healthy patients with mild accidental hypothermia may not require laboratory investigation. 
  • Patients with moderate or severe hypothermia may need the following studies:
    • Blood chemistry:
      • Glucose level for hypo- or hyperglycemia
      • Hyperkalemia or hypokalemia
      • Blood urea nitrogen (BUN) and creatinine may indicate renal injury.
      • Lactic acidosis
      • Creatine phosphokinase (CPK) (for possible rhabdomyolysis)
      • Electrolytes need to be monitored during rewarming.
    • Lipase for cold-induced ischemic pancreatitis
    • Complete blood count (CBC):
      • Hematocrit can be elevated from hypovolemia.
      • Low platelet count or white blood cells from splenic or liver sequestration
    • Coagulation profile for coagulopathy
    • Ethanol level
    • Arterial blood gas (ABG) for metabolic acidosis

Other studies

  • Chest X-ray (CXR) to evaluate for pulmonary edema
  • Electrocardiogram (ECG):
    • Bradycardia
    • Ventricular arrhythmias
    • Osborn waves (J point elevation):
      • Classic ECG finding in severe hypothermia
      • Deflection with a dome or hump configuration occurring at the R-ST junction
      • Often mistaken for ST-segment elevation myocardial infarction (STEMI)
      • No prognostic significance
      • Resolves with rewarming

Osborn waves (J point elevation) associated with hypothermia

Image: “Osborn waves” by the First Department of Internal Medicine, Nippon Medical School, Tokyo, Japan. License: CC BY 2.5, edited by Lecturio.

Management

Mild hypothermia (32°C–35°C (89.6°F–95°F))

  •  Passive external rewarming:
    • Remove wet clothes.
    • Cover the patient with an insulating material (blanket).
    • Bring to a warm environment.
  • More aggressive techniques may be considered if the rise in body temperature < 0.5°/hour.

Moderate hypothermia (29°C–32°C (84.2°F–89.6°F))

  • Passive external rewarming
  • Active external rewarming:
    • Use heated blankets/pads.
    • Warm baths
    • Forced air systems
    • Radiant heat lamps
  • The aim is to warm the torso > limbs to prevent the loss of the core temperature

Severe (22°C–28°C (71.6°F–82.4°F)) and profound (< 22°C (< 71.6°F)) hypothermia

  • Passive external rewarming
  • Active external rewarming
  • Active core rewarming:
    • Usage of warmed intravenous (IV) fluids
    • Warmed humidified oxygen
    • Bladder/gastric irrigation 
    • Peritoneal dialysis 
    • Closed thoracic lavage 
    • Extracorporeal blood rewarming

Failure to rewarm

  • Continue rewarming efforts.
  • Consider and address contributing factors:
    • Infection/sepsis (empiric IV antibiotics should be started if the body temperature rises < 0.67°/hour)
    • Adrenal insufficiency
    • Hypoglycemia
    • Hypothyroidism

Complications of rewarming

  • Hypotension (from peripheral vasodilation)
  • Electrolyte abnormalities
  • Arrhythmias
  • Rhabdomyolysis
  • Delayed pulmonary, renal, or neurologic complications

Cardiac arrest in hypothermic patients

  • “You’re not dead until you are warm and dead.”
  • Efforts should be continued if practical until body temperature is 32°C–35°C (89.6°F–95°F).
  • Airway/breathing:
    • Intubate as usual for coma or respiratory depression.
    • Provide warmed O2 via ventilator or bag-valve-mask device.
    • Pulse oximeter may not be able to detect a waveform.
  • Circulation:
    • Some bradycardia is physiologic in hypothermia.
    • Allow a full 60 seconds for a pulse check.
    • Handheld Doppler may be useful to verify the presence of a pulse.
    • If there is cardiac activity on the monitor, consider withholding chest compressions even if no pulse can be felt.
    • Look for cardiac motion on an ultrasound, if available, before beginning chest compressions.
  • Defibrillation:
    • Proceed as usual.
    • May not work until core temperature > 30°C (86°F)

Prognosis

  • Factors associated with death within 24 hours:
    • Prehospital cardiac arrest
    • Low or absent blood pressure
    • Need for endotracheal intubation
    • Elevated BUN
  • Hypothermia with asphyxia carries a worse prognosis:
    • Drowning
    • Avalanche burial
  • Hypothermia with cardiac arrest:
    • 50% neurologically intact survival if extracorporeal circulation is used
    • < 37% neurologically intact survival with other methods

Differential Diagnosis

  • Hypothyroidism: deficiency of T3 and T4 thyroid hormones. Clinical features of hypothyroidism are primarily due to the accumulation of matrix substances and a decreased metabolic rate. Severe hypothyroidism is associated with hypothermia secondary to decreased metabolic heat production.
  • Adrenal insufficiency: inadequate production of adrenocortical hormones (glucocorticoids, mineralocorticoids, and adrenal androgens). Primary adrenal insufficiency (Addison’s disease) is caused by diseases in the gland itself. Secondary adrenal insufficiency occurs due to decreased production of ACTH either from prolonged glucocorticoid therapy or disease in the pituitary/hypothalamic glands. Both scenarios put the patient at risk of developing hypothermia. 
  • Sepsis: bacteremia associated with signs of systemic toxicity and progression to multi-organ failure. Late sepsis can be associated with hypothermia. Vital signs (e.g., tachycardia) that are inconsistent with the degree of accidental hypothermia should raise the suspicion of an alternative diagnosis.

References

  1. Corneli, H. M., & Kadish, H. (2020). Hypothermia in children: Clinical manifestations and diagnosis. Retrieved January 2021, from https://www.uptodate.com/contents/hypothermia-in-children-clinical-manifestations-and-diagnosis
  2. Zafren, K., & Mechem, C.C. (2020). Accidental hypothermia in adults. Retrieved 15 January 2021, from https://www.uptodate.com/contents/accidental-hypothermia-in-adults

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