Hypothermia

Hypothermia as a Critical Environmental Emergency

Hypothermia is a life-threatening environmental emergency defined as a core body temperature below 35°C (95°F). It results from the body losing heat faster than it can produce it, overwhelming thermoregulatory mechanisms.^[1] On the Certified Emergency Nurse (CEN) exam, hypothermia is a high-yield topic because it requires rapid recognition, understanding of pathophysiological changes, and knowledge of rewarming strategies that differ by severity. Emergency nurses must be prepared to manage hypothermia in diverse settings, from wilderness accidents to trauma, drowning, and exposure in vulnerable populations (e.g., elderly, intoxicated patients).^[2]

Hypothermia Severity Stages and Related Phenomena

• Core temperature: The temperature of the body’s internal organs, distinct from peripheral (skin) temperature.
• Thermoregulation: The hypothalamus maintains core temperature via shivering, vasoconstriction, and behavioral changes.
• Afterdrop: Continued decline in core temperature after removal from cold, due to return of cold peripheral blood to the core.
• Rewarming shock: Hypotension from vasodilation and fluid shifts during rewarming.
• Severity stages:
  • Mild hypothermia: 32–35°C (89.6–95°F)
  • Moderate hypothermia: 28–32°C (82.4–89.6°F)
  • Severe hypothermia: <28°C (82.4°F)

Heat Loss Mechanisms and Compensatory Responses

Heat Loss Mechanisms

• Conduction: Direct contact with cold surfaces (e.g., wet ground).
• Convection: Heat loss to moving air or water (wind chill, cold water immersion).
• Radiation: Heat emitted from the body to cooler surroundings (primary loss in air).
• Evaporation: Sweat or wet clothing increases cooling.
• Respiration: Warm air exhaled, cold air inhaled.

Physiologic Response to Cold

1. Peripheral vasoconstriction: Redirects blood to core to preserve organ function.^[3]
2. Shivering: Involuntary muscle contractions generate heat (can increase metabolism 2–5x).
3. Cold diuresis: Vasoconstriction increases central blood volume, suppressing ADH and increasing urine output → volume depletion.
4. Progressive organ dysfunction: As temperature drops, enzyme systems slow, leading to bradycardia, decreased cardiac output, coagulopathy, and altered consciousness.

Clinical Presentation by Temperature Stage

Note: The “cold, dead” appearance can mimic death; always assume hypothermia is reversible until core temperature is corrected.^[4]

Diagnostic Approaches and Laboratory Indicators

• Core temperature measurement: Low-reading rectal thermometer or esophageal probe (preferred) is essential. Tympanic, oral, axillary methods are unreliable.^[5]
• ECG changes (especially in moderate/severe):
  • Osborn (J) wave: Deflection at the J point (QRS-ST junction), seen in leads II, V5, V6; pathognomonic but not always present.
  • Prolonged PR, QRS, QT intervals
  • Bradycardia, atrial fibrillation (common), ventricular fibrillation (at <28°C)
• Laboratory findings:
  • Hemoconcentration (from cold diuresis and fluid shift)
  • Hyperkalemia (cellular shift) or hypokalemia (diuresis)
  • Coagulopathy (PT/PTT prolonged due to enzyme dysfunction)
  • Metabolic acidosis (lactic acidosis from hypoperfusion)
  • Arterial blood gas: usually shows mixed acidosis; corrected for temperature (Alpha-stat vs. pH-stat – see exam tip below)

Rewarming Strategies and ACLS Adaptations

General Principles

• Handle patient gently: Rough movement can precipitate ventricular fibrillation (VF).^[6]
• Remove wet clothing and cover with warm blankets.
• Warm IV fluids (0.9% NS) to 40–42°C.
• Warm, humidified oxygen (42°C) via mask or ventilator.
• Rewarming method depends on severity.

Classification of Rewarming

1. Passive external rewarming: For mild hypothermia. Insulate with blankets, allow endogenous heat production. Do not rub or massage extremities.
2. Active external rewarming: For mild to moderate hypothermia. Apply forced-air warming devices (e.g., Bair Hugger), warm water packs (to axillae, groin, neck). Avoid direct heat to extremities (causes vasodilation → afterdrop).
3. Active internal (core) rewarming: For moderate to severe hypothermia. Modalities include:
   • Warmed IV fluids (0.9% NS)
   • Warmed, humidified oxygen
   • Peritoneal lavage with warmed dialysate (1.5% dextrose, 40–42°C)
   • Bladder irrigation with warmed saline
   • Extracorporeal rewarming: Cardiopulmonary bypass (CPB) or ECMO (most effective for severe hypothermia with cardiac arrest)^[7]

Advanced Cardiac Life Support (ACLS) Modifications

• If patient is in cardiac arrest or VF/VT, follow standard ACLS algorithms but with modifications:
  • Check pulse for up to 60 seconds before confirming cardiac arrest (bradycardia may be profound).
  • Defibrillation: For VF/VT, give one shock at maximum energy (200 J biphasic or 360 J monophasic). If no response, delay further shocks until core temperature >30°C.
  • Medications: Epinephrine and amiodarone may be ineffective at low temperatures. Space doses to longer intervals (e.g., epinephrine every 6–10 minutes). Once core >30°C, resume standard intervals.
  • Do not pronounce death until patient is rewarmed to >32–35°C and still no signs of life.^[8]

Potential Adverse Events and Preventive Measures

• Afterdrop: Prevent by using active core rewarming before peripheral vasodilation occurs.
• Rewarming shock: Aggressive volume resuscitation with warm fluids and vasopressors (if needed) after rewarming begins.
• Arrhythmias: VF is easily triggered by movement, central line placement, or endotracheal intubation; use gentle technique.
• Coagulopathy: Avoid invasive procedures until coagulopathy corrects with rewarming; use warmed blood products if hemorrhaging.
• Infection: Hypothermia suppresses immune response; monitor for pneumonia, sepsis.
• Hypoglycemia: Check glucose frequently; provide warm D50 if needed.

Test-Ready Insights for Hypothermia Questions

• Osborn wave is classic for CEN: Remember it as a positive deflection at the J point, often best seen in precordial leads V3–V6.
• Shivering stops at ~32°C: If a patient stops shivering and becomes confused, suspect moderate hypothermia.
• Paradoxical undressing: In moderate hypothermia, peripheral vasodilation from failed shivering can give a false sense of warmth; patient may remove clothing.
• “You’re not dead until you’re warm and dead”: Classic exam phrase – resuscitation efforts continue until core temp >35°C.
• ACLS modification summary:
  • Pulse check: 60 seconds
  • Defibrillation: Max shock, then warm before repeating
  • Medications: Space doses, avoid until >30°C if possible
  • No termination of resuscitation until rewarmed
• ABG temperature correction: Two schools – Alpha-stat (uncorrected, preferred for hypothermia by many clinicians) vs. pH-stat (corrected, used in deep hypothermic cardiac surgery). CEN usually follows Alpha-stat: do not correct PaO2/PaCO2/pH for temperature; interpret as standard. However, be aware that uncorrected values overestimate PaO2 and underestimate PaCO2. Exam questions typically follow Alpha-stat unless specified.
• Worst-case scenario: Cold-water drowning with hypothermia – prognosis can be good even after prolonged submersion (especially children). Full neurological recovery possible due to decreased metabolic demands.

References

1. Centers for Disease Control and Prevention. Hypothermia-Related Deaths — United States, 2003–2018. MMWR Morb Mortal Wkly Rep. 2020;69(4):99–104. https://doi.org/10.15585/mmwr.mm6904a1
2. Emergency Nurses Association. (2020). Sheehy's Emergency Nursing: Principles and Practice (7th ed.). Elsevier. Chapter 36: Environmental Emergencies. ISBN: 9780323461863.
3. Kurz A, Sessler DI, Christensen R, Dechert M. (1995). Heat balance and distribution during the core-temperature plateau in anesthetized humans. Anesthesiology. 83(3):491–499. https://doi.org/10.1097/00000542-199509000-00007
4. Brown DJ, Brugger H, Boyd J, Paal P. (2012). Accidental hypothermia. N Engl J Med. 367(20):1930–1938. https://doi.org/10.1056/NEJMra1114208
5. Zafren K. (2021). Hypothermia: Diagnosis and Management. In: Walls RM, Hockberger RS, Gausche-Hill M, eds. Rosen's Emergency Medicine (10th ed.). Elsevier. https://doi.org/10.1016/B978-0-323-75789-4.00185-9
6. Vanden Hoek TL, Morrison LJ, Shuster M, et al. (2010). Part 12: Cardiac Arrest in Special Situations: 2010 American Heart Association Guidelines for Cardiopulmonary Resuscitation and Emergency Cardiovascular Care. Circulation. 122(18 Suppl 3):S829–S861. https://doi.org/10.1161/CIRCULATIONAHA.110.971069
7. Paal P, Gordon L, Strapazzon G, et al. (2016). Accidental hypothermia–an update: the Scandinavian approach to treatment. Scand J Trauma Resusc Emerg Med. 24(1):111. https://doi.org/10.1186/s13049-016-0303-7
8. Soar J, Perkins GD, Abbas G, et al. (2010). European Resuscitation Council Guidelines for Resuscitation 2010 Section 8. Cardiac arrest in special circumstances: Electrolyte abnormalities, poisoning, drowning, accidental hypothermia, hyperthermia, asthma, anaphylaxis, cardiac surgery, trauma, pregnancy, electrocution. Resuscitation. 81(10):1400–1433. https://doi.org/10.1016/j.resuscitation.2010.08.015