Hypothermia

Accidental hypothermia is defined as an involuntary drop in core temperature below 35 degrees

May be primary (environmental exposure) or secondary to due to impaired thermoregulation.

Modified Staging system (combination Swiss and classical systems)

Secondary hypothermia

• Increased heat loss
  • Burns
  • Iatrogenic (cold fluids)
  • Recent birth
• Impaired thermogenesis
  • Advanced or very young age
  • Malnutrition
  • Physical exhaustion
  • Neuromuscular disease
• Multifactorial
  • Alcohol, sedatives, narcotics, vasodilators
  • Alcohol or diabetic ketoacidosis, Addisonian, hypothyroidism, hypopituitarism, hypoglycaemia, Wernicke’s encephalopathy
  • Stroke, spinal cord injury
  • Sepsis (elderly, cachectic usually)
  • Shock
  • Trauma

Cold physiology

• Paradoxical undressing occurs in up to 30% of fatal hypothermia cases
• Below 29 degrees the pupils become dilated and fixed
• Below 23 degrees corneal reflexes are absent
• Cardiorespiratory
  • Profound peripheral vasoconstriction
  • Initial tachycardia and hypertension -> Progressive bradycardia, hypotension and myocardial irritability
  • Below 32 -> Malignant arrhythmias more common
  • AF and A Flutter are common and no necessarily markers of cardiac instability
  • ECG changes: Bradycardia, prolonged PR, QRS widening, prolonged QTc, shivering artefact and almost any heart block, atrial or ventricular arrhythmia
    • Classic Osborn J waves may also be due to raised intracranial pathology or sepsis
  • Respiratory changes include initial tachypnoea, progressive respiratory depression
• Metabolic
  • Cold diuresis
  • Rhabdomyolysis
  • Pseudo-rigor mortis
  • Coagulopathy can be profound but is often missed on labs due to warming of blood samples prior to analysis
  • Hypercoagulable also due to increased viscosity, haemoconcentration and an inflammatory cascade similar to DIC
  • Cellular oxygen requirements dramatically reduce with worsening hypothermia

Diagnosis

• Practical approach is to focus differential diagnosis using expected degree of dysfunction for clinical stage and measured core temperature -> If the temperature is not low enough to explain the physiological dysfunction, perhaps a secondary hypothermia at play
• Similarly, if the core temperature is well below what would be expected from environmental conditions and history, then secondary hypothermia is likely
• A clear history of normothermic arrest with subsequent hypothermia rules out hypothermia as the cause of arrest

Declaration of Death

• Hypothermia alone can cause dilated fixed pupils, rigor mortis, absent reflexes and respiratory arrest
• If obvious signs of irreversible death, frozen solid, history of normothermic arrest, persistent arrest despite rewarming to >32 or K >12mmol/L on arrival-> Can cease resuscitation efforts
• One exception to this is a child with simultaneous arrest and rapid cooling (e.g. pinned in icy creek) based on case series -> Prolonged resuscitation efforts would be suitable here

Hypothermia effects on laboratory values

• Most lab values will normalise with sample rewarming; therefore treat the patient, not the numbers

Treatment

• Minimally invasive rewarming
  • Hypothermia burrito
    • Outer wind and waterproof wrap
    • Insulation or heating pad
    • Replace wet clothes if practical, if not wrap in plastic
    • Forced air heating device
    • Insulating blanket
    • Insulate the head (unless in cardiac arrest)
  • Minimally invasive rewarming
    • Bladder lavage 40 degrees saline – 2-4L/hr by gravity
    • Warm IV fluids 40 degrees
• Avoid vasopressors early in resuscitation as profound vasoconstriction is physiological initially
• ECMO and cardiopulmonary bypass have largely replaced body cavity lavage techniques
  • Well-selected patients have a 50-100% survival rate compared to 10-30% in non-ECMO centres
  • Perhaps one of the more important steps here is recognising the patient who may arrest and getting them to an ECMO centre as soon as possible
• Warm humidified gases are recommended but contribute minimally to rewarming
• Treat acid-base disturbances by rewarming and reassessment
  • Bicarbonate should only be given for other specific indications i.e. specific toxidromes
  • Target an uncorrected PCO2 of 40mmHg rather than utilising any correction factors for the patient and blood gas analysis temperatures
• Priorities once vital signs lost:
  • Rewarming
  • Good CPR
  • Transfer to ECMO centre if appropriate
• Target temperature is >32 degrees Celcius
• Adrenaline and defibrillation
  • Guidelines differ
  • Tintinalli describes using standard protocols for 3 cycles and then defer further defibrillation and adrenaline until core temperature has increased significantly or the patients clinical status changes
  • ERC recommend up to 3 defibrillation attempts, to withold adrenalien until core temperature >30 and to doubel the dose frequency until temp >35
  • AHA states to use normal ALS algorithms
  • ARC does not mention this topic

Last Updated on August 28, 2023 by Andrew Crofton