hypothermia

see also:

• frostbite
• http://lifeinthefastlane.com/ccc/hypothermia/
• hyperthermia

• many animals are able to enter a hypothermic physiologic state of torpor or hibernation at times of starvation and this appears to be activated at least in mice, by avMLPA neurons in the hypothalamus¹⁾
• humans do not exhibit this phenomenon and when temperature homeostasis (endothermy) fails they are at risk of potentially lethal pathophysiologic hypothermia
• in addition to hypothermia, cold exposure can also cause:
  • exacerbation of joint pains in hands and feet in particular
  • cramps, especially in feet
  • cold urticaria
  • cold-induced bronchospasm
  • if prolonged, reduced nail and hair growth, dermatitis
  • frostbite
  • precipitation of flash acute pulmonary oedema (APO) in those with underlying cardiac failure (redistribution of blood volume away from cold skin to overload central circulation)
  • exacerbations of cold-related conditions such as:
    • Raynaud's phenomenon
    • cryoglobulinaemia - rare, mainly in those with underlying conditions
      • skin effects such as: palpable purpura, necrosis
      • arthralgias/arthritis: esp. MCP/PIP jts of hands and feet
      • renal injury
      • lung injury - cough, SOB
      • other organ injury
    • cold agglutinin disease - rare, agglutination of red blood cells in response to cold
    • cryofibrinogenemia or cryofibrinogenemic disease, conditions which involve the cold-induced intravascular deposition of circulating native fibrinogens which precipitate at lower peripheral temperatures of around 4°C
• WHO recommends indoor temperatures be above 18degC
  • Australia has amongst the coldest indoor temperatures in Winter of any country in the world, in part due to not building them for efficient heating in winter and in part due to a stoic culture of a tendency to under heat homes. A study showed that 85% of Australian homes are below 18degC in winter and that 5% are below 11degC in Victoria in winter. ²⁾
  • children with asthma have a proportion decline in lung function as bedroom temperatures fall
  • adults with COPD, especially those who smoke also have a decline in lung function with falling temperature
  • falling temperature also raises BP by 0.5mmHg for each degree it falls below 18degC in those over 60yrs - and much more in those with limited cardiac reserve who are at risk of flash APO

• predisposing factors for hypothermia
  • decreased heat production
    • inactivity
    • endocrine disorders
  • increased heat loss
    • environmental - cold weather, wet, wind chill, etc
    • erythrodermas:
      • sunburn, burns, psoriasis, etc
    • iatrogenic
      • emergency birth
      • cold infusions
      • heat illness treatment
  • impaired thermoregulation
    • trauma
    • acute spinal cord injury
    • neuropathies
    • CNS pathology (eg. stroke, SAH, MS, Parkinsons)
    • pharmacologic / drug overdose
  • miscellaneous
    • sepsis
    • renal failure
    • neoplasia / cancer / tumours

Exposure time for core temperature to fall to 35.5°C

NB. times are half that (except for neck) if it is raining; those with less body fat or higher surface area may have less times;

• Cerebral metabolism decreases by 6% for every 1 °C fall in core temperature
• Moderate or severe hypothermia is often associated with volume depletion, and increased blood viscosity may make patients susceptible to thromboembolism