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hypothermia

hypothermia

Introduction

  • 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
    • 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:
      • 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

Aetiology

  • 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

Hypothermia due to immersion in cold water

Exposure time for core temperature to fall to 35.5°C

Water temp ankle deep knee deep waist deep neck deep
10-12degC 7hrs 5hrs 1.5hrs 5 minutes
13-15degC 8hrs 7hrs 2hrs 5 minutes
15.5-18degC 9hrs 8hrs 3.5hrs 10 minutes
18-20.5degC 12hrs 12hrs 6hrs 10 minutes
>21deg C no limit no limit no limit 30 minutes

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;

Clinical features

  • 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
core temperature clinical features
35degC maximal shivering; increased BP
34degC amnesia, dysarthria, impaired judgment, behaviour change
33degC ataxia, apathy
32degC stupor
31degC shivering ceases; pupils dilate
30degC cardiac arrhythmias; decreased cardiac output
29degC unconsciousness
28degC VF is likely; hypoventilation
27degC loss of reflexes and voluntary motion
26degC acid-base disturbances; no response to pain
25degC reduced cerebral blood flow
24degC hypotension, bradycardia, pulmonary oedema
23degC no corneal reflexes, aphasia
19degC EEG silence
18degC asystole
hypothermia.txt · Last modified: 2024/06/11 12:07 by gary1

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