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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

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: 2020/08/01 22:15 by gary1