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

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

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;

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

¹⁾