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- hypothermia = core temperature of 35^o C or less
- mild hypothermia 32-35^o C
- moderate 28-32^o C
- severe <28^o C

Induced hypothermia. Body temperature has been deliberately lowered as part of a therapeutic regime
Accidental hypothermia
Primary - thermoregulation normal but cold stress overwhelming (eg immersion)

Secondary - thermoregulation impaired so that mild/moderate cold exposure leads to hypothermia. Underlying causes include:
- endocrine:

• hypothyroidism
• hypopituitarism
• diabetic ketoacidosis
• hyperosmolar non-ketotic diabetic coma
• hypoglycaemia
• hypoadrenalism

- CNS:

• CVA
• head injury
• brain tumours
• Wernicke's
• Parkinsonism
• mental illness
• hypothalamic lesions
• cord transection

- Infections:

• bronchopneumonia
• systemic sepsis

- Circulatory:

• myocardial infarction
• pulmonary embolus

- Drugs:

• phenothiazines
• hypnotics
• tranquillizers
• anti-depressants
• alcohol

- Skin lesions:

• burns
• exfoliative dermatitis

- Miscellaneous:

• major trauma
• debility
• steatorrhoea
• extensive Paget's disease

- grey colour due to combination of pallor and cyanosis
- skin cold to touch even in areas not usually exposed to cold
- +/- puffy face

- mild hypothermia associated with increased cardiac output, peripheral vasoconstriction and tachycardia due to sympathetic stimulation
- progressive CVS depression with increasing hypothermia and hence decreasing tissue perfusion
- rhythm disturbance: bradycardia and increasing degrees of heart block with increasing severity of hypothermia. 1st degree HB common <33^oC, complete heart block may been seen at 20^oC. AF common <34^o, VF <28^oC and asystole <20^o. Fibrillation can occur earlier in presence of diseased or ischaemic myocardium, or in association with stimuli such as CV line insertion or intubation
- +/- J wave (pathognomonic deflection at the junction of the QRS complex and ST segment segment) and T inversion <33^o

- hypotension associated with poor prognosis

- initial stimulation of respiration followed by progressive depression
- resp drive ceases at about 24^o
- depression of cough reflex increases risk of aspiration
- bronchorrhoea - may increase difficulty of airways management
- oxyHb dissociation curve shifted to left with resultant decrease in oxygen delivery to tissues. Balanced by right shift due to acidosis
- hypoxaemia. NB values measured at 37o may be substantially higher than actual value. Correct by decreasing PaO₂ (measured at 37^o) by 7.2% for every degree that patient's temp is below 37^o
- signs of pneumonia may be masked
- basal crackles often present. May be due to alveolar injury
- although oxygen consumption falls, it is only at about 20° that hypothermia provides significant protection against hypoxic organ damage
- combination of decreased ventilation and tissue hypoxia leads to mixed acidosis

- progressive hyporeflexia
- shivering is replaced by muscular rigidity at about 33^o. Rigor mortis-like appearance at 24^o
- +/- involuntary flapping tremor
- generalized cerebral depression. Confusion may lead to illogical behaviour
- CBF decreases by 7%/^o C with corresponding decrease in CMRO₂
- EEG usually flat <20^o

- gastric dilatation common
- acute GU; may lead to GI bleed
- acute pancreatitis common but few of the typical signs are present
- pancreatic exocrine and endocrine function suppressed. <30o insulin secretion decreases and tissue insulin resistance increases. Hyperglycaemia is exacerbated by glycogenolysis and steroid secretion. Prolonged hypothermia may exhaust glycogen stores resulting in hypoglycaemia
- ileus common
- depressed liver function

- initial diuresis due to shunting from blood from peripheries in response to peripheral vasoconstriction
- followed by decreased RBF and GFR due to decreased cardiac output
- oliguria common and ATN may occur. May be due to combination of ischaemia and a direct effect of cold on the kidneys

- increased susceptibility to infection. Cause not clear

- may be difficult to differentiate from death, especially in the immersion victim. Death should not be assumed until resuscitation has failed in an adequately rewarmed patient (at least 35^o)

- increased PCV and Hb due to haemoconcentration
- leukocytosis common but may be leukopaenia and thrombocytopaenia due to splenic sequestration
- coagulopathy due to interference with intrinsic clotting cascade
- DIC may occur in severe cases

- moderate rise in urea
- electrolyte pattern depends on degree of renal and respiratory failure
- glucose raised. In absence of diabetes usually reverts to normal as patients temperature rises
- raised transaminases, CPK and HBD. Very high levels suggestive of hypothyroidism
- thyroid function tests
- amylase

CXR
ECG: J waves pathognomonic, rhythm disturbance

- CPR during hypothermia may induce VF
- if core temperature unknown or >28o and patient has apparently arrested start CPR
- if core temperature <28^o and ECG does not show asystole or VF avoid cardiac massage as this may precipitate VF
- DC shock may not be effective until temperature >30^o
- patients in respiratory failure should be ventilated with warm oxygen
- avoid lactated Ringer's solution as liver may not be able to metabolize lactate to bicarbonate
- use warm IV fluids
- relatively few drugs useful during severe hypothermia because of diminished effectiveness. Among inotropes dopamine known to be effective
- avoid epinephrine, isoproterenol, metaraminol because of their arrhythmogenicity
- atrial arrhythmias and heart block generally resolve spontaneously on rewarming

Mild to moderate hypothermia
- passive external rewarming. Lightly covered patient should be nursed in a room with an ambient temperature of 25-30^o. Allow core temperature to rise by 0.5^o/h. Patient must be stable and capable of generating heat
- active external rewarming. Involves the use of external heating devices. Suitable for alert, young, mildly/moderately hypothermic patients. Can also be used as adjunctive treatment in severely hypothermic patients. Associated hazards:

• "afterdrop". Peripheral vasodilatation may cause increased flow of cold blood from periphery to central circulation. This may trigger fatal arrhythmias
• "rewarming shock". Due to vasodilatation in a hypovolaemic patient. Can be mitigated by leaving limbs cool while warming trunk

Severe hypothermia: use active core rewarming

- airway rewarming. Heat and humidify inspired gas up to 40^o. Raises core temperature 1.5^o/h
- GI rewarming. Warm gastric lavage or oesophageal balloon can provide considerable heat. Risk of VF during tube placement, aspiration and gut perforation
- peritoneal lavage. Diasylate warmed to 40-42^o. Rewarming is rapid
- haemodialysis using a blood warmer may be useful in drug overdose cases
- cardiopulmonary bypass. Method of choice for profound hypothermia, especially in cases presenting with cardiac arrest. Associated risks of air embolism, haemolysis and heparinization

- do NOT give thyroid hormone unless there is strongly suggestive history or if there is laboratory evidence of hypothyroidism
- routine use of hydrocortisone is not indicated as levels of 11-hydroxycorticosterone are raised anyway and utilization of cortisol is impaired

© Charles Gomersall December 1999

©Charles Gomersall, May, 2008 unless otherwise stated. The author, editor and The Chinese University of Hong Kong take no responsibility for any adverse event resulting from the use of this webpage.
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