Hypothermia and hyperthermia

Hypothermia

Hypothermia is defined as a core body temperature <35°C

Thermoregulation occurs below the interthreshold range.

37 +/- 0.2°C in normal adult (i.e. 0.4°C)
37 +/- 2°C in anaesthetised adult (i.e. 4°C)
below this temperature, cutaneous vasoconstriction occurs to minimise heat loss through the skin

Basal metabolic rate (BMR) increases below the thermoneutral zone (27-31°C in naked adult). This results in heat production through:

Behavioural responses also occur:

Normal thermoregulation is impaired below a core body temperature of 35°C.

System	Effects
Cardiovascular	ECG changes - long PR/QRS, J wave Arrhythmia - bradycardia, VF Cardiac output ↓ due to ↓HR/inotropy Vasoconstriction response is impaired in severe hypothermia Blood pressure may increase in early/mild hyopthermia due to vasoconstriction hypotension occurs in more severe hypothermia due to ↓cardiac output and impaired vasoconstriction response Organ perfusion is impaired caused by ↑ blood viscosity vasoconstriction ↓ cardiac output and blood pressure which causes any end organ ischaemia (e.g. myocardial) ↓ organ activity ↓ GI absorption ↓ hepatic clearance ↓ GFR
Respiratory	↓ RR and minute ventilation due to CNS depression Left-shift of oxy-Hb dissociation curve (↓ O2 offloading) → ↓oxygen supply to tissues
Neurological	Mild hypothermia: confusion and irritability Moderate-severe hypothermia: CNS depression and coma
Metabolic	↓ BMR as thermoregulation becomes impaired hyperglycaemia due to ↓ cellular uptake of glucose ↓ H+ and CO2 production due to less metabolic activity causes an alkalosis despite the changes to minute ventilation described above ↓ Enzyme activity → impaired drug metabolism → prolonged anaesthesia and neuromuscular blockade
Renal	Suppression of ADH → ‘cold diuresis’
Haematological	Platelet and clotting dysfunction

System	Effects
Cardiovascular	↑ HR and cardiac output cutaneous vasodilation → ↑SVR BP may not change, or may develop hypotension due to dehydration
Respiratory	↑ O2 consumption due to increased BMR ↑ RR and minute ventilation ↑ heat dissipation by convection and evaporation ↑ CO2 elimination in response to ↑ BMR right-shift of oxy-Hb dissociation curve (↑ O2 offloading)
Neurological	Confusion and irritability with mild hyperthermia Coma occurs at 42°C Loss of thermoregulation in severe hyperthermia
Metabolic	↓ BMR higher temperatures increase the rate of chemical reactions and hence cellular activity ↑ H+ and CO2 production due to less metabolic activity causes an alkalosis despite the changes to minute ventilation described above Sweating salt loss → hyponatraemia water loss → hypovolaemia and hypotension Enzyme activity some metabolic enzymes are enhanced by hyperthermia, others are inhibited
End organs	Parenchymal haemorrhage and degeneration - particularly in the brain
Haematological	Platelet and clotting dysfunction

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