Accidental hypothermia is defined as an unintentional decline in core temperature below 35°C (95°F).
•Primary hypothermia occurs because of accidental exposure to cold.
The difference between ambient and core temperature does not have to be great. Since most heat generation occurs through muscle activity, as long as the level of muscle activity required to keep up with heat loss is sustained, the core temperature is maintained. As the ambient temperature remains low, fatigue eventually occurs and muscle activity declines or ceases, and the core temperature falls. Physical conditioning, dehydration, and lack of caloric intake necessary to feed the required muscle activity are examples of factors that exacerbate the problem (this often occurs in recreational situations by accident, misfortune, or stupidity). Medical conditions such as strokes or other injuries may prevent muscle activity or behavioral responses to cold, causing or worsening hypothermia.
•Secondary hypothermia occurs when a disease state causes failure of thermoregulatory function. A high index of suspicion is necessary to accurately diagnose and treat secondary hypothermia since many causes are possible, and treatment is predicated on identification and correction of the underlying abnormality.
At temperatures < 35°C (95°F), the patient becomes less capable of generating heat, and body temperature continues to fall unless some action is taken.
At a core temperature < 30°C (86°F), the body assumes the temperature of the surrounding environment.
Heat is lost or gained through several physical mechanisms, including radiation, conduction, convections, and evaporation. Radiation may account for 55% of loss, evaporation 30%, and conduction 15%, with convection being a relatively minor component. Being wet or immersed in water causes more rapid heat loss.
Air < water (25 X) < ground < Concrete/stone ( 100 X)
A physiologic heat balance is a result of many variables, including the ability to generate heat, body size, age, insulation in the form of clothing, and the temperature of the environment to which the patient is exposed. Heat always flows from a warmer object to a colder object. Under most circumstances the body is warmer than the surrounding environment; thus, the natural flow of heat is out of the body.
3 mechanisms contributing(alone or together) to hypothermia
Reduction in heat production
Endocrine disorders
Myxoedema, Addison’s disease, hypopituitarism, diabetic ketoacidosis, hypoglycaemia, lactic acidosis
Severe infection
Septicaemia, pneumonia, peritonitis, pyelonephritis, meningitis
Malnutrition
Wernicke’s encephalopathy
Uraemia
Cirrhosis
Pancreatitis
Increased heat loss
Cardiovascular accident
Major surgery
Burns, exfoliative dermatitis
Trauma
Massive transfusion
Renal replacement therapy, dialysis
Drugs : ethanol, opiates, barbiturates, benzodiazepines, phenothiazines, tricyclics
Loss of Thermoregulation
Clinical
History
Because hypothermia may accompany a more obvious condition, considering hypothermia in the differential diagnosis in any patient is critical, especially those who present with unexplained symptoms. Remembering that symptoms of a primary condition, such as a stroke, may overshadow the symptoms of hypothermia is also important.
•Symptoms are vague
•Clinical manifestations
•Risk factors include recreational exposure to a cold environment
•Trauma
•Drug use or an overdose
•Inadequate clothing for ambient temperatures
•Hypothermia as a complication of underlying diseases
Physical
Mild hypothermia (32-35°C or 89.6-95°F) : In normal individuals shivering usually occurs when the core temperature is reduced by 0.7°C (increasing the metabolic rate by up to 5 times). When the temperature is reduced to 35°C the patient usually shivers uncontrollably. The reduction in temperature also produces dermal vasoconstriction, tachycardia, elevation of the cardiac output, elevation of plasma catecholamine levels, a ‘cold’ diuresis and hyper-glycaemia. If glycogen stores are depleted, hypo-glycaemia may occur which inhibits shivering. The plasma levels of thyrotropin releasing hormone (TRH), triiodothyronine (T3), L-thyroxine (T4), growth hormone, thyroid-stimulating hormone (TSH) and adrenocorticotropic hormone (ACTH) stimulation tests are normal, suggesting normal pituitary, adrenal and thyroid function during mild hypothermia. other features are Ataxia, Dysarthria, Loss of fine motor coordination, Lethargy, apathy, confusion, impaired judgment
Moderate hypothermia (28-32°C or 82.4-89.6°F) : At temperatures below 33°C, shivering gradually decreases, muscle and joints become stiff and there is a delayed relaxation phase of the stretch reflexes. The patient becomes lethargic, drowsy and often falls asleep. Unconsciousness rarely occurs at temperatures above 28°C, so another cause of coma should be sought if coma exists at levels above this temperature. The pulse, blood pressure and respiratory rate are usually depressed. J waves on ECG and Cold diuresis.
Severe hypothermia (<28°C or 82.4°F) : At a temperature below 30°C, the body loses its ability to spontaneously return to a normal temperature (i.e. the patient becomes poikilothermic); thus active rewarming must be performed. At a temperature below 28°C, the patient is unconscious, areflexic with fixed and dilated pupils, and life may be difficult to detect. Bradycardia and atrial fibrillation occur at temperatures below 30°C and ventricular fibrillation (VF) may occur at temperatures below 28°C. At 20°C asystole is more common than VF. Respiratory frequency maybe reduced to 1 - 2 breaths/min. Bronchorrhoea is common (probably due to deficient ciliary function), whereas pulmonary oedema is rare. Circulatory arrests for 10 min at 30°C, 25 min at 25°C, 45 min at 20°C and 60 min at 16°C are often quoted as being the limits at which cerebral function may return to normal, although case studies indicate that these limits can be extended.The lowest temperature recorded in a person who has subsequently survived is 16.4°C, although under controlled hypothermic conditions for surgical procedures, temperatures down to 10°C are used, and temperatures as low as 6°C have been recorded.
Onset of hypothermia can be triggered by events similar to the generation of primary hypothermia, and primary and secondary hypothermia may exist concurrently. For example, when a patient has a stroke (disruption of thermoregulation due to CNS injury) and falls to a concrete floor (heat loss through conduction) and cannot get up (failure of behavioral adaptations to cold), hypothermia may ensue.
