Pathophysiology
Clinical meaning
Hypothermia is defined as a core body temperature below 35 degrees Celsius (95 degrees Fahrenheit) and occurs when heat loss exceeds the body's capacity for heat production and conservation. The hypothalamus serves as the body's thermoregulatory center. When core temperature falls, compensatory responses include vasoconstriction, shivering thermogenesis, increased metabolic rate, and behavioral responses. Heat loss occurs through conduction, convection, radiation (largest source ~60%), and evaporation. Hypothermia is classified by severity: mild (32-35°C) where shivering is vigorous with tachycardia and confusion; moderate (28-32°C) where shivering ceases (dangerous sign), heart rate decreases, and consciousness declines; severe (<28°C) where the patient appears clinically dead with fixed dilated pupils, absent reflexes, and VF/asystole risk. The J-wave (Osborn wave) on ECG is pathognomonic. Treatment escalates from passive external rewarming (mild) to active external and core rewarming (moderate-severe). The critical nursing principle is 'no one is dead until they are warm and dead' — hypothermia provides neuroprotection allowing survival after prolonged cardiac arrest.