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Thermoregulation Heat loss occurs through five mechanisms: radiation (55–65% of heat loss), conduction (10–15% of heat loss, but much greater in cold water), convection (increased in the wind), respiration, and evaporation (which are affected by the ambient temperature and the relative humidity). The preoptic anterior hypothalamus normally orchestrates thermoregulation (Chap. 17). The immediate defense of thermoneutrality is via the autonomic nervous system, whereas delayed control is mediated by the endocrine system. Autonomic nervous system responses include the release of norepinephrine, increased muscle tone, and shivering, leading to thermogenesis and an increase in the basal metabolic rate. Cutaneous cold thermoreception causes direct. | Chapter 020. Hypothermia and Frostbite Part 2 Thermoregulation Heat loss occurs through five mechanisms radiation 55-65 of heat loss conduction 10-15 of heat loss but much greater in cold water convection increased in the wind respiration and evaporation which are affected by the ambient temperature and the relative humidity . The preoptic anterior hypothalamus normally orchestrates thermoregulation Chap. 17 . The immediate defense of thermoneutrality is via the autonomic nervous system whereas delayed control is mediated by the endocrine system. Autonomic nervous system responses include the release of norepinephrine increased muscle tone and shivering leading to thermogenesis and an increase in the basal metabolic rate. Cutaneous cold thermoreception causes direct reflex vasoconstriction to conserve heat. Prolonged exposure to cold also stimulates the thyroid axis leading to an increased metabolic rate. Clinical Presentation In most cases of hypothermia the history of exposure to environmental factors such as prolonged exposure to the outdoors without adequate clothing makes the diagnosis straightforward. In urban settings however the presentation is often more subtle and other disease processes toxin exposures or psychiatric diagnoses should be considered. After initial stimulation by hypothermia there is progressive depression of all organ systems. The timing of the appearance of these clinical manifestations varies widely Table 20-2 . Without knowing the core temperature it can be difficult to interpret other vital signs. For example a tachycardia disproportionate to the core temperature suggests secondary hypothermia resulting from hypoglycemia hypovolemia or a toxin overdose. Because carbon dioxide production declines progressively the respiratory rate should be low persistent hyperventilation suggests a central nervous system CNS lesion or one of the organic acidoses. A markedly depressed level of consciousness in a patient with mild hypothermia should .
