Chapter 49

As humans, we are homeothermic mammals who maintain our core temperature within a very narrow range, with a normal diurnal variation of about 1.5°C.1 Temperate climates present a range of temperatures that cause rapid loss of body heat to the environment, and three specific behaviors—fabrication of clothing, shelter-building, and control of fire—have allowed us to populate the earth. But hypothermia occurs when external heat loss exceeds the rate of endogenous thermogenesis. In the initial stages there are no irreversible changes as the metabolic rate slows about 50% for each 10°C drop in core temperature (Arrhenius’ law).2 Under rigidly controlled anesthesia, asystolic arrest occurs between 15–20°C, and a brief state of “suspended animation” can facilitate the completion of otherwise impossible surgical procedures in a relatively bloodless field.3

### Primary Accidental Hypothermia

Accidental nontraumatic hypothermia, with a core temperature below 35°C, occurs if a normal person is exposed to cool temperatures with inadequate clothing or shelter, and is most common during winter months.4 Initially there is an intense cutaneous vasoconstriction to reduce heat loss, and the exposed skin can rapidly cool to the ambient temperature. The patient experiences thermogenic shivering, which markedly increases oxygen consumption and depletes glycogen stores.5 As the temperature drops, metabolism progressively slows and shivering ceases. The patient becomes confused, lethargic, and cold to the touch. Urine production is profuse as the kidneys lose the ability to transport sodium and other ions. The patient exhibits bradycardia, hypotension, hypovolemia, and metabolic acidosis with elevated blood lactate. Agitation, irrational behavior, and combativeness are replaced by obtundation and finally coma. When cardiac arrest occurs, death is not immediate, but is inevitable without medical intervention.

Cooling is accelerated in windy conditions, but immersion in cold water will remove heat up to 25 times faster than air at the same temperature. Although exceptional athletes can swim for hours in cold water, an unconditioned person may become unconscious within 30 minutes of immersion in 4°C water. Children immersed in cold water quickly become hypothermic and comatose, but if the glottis closes and excludes water from the lungs, they may be resuscitated even if vital signs are undetectable. If a rescued child is given cardiopulmonary resuscitation (CPR) and rapidly rewarmed, survival without neurologic impairment is possible.6

For the responsive hypothermic patient with a core temperature above 34°C, passive rewarming measures with convective warming blankets and warm fluids are adequate.5 Severely hypothermic patients require warmed parenteral fluids to correct the cold-induced diuresis. Urine output will remain brisk during rewarming and is NOT an indicator of adequate intravascular volume. Subclavian or internal jugular catheter placement is avoided because the guide wire can easily trigger ventricular fibrillation of the cold myocardium. A Foley catheter with an integral temperature probe is optimal to monitor the core temperature. Comatose patients require endotracheal and nasogastric tubes to protect the airway and prevent aspiration. If any perfusing rhythm can be detected, ...

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