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KEY POINTS

KEY POINTS

  • The standard definitions of hypothermia are mild (>32°C), moderate (28–32°C), and severe (<28°C).

  • Warming techniques to reverse hypothermia include passive, active and noninvasive, and active and invasive.

  • Depressed platelet function, impaired platelet delivery, slowed activation of coagulation enzymes, activation of protein C, and activation of fibrinolysis occur in hypothermic patients.

  • Initial hospital management of frostbite should include rapid rewarming of the affected body part, in a 38°C to 40°C water bath.

  • Surgical debridement and amputation should be delayed until demarcation has occurred after a frostbite injury, unless the patient develops wet gangrene, overwhelming infection, or a necrotizing soft tissue infection.

  • The normal core body temperature is maintained at approximately 37°C by conduction, convection, evaporation, and radiation.

  • As heat exhaustion progresses, the cutaneous blood vessels paradoxically vasoconstrict and sweating stops in many, but not all, patients.

  • In patients with heat stroke, rapid cooling improves morbidity and mortality.

  • Cellular changes, including protein denaturation, begin to take place at approximately 41.6°C to 42.3°C.

  • Malignant hyperthermia is caused by exposure of susceptible individuals with a unique genetic composition to halogenated anesthetic agents.

INTRODUCTION

Humans as homeothermic mammals must maintain a stable internal body temperature within narrow range to allow function of enzymes. Regional and seasonal variations in the environment mandate the ability to either lose or generate heat in order to maintain temperature within this range. Mechanisms to either conserve heat (surface vasoconstriction, shivering, and piloerection) or lose heat (surface vasodilation, sweating with evaporation) allow adaptation to a colder or warmer environment. Three specific adaptive behaviors permit existence in a cold environment and have allowed the spread of humankind across six of the seven continents. These are fabrication of clothing, shelter-building, and control of fire. However, environmental extremes, abnormal endocrine function, or infection and injury can result in the inability of the organism to maintain body temperature within the normal range, leading to significant functional disturbances.

HYPOTHERMIA

It is important to understand the distinction between environmental hypothermia (due to exposure to cold) and hypothermia associated with injury. Hypothermia due to exposure can be lethal when it is severe and ongoing (consider Napoleon’s army during the retreat from Moscow1), but with modern medical care, hypothermia without injury has a significant survival rate even in the setting of cardiac arrest (50%).2 In patients after injury, however, the effects of hypothermia are profound, with hypothermia being an important component of the “bloody vicious cycle” first described by Kashuk and colleagues.3 In one large historical series, there were no survivors of hypothermia and serious trauma if initial core body temperature was less than 32°C.4 This distinction mandates a different approach for patients with hypothermia and injury.

Definitions

The normal core body temperature for humans is 37°C, with a circadian variation of approximately 0.5°C to 1°C.5 The standard definition of hypothermia, ...

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