Humans as homeothermic mammals must maintain a stable internal body temperature within narrow range to allow function of enzymes within this narrow range. Regional and seasonal variations in the environment mandate the ability to either loose 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 human kind 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.
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.
The normal core body temperature for humans is 37°C, with a circadian variation of approximately 0.5–1°C.5 The standard definition of hypothermia, developed for environmental hypothermia, defines mild hypothermia above 32°C, moderate from 28 to 32°C, and a core body temperature of less than 28°C as severe. The lowest reported temperature for an adult survivor of hypothermia is 13.7°C. In trauma patients with hypothermia, the scale is shifted due to significant changes in mortality, with temperatures of 34–36°C defined as mild, 32–34°C as moderate, and less than 32°C as severe.
Measurement of temperature should be performed using a reliable technique. The most readily available and accurate techniques include a bladder catheter with thermistor tip, tympanic thermometer (although many tympanic thermometers do not read below 34°C), or esophageal monitoring (not as widely available). Rectal temperature measurements are not as responsive to changes in core body temperature as the previously noted techniques. Oral and axillary temperature measurements are not reliable in hypothermic patients and should not be utilized.
Environmental hypothermia (also known ...